@article{kilinc_dreifuerst_kim_heath_2024, title={Beam Training in mmWave Vehicular Systems: Machine Learning for Decoupling Beam Selection}, ISBN={["979-8-3503-5186-6"]}, ISSN={["2375-8236"]}, DOI={10.1109/BLACKSEACOM61746.2024.10646235}, journal={2024 IEEE INTERNATIONAL BLACK SEA CONFERENCE ON COMMUNICATIONS AND NETWORKING, BLACKSEACOM 2024}, author={Kilinc, Ibrahim and Dreifuerst, Ryan M. and Kim, Junghoon and Heath, Robert W., Jr.}, year={2024}, pages={54–59} }
 @article{dreifuerst_kilinc_heath_2024, title={Context-aware Codebook Design for 6G Extreme MIMO Systems}, ISBN={["979-8-3503-9319-4"]}, ISSN={["1948-3244"]}, DOI={10.1109/SPAWC60668.2024.10694030}, journal={2024 IEEE 25TH INTERNATIONAL WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS, SPAWC 2024}, author={Dreifuerst, Ryan M. and Kilinc, Ibrahim and Heath, Robert W., Jr.}, year={2024}, pages={121–125} }
 @article{esfandiari_vorobyov_heath_2023, title={ADMM-based Solution for mm Wave UL Channel Estimation with One-bit ADCs via Sparsity Enforcing and Toeplitz Matrix Reconstruction}, DOI={10.1109/ICC45041.2023.10279217}, abstractNote={Low-power millimeter wave (mmWave) multi-input multi-output communication systems can be enabled with the use of one-bit analog-to-digital converters. Owing to the extreme quantization, conventional signal processing tasks such as channel estimation are challenging, making uplink (UL) multiuser receivers difficult to implement. To address this issue, we first reformulate the UL channel estimation problem, and then combine the idea of $\ell_{1}$ regularized logistic regression classification and Toeplitz matrix reconstruction in a properly designed optimization problem. Our new method is referred to as $\ell_{1}$ regularized logistic regression with Toeplitz matrix reconstruction (L1-RLR-TMR). In addition, we develop a computationally efficient alternating direction method of multi-pliers (ADMM)-based implementation for the L1-RLR-TMR method. Numerical results demonstrate the performance of the L1-RLR-TMR method in comparison with other existing methods.}, journal={ICC 2023 - IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS}, author={Esfandiari, Majdoddin and Vorobyov, Sergiy A. and Heath, Robert W., Jr.}, year={2023}, pages={1338–1343} }
 @article{akrout_shyianov_bellili_mezghani_heath_2023, title={Bandwidth Gain: the Missing Gain of Massive MIMO}, DOI={10.1109/ICC45041.2023.10278592}, abstractNote={We present a unified model for connected antenna arrays with a large number of tightly integrated (i.e., coupled) antennas in a compact space within the context of massive multiple-input multiple-output (MIMO) communication. We refer to this system as tightly-coupled massive MIMO. From an information-theoretic perspective, scaling the design of tightly-coupled massive MIMO systems in terms of the number of antennas, the operational bandwidth, and form factor was not addressed in prior art. We investigate this open research problem using a physically consistent modeling approach for far-field (FF) MIMO communication based on multi-port circuit theory. In doing so, we turn mutual coupling (MC) from a foe to a friend of MIMO systems design, thereby challenging a basic percept in antenna systems engineering that promotes MC mitigation/compensation. We show that tight MC widens the operational bandwidth of antenna arrays thereby unleashing a missing MIMO gain that we coin “bandwidth gain”. Furthermore, we derive analytically the asymptotically optimum spacing-to-antenna-size ratio by establishing a condition for tight coupling in the limit of large-size antenna arrays with quasi-continuous apertures. We also optimize the antenna array size while maximizing the achievable rate under fixed transmit power and inter-element spacing. Then, we study the impact of MC on the achievable rate of MIMO systems under line-of-sight (LoS) and Rayleigh fading channels. These results reveal new insights into the design of tightly-coupled massive antenna arrays as opposed to the widely-adopted “disconnected” designs that disregard MC by putting faith in the half-wavelength spacing rule.}, journal={ICC 2023 - IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS}, author={Akrout, Mohamed and Shyianov, Volodymyr and Bellili, Faouzi and Mezghani, Amine and Heath, Robert W.}, year={2023}, pages={5997–6003} }
 @article{lee_heath_2023, title={Call for Papers}, volume={61}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.2023.10298095}, number={10}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Lee, Namyoon and Heath, Robert W.}, year={2023}, month={Oct}, pages={69–69} }
 @article{zeulin_galinina_himayat_andreev_heath jr_2023, title={Dynamic Network-Assisted D2D-Aided Coded Distributed Learning}, volume={71}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2023.3259442}, abstractNote={Today, numerous machine learning (ML) applications offer continuous data processing and real-time data analytics at the edge of wireless networks. Distributed real-time ML solutions are highly susceptible to the so-called straggler effect caused by resource heterogeneity, which can be mitigated by various computation offloading mechanisms that severely impact communication efficiency, especially in large-scale scenarios. To reduce the communication overhead, we leverage device-to-device (D2D) connectivity, which enhances spectrum utilization and allows for efficient data exchange between proximate devices. In particular, we design a novel D2D-aided coded distributed learning method named D2D-CDL for efficient load balancing across devices. The proposed solution captures system dynamics, including data (time-varying learning model, irregular intensity of data arrivals), device (diverse computational resources and volume of training data), and deployment (different locations and D2D graph connectivity). To decrease the number of communication rounds, we derive an optimal compression rate, which minimizes the processing time. The resulting optimization problem provides suboptimal compression parameters that improve the total training time. Our proposed method is particularly beneficial for real-time collaborative applications, where users continuously generate training data.}, number={6}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Zeulin, Nikita and Galinina, Olga and Himayat, Nageen and Andreev, Sergey and Heath Jr, Robert W.}, year={2023}, month={Jun}, pages={3352–3367} }
 @article{castellanos_heath_2023, title={Electromagnetic-based signal processing for arbitrary arrays}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF59524.2023.10476827}, abstractNote={We recently proposed an electromagnetic manifold that characterizes arrays through the antenna current distributions and can be leveraged to model arbitrary antennas in MIMO systems. The manifold represents arrays with many Hertzian dipoles, yielding an analytically tractable representation that accounts for nonideal effects such as mutual coupling, polarization, near-field effects, and antenna patterns. We use this electromagnetic manifold to model a transceiver using arbitrary arrays. We highlight how array properties, such as coupling and polarization, affect signal processing at both the transmitter and receiver. We also demonstrate how the proposed model can be applied to diverse arrays with varied elements. Our simulation results in a realistic wireless environment show how the proposed transmission methods achieve higher spectral efficiency than approaches that assume ideal conditions.}, journal={FIFTY-SEVENTH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS & COMPUTERS, IEEECONF}, author={Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2023}, pages={1584–1588} }
 @article{drago_gurses_heath_sichitiu_zorzi_2023, title={End-to-end Full-Stack Drone Measurements: A Case Study Using AERPAW}, ISSN={["2164-7038"]}, url={http://dx.doi.org/10.1109/iccworkshops57953.2023.10283735}, DOI={10.1109/ICCWORKSHOPS57953.2023.10283735}, abstractNote={While $a$ lot of studies have been made to include drone communications in the 5th Generation of Mobile Networks (5G), it is still arguable how reliably current air-to-ground infrastructures can perform. To apply a further boost to this research direction, the National Science Foundation (NSF) recently funded the Aerial Experimentation Research Platform for Advanced Wireless (AERPAW) for the creation of a high-end publicly available testbed. Considering the current lack in the literature of experimental studies carried out with open testbeds, in this paper we target two contributions. First, we use AERPAW for the end-to-end evaluation of the performance of an emulated Uplink (UL) traffic between an Unmanned Aerial Vehicle (UAV) and a Fixed Node (FN), connected through an open source LTE network software (srsRAN). Second, in addition to providing a thorough analysis of the results obtained from our experiments, we made our testbed's configuration files and collected dataset available to the public, to provide a reference for future research on UAV communication, enabled by AERPAW.}, journal={2023 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS WORKSHOPS, ICC WORKSHOPS}, author={Drago, Matteo and Gurses, Anil and Heath, Robert W., Jr. and Sichitiu, Mihail L. and Zorzi, Michele}, year={2023}, pages={1422–1427} }
 @article{castellanos_carlson_heath_2023, title={Energy-efficient tri-hybrid precoding with dynamic metasurface antennas}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF59524.2023.10476911}, abstractNote={Dynamic metasurface antennas (DMA) are smart leaky-wave antennas that enable low-power beamforming, albeit with particular constraints. In this paper, we develop algorithms to configure a tri-hybrid architecture that combines digital, analog, and DMA precoding. Our optimization prioritizes low power consumption to determine the most energy-efficient configurations of precoding weights. Further, we integrate the unique characteristics of the DMA architecture, such as the limited beamforming weight distribution and waveguide propagation, to generate jointly optimized precoding weights for a realistic DMA design. The key metrics of analysis are spectral and energy efficiency results to determine tradeoffs in performance between increased data rates and power consumption.}, journal={FIFTY-SEVENTH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS & COMPUTERS, IEEECONF}, author={Castellanos, Miguel R. and Carlson, J. and Heath, Robert W., Jr.}, year={2023}, pages={1625–1630} }
 @article{wu_zhang_huang_heath_guo_2023, title={Green Joint Communications and Sensing Employing Analog Multi-Beam Antenna Arrays}, volume={61}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.007.2200495}, abstractNote={Joint communications and sensing (JCAS) is potentially a hallmark technology for the sixth generation mobile network (6G). Most existing JCAS designs are based on digital arrays, analog arrays with tunable phase shifters, or hybrid arrays, which are effective but are generally complicated to design and power inefficient. This article introduces the energy-efficient and easy-to-design multi-beam antenna arrays (MBAAs) for JCAS. Using pre-designed and fixed analog devices, such as lens or Butler matrix, MBAA can simultaneously steer multiple beams yet with negligible power consumption compared with other techniques. Moreover, MBAAs enable flexible beam synthesis, accurate angle-of-arrival estimation, and easy handling/utilization of the beam squint effect. All these features have not been well captured by the JACS community yet. To promote the awareness of them, we intuitively illustrate them and also exploit them for constructing a multi-beam JCAS framework. Finally, the challenges and opportunities are discussed to foster the development of green JCAS systems.}, number={7}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Wu, Kai and Zhang, J. Andrew and Huang, Xiaojing and Heath, Robert W. and Guo, Y. Jay}, year={2023}, month={Jul}, pages={172–178} }
 @article{kim_castellanos_heath_2023, title={Joint Relay Selection and Beam Management Based on Deep Reinforcement Learning for Millimeter Wave Vehicular Communication}, volume={72}, ISSN={["1939-9359"]}, DOI={10.1109/TVT.2023.3274763}, abstractNote={Cooperative relays improve reliability and coverage in wireless networks by providing multiple paths for data transmission. Relaying will play an essential role in vehicular networks at higher frequency bands, where mobility and frequent signal blockages cause link outages. To ensure connectivity in a relay-aided vehicular network, the relay selection policy should be designed to efficiently find unblocked relays. Inspired by recent advances in beam management in mobile millimeter wave (mmWave) networks, this article addresses the question: how can the best relay be selected with minimal overhead from beam management? In this regard, we formulate a sequential decision problem to jointly optimize relay selection and beam management. We propose a joint relay selection and beam management policy based on deep reinforcement learning (DRL) using the Markov property of beam indices and beam measurements. The proposed DRL-based algorithm learns time-varying thresholds that adapt to the dynamic channel conditions and traffic patterns. Numerical experiments demonstrate that the proposed algorithm outperforms baselines without prior channel knowledge. Moreover, the DRL-based algorithm can maintain high spectral efficiency under fast-varying channels.}, number={10}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, author={Kim, Dohyun and Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2023}, month={Oct}, pages={13067–13080} }
 @misc{dreifuerst_heath_2023, title={Massive MIMO in 5G: How Beamforming, Codebooks, and Feedback Enable Larger Arrays}, volume={61}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.001.2300064}, abstractNote={Massive multiple-input multiple-output (MIMO) is an important technology in fifth generation (5G) cellular networks and beyond. To help design the beamforming at the base station, 5G has introduced new support in the form of flexible feedback and configurable antenna array geometries that allow for arbitrarily massive physical arrays. In this article, we present an overview of MIMO throughout the mobile standards, highlight the new beam-based feedback system in 5G NR, and describe how this feedback system enables massive MIMO through beam management. Finally, we conclude with challenges related to massive MIMO in 5G.}, number={12}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Dreifuerst, Ryan M. and Heath, Robert W., Jr.}, year={2023}, month={Dec}, pages={18–23} }
 @article{zhang_heath jr_2023, title={Multi-Armed Bandit for Link Configuration in Millimeter-Wave Networks: An Approach for Solving Sequential Decision-Making Problems}, volume={2}, ISSN={["1556-6080"]}, DOI={10.1109/MVT.2023.3237940}, abstractNote={Establishing and maintaining millimeter-wave (mm-wave) links are challenging due to the changing environment and the high sensibility of mm-wave signals to user mobility and channel conditions. mm-Wave link configuration problems often involve a search for optimal system parameter(s) under environmental uncertainties from a finite set of alternatives that are supported by the system hardware and protocol. For example, beam sweeping aims at identifying the optimal beam(s) for data transmission from a discrete codebook. Selecting parameters such as the beam sweeping period and the beamwidth is crucial to achieving high overall system throughput. In this article, we motivate the use of the multi-armed bandit (MAB) framework to intelligently search out the optimal configuration when establishing the mm-wave links. MAB is a reinforcement learning framework that guides a decision maker to sequentially select one action from a set of actions. As an example, we show that within the MAB framework, the optimal beam sweeping period, beamwidth, and beam directions could be dynamically learned with sample-computational-efficient bandit algorithms. We conclude by highlighting some future research directions on enhancing mm-wave link configuration design with MAB.}, journal={IEEE VEHICULAR TECHNOLOGY MAGAZINE}, author={Zhang, Yi and Heath Jr, Robert W.}, year={2023}, month={Feb} }
 @article{mazher_mezghani_heath_2023, title={Multi-User Downlink Beamforming Using Uplink Downlink Duality With CEQs for Frequency Selective Channels}, volume={22}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2023.3273197}, abstractNote={High-resolution fully digital transceivers are infeasible at millimeter-wave (mmWave) due to their increased power consumption, cost, and hardware complexity. The use of low-resolution converters is one possible solution to realize fully digital architectures at mmWave. In this paper, we consider a setting in which a fully digital base station with constant envelope quantized (CEQ) digital-to-analog converters on each radio frequency chain communicates with multiple single antenna users with individual signal-to-quantization-plus-interference-plus-noise ratio (SQINR) constraints over frequency selective channels. We first establish uplink downlink duality for the system with CEQ hardware constraints and OFDM-based transmission considered in this paper. Based on the uplink downlink duality principle, we present a solution to the multi-user multi-carrier beamforming and power allocation problem that maximizes the minimum SQINR over all users and sub-carriers. We then present a per sub-carrier version of the originally proposed solution that decouples all sub-carriers of the OFDM waveform resulting in smaller sub-problems that can be solved in a parallel manner. Our numerical results based on 3GPP channel models generated from Quadriga demonstrate improvements in terms of ergodic sum rate and ergodic minimum rate over state-of-the-art linear solutions. We also show improved performance over non-linear solutions in terms of the coded bit error rate with the increased flexibility of assigning individual user SQINRs built into the proposed framework.}, number={12}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Mazher, Khurram Usman and Mezghani, Amine and Heath, Robert W.}, year={2023}, month={Dec}, pages={9688–9702} }
 @article{amir_castellanos_heath_2023, title={Multi-node joint communication and radar using synchronization signals in 5G}, DOI={10.1109/CAMSAP58249.2023.10403526}, abstractNote={Joint communication and radar (JCR) provide the mutual benefits of sensing and communication with shared hardware and spectral resources. We propose a JCR system with multiple nodes that leverages the 5G communication waveform for range estimation. We analyze a multi-node scenario where each node obtains an individual range estimate and then shares the estimate with other nodes to estimate each node's distance from the target jointly. We propose a linear minimum mean squared error (LMMSE) based estimate that combines the information from all nodes. The estimate can also be adapted to account for quantized feedback. The analysis and simulations for the model show that the proposed method outperforms the baseline models for both quantized and non-quantized methods. The distributed JCR results show that the transceivers in the system can detect the target more robustly, reducing the error in detection by 75%, as compared to the conventional approach.}, journal={2023 IEEE 9TH INTERNATIONAL WORKSHOP ON COMPUTATIONAL ADVANCES IN MULTI-SENSOR ADAPTIVE PROCESSING, CAMSAP}, author={Amir, Shehla and Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2023}, pages={161–165} }
 @article{cuvelier_tanaka_heath_2023, title={Online variable-length source coding for minimum bitrate LQG control}, ISSN={["2576-2370"]}, DOI={10.1109/CDC49753.2023.10383920}, abstractNote={We propose an adaptive coding approach to achieve linear-Quadratic-Gaussian (LQG) control with near-minimum bitrate prefix-free feedback. Our approach combines a recent analysis of a quantizer design for minimum rate LQG control with work on universal lossless source coding for sources on countable alphabets. It was recently demonstrated that the aforementioned quantizer's outputs are an asymptotically stationary, ergodic process. To enable LQG control with provably near-minimum bitrate, the quantizer outputs must be encoded into binary codewords efficiently. This is possible given knowledge of the quantizations' probability distributions, or of their limiting distribution. Obtaining such knowledge is challenging; the distributions do not readily admit closed form descriptions. This motivates the application of universal source coding. Our main theoretical contribution in this work is a proof that (after an invertible transformation), the quantizer outputs are random variables that fall within an exponential or power-law envelope class (depending on the plant dimension). Using ideas from universal coding on envelope classes, we develop a practical, zero-delay, fixed precision source code for the quantizer outputs. We evaluate the performance of this approach numerically, and demonstrate competitive results with respect to fundamental tradeoffs between bitrate and LQG control performance.}, journal={2023 62ND IEEE CONFERENCE ON DECISION AND CONTROL, CDC}, author={Cuvelier, Travis C. and Tanaka, Takashi and Heath, Robert W., Jr.}, year={2023}, pages={3917–3924} }
 @article{loukil_castellanos_bhuyan_heath jr_2023, title={Physical Layer Security at a Point-to-Point MIMO System With 1-Bit DACs and ADCs}, volume={12}, ISSN={["2162-2345"]}, DOI={10.1109/LWC.2023.3277718}, abstractNote={Low-resolution converters reduce the power consumption of wireless devices and help enable large energy-efficient arrays. In this letter, we evaluate the secrecy rate for the extreme case of 1-bit single-user multiple-input multiple-output systems following two signaling approaches. In the first method, the system uses discrete signaling. In the second, the system combines Gaussian signaling with artificial noise alongside power allocation. In the first case, we give a closed-form expression of the secrecy rate and propose an algorithm that provides near-optimal results for low signal-to-noise ratios (SNRs). In the second scenario, depending on the channel state information, we suggest different precoding plans and derive a secrecy rate lower bound. At low SNR, power is allocated to the main signal. Only at high SNR is optimal power allocation effective. Moreover, knowing the eavesdropper channel greatly improves the system’s secrecy.}, number={8}, journal={IEEE WIRELESS COMMUNICATIONS LETTERS}, author={Loukil, Mohamed Habib and Castellanos, Miguel R. and Bhuyan, Arupjyoti and Heath Jr, Robert W. W.}, year={2023}, month={Aug}, pages={1439–1443} }
 @article{jo_shim_chae_kim_heath_2024, title={Sparse RF Lens Antenna Array Design for AoA Estimation in Wideband Systems: Placement Optimization and Performance Analysis}, volume={23}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2023.3303462}, abstractNote={In this paper, we propose a novel architecture for a lens antenna array (LAA) designed to work with a small number of antennas and enable angle-of-arrival (AoA) estimation for advanced 5G vehicle-to-everything (V2X) use cases that demand wider bandwidths and higher data rates. We derive a received signal in terms of optical analysis to consider the variability of the focal region for different carrier frequencies in a wideband multi-carrier system. By taking full advantage of the beam squint effect for multiple pilot signals with different frequencies, we propose a novel reconfiguration of antenna array (RAA) for the sparse LAA and a max-energy antenna selection (MS) algorithm for the AoA estimation. In addition, this paper presents an analysis of the received power at the single antenna with the maximum energy and compares it to simulation results. In contrast to previous studies on LAA that assumed a large number of antennas, which can require high complexity and hardware costs, the proposed RAA with MS estimation algorithm is shown that meets the requirements of 5G V2X in a vehicular environment while utilizing limited RF hardware and has low complexity.}, number={4}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Jo, Joo-Hyun and Shim, Jae-Nam and Chae, Chan-Byoung and Kim, Dong Ku and Heath, Robert W.}, year={2024}, month={Apr}, pages={2869–2883} }
 @article{akrout_shyianov_bellili_mezghani_heath jr_2023, title={Super-Wideband Massive MIMO}, volume={41}, ISSN={["1558-0008"]}, DOI={10.1109/JSAC.2023.3288269}, abstractNote={We present a unified model for connected antenna arrays with a large number of tightly integrated (i.e., coupled) antennas in a compact space within the context of massive multiple-input multiple-output (MIMO) communication. We refer to this system as tightly-coupled massive MIMO. From an information-theoretic perspective, scaling the design of tightly-coupled massive MIMO systems in terms of the number of antennas, the operational bandwidth, and form factor was not addressed in prior art. We investigate this open research problem using a physically consistent modeling approach for far-field (FF) MIMO communication based on multi-port circuit theory. In doing so, we turn mutual coupling (MC) from a foe to a friend of MIMO systems design, thereby challenging a basic percept in antenna systems engineering that promotes MC mitigation/compensation. We show that tight MC widens the operational bandwidth of antenna arrays thereby unleashing a missing MIMO gain that we coin “bandwidth gain”. Furthermore, we derive analytically the asymptotically optimum spacing-to-antenna-size ratio by establishing a condition for tight coupling in the limit of large-size antenna arrays with quasi-continuous apertures. We also optimize the antenna array size while maximizing the achievable rate under fixed transmit power and inter-element spacing. Then, we study the impact of MC on the achievable rate of MIMO systems under line-of-sight (LoS) and Rayleigh fading channels. These results reveal new insights into the design of tightly-coupled massive antenna arrays as opposed to the widely-adopted “disconnected” designs that disregard MC by putting faith in the half-wavelength spacing rule.}, number={8}, journal={IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS}, author={Akrout, Mohamed and Shyianov, Volodymyr and Bellili, Faouzi and Mezghani, Amine and Heath Jr, Robert W. W.}, year={2023}, month={Aug}, pages={2414–2430} }
 @article{elbir_mishra_vorobyov_heath jr_2023, title={Twenty-Five Years of Advances in Beamforming: From convex and nonconvex optimization to learning techniques}, volume={40}, ISSN={["1558-0792"]}, DOI={10.1109/MSP.2023.3262366}, abstractNote={Beamforming is a signal processing technique to steer, shape, and focus an electromagnetic (EM) wave using an array of sensors toward a desired direction. It has been used in many engineering applications, such as radar, sonar, acoustics, astronomy, seismology, medical imaging, and communications. With the advent of multiantenna technologies in, say, radar and communication, there has been a great interest in designing beamformers by exploiting convex or nonconvex optimization methods. Recently, machine learning (ML) is also leveraged for obtaining attractive solutions to more complex beamforming scenarios. This article captures the evolution of beamforming in the last 25 years from convex to nonconvex optimization and optimization to learning approaches. It provides a glimpse into these important signal processing algorithms for a variety of transmit–receive architectures, propagation zones, propagation paths, and multidisciplinary applications.}, number={4}, journal={IEEE SIGNAL PROCESSING MAGAZINE}, author={Elbir, Ahmet M. and Mishra, Kumar Vijay and Vorobyov, Sergiy A. and Heath Jr, Robert W. W.}, year={2023}, month={Jun}, pages={118–131} }
 @article{cuvelier_tanaka_heath_2022, title={A Lower-Bound for Variable-Length Source Coding in Linear-Quadratic-Gaussian Control With Shared Randomness}, volume={6}, ISSN={["2475-1456"]}, DOI={10.1109/LCSYS.2022.3180402}, abstractNote={In this letter, we consider a Linear Quadratic Gaussian (LQG) control system where feedback occurs over a noiseless binary channel and derive lower bounds on the minimum communication cost (quantified via the channel bitrate) required to attain a given control performance. We assume that at every time step an encoder can convey a packet containing a variable number of bits over the channel to a decoder at the controller. Our system model provides for the possibility that the encoder and decoder have shared randomness, as is the case in systems using dithered quantizers. We define two extremal prefix-free requirements that may be imposed on the message packets; such constraints are useful in that they allow the decoder, and potentially other agents to uniquely identify the end of a transmission in an online fashion. We then derive a lower bound on the rate of prefix-free coding in terms of directed information; in particular we show that a previously known bound still holds in the case with shared randomness. We generalize the bound for when prefix constraints are relaxed, and conclude with a rate-distortion formulation.}, journal={IEEE CONTROL SYSTEMS LETTERS}, author={Cuvelier, Travis C. and Tanaka, Takashi and Heath, Robert W., Jr.}, year={2022}, pages={2918–2923} }
 @article{deshpande_castellanos_khosravirad_du_viswanathan_heath jr_2023, title={A Wideband Generalization of the Near-Field Region for Extremely Large Phased-Arrays}, volume={12}, ISSN={["2162-2345"]}, DOI={10.1109/LWC.2022.3233011}, abstractNote={The narrowband and far-field assumption in conventional wireless system design leads to a mismatch with the optimal beamforming required for wideband and near-field systems. This discrepancy is exacerbated for larger apertures and bandwidths. To characterize the behavior of near-field and wideband systems, we derive the beamforming gain expression achieved by a frequency-flat phased array designed for plane-wave propagation. To determine the far-field to near-field boundary for a wideband system, we propose a frequency-selective distance metric. The proposed far-field threshold increases for frequencies away from the center frequency. The analysis results in a fundamental upper bound on the product of the array aperture and the system bandwidth. We present numerical results to illustrate how the gain threshold affects the maximum usable bandwidth for the n260 and n261 5G NR bands.}, number={3}, journal={IEEE WIRELESS COMMUNICATIONS LETTERS}, author={Deshpande, Nitish and Castellanos, Miguel R. R. and Khosravirad, Saeed R. R. and Du, Jinfeng and Viswanathan, Harish and Heath Jr, Robert W. W.}, year={2023}, month={Mar}, pages={515–519} }
 @article{akrout_shyianov_bellili_mezghani_heath_2023, title={Achievable Rate of Near-Field Communications Based on Physically Consistent Models}, volume={22}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2022.3203963}, abstractNote={This paper introduces a novel information-theoretic approach for studying the effects of mutual coupling (MC), between the transmit and receive antennas, on the overall performance of single-input-single-output (SISO) near-field communications (NFC). By incorporating the finite antenna size constraint using Chu’s theory and under the assumption of canonical-minimum scattering (CMS), we derive the MC between two radiating volumes of fixed sizes. Expressions for the self and mutual impedances are obtained by the use of the reciprocity theorem. Based on a circuit-theoretic two-port model for SISO radio communication systems, we first establish its input-output relationship where the noise depends on the self/mutual impedances of the antennas, unlike the conventional assumption of independent additive white Gaussian noise. We then characterise the achievable data rate for a given pair of transmit and receive antenna sizes, thereby providing an upper bound on the system performance under physical size constraints. Through the lens of these findings, we shed new light on the influence of MC on the information-theoretic limits of near-field communications using compact antennas.}, number={2}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Akrout, Mohamed and Shyianov, Volodymyr and Bellili, Faouzi and Mezghani, Amine and Heath, Robert W. W.}, year={2023}, month={Feb}, pages={1266–1280} }
 @article{akrout_shyianov_bellili_mezghani_heath_2022, title={Achievable Rate of Near-Field Communications Based on Physically Consistent Models}, ISSN={["1550-3607"]}, DOI={10.1109/ICC45855.2022.9839044}, abstractNote={This paper introduces a novel information-theoretic approach for studying the effects of mutual coupling (MC), between the transmit and receive antennas, on the overall performance of single-input-single-output (SISO) near-field communications. By incorporating the finite antenna size constraint using Chu’s theory and under the assumption of canonical-minimum scattering, we derive the MC between two radiating volumes of fixed sizes. Expressions for the self and mutual impedances are obtained by the use of the reciprocity theorem. Based on a circuit-theoretic two-port model for SISO radio communication systems, we establish the achievable rate for a given pair of transmit and receive antenna sizes, thereby providing an upper bound on the system performance under physical size constraints. Through the lens of these findings, we shed new light on the influence of MC on the information-theoretic limits of near-field communications using compact antennas.}, journal={IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC 2022)}, author={Akrout, Mohamed and Shyianov, Volodymyr and Bellili, Faouzi and Mezghani, Amine and Heath, Robert W.}, year={2022}, pages={938–943} }
 @article{saab_abu surra_xu_heath_2022, title={An Inverted L-Shaped Multi-Layer Reconfigurable Intelligent Surface for THz Communications}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF56349.2022.10052030}, abstractNote={Realizing the gains of reconfigurable intelligent surfaces (RISs) for terahertz (THz) frequencies requires the practical design of reconfigurable surfaces. In this paper, we propose the design of a compact reconfigurable surface composed of meta-materials equipped with a voltage-controlled tunable dielectric substrate. We show that controlling the tunable dielectric substrate's dielectric properties is conducted by varying the voltage source at each unit cell. We show that the use of independent voltage sources at each unit surface introduces a linear and progressive phase variation to the reflected signal from the surface of the RIS. The reflection coefficient performance at each voltage level conveys the phase shifts for different dielectric constants. As a result, we derive the maximum beam-steering angle from the phase shifts to redirect the signal in the intended direction.}, journal={2022 56TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Saab, Sandy and Abu Surra, Shadi and Xu, Gary and Heath, Robert, Jr.}, year={2022}, pages={1060–1063} }
 @article{jeon_kim_hong_lee_heath_2022, title={Artificial Intelligence for Physical-Layer Design of MIMO Communications with One-Bit ADCs}, volume={60}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.007.2200002}, abstractNote={Wireless systems continue to go toward higher carrier frequencies, including terahertz bands, to take advantage of higher bandwidth channels. At the same time, antenna arrays remain important with continued increases in array elements. However, the power consumption of RF and digital circuits can increase proportionally to both the amount of signal bandwidth and the number of antennas. The use of one-bit analog-to-digital converters at the receiver is a cost- and power-efficient solution for wideband and/or massive antenna wireless systems. The nonlinearity of one-bit received signals brings challenges in physical-layer (PHY) design at the receiver. At the same time, the binary nature of these signals opens new opportunities for artificial intelligence (AI)-based PHY design. This article covers recent progress in incorporating AI into the design of classical PHY techniques and emerging studies on establishing AI-inspired frameworks that fundamentally replace classical model-driven techniques with data-driven AI techniques. It concludes with a discussion, including practical challenges and future research directions.}, number={7}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Jeon, Yo-Seb and Kim, Daeun and Hong, Song-Nam and Lee, Namyoon and Heath, Robert W., Jr.}, year={2022}, month={Jul}, pages={76–81} }
 @article{kocharlakota_vorobyov_heath_2022, title={Attention Neural Network for Downlink Cell-Free Massive MIMO Power Control}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF56349.2022.10051863}, abstractNote={The downlink power control is challenging in a cell-free massive multiple-input multiple-output (CFmMIMO) system because of the non-convexity of the problem. This paper proposes a computationally efficient deep-learning algorithm to solve the max-min power control optimization problem subject to power constraints. To solve this problem, it presents an attention neural network(ANN) composed using the masked multi-head attention network modules, which are building blocks of the popular transformer neural network. The ANN solves the downlink power control problem of CFmMIMO in the presence of pilot contamination (non-orthogonal pilot sequences). The paper first translates the constrained optimization problem to an unconstrained one parameterized by the weights of the ANN. These weights are trained in an unsupervised fashion. The performance of the ANN power control algorithm is demonstrated using numerical simulations. The paper also provides a computational complexity analysis of the method.}, journal={2022 56TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Kocharlakota, Atchutaram K. and Vorobyov, Sergiy A. and Heath, Robert W., Jr.}, year={2022}, pages={738–743} }
 @article{patel_myers_heath jr_2023, title={Circulant Shift-Based Beamforming for Secure Communication With Low-Resolution Phased Arrays}, volume={22}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2022.3210649}, abstractNote={Millimeter wave (mmWave) technology can achieve high-speed communication due to the large available spectrum. Furthermore, the use of directional beams in mmWave system provides a natural defense against physical layer security attacks. In practice, however, the beams are imperfect due to mmWave hardware limitations such as the low-resolution of the phase shifters. These imperfections in the beam pattern introduce an energy leakage that can be exploited by an eavesdropper. To defend against such eavesdropping attacks, we propose a directional modulation-based defense technique where the transmitter applies random circulant shifts of a beamformer. We show that the use of random circulant shifts together with appropriate phase adjustment induces (APN) in the directions different from that of the target receiver. Our method corrupts the phase at the eavesdropper without affecting the communication link of the target receiver. We also experimentally verify the APN induced due to circulant shifts, using channel measurements from a 2-bit mmWave phased array testbed. Using simulations, we study the performance of the proposed defense technique against a greedy eavesdropping strategy in a vehicle-to-infrastructure scenario. The proposed technique achieves better defense than the antenna subset modulation, without compromising on the communication link with the target receiver.}, number={4}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Patel, Kartik and Myers, Nitin Jonathan and Heath Jr, Robert W.}, year={2023}, month={Apr}, pages={2295–2310} }
 @article{olson_andrews_heath_2022, title={Coverage and Capacity of Terahertz Cellular Networks With Joint Transmission}, volume={21}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2022.3179963}, abstractNote={Beamforming with high dimensional antenna arrays provides the gain needed to enable high bandwidth communication in the sub-terahertz (THz) band. The resulting narrow beams, however, come at the cost of increased sensitivity to beam alignment errors. A potential remedy to this problem is to introduce a form a macrodiversity through non-coherent joint transmission (NC-JT). We employ a stochastic geometry framework to analyze the performance of a THz network employing user-centric base station clustering and NC-JT. We derive semi-closed form lower bounds for the coverage probability and ergodic capacity experienced by a typical user in the network. Our model includes THz networks without joint transmission as a special case. Overall, our analysis indicates that joint transmission from a few base stations improves coverage and capacity by mitigating the impact of beam misalignment. Moreover, in certain settings, NC-JT is a more efficient usage a network access points compared to the non-cooperative case, and reduces the sensitivity of coverage and capacity to the choice of user beamwidth.}, number={11}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Olson, Nicholas R. and Andrews, Jeffrey G. and Heath, Robert W., Jr.}, year={2022}, month={Nov}, pages={9865–9878} }
 @article{lopes_nahum_dreifuerst_batista_klautau_cardoso_heath jr_2022, title={Deep Reinforcement Learning-Based Scheduling for Multiband Massive MIMO}, volume={10}, ISSN={["2169-3536"]}, DOI={10.1109/ACCESS.2022.3224808}, abstractNote={Fifth-generation (5G) cellular communication systems have embraced massive multiple-input-multiple-output (MIMO) in the low- and mid-band frequencies. In a multiband system, the base station can serve different users in each band, while the user equipment can operate only in a single band simultaneously. This paper considers a massive MIMO system where channels are dynamically allocated in different frequency bands. We treat multiband massive MIMO as a scheduling and resource allocation problem and propose deep reinforcement learning (DRL) agents to perform user scheduling. The DRL agents use buffer and channel information to compose their observation space, and the agent’s reward function maximizes the transmitted throughput and minimizes the packet loss rate. We compare the proposed DRL algorithms with traditional baselines, such as maximum throughput and proportional fairness. The results show that the DRL models outperformed baselines obtaining a 20% higher network sum rate and an 84% smaller packet loss rate. Moreover, we compare different DRL algorithms focusing on training time to assess the online implementation of the DRL agents, showing that the best agent needs about 50K training steps to converge.}, journal={IEEE ACCESS}, author={Lopes, Victor Hugo L. and Nahum, Cleverson Veloso and Dreifuerst, Ryan M. and Batista, Pedro and Klautau, Aldebaro and Cardoso, Kleber Vieira and Heath Jr, Robert W.}, year={2022}, pages={125509–125525} }
 @article{alammouri_mo_ratnam_ng_heath_lee_zhang_2022, title={Extending Uplink Coverage of mmWave and Terahertz Systems Through Joint Phase-Time Arrays}, volume={10}, ISSN={["2169-3536"]}, DOI={10.1109/ACCESS.2022.3200334}, abstractNote={In this paper, we establish the potential of joint phase-time arrays (JPTAs) for uplink coverage extension in cellular systems. JPTA allows the base station (BS) to create frequency-dependent wide-beams without sacrificing the array gain. In this work, we propose a novel use case of JPTA where the BS exploits the frequency-dependent beams to serve multiple users’ equipment (UEs) at different directions simultaneously with the full array gain and with no inter-UE interference. This is achieved by assigning each UE a corresponding bundle of sub-carriers that benefit from the full array-gain. A key feature of this scheme is the prolonged channel access for uplink communication for each user, due to the BS’s ability to serve multiple UEs at the same time. We focus on two performance metrics: uplink coverage and uplink throughput. Our results show that using JPTA can extend the uplink coverage range by $3\times $ while boosting the uplink throughput by providing more flexibility for the BS in resource allocation. These results are based on both theoretical analysis and 3GPP spec-compliant simulations with a sub-terahertz transceiver prototype.}, journal={IEEE ACCESS}, author={Alammouri, Ahmad and Mo, Jianhua and Ratnam, Vishnu V. and Ng, Boon Loong and Heath, Robert W. and Lee, Juho and Zhang, Jianzhong}, year={2022}, pages={88872–88884} }
 @article{zhang_wu_huang_guo_zhang_heath_2022, title={Integration of Radar Sensing into Communications with Asynchronous Transceivers}, volume={60}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.003.2200096}, abstractNote={Clock asynchronism is a critical issue in integrating radar sensing into communication networks. It can cause ranging ambiguity and prevent coherent processing of discontinuous measurements in integration with asynchronous transceivers. Should it be resolved, sensing can be efficiently realized in communication networks, requiring few network infrastructure and hardware changes. This article provides a systematic overview of existing and potential new techniques for tackling this fundamental problem. We first review existing solutions, including using a finetuned global reference clock, and single-node-based and network-based techniques. We then examine open problems and research opportunities, offering insights into what may be better realized in each of the three solution areas.}, number={11}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Zhang, J. Andrew and Wu, Kai and Huang, Xiaojing and Guo, Y. Jay and Zhang, Daqing and Heath, Robert W.}, year={2022}, month={Nov}, pages={106–112} }
 @article{kim_castellanos_heath_2022, title={Joint Beam Management and Relay Selection Using Deep Reinforcement Learning for MmWave UAV Relay Networks}, ISSN={["2155-7578"]}, DOI={10.1109/MILCOM55135.2022.10017754}, abstractNote={Unmanned aerial vehicle (UAV) relays are useful in tactical millimeter wave (mmWave) networks to overcome blockages and improve link resilience. Getting the most benefits from relays, though, requires efficient strategies for relay selection and for beam management. In this paper, we use deep reinforcement learning (DRL) to jointly select unblocked UAV relays and to perform beam management. The proposed DRL-based algorithm uses rate feedback from the receiver to learn a policy that adapts to the dynamic channel conditions. We show with numerical evaluation that the proposed method outperforms baselines without prior channel knowledge. Moreover, the DRL-based algorithm can maintain high spectral efficiency even under frequent blockages.}, journal={2022 IEEE MILITARY COMMUNICATIONS CONFERENCE (MILCOM)}, author={Kim, Dohyun and Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2022} }
 @article{dreifuerst_heath_yazdan_2022, title={Massive MIMO Beam Management in Sub-6 GHz 5G NR}, DOI={10.1109/VTC2022-Spring54318.2022.9860458}, abstractNote={Beam codebooks are a new feature of massive multiple-input multiple-output (M-MIMO) in 5G new radio (NR). Codebooks comprised of beamforming vectors are used to transmit reference signals and obtain limited channel state information (CSI) from receivers via the codeword index. This enables large arrays that cannot otherwise obtain sufficient CSI. The performance, however, is limited by the codebook design. In this paper, we show that machine learning can be used to train site-specific codebooks for initial access. We design a neural network based on an autoencoder architecture that uses a beamspace observation in combination with RF environment characteristics to improve the synchronization signal (SS) burst codebook. We test our algorithm using a flexible dataset of channels generated from QuaDRiGa. The results show that our model outperforms the industry standard (DFT beams) and approaches the optimal performance (perfect CSI and singular value decomposition (SVD)-based beamforming), using only a few bits of feedback.}, journal={2022 IEEE 95TH VEHICULAR TECHNOLOGY CONFERENCE (VTC2022-SPRING)}, author={Dreifuerst, Ryan M. and Heath, Robert W., Jr. and Yazdan, Ali}, year={2022} }
 @article{mezghani_heath_2022, title={Massive MIMO Precoding and Spectral Shaping With Low Resolution Phase-Only DACs and Active Constellation Extension}, volume={21}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2021.3139034}, abstractNote={Nonlinear precoding and pulse shaping are jointly considered in multi-user massive multiple-input multiple-output (MIMO) systems with low resolution D/A-converters (DACs) in terms of algorithmic approach as well as large system performance. Two design criteria are investigated: the mean squared error (MSE) with active constellation extension (ACE) and the symbol error rate (SER). Both formulations are solved based on a modified version of the generalized approximate message passing (GAMP) algorithm. Furthermore, theoretical performance results are derived based on the state evolution analysis of the GAMP algorithm. The MSE based technique is extended to jointly perform over-the-air (OTA) spectral shaping and precoding for frequency-selective channels, in which the spectral performance is characterized at the transmitter and at the receiver. Simulation and analytical results demonstrate that the MSE based approach yields the same performance as the SER based formulation in terms of uncoded SER. The analytical results provide good performance predictions up to medium SNR. Substantial improvements in detection, as well as spectral performance, are obtained from the proposed combined pulse shaping and precoding approach compared to standard linear methods.}, number={7}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Mezghani, Amine and Heath, Robert W., Jr.}, year={2022}, month={Jul}, pages={5265–5278} }
 @article{mazher_mezghani_heath_2022, title={Multi-User Downlink Beamforming Using Uplink Downlink Duality With 1-Bit Converters for Flat Fading Channels}, volume={71}, ISSN={["1939-9359"]}, DOI={10.1109/TVT.2022.3196603}, abstractNote={The increased power consumption of high-resolution data converters at higher carrier frequencies and larger bandwidths is becoming a bottleneck for communication systems. In this paper, we consider a fully digital base station equipped with 1-bit analog-to-digital (in uplink) and digital-to-analog (in downlink) converters on each radio frequency chain. The base station communicates with multiple single antenna users with individual SINR constraints. We first establish the uplink downlink duality principle under 1-bit hardware constraints under an uncorrelated quantization noise assumption. We then present a linear solution to the multi-user downlink beamforming problem based on the uplink downlink duality principle. The proposed solution takes into account the hardware constraints and jointly optimizes the downlink beamformers and the power allocated to each user. Optimized dithering obtained by adding dummy users to the true system users ensures that the uncorrelated quantization noise assumption is true under realistic settings. Detailed simulations carried out using 3GPP channel models generated from Quadriga show that our proposed solution outperforms state of the art solutions in terms of the ergodic sum and minimum rate especially when the number of users is large. We also demonstrate that the proposed solution significantly reduces the performance gap from non-linear solutions in terms of the uncoded bit error rate at a fraction of the computational complexity.}, number={12}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, author={Mazher, Khurram Usman and Mezghani, Amine and Heath, Robert W.}, year={2022}, month={Dec}, pages={12885–12900} }
 @article{deshpande_castellanos_heath_2022, title={Nonuniform true time delay precoding in wideband MISO systems}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF56349.2022.10051948}, abstractNote={We focus on true time delay (TTD) precoding which is a reduced hardware complexity alternative to fully-digital frequency selective beamforming in wide band systems. It is challenging to select the delays for TTD beamforming architectures for a channel with multi-path and near-field effects because of the nonuniform delay variation across the array. We design the TTD beamformer by maximizing the total energy in the filtered channel response. Based on the idea of temporal focusing, we reformulate the TTD constraint by including a sparsity regularizer penalty term in the objective. We then propose a procedure to compute the delays using the precoder response obtained after solving a sparse eigenvalue problem for maximizing the total energy. Incorporating sparsity in the problem formulation enables us to obtain a response that closely resembles the TTD precoder response in time domain. The proposed approach outperforms the time-reversal precoder based TTD design and the phase-shifter implementation in terms of the wideband mutual information.}, journal={2022 56TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Deshpande, Nitish Vikas and Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2022}, pages={1233–1237} }
 @article{tian_gonzalez-prelcic_heath_2022, title={Optimizing the Deployment of Reconfigurable Intelligent Surfaces in MmWave Vehicular Systems}, ISSN={["2576-6813"]}, DOI={10.1109/GLOBECOM48099.2022.10001015}, abstractNote={Millimeter wave (MmWave) systems are vulnerable to blockages, which cause signal drop and link outage. One solution is to deploy reconfigurable intelligent surfaces (RISs) to add a strong non-line-of-sight path from the transmitter to receiver. To achieve the best performance, the location of the deployed RIS should be optimized for a given site, considering the distribution of potential users and possible blockers. In this paper, we find the optimal location, height and downtilt of RIS working in a realistic vehicular scenario. Because of the proximity between the RIS and the vehicles, and the large electrical size of the RIS, we consider a 3D geometry including the elevation angle and near-field beamforming. We provide results on RIS configuration in terms of both coverage ratio and area-averaged rate. We find that the optimized RIS improves the area-averaged rate fifty percent over the case without a RIS, as well as further improvements in the coverage ratio.}, journal={2022 IEEE GLOBAL COMMUNICATIONS CONFERENCE (GLOBECOM 2022)}, author={Tian, Xiaowen and Gonzalez-Prelcic, Nuria and Heath, Robert W., Jr.}, year={2022}, pages={5261–5266} }
 @article{patel_myers_heath_2022, title={Physical Layer Defense against Eavesdropping Attacks on Low-Resolution Phased Arrays}, ISSN={["1550-3607"]}, DOI={10.1109/ICC45855.2022.9838571}, abstractNote={Eavesdropping attacks are a severe threat to millimeter-wave (mmWave) networks that use low-resolution phased arrays. Although directional beamforming in mmWave phased arrays provides natural defense against eavesdropping, the use of low-resolution phase shifters induces energy leakage into unintended directions. This energy leakage can be exploited by the adversaries. In this paper, we propose a directional modulation (DM)-based defense against eavesdropping attacks on low-resolution phased arrays. Our defense technique applies random circulant shifts to the beamformer for every symbol transmission. By appropriately adjusting the phase of the transmitted symbol, the transmitter (TX) can maintain a high-quality link with the receiver while corrupting the symbols transmitted along unintended directions. We theoretically analyze the secrecy mutual information (SMI) achieved by the proposed defense mechanism and show that our defense induces artificial phase noise (APN) along unintended directions, which increases the SMI of the system. Finally, we numerically show the superiority of the proposed defense technique over the state-of-the-art defense techniques.}, journal={IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC 2022)}, author={Patel, Kartik and Myers, Nitin Jonathan and Heath, Robert W., Jr.}, year={2022}, pages={492–497} }
 @article{han_choi_heath_2022, title={Radar Imaging Based on IEEE 802.11ad Waveform in V2I Communications}, volume={70}, ISSN={["1941-0476"]}, DOI={10.1109/TSP.2022.3213488}, abstractNote={Since most of vehicular radar systems are already exploiting millimeter-wave (mmWave) spectra, it would become much more feasible to implement a joint radar and communication system by extending communication frequencies into the mmWave band. In this paper, an IEEE 802.11ad waveform-based radar imaging technique is proposed for vehicular settings. A roadside unit (RSU) transmits the IEEE 802.11ad waveform to a vehicle for communications while the RSU also listens to the echoes of transmitted waveform to perform inverse synthetic aperture radar (ISAR) imaging. To obtain high-resolution images of the vehicle, the RSU needs to accurately estimate round-trip delays, Doppler shifts, and velocity of vehicle. The proposed ISAR imaging first estimates the round-trip delays using a good correlation property of Golay complementary sequences in the IEEE 802.11ad preamble. The Doppler shifts are then obtained using least square estimation from the echo signals and refined to compensate phase wrapping caused by phase rotation. The velocity of vehicle is determined using an equation of motion and the estimated Doppler shifts. Simulation results verify that the proposed technique is able to form high-resolution ISAR images from point scatterer models of realistic vehicular settings with different viewpoints. The proposed ISAR imaging technique can be used for various vehicular applications, e.g., traffic condition analyses or advanced collision warning systems.}, journal={IEEE TRANSACTIONS ON SIGNAL PROCESSING}, author={Han, Geonho and Choi, Junil and Heath, Robert W.}, year={2022}, pages={4981–4996} }
 @article{zhang_heath jr_2023, title={Reinforcement Learning-Based Joint User Scheduling and Link Configuration in Millimeter-Wave Networks}, volume={22}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2022.3215922}, abstractNote={In this paper, we develop algorithms for joint user scheduling and three types of mmWave link configuration: relay selection, codebook optimization, and beam tracking in millimeter wave (mmWave) networks. Our goal is to design an online controller that dynamically schedules users and configures their links to minimize system delay. To solve this complex scheduling problem, we model it as a dynamic decision-making process and develop two reinforcement learning-based solutions. The first solution is based on deep reinforcement learning (DRL), which leverages the proximal policy optimization to train a neural network-based solution. Due to the potential high sample complexity of DRL, we also propose an empirical multi-armed bandit (MAB)-based solution, which decomposes the decision-making process into a sequential of sub-actions and exploits classic maxweight scheduling and Thompson sampling to decide those sub-actions. Our evaluation of the proposed solutions confirms their effectiveness in providing acceptable system delay. It also shows that the DRL-based solution has better delay performance while the MAB-based solution has a faster training process.}, number={5}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Zhang, Yi and Heath Jr, Robert W. W.}, year={2023}, month={May}, pages={3038–3054} }
 @article{esfandiari_vorobyov_heath_2022, title={SPARSITY ENFORCING WITH TOEPLITZ MATRIX RECONSTRUCTION METHOD FOR MMWAVE UL CHANNEL ESTIMATION WITH ONE-BIT ADCS}, DOI={10.1109/SAM53842.2022.9827806}, abstractNote={One-bit analog-to-digital converters enable digital beamforming in millimeter wave (mmWave) multi-input multi-output communication systems with low power consumption. Con-ventional signal processing tasks like channel estimation, though, are challenging due to the extreme quantization, making it challenging to implement uplink (UL) multiuser re-ceivers. We reformulate the UL channel estimation prob-lem as a multiplication of two specific matrices, and then we leverage Toeplitz matrix reconstruction in conjunction with the angular domain sparsity of the UL channel to recover UL channel via solving a properly designed optimization prob-lem. Our new approach is called the sparsity enforcing with Toeplitz matrix reconstruction (SE-TMR) method. Numerical simulations are carried out to showcase the advantages of SE-TMR over existing competitive methods in terms of normal-ized mean squared error in clustered narrowband channels.}, journal={2022 IEEE 12TH SENSOR ARRAY AND MULTICHANNEL SIGNAL PROCESSING WORKSHOP (SAM)}, author={Esfandiari, Majdoddin and Vorobyov, Sergiy A. and Heath, Robert W., Jr.}, year={2022}, pages={141–145} }
 @article{dreifuerst_heath_2022, title={SignalNet: A Low Resolution Sinusoid Decomposition and Estimation Network}, volume={70}, ISSN={["1941-0476"]}, DOI={10.1109/TSP.2022.3201336}, abstractNote={The detection and estimation of sinusoids is a fundamental signal processing task for many applications related to sensing and communications. While algorithms have been proposed for this setting, quantization is a critical, but often ignored modeling effect. In wireless communications, estimation with low resolution data converters is relevant for reduced power consumption in wideband receivers. Similarly, low resolution sampling in imaging and spectrum sensing allows for efficient data collection. In this work, we propose SignalNet, a neural network architecture that detects the number of sinusoids and estimates their parameters from quantized in-phase and quadrature samples. We incorporate signal reconstruction internally as domain knowledge within the network to enhance learning and surpass traditional algorithms in mean squared error and Chamfer error. We introduce a worst-case learning threshold for comparing the results of our network relative to the underlying data distributions. This threshold provides insight into why neural networks tend to outperform traditional methods and into the learned relationships between the input and output distributions. In simulation, we find that our algorithm is always able to surpass the threshold for three-bit data but often cannot exceed the threshold for one-bit data. We use the learning threshold to explain, in the one-bit case, how our estimators learn to minimize the distributional loss, rather than learn features from the data.}, journal={IEEE TRANSACTIONS ON SIGNAL PROCESSING}, author={Dreifuerst, Ryan M. and Heath, Robert W., Jr.}, year={2022}, pages={4454–4467} }
 @article{saab_mezghani_heath_2021, title={A Novel Antenna Matching Technique for Joint Wireless Communication and Energy Harvesting}, DOI={10.1109/APWC52648.2021.9539606}, abstractNote={Summary form only given. Merging the fields of digital communication, energy harvesting (EH), sensing, and RF system design is fundamental for achieving the best trade-off between the competing performance metrics. Matching the Low-Noise Amplifier (LNA) or the energy harvester input to the antenna source impedance is critical for maximizing the achievable rate or the power transfer efficiency. Therefore, this maximization needs to be optimized using joint performance metrics. In this paper, we introduce an optimized antenna matching technique for joint wireless communication and energy harvesting. The enhanced antenna matching process integrates the information theoretic capacity equation and Maxwell's equations. The resulting system substantially improves the trade-off between the harvested power and spectral efficiency based on our joint optimization approach. We design a 24 x 26 m2 compact wideband elliptical planar antenna using an FR4 substrate of 1.6mm thickness and a 4.4 dielectric constant. We optimize a lossless common matching network that matches the designed antenna to a LNA and an EH Schottky diode. The common matching network constitutes an ideal transformer in parallel with an inductor. The designed wideband antenna operates between 5.5-8 GHz.}, journal={2021 IEEE-APS TOPICAL CONFERENCE ON ANTENNAS AND PROPAGATION IN WIRELESS COMMUNICATIONS (APWC)}, author={Saab, Sandy and Mezghani, Amine and Heath, Robert W., Jr.}, year={2021}, pages={36–36} }
 @article{shyianov_akrout_bellili_mezghani_heath_2022, title={Achievable Rate With Antenna Size Constraint: Shannon Meets Chu and Bode}, volume={70}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2021.3099842}, abstractNote={Using ideas from Chu and Bode/Fano theories, we characterize the maximum achievable rate over the single-input single-output wireless communication channels under a restriction on the antenna size at the receiver. By employing circuit-theoretic multiport models for radio communication systems, we derive the information-theoretic limits of compact antennas. We first describe an equivalent Chu’s antenna circuit under the physical realizability conditions of its reflection coefficient. Such a design allows us to subsequently compute the achievable rate for a given receive antenna size thereby providing a physical bound on the system performance that we compare to the standard size-unconstrained Shannon capacity. We also determine the effective signal-to-noise ratio (SNR) which strongly depends on the antenna size and experiences an apparent finite-size performance degradation where only a fraction of Shannon capacity can be achieved. We further determine the optimal signaling bandwidth which shows that impedance matching is essential in both narrowband and broadband scenarios. We also examine the achievable rate in presence of interference showing that the size constraint is immaterial in interference-limited scenarios. Finally, our numerical results of the derived achievable rate as function of the antenna size and the SNR reveal new insights for the physically consistent design of radio systems.}, number={3}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Shyianov, Volodymyr and Akrout, Mohamed and Bellili, Faouzi and Mezghani, Amine and Heath, Robert W., Jr.}, year={2022}, month={Mar}, pages={2010–2024} }
 @article{kumari_myers_heath_2021, title={Adaptive and Fast Combined Waveform-Beamforming Design for MMWave Automotive Joint Communication-Radar}, volume={15}, ISSN={["1941-0484"]}, DOI={10.1109/JSTSP.2021.3071592}, abstractNote={Millimeter-wave (mmWave) joint communication-radar (JCR) will enable high data rate communication and high-resolution radar sensing for applications such as autonomous driving. Prior JCR systems that are based on the mmWave communications hardware, however, suffer from a limited angular field-of-view and low estimation accuracy for radars due to the employed directional communication beam. In this paper, we propose an adaptive and fast combined waveform-beamforming design for the mmWave automotive JCR with a phased-array architecture that permits a trade-off between communication and radar performances. To rapidly estimate the mmWave automotive radar channel in the Doppler-angle domain with a wide field-of-view, our JCR design employs circulant shifts of the transmit beamformer to acquire radar channel measurements and uses two-dimensional compressed sensing (CS) in the space-time dimension. We optimize these circulant shifts to minimize the coherence of the CS matrix, under the space-time sampling constraints in our problem. We evaluate the JCR performance trade-offs using a normalized mean square error (MSE) metric for radar estimation and a distortion MSE metric for data communication, which is analogous to the distortion metric in the rate-distortion theory. Additionally, we develop a MSE-based weighted average optimization problem for the adaptive JCR combined waveform-beamforming design. Numerical results demonstrate that our proposed JCR design enables the estimation of short- and medium-range radar channels in the Doppler-angle domain with a low normalized MSE, at the expense of a small degradation in the communication distortion MSE.}, number={4}, journal={IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING}, author={Kumari, Preeti and Myers, Nitin Jonathan and Heath, Robert W.}, year={2021}, month={Jun}, pages={996–1012} }
 @article{zhang_liu_masouros_heath_feng_zheng_petropulu_2021, title={An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing}, volume={15}, ISSN={["1941-0484"]}, DOI={10.1109/JSTSP.2021.3113120}, abstractNote={Joint communication and radar sensing (JCR) represents an emerging research field aiming to integrate the above two functionalities into a single system, by sharing the majority of hardware, signal processing modules and, in a typical case, the transmitted signal. The close cooperation of the communication and sensing functions can enable significant improvement of spectrum efficiency, reduction of device size, cost and power consumption, and improvement of performance of both functions. Advanced signal processing techniques are critical for making the integration efficient, from transmission signal design to receiver processing. This paper provides a comprehensive overview of the state-of-the-art on JCR systems from the signal processing perspective. A balanced coverage on both transmitter and receiver is provided for three types of JCR systems, namely, communication-centric, radar-centric, and joint design and optimization.}, number={6}, journal={IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING}, author={Zhang, J. Andrew and Liu, Fan and Masouros, Christos and Heath, Robert W. and Feng, Zhiyong and Zheng, Le and Petropulu, Athina}, year={2021}, month={Nov}, pages={1295–1315} }
 @article{zhang_love_krogmeier_anderson_heath_buckmaster_2021, title={Challenges and Opportunities of Future Rural Wireless Communications}, volume={59}, ISSN={["1558-1896"]}, DOI={10.1109/MCOM.001.2100280}, abstractNote={Broadband access is key to ensuring robust economic development and improving quality of life. Unfortunately, the communication infrastructure deployed in rural areas throughout the world lags behind its urban counterparts due to low population density and economics. This article examines the motivations and challenges of providing broadband access over vast rural regions, with an emphasis on the wireless aspect in view of its irreplaceable role in closing the digital gap. Applications and opportunities for future rural wireless communications are discussed for a variety of areas, including residential welfare, digital agriculture, and transportation. This article also comprehensively investigates current and emerging wireless technologies that could facilitate rural deployment. Although there is no simple solution, there is an urgent need for researchers to work on coverage, cost, and reliability of rural wireless access.}, number={12}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Zhang, Yaguang and Love, David J. and Krogmeier, James V and Anderson, Christopher R. and Heath, Robert W. and Buckmaster, Dennis R.}, year={2021}, month={Dec}, pages={16–22} }
 @article{olson_andrews_heath_2021, title={Coverage in Terahertz Cellular Networks with Imperfect Beam Alignment}, ISSN={["2576-6813"]}, DOI={10.1109/GLOBECOM46510.2021.9685773}, abstractNote={We develop a novel stochastic geometry framework to quantify the SINR coverage probability of a Terahertz (THz) cellular network. THz frequencies will require highly directional beams, leading to inevitably imperfect beam alignment. We derive a tractable and accurate semi-closed form lower bound for the coverage probability of a typical user in the network, introducing a novel approach of characterizing non line-of-sight (NLOS) links as equivalent LoS links using a non-homogeneous Poisson Point Process. We use the coverage bound to investigate the SINR scaling trends with base station density and array directivity at the base station and user. Dense base station deployments are required to achieve sufficient coverage and our analysis exposes a tradeoff between directivity (array gain) and loss in SINR due to misalignment.}, journal={2021 IEEE GLOBAL COMMUNICATIONS CONFERENCE (GLOBECOM)}, author={Olson, Nicholas and Andrews, Jeffrey G. and Heath, Robert W., Jr.}, year={2021} }
 @article{annaluru_mazher_heath_2021, title={DEEP LEARNING BASED RANGE AND DOA ESTIMATION USING LOWRESOLUTION FMCW RADARS}, DOI={10.1109/SSP49050.2021.9513759}, abstractNote={We present a deep learning model for simultaneous range and direction of arrival estimation of frequency modulated continuous waveform radars using 1-bit analog-to-digital converters. Standard fast Fourier transform based processing of 1-bit signals can suffer from various signal distortions. To combat this, we train a neural network on vectorized covariance matrices of simulated targets and test it under realistic settings in terms of signal-to-noise ratio and computation. We also test under off-grid settings not seen during training and demonstrate that this type of model can indeed generalize. Furthermore, we present results from data collected from a 77 GHz automotive radar under low resolution settings and can accurately detect the position of a vehicle within the field of view.}, journal={2021 IEEE STATISTICAL SIGNAL PROCESSING WORKSHOP (SSP)}, author={Annaluru, Ramakrishna Sai and Mazher, Khurram Usman and Heath, Robert W.}, year={2021}, pages={366–370} }
 @article{mazher_mezghani_heath_2021, title={Improved CRB for Millimeter-Wave Radar With 1-Bit ADCs}, volume={2}, ISSN={["2644-1322"]}, DOI={10.1109/OJSP.2021.3080213}, abstractNote={Millimeter-wave is widely used for consumer radar applications like driver assistance systems in automated vehicles and gesture recognition in touch-free interfaces. To cope with the increased hardware complexity, higher costs and power consumption of wideband systems at millimeter-wave frequencies, we propose a fully digital architecture with low-resolution analog-to-digital converters (ADCs) on each radio-frequency chain. The effect of the low-resolution ADCs on radar parameter estimation is characterized by the Cramér-Rao bound (CRB) under the proposed hardware constraints. Prior work has shown that at low signal-to-noise ratio, a radar system with 1-bit ADCs suffers a performance loss of 2 dB in parameter estimation compared to a system with ideal infinite resolution ADCs. In this paper, we design an analog preprocessing unit that beamforms in a particular direction and improves the system performance in terms of the achievable CRB. We optimize the proposed preprocessing architecture and show that the optimized network is realizable through low-cost low-resolution phase-shifters. With the optimized preprocessor network in the system, we reduce the gap to 1.16 dB compared to a system with ideal ADCs. We demonstrate the potential of the proposed architecture to meet the requirements of high-resolution sensing through analytical derivation and numerical computation of an improved CRB and show its achievability through a correlation-based estimator.}, journal={IEEE OPEN JOURNAL OF SIGNAL PROCESSING}, author={Mazher, Khurram Usman and Mezghani, Amine and Heath, Robert W.}, year={2021}, pages={318–335} }
 @article{myers_heath_2022, title={InFocus: A Spatial Coding Technique to Mitigate Misfocus in Near-Field LoS Beamforming}, volume={21}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2021.3110011}, abstractNote={Phased arrays, commonly used in IEEE 802.11ad and 5G radios, are capable of focusing radio frequency signals in a specific direction or a spatial region. Beamforming achieves such directional or spatial concentration of signals and enables phased array-based radios to achieve high data rates. Designing beams for millimeter wave and terahertz communication using massive phased arrays, however, is challenging due to hardware constraints and the wide bandwidth in these systems. For example, beams which are optimal at the center frequency may perform poor in wideband communication systems where the radio frequencies differ substantially from the center frequency. The poor performance in such systems is due to differences in the optimal beamformers corresponding to distinct radio frequencies within the wide bandwidth. Such a mismatch leads to a misfocus effect in near-field systems and the beam squint effect in far-field systems. In this paper, we investigate the misfocus effect and propose InFocus, a low complexity technique to construct beams that are well suited for massive wideband phased arrays. The beams are constructed using a carefully designed frequency modulated waveform in the spatial dimension. InFocus mitigates beam misfocus and beam squint when applied to near-field and far-field systems.}, number={4}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Myers, Nitin Jonathan and Heath, Robert W., Jr.}, year={2022}, month={Apr}, pages={2193–2209} }
 @article{kumari_mezghani_heath_2021, title={JCR70: A Low-Complexity Millimeter-Wave Proof-of-Concept Platform for a Fully-Digital SIMO Joint Communication-Radar}, volume={2}, ISSN={["2644-1330"]}, DOI={10.1109/OJVT.2021.3069946}, abstractNote={A fully-digital wideband joint communication-radar (JCR) with a single-input-multiple-output (SIMO) architecture at the millimeter-wave (mmWave) band will enable high joint communication and radar performance with enhanced design flexibility. A quantized receiver with few-bit analog-to-digital converters (ADCs) will enable a practical JCR solution with reduced power consumption for futuristic portable devices and autonomous vehicles. In this paper, we present a joint communication-radar proof-of-concept platform, named JCR70, to evaluate and demonstrate the performance of these JCR systems using real channel measurements in the 71–76 GHz band. We develop this platform by extending a mmWave communication set-up with an additional full-duplex radar receiver and by capturing the SIMO JCR channel using a moving antenna on a sliding rail. To characterize the JCR performance of our developed testbed, we conduct several indoor and outdoor experiments and apply traditional as well as advanced processing algorithms on the measured data. The experimental results show that the quantized receiver with 2–4 b ADCs generally performs quite close to the high-resolution ADC for a signal-to-noise ratio of up to 5 dB. Additionally, we compare the performance of our JCR70 platform with the INRAS Radarbook, which is a state-of-the-art automotive radar evaluation platform at 77 GHz.}, journal={IEEE OPEN JOURNAL OF VEHICULAR TECHNOLOGY}, author={Kumari, Preeti and Mezghani, Amine and Heath, Robert W.}, year={2021}, pages={218–234} }
 @article{ullah_tewfik_heath_2021, title={Leveraging Waveform Structure to Develop a Power Scalable AoA Estimation}, volume={2}, ISSN={["2644-125X"]}, DOI={10.1109/OJCOMS.2021.3134812}, abstractNote={In a mmWave mobile device, power consumption resulting from the high sampling rate is a primary concern for angle of arrival (AoA) estimation solutions. In this paper, we provide a power scalable solution for AoA estimation with structured waveforms in a narrowband channel. We design a set of pilot sequences that maintain orthogonality in sub-Nyquist sampling domains. We leverage the sequences’ structure to develop a variable rate decoupling algorithm to separate multiple sources at the receiver using partial knowledge about the pilots. The decoupling enables a feasible, low-complexity AoA estimation for digital architectures with flexible antenna array design. In this paper, we provide one such AoA estimation solution named ADELA for a linear antenna array design. Simulation results show that AoA estimation performance reaches the Cramer-Rao-Bounds (CRBs) for a range of SNRs. The proposed estimator with subsampling factors of 8 or less outperforms two examples of full rate virtual array AoA estimators for unknown waveforms: 2-level nested array and coprime filter bank estimators. Compared to these two examples of virtual array, our method offers an 8 times lower ADC power consumption, and a significantly lower computational complexity.}, journal={IEEE OPEN JOURNAL OF THE COMMUNICATIONS SOCIETY}, author={Ullah, Mahbuba Sheba and Tewfik, Ahmed and Heath, Robert W.}, year={2021}, pages={2739–2759} }
 @article{gapeyenko_moltchanov_andreev_heath_2021, title={Line-of-Sight Probability for mmWave-Based UAV Communications in 3D Urban Grid Deployments}, volume={20}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2021.3075099}, abstractNote={The network operators will soon be accommodating a new type of users: unmanned aerial vehicles (UAVs). 5G New Radio (NR) technology operating in the millimeter-wave (mmWave) frequency bands can support the emerging bandwidth-hungry applications facilitated by such aerial devices. To reliably integrate UAVs into the NR-based network infrastructure, new system models that capture the features of UAVs in urban environments are required. As city building blocks constitute one of the primary sources of blockage on the links from the UAV to its serving base station (BS), the corresponding line-of-sight (LoS) probability models are essential for accurate performance evaluation in realistic scenarios. We propose a LoS probability model in UAV communication setups over regular urban grid deployments, which is based on a Manhattan Poisson line process. Our approach captures different building height distributions as well as their dimensions and densities. Under certain characteristic distributions, closed-form expressions for the LoS probability are offered. Our numerical results demonstrate the importance of accounting for the building height distribution type as well as the orientation of the UAV with respect to its BS. By comparing our model with the standard ITU and 3GPP formulations, we establish that the latter provide an overly optimistic approximation for various deployments.}, number={10}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Gapeyenko, Margarita and Moltchanov, Dmitri and Andreev, Sergey and Heath, Robert W., Jr.}, year={2021}, month={Oct}, pages={6566–6579} }
 @article{hriba_valenti_heath_2021, title={Optimization of a Millimeter-Wave UAV-to-Ground Network in Urban Deployments}, ISSN={["2155-7578"]}, DOI={10.1109/MILCOM52596.2021.9653132}, abstractNote={An urban tactical wireless network is considered wherein the base stations are situated on unmanned aerial vehicles (UAVs) that provide connectivity to ground assets such as vehicles located on city streets. The UAVs are assumed to be randomly deployed at a fixed height according to a two-dimensional point process. Millimeter-wave (mmWave) frequencies are used to avail of large available bandwidths and spatial isolation due to beamforming. In urban environments, mmWave signals are prone to blocking of the line-of-sight (LoS) by buildings. While reflections are possible, the desire for consistent connectivity places a strong preference on the existence of an unblocked LoS path. As such, the key performance metric considered in this paper is the connectivity probability, which is the probability of an unblocked LoS path to at least one UAV within some maximum transmission distance. By leveraging tools from stochastic geometry, the connectivity probability is characterized as a function of the city type (e.g., urban, dense urban, suburban), density of UAVs (average number of UAVs per square km), and height of the UAVs. The city streets are modeled as a Manhattan Poisson Line Process (MPLP) and the building heights are randomly distributed. The analysis first finds the connectivity probability conditioned on a particular network realization (location of the UAVs) and then removes the conditioning to uncover the distribution of the connectivity; i.e., the fraction of network realizations that will fail to meet an outage threshold. While related work has applied an MPLP to networks with a single UAV, the contributions of this paper are that it (1) considers networks of multiple UAVs, (2) characterizes the performance by a connectivity distribution, and (3) identifies the optimal altitude for the UAVs.}, journal={2021 IEEE MILITARY COMMUNICATIONS CONFERENCE (MILCOM 2021)}, author={Hriba, Enass and Valenti, Matthew C. and Heath, Robert W., Jr.}, year={2021} }
 @article{dreifuerst_daulton_qian_varkey_balandat_kasturia_tomar_yazdan_ponnampalam_heath_2021, title={Optimizing Coverage and Capacity in Cellular Networks using Machine Learning}, DOI={10.1109/ICASSP39728.2021.9414155}, abstractNote={Wireless cellular networks have many parameters that are normally tuned upon deployment and re-tuned as the network changes. Many operational parameters affect reference signal received power (RSRP), reference signal received quality (RSRQ), signal-to-interference-plus-noise-ratio (SINR), and, ultimately, throughput. In this paper, we develop and compare two approaches for maximizing coverage and minimizing interference by jointly optimizing the transmit power and downtilt (elevation tilt) settings across sectors. To evaluate different parameter configurations offline, we construct a realistic simulation model that captures geographic correlations. Using this model, we evaluate two optimization methods: deep deterministic policy gradient (DDPG), a reinforcement learning (RL) algorithm, and multi-objective Bayesian optimization (BO). Our simulations show that both approaches significantly outperform random search and converge to comparable Pareto frontiers, but that BO converges with two orders of magnitude fewer evaluations than DDPG. Our results suggest that data-driven techniques can effectively self-optimize coverage and capacity in cellular networks.}, journal={2021 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING (ICASSP 2021)}, author={Dreifuerst, Ryan M. and Daulton, Samuel and Qian, Yuchen and Varkey, Paul and Balandat, Maximilian and Kasturia, Sanjay and Tomar, Anoop and Yazdan, Ali and Ponnampalam, Vish and Heath, Robert W.}, year={2021}, pages={8138–8142} }
 @article{saab_mezghani_heath_2022, title={Optimizing the Mutual Information of Frequency-Selective Multi-Port Antenna Arrays in the Presence of Mutual Coupling}, volume={70}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2021.3133435}, abstractNote={As larger bandwidths are used in multiple antenna wireless systems, the frequency selectivity of the antenna arrays starts to impact rate. Motivated by optimizing the achievable rate in compact antenna arrays, we present a system model that incorporates the effects of mutual coupling (MC) of wideband physically realizable single-input multiple-output (SIMO) and multiple-input single-output (MISO) antenna systems. For the SIMO system setup, we extract the noise correlation matrices for two different antenna array configurations (parallel and co-linear). We optimize the inter-element spacing in each alignment while maximizing the achievable rate and fixing the transmit power. Then, we compare the two compact antenna designs to a perfectly matched single omni-directional antenna while accounting for MC. Likewise, for the MISO antenna system, we derive the optimal beamformer that maximizes the achievable rate using the same antenna configurations as the SIMO system. Then, we study the impact of MC and develop a new single-port matching technique for wideband antenna arrays. Finally, we provide reciprocity plots to compare the performance of the SIMO-MISO systems using different channel models.}, number={3}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Saab, Sandy and Mezghani, Amine and Heath, Robert W., Jr.}, year={2022}, month={Mar}, pages={2072–2084} }
 @article{ullah_tewfik_heath_2021, title={Power Scalable Angle of Arrival Estimation Using Pilot Design With Orthogonal Subsequences}, volume={2}, ISSN={["2644-125X"]}, DOI={10.1109/OJCOMS.2021.3095384}, abstractNote={Energy efficient array processing is critical to implement feasible solutions for directional communication in a mmWave channel. MmWave channels are highly susceptible to blockage and require frequent angle of arrival (AoA) estimation. An AoA estimation solutions with a fully digital architecture offers a low latency, high performance and flexible solution suitable for the stringent requirements of $\mathbf {5}\text{G}$ . However, the large number of high speed converters in a digital receiver are the dominant power consuming elements. Alternative analog or hybrid architectures use fewer high speed converters, but require sweeping measurements to estimate AoAs over the angular space, and thus adds latency to the estimation process. In this paper, we present a variable rate sub-Nyquist decoupling solution that leverages pilot design. The pilot’s subsequence properties allow decoupling the source waveforms at fractions of the Nyquist rate. We leverage this concept to scale power consumption by the converters. We preprocess the received signals at the antenna array with the variable rate sub-Nyquist decoupling algorithm and use a few well known digital estimators for AoA estimation including DEML, KR-MUSIC based two level nested array and coprime filter bank. In addition to scalable power consumption, our research indicates some other benefits of the decoupling, including reduced complexity algorithm implementation and improved performance estimation for the non maximum likelihood estimators.}, journal={IEEE OPEN JOURNAL OF THE COMMUNICATIONS SOCIETY}, author={Ullah, Mahbuba S. and Tewfik, Ahmed and Heath, Robert W., Jr.}, year={2021}, pages={1690–1709} }
 @article{cuvelier_lanham_la cour_heath_2021, title={Quantum Codes in Classical Communication: A Space-Time Block Code From Quantum Error Correction}, volume={2}, ISSN={["2644-125X"]}, DOI={10.1109/OJCOMS.2021.3121183}, abstractNote={We propose a general framework for noncoherent communication using techniques from the field of quantum error correction (QEC). We first propose an approach for analyzing a classical communication channel as a quantum channel, and develop an extension of the QEC conditions to the classical case. We derive a quantum analogue of the noncoherent multiantenna wireless channel. Restricting to the case in which the number of transmitter and receiver antennas are equal and a power of two, we use the framework to develop a family of space-time block codes for noncoherent multiple-input multiple-output (MIMO) communication. Under a Rayleigh fading assumption, we derive the optimal decoder and bound the probability of symbol detection error. We compare our performance to comparable coherent and noncoherent approaches and achieve competitive performance for various antenna geometries and rates.}, journal={IEEE OPEN JOURNAL OF THE COMMUNICATIONS SOCIETY}, author={Cuvelier, Travis C. and Lanham, S. Andrew and La Cour, Brian R. and Heath, Robert W., Jr.}, year={2021}, pages={2383–2412} }
 @article{buckley_heath_2021, title={Signal Conditioning and Prototyping for Selective OFDM Systems with Simultaneous Wireless Information and Power Transfer}, DOI={10.1109/APWC52648.2021.9539718}, abstractNote={Simultaneous Wireless Information and Power Transfer (SWIPT) is the use of radio signals for both the transfer of energy and information. To enable SWIPT transmissions, receiver architectures often incorporate co-receiver implementations. In this paper, a selective OFDM system is introduced which leverages the OFDM signal structure for energy harvesting.}, journal={2021 IEEE-APS TOPICAL CONFERENCE ON ANTENNAS AND PROPAGATION IN WIRELESS COMMUNICATIONS (APWC)}, author={Buckley, Raquel F. and Heath, Robert W., Jr.}, year={2021}, pages={61–61} }
 @article{buckley_heath_2021, title={Signal Conditioning for Selective OFDM SWIPT Systems}, volume={2}, ISSN={["2644-125X"]}, DOI={10.1109/OJCOMS.2021.3101010}, abstractNote={The conversion efficiency of a harvesting component can hinder device self-sustainability. Countering this inefficiency can be a rigorous task that requires either a modified input waveform at the harvesting component or an improved design of the harvesting component itself. This paper considers a selective orthogonal frequency division multiplexing (OFDM) system that is designed for simultaneous wireless information and power transfer (SWIPT). This paper introduces an enhanced transmitter architecture designed to condition the transmit signal for improved conversion efficiency. Leveraging the structure of the OFDM signal, the cyclic prefix plus a portion of the information signal are conditioned and used for harvesting. Then, a rectifier model and prototypes are designed to further improve conversion efficiency. System performance is evaluated with the analytical derivation of the rate-energy (R-E) tradeoff and an optimized transmission strategy for system self-sustainability. Monte Carlo simulations show increased energy gains and full self-sustainability with the introduction of signal conditioning.}, journal={IEEE OPEN JOURNAL OF THE COMMUNICATIONS SOCIETY}, author={Buckley, Raquel F. and Heath, Robert W., Jr.}, year={2021}, pages={1886–1900} }
 @article{olson_andrews_heath_2022, title={Single Channel Equivalent Point Processes of Poisson Networks With Multiple Channel Laws}, volume={26}, ISSN={["1558-2558"]}, DOI={10.1109/LCOMM.2021.3136462}, abstractNote={We present the construction and characterization of novel single channel equivalent representations of a Poisson Point Process (PPP) whose points follow multiple channel laws with respect to the origin. Two variants are presented. The first, called the Type 1 Equivalent Process (EPP), allows for IID fading while maintaining equivalence in terms of path loss and fading with respect to a single channel law. The second, called the Type 2 EPP, also maintains equivalence with respect to a single channel law, but requires the fading distribution to be dependent on the underlying point process. The Type 1 EPP only exists in certain settings, while the Type 2 EPP always exists.}, number={3}, journal={IEEE COMMUNICATIONS LETTERS}, author={Olson, Nicholas R. and Andrews, Jeffrey G. and Heath, Robert W., Jr.}, year={2022}, month={Mar}, pages={711–715} }
 @article{saab_mezghani_heath_2020, title={Beamforming optimization of wideband MISO systems in the presence of mutual coupling}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF51394.2020.9443408}, abstractNote={We introduce a mutual information based optimization for a two-port multiple-input single-output (MISO) antenna system. We develop a complete circuit-level analysis of a compact MISO system in the wideband regime. We design a physically realizable antenna array and study the impact of mutual coupling on the spectral efficiency. Then, we maximize the system’s mutual information by optimizing the beamformer under two different power constraints, namely the total dissipated power and the available power of the amplifiers. By varying the inter-element antenna spacing, we present results for the achievable spectral efficiency under different power amplifier constraints.}, journal={2020 54TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Saab, Sandy and Mezghani, Amine and Heath, Robert W., Jr.}, year={2020}, pages={1338–1341} }
 @article{zhang_doshi_liston_tan_zhu_andrews_heath_2021, title={DeepWiPHY: Deep Learning-Based Receiver Design and Dataset for IEEE 802.11ax Systems}, volume={20}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2020.3034610}, abstractNote={In this work, we develop DeepWiPHY, a deep learning-based architecture to replace the channel estimation, common phase error (CPE) correction, sampling rate offset (SRO) correction, and equalization modules of IEEE 802.11ax based orthogonal frequency division multiplexing (OFDM) receivers. We first train DeepWiPHY with a synthetic dataset, which is generated using representative indoor channel models and includes typical radio frequency (RF) impairments that are the source of nonlinearity in wireless systems. To further train and evaluate DeepWiPHY with real-world data, we develop a passive sniffing-based data collection testbed composed of Universal Software Radio Peripherals (USRPs) and commercially available IEEE 802.11ax products. The comprehensive evaluation of DeepWiPHY with synthetic and real-world datasets (110 million synthetic OFDM symbols and 14 million real-world OFDM symbols) confirms that, even without fine-tuning the neural network’s architecture parameters, DeepWiPHY achieves comparable performance to or outperforms the conventional WLAN receivers, in terms of both bit error rate (BER) and packet error rate (PER), under a wide range of channel models, signal-to-noise (SNR) levels, and modulation schemes.}, number={3}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Zhang, Yi and Doshi, Akash and Liston, Rob and Tan, Wai-Tian and Zhu, Xiaoqing and Andrews, Jeffrey G. and Heath, Robert W., Jr.}, year={2021}, month={Mar}, pages={1596–1611} }
 @article{dreifuerst_heath_kulkarni_zhang_2020, title={Frequency Synchronization for Low Resolution Millimeter-Wave}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF51394.2020.9443378}, abstractNote={Low resolution data converters can enable power efficient high bandwidth communication at millimeter-wave and terahertz frequencies. Synchronization of such systems is a critical step in accurate decoding, yet current approaches require long block lengths or fail to reach the Cramer Rao Bound (CRB).´ Prior solutions have traditionally been divided into two distinct focuses: algorithms and designed sequences for synchronization. In this paper, we develop a jointly optimized neural architecture for frequency synchronization from configurable sequences and estimators. Our proposed technique uses two neural networks to generate sequences and determine the carrier frequency offset of the sequence after propagating through a channel and applying one-bit quantization. Our simulations show that we can improve estimation performance at low signal to noise ratio (SNR) by up to 8dB at little cost compared to the same estimator without the sequence generator. Our proposed system is fast, efficient, and easily updated, allowing it to handle time-varying systems. In conclusion, we believe further investigation in jointly optimized pilot sequences and estimators will be fundamental to handling signal processing techniques with low resolution data converters.}, journal={2020 54TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Dreifuerst, Ryan M. and Heath, Robert W., Jr. and Kulkarni, Mandar N. and Zhang, Jianzhong Charlie}, year={2020}, pages={314–318} }
 @article{ali_gonzalez-prelcic_heath_ghosh_2020, title={Leveraging Sensing at the Infrastructure for mmWave Communication}, volume={58}, ISSN={["1558-1896"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85089539802&partnerID=MN8TOARS}, DOI={10.1109/MCOM.001.1900700}, abstractNote={V2X communication in the mmWave band is one way to achieve high data rates for applications like infotainment, cooperative perception, augmented reality assisted driving, and so on. MmWave communication relies on large antenna arrays, and configuring these arrays poses high training overhead. In this article, we motivate the use of infrastructure mounted sensors (which will be part of future smart cities) to aid establishing and maintaining mmWave vehicular communication links. We provide numerical and measurement results to demonstrate that information from these infrastructure sensors reduces the mmWave array configuration overhead. Finally, we outline future research directions to help materialize the use of infrastructure sensors for mmWave communication.}, number={7}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Ali, Anum and Gonzalez-Prelcic, Nuria and Heath, Robert W. and Ghosh, Amitava}, year={2020}, month={Jul}, pages={84–89} }
 @article{saxena_mezghani_heath_2020, title={Linear CE and 1-bit Quantized Precoding With Optimized Dithering}, volume={1}, ISSN={["2644-1322"]}, DOI={10.1109/OJSP.2020.3040590}, abstractNote={High power amplifiers (HPA), used at transmission, add nonlinear impairments to the output signals. Through Constant envelope (CE) transmission, distortion in the signal can be avoided without wasting power on PA linearization. A more restricted form of CE transmission, 1-bit quantized transmission, further simplifies the RF chain and reduces the DAC power consumption. In this paper, for CE transmission and 1-bit quantized transmission at the BS antennas, we analyze downlink transmission for low complexity linear precoding. We observe that for small numbers of users in the downlink, correlation among the quantization error components across BS antennas is high, deteriorating the performance rapidly as number of users become smaller. To improve performance for smaller numbers of downlink users, we propose the addition of correlated Gaussian dither to the precoded signal before quantization and subsequent transmission. We observe that the receive SQINR peaks for finite non-trivial dither power. For given value of transmit power, number of BS antennas and number of users, SQINR is maximized analytically by the transmitter, to find the optimum dither power, using the Bussgang decomposition. We observe that with the implementation of optimized dithering, the error floor in the coded BER at high transmit power, for CE and 1-bit quantized transmissions, is pushed down significantly. We also observe that optimum dither power increases monotonically with transmit power, with rate of increase decreasing with increasing transmit power. Further, the optimum dither power strictly increases with number of BS antennas.}, journal={IEEE OPEN JOURNAL OF SIGNAL PROCESSING}, author={Saxena, Amodh Kant and Mezghani, Amine and Heath, Robert W.}, year={2020}, pages={310–325} }
 @article{zhang_rahman_huang_chen_guo_heath_2021, title={Perceptive Mobile Networks: Cellular Networks With Radio Vision via Joint Communication and Radar Sensing}, volume={16}, ISSN={["1556-6080"]}, DOI={10.1109/MVT.2020.3037430}, abstractNote={Joint communication and radar/radio sensing (JCAS), also known as dual-function radar communications, enables the integration of communication and radio sensing into one system, sharing a single transmitted signal. The perceptive mobile network (PMN) is a natural evolution of JCAS from simple point-to-point links to a mobile/cellular network with integrated radio-sensing capability. In this article, we present a system architecture that unifies three types of sensing, investigate the required modifications to existing mobile networks, and exemplify the signals applicable to sensing. We also provide a review to stimulate research problems and potential solutions, including mutual information, joint design and optimization for waveform and antenna grouping, clutter suppression, sensing parameter estimation and pattern recognition, and networked sensing under the cellular topology.}, number={2}, journal={IEEE VEHICULAR TECHNOLOGY MAGAZINE}, author={Zhang, J. Andrew and Rahman, Md. Lushanur and Huang, Xiaojing and Chen, Shanzhi and Guo, Yingjie Jay and Heath, Robert W., Jr.}, year={2021}, month={Jun}, pages={20–30} }
 @article{heath_2020, title={Revisiting Research on Signal Processing for Communications in a Pandemic [From the Editor]}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85084545450&partnerID=MN8TOARS}, DOI={10.1109/MSP.2020.2975306}, abstractNote={Presents the introductory editorial for this issue of the publication.}, number={3}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2020}, pages={3–5} }
 @article{jurdi_andrews_heath_2021, title={Scheduling Observers Over a Shared Channel With Hard Delivery Deadlines}, volume={69}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2020.3032172}, abstractNote={We abstract the core logical functions from applications that require ultra-low-latency wireless communications to provide a novel definition for reliability. Real-time applications — such as intelligent transportation, remote surgery, and industrial automation — involve a significant element of control and decision making. Such systems involve three logical components: observers (e.g. sensors) measuring the state of an environment or dynamical system, a centralized executive (e.g. controller) deciding on the state, and agents (e.g. actuators) that implement the executive’s decisions. The executive harvests the observers’ measurements and decides on the short-term trajectory of the system by instructing its agents to take appropriate actions. All observation packets (typically uplink) and action packets (typically downlink) must be delivered by hard deadlines to ensure the proper functioning of the controlled system. In-full on-time delivery cannot be guaranteed in wireless systems due to inherent uncertainties in the channel such as fading and unpredictable interference; accordingly, the executive will have to drop some packets. We develop a novel framework to formulate the Observer Selection Problem (OSP) through which the executive schedules a sequence of observations that maximize its knowledge about the current state of the system. To solve this problem efficiently yet optimally, we devise a branch-and-bound algorithm that systematically prunes the search space. Our work is different from existing work on real-time communications in that communication reliability is not conveyed by packet loss or error rate, but rather by the extent of the executive’s knowledge about the state of the system it controls.}, number={1}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Jurdi, Rebal and Andrews, Jeffrey G. and Heath, Robert W., Jr.}, year={2021}, month={Jan}, pages={133–148} }
 @article{heath_2020, title={Submitting Columns and Forums to SPM [From the Editor]}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078531946&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2952964}, abstractNote={Reports on the process of submitting columns and forums to the publication and explains the content that is included therein.}, number={1}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2020}, pages={3–4} }
 @article{sakaguchi_fukatsu_yu_fukuda_mahler_heath_fujii_takahashi_khoryaev_nagata_et al._2021, title={Towards mmWave V2X in 5G and Beyond to Support Automated Driving}, volume={E104B}, ISSN={["1745-1345"]}, DOI={10.1587/transcom.2020EBI0001}, abstractNote={Millimeter wave provides high data rates for Vehicle-to-Everything (V2X) communications. This paper motivates millimeter wave to support automated driving and begins by explaining V2X use cases that support automated driving with references to several standardi-zation bodies. The paper gives a classification of existing V2X stand-ards: IEEE802.11p and LTE V2X, along with the status of their com-mercial deployment. Then, the paper provides a detailed assessment on how millimeter wave V2X enables the use case of cooperative perception. The explanations provide detailed rate calculations for this use case and show that millimeter wave is the only technology able to achieve the requirements. Furthermore, specific challenges related to millimeter wave for V2X are described, including cover-age enhancement and beam alignment. The paper concludes with some results from three studies, i.e. IEEE802.11ad (WiGig) based V2X, extension of 5G NR (New Radio) toward mmWave V2X, and prototypes of intelligent street with mmWave V2X.}, number={6}, journal={IEICE TRANSACTIONS ON COMMUNICATIONS}, author={Sakaguchi, Kei and Fukatsu, Ryuichi and Yu, Tao and Fukuda, Eisuke and Mahler, Kim and Heath, Robert and Fujii, Takeo and Takahashi, Kazuaki and Khoryaev, Alexey and Nagata, Satoshi and et al.}, year={2021}, month={Jun}, pages={587–603} }
 @article{wu_zhang_huang_guo_heath_2021, title={Waveform Design and Accurate Channel Estimation for Frequency-Hopping MIMO Radar-Based Communications}, volume={69}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2020.3034357}, abstractNote={Frequency-hopping (FH) MIMO radar-based dual-function radar communication (FH-MIMO DFRC) enables communication symbol rate to exceed radar pulse repetition frequency, which requires accurate estimations of timing offset and channel parameters. The estimations, however, are challenging due to unknown, fast-changing hopping frequencies and the multiplicative coupling between timing offset and channel parameters. In this article, we develop accurate methods for a single-antenna communication receiver to estimate timing offset and channel for FH-MIMO DFRC. First, we design a novel FH-MIMO radar waveform, which enables a communication receiver to estimate the hopping frequency sequence (HFS) used by radar, instead of acquiring it from radar. Importantly, the novel waveform incurs no degradation to radar ranging performance. Then, via capturing distinct HFS features, we develop two estimators for timing offset and derive mean squared error lower bound of each estimator. Using the bounds, we design an HFS that renders both estimators applicable. Furthermore, we develop an accurate channel estimation method, reusing the single hop for timing offset estimation. Validated by simulations, the accurate channel estimates attained by the proposed methods enable the communication performance of DFRC to approach that achieved based on perfect timing and ideal knowledge of channel.}, number={2}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Wu, Kai and Zhang, J. Andrew and Huang, Xiaojing and Guo, Y. Jay and Heath, Robert W., Jr.}, year={2021}, month={Feb}, pages={1244–1258} }
 @article{heath_2020, title={What Does an Editor-in-Chief of IEEE Signal Processing Magazine Do, Anyway? [From the Editor]}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85080942350&partnerID=MN8TOARS}, DOI={10.1109/MSP.2020.2971121}, abstractNote={Reports on the role and responsibilities of the Editor-in-Chief of the IEEE Signaling Processing Magazine.}, number={2}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2020}, pages={3–4} }
 @inproceedings{kumari_myers_vorobyov_heath_2019, title={A Combined Waveform-Beamforming Design for Millimeter-Wave Joint Communication-Radar}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083315613&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9049020}, abstractNote={Millimeter-wave (mmWave) joint communication- radar (JCR) simultaneously realizes a high data rate communication and a high-resolution radar sensing for applications such as autonomous driving. Prior JCR systems that are based on the state-of-the-art mmWave communications hardware, however, suffer from a limited angular field-of-view (FoV) and low detection rate for radars due to the employed directional beam. To address this limitation, we propose an adaptive and fast combined waveform-beamforming design for mmWave JCR with a phased-array architecture. We present a JCR beamformer design algorithm that permits a trade-off between communication data rate and radar successful recovery rate in the angular domain. We show that distinct radar measurements can be obtained with circulant shifts of the designed JCR beamformer for compressed radar sensing. Numerical results demonstrate that our JCR design enables the angle-of-arrival/departure estimation of short- range radar targets with a high successful recovery rate and a wide FoV at the expense of a slight loss in the communication rate.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Kumari, P. and Myers, N.J. and Vorobyov, S.A. and Heath, R.W.}, year={2019}, pages={1422–1426} }
 @inproceedings{kaleva_myers_tölli_heath_2019, title={A Geometry-aided Message Passing Method for AoA-Based Short Range MIMO Channel Estimation}, volume={2019-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072322341&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2019.8815555}, abstractNote={Short range channels commonly arise in millimeter wave (mmWave) wearable settings, where the length of the antenna arrays can be comparable to the distance between the radios. Conventional mmWave MIMO channel estimation techniques based on the far field assumption may perform poorly in short range settings due to the large angular spread and, hence, high available rank. We propose a geometry-aided message passing algorithm that exploits structure in short range line-of-sight (LoS) channels for spatial sub-Nyquist channel estimation. Our approach parametrizes the channel using angle-of-arrivals (AoAs) that are locally defined for subarrays of an antenna array. Furthermore, it leverages the dependencies between the local AoAs using factors based on the array geometry. We show that the LoS MIMO channel can be reconstructed using the derived local AoA estimates and the known transceiver geometry. The proposed approach achieves a reasonable rate with greatly reduced pilot transmissions when compared to exhaustive beam search-based local AoA estimation.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Kaleva, J. and Myers, N.J. and Tölli, A. and Heath, R.W.}, year={2019} }
 @inproceedings{lanham_cuvelier_ostrove_la cour_ott_heath_2019, title={A Noncoherent Space-Time Code from Quantum Error Correction}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85065181093&partnerID=MN8TOARS}, DOI={10.1109/CISS.2019.8692830}, abstractNote={In this work, we develop a space-time block code for noncoherent communication using techniques from the field of quantum error correction. We decompose the multiple-input multiple-output (MIMO) channel into operators from quantum mechanics, and design a non-coherent space time code using the quantum stabilizer formalism. We derive an optimal decoder, and analyze the former through a quantum mechanical lens. We compare our approach to a comparable coherent approach and a noncoherent differential approach, achieving comparable or better performance.}, booktitle={2019 53rd Annual Conference on Information Sciences and Systems, CISS 2019}, author={Lanham, S.A. and Cuvelier, T.C. and Ostrove, C. and La Cour, B. and Ott, G. and Heath, R.}, year={2019} }
 @inproceedings{mo_alkhateeb_abu-surra_heath_2019, title={Achievable rates of hybrid architectures with few-bit ADC receivers}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073566890&partnerID=MN8TOARS}, booktitle={WSA 2016 - 20th International ITG Workshop on Smart Antennas}, author={Mo, J. and Alkhateeb, A. and Abu-Surra, S. and Heath, R.W.}, year={2019}, pages={194–201} }
 @article{kumari_vorobyov_heath_2020, title={Adaptive Virtual Waveform Design for Millimeter-Wave Joint Communication-Radar}, volume={68}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85079812267&partnerID=MN8TOARS}, DOI={10.1109/TSP.2019.2956689}, abstractNote={Joint communication and radar (JCR) waveforms with fully digital baseband generation and processing can now be realized at the millimeter-wave (mmWave) band. Prior work has developed a mmWave wireless local area network (WLAN)-based JCR that exploits the WLAN preamble for radars. The performance of target velocity estimation, however, was limited. In this paper, we propose a virtual waveform design for an adaptive mmWave JCR. The proposed system transmits a few non-uniformly placed preambles to construct several receive virtual preambles for enhancing velocity estimation accuracy, at the cost of only a small reduction in the communication data rate. We evaluate JCR performance trade-offs using the Cramér-Rao Bound (CRB) metric for radar estimation and a novel distortion minimum mean square error (MMSE) metric for data communication. Additionally, we develop three different MMSE-based optimization problems for the adaptive JCR waveform design. Simulations show that an optimal virtual (non-uniform) waveform achieves a significant performance improvement as compared to a uniform waveform. For a radar CRB constrained optimization, the optimal radar range of operation and the optimal communication distortion MMSE (DMMSE) are improved. For a communication DMMSE constrained optimization with a high DMMSE constraint, the optimal radar CRB is enhanced. For a weighted MMSE average optimization, the advantage of the virtual waveform over the uniform waveform is increased with decreased communication weighting. Comparison of MMSE-based optimization with traditional virtual preamble count-based optimization indicated that the conventional solution converges to the MMSE-based one only for a small number of targets and a high signal-to-noise ratio.}, journal={IEEE Transactions on Signal Processing}, author={Kumari, P. and Vorobyov, S.A. and Heath, R.W.}, year={2020}, pages={715–730} }
 @inproceedings{petrov_moltchanov_andreev_heath_2019, title={Analysis of intelligent vehicular relaying in urban 5G+ millimeter-wave cellular deployments}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077491587&partnerID=MN8TOARS}, DOI={10.1109/GLOBECOM38437.2019.9013636}, abstractNote={The capability of smarter networked devices to dynamically select appropriate radio connectivity options is especially important in the emerging millimeter-wave (mmWave) systems to mitigate abrupt link blockage in complex environments. To enrich the levels of diversity, mobile mmWave relays can be employed for improved connection reliability. These are considered by 3GPP for on-demand densification on top of the static mmWave infrastructure. However, performance dynamics of mobile mmWave relaying is not nearly well explored, especially in realistic conditions, such as urban vehicular scenarios. In this paper, we develop a mathematical framework for the performance evaluation of mmWave vehicular relaying in a typical street deployment. We analyze and compare alternative connectivity strategies by quantifying the performance gains made available to smart devices in the presence of mmWave relays. We identify situations where the use of mmWave vehicular relaying is particularly beneficial. Our methodology and results can support further standardization and deployment of mmWave relaying in more intelligent 5G+ "all-mmWave" cellular networks.}, booktitle={2019 IEEE Global Communications Conference, GLOBECOM 2019 - Proceedings}, author={Petrov, V. and Moltchanov, D. and Andreev, S. and Heath, R.W.}, year={2019} }
 @inproceedings{saxena_mezghani_bendlin_nammi_heath_andrews_chopra_2019, title={Asymptotic Performance of Downlink Massive MIMO with 1-bit Quantized Zero-Forcing Precoding}, volume={2019-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072311301&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2019.8815476}, abstractNote={The use of 1-bit DACs for downlink massive MIMO system can significantly reduce base station energy consumption and hardware complexity with the tradeoff being the introduction of nonlinear impairments in the transmitted signal. We analyze the corresponding received signal at the single antenna users through Bussgang decomposition and spectral analysis up to third order harmonics. Through mathematical investigation and simulations, we also evaluate the spectral emissions, signal to quantization and interference ratio (SQINR), along with the achievable rate at the receivers. We observe that SQINR and achievable rate per user achieve a maximum for a nontrivial optimum number of users, which we estimate through our analyses.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Saxena, A.K. and Mezghani, A. and Bendlin, R. and Nammi, S. and Heath, R.W. and Andrews, J.G. and Chopra, A.}, year={2019} }
 @inproceedings{saab_mezghani_heath_2019, title={Capacity Based Optimization of Compact Wideband Antennas}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074355213&partnerID=MN8TOARS}, DOI={10.1109/APWC.2019.8870378}, abstractNote={In this paper, we propose a framework for designing and optimizing compact wideband antennas using the Shannon capacity formula. We present a new single port Gamma matching network that enhances the system’s spectral efficiency apart from satisfying the antenna’s figures of merit. We provide an in-depth analysis of the designed antenna by using a circuit theoretic approach. To optimize the allocated power across the frequency range of interest, we implement the waterfilling method. Finally, we develop interesting insights on the importance of optimizing impedance matching networks from an information theoretic perspective.}, booktitle={Proceedings of the 2019 9th IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications, APWC 2019}, author={Saab, S. and Mezghani, A. and Heath, R.W.}, year={2019}, pages={322–325} }
 @inproceedings{saab_mezghani_heath_2019, title={Capacity based analysis of a wideband SIMO system in the presence of mutual coupling}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85081981134&partnerID=MN8TOARS}, DOI={10.1109/GLOBECOM38437.2019.9013254}, abstractNote={In this paper we present a novel capacity optimization methodology for a two-element multi-band single-input multiple-output (SIMO) antenna. We first introduce a system model that incorporates antenna mutual coupling in the wideband regime. Then, we extract the noise correlation matrices from two different antenna configurations (parallel and co-linear). We optimize the spacing in each scenario while maximizing the Shannon capacity under transmit power constraint. Consequently, we compare the two compact planar antenna designs to the AWGN capacity baseline at different inter- element spacing while accounting for mutual coupling. Finally, we introduce a matching network that maximizes the capacity of the wideband SIMO system.}, booktitle={2019 IEEE Global Communications Conference, GLOBECOM 2019 - Proceedings}, author={Saab, S. and Mezghani, A. and Heath, R.W.}, year={2019} }
 @inproceedings{shen_dai_han_chih-lin_heath_2019, title={Channel Estimation for Orthogonal Time Frequency Space (OTFS) Massive MIMO}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85070230784&partnerID=MN8TOARS}, DOI={10.1109/ICC.2019.8761362}, abstractNote={Orthogonal time frequency space (OTFS) modulation outperforms orthogonal frequency division multiplexing (OFDM) in high-mobility scenarios. One challenge for OTFS massive MIMO is downlink channel estimation due to the required high pilot overhead. In this paper, we propose a 3D structured orthogonal matching pursuit (3D-SOMP) algorithm based channel estimation technique. First, we show that the OTFS MIMO channel exhibits 3D structured sparsity: normal sparsity along the delay dimension, block sparsity along the Doppler dimension, and burst sparsity along the angle dimension. Based on the 3D structured channel sparsity, we then formulate the downlink channel estimation problem as a sparse signal recovery problem. Simulation results show that the proposed 3D-SOMP algorithm can achieve accurate channel state information with low pilot overhead.}, booktitle={IEEE International Conference on Communications}, author={Shen, W. and Dai, L. and Han, S. and Chih-Lin, I. and Heath, R.W.}, year={2019} }
 @article{shen_dai_an_fan_heath_2019, title={Channel Estimation for Orthogonal Time Frequency Space (OTFS) Massive MIMO}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85069783114&partnerID=MN8TOARS}, DOI={10.1109/TSP.2019.2919411}, abstractNote={Orthogonal time frequency space (OTFS) modulation outperforms orthogonal frequency division multiplexing (OFDM) in high-mobility scenarios. One challenge for OTFS massive MIMO is downlink channel estimation due to the large number of base station antennas. In this paper, we propose a 3D structured orthogonal matching pursuit algorithm based channel estimation technique to solve this problem. First, we show that the OTFS MIMO channel exhibits 3D structured sparsity: normal sparsity along the delay dimension, block sparsity along the Doppler dimension, and burst sparsity along the angle dimension. Based on the 3D structured channel sparsity, we then formulate the downlink channel estimation problem as a sparse signal recovery problem. Simulation results show that the proposed algorithm can achieve accurate channel state information with low pilot overhead.}, number={16}, journal={IEEE Transactions on Signal Processing}, author={Shen, W. and Dai, L. and An, J. and Fan, P. and Heath, R.W.}, year={2019}, pages={4204–4217} }
 @inproceedings{ribero_heath_vikalo_chizhik_valenzuela_2019, title={Deep Learning Propagation Models over Irregular Terrain}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85068968797&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2019.8682491}, abstractNote={Accurate path gain models are critical for coverage prediction and radio frequency (RF) planning in wireless communications. In many settings irregular terrain induces blockages and scattering making it difficult to predict the path gain. Current solutions are either computationally expensive or slope-intercept fits that do not capture local deviations due to terrain variation, leading to large prediction errors. We propose to use machine learning to learn path gain based on terrain elevation as features. We implement different neural network architectures with dense and convolutional layers that could include effects difficult to describe with traditional models (e.g. back scatter). We test our framework on an extensive set of measured path gain data and consistently predict with 5 dB Root Mean Squared Error, an 8 dB improvement over traditional slope-intercept solutions.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Ribero, M. and Heath, R.W. and Vikalo, H. and Chizhik, D. and Valenzuela, R.A.}, year={2019}, pages={4519–4523} }
 @article{zhu_bendlin_akoum_ghosh_heath_2019, title={Directional Frame Timing Synchronization in Wideband Millimeter-Wave Systems with Low-Resolution ADCs}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077333036&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2935733}, abstractNote={In this paper, we propose and evaluate a novel beamforming strategy for directional frame timing synchronization in wideband millimeter-wave (mmWave) systems operating with low-resolution analog-to-digital converters (ADCs). In the employed system model, we assume multiple radio frequency chains equipped at the base station to simultaneously form multiple synchronization beams in the analog domain. We formulate the corresponding directional frame timing synchronization problem as a max-min multicast beamforming problem under low-resolution quantization. We first show that the formulated problem cannot be effectively solved by conventional single-stream beamforming based approaches due to large quantization loss and limited beam codebook resolution. We then develop a new multi-beam probing based directional synchronization strategy, targeting at maximizing the minimum received synchronization signal-to-quantization-plus-noise ratio (SQNR) among all users. Leveraging a common synchronization signal structure design, the proposed approach synthesizes an effective composite beam from the simultaneously probed beams to better trade off the beamforming gain and the quantization distortion. Numerical results reveal that for wideband mmWave systems with low-resolution ADCs, the timing synchronization performance of our proposed method outperforms the existing approaches due to the improvement in the received synchronization SQNR.}, number={11}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, D. and Bendlin, R. and Akoum, S. and Ghosh, A. and Heath, R.W.}, year={2019}, pages={5350–5366} }
 @article{zhu_bendlin_akoum_ghosh_heath_2020, title={Double-Sequence Frequency Synchronization for Wideband Millimeter-Wave Systems with Few-Bit ADCs}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85079767034&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2953263}, abstractNote={In this paper, we propose and evaluate a novel double-sequence low-resolution frequency synchronization method in millimeter-wave (mmWave) systems. In our system model, the base station uses analog beams to send the synchronization signal with infinite-resolution digital-to-analog converters. The user equipment employs a fully digital front end to detect the synchronization signal with low-resolution analog-to-digital converters (ADCs). The key ingredient of the proposed method is the custom designed synchronization sequence pairs, from which there exists an invertible function (a ratio metric) of the carrier frequency offset (CFO) to be estimated. We use numerical examples to show that the ratio metric is robust to the quantization distortion. To implement our proposed method in practice, we propose to optimize the double-sequence design parameters such that: (i) for each individual user, the impact of the quantization distortion on the CFO estimation accuracy is minimized, and (ii) the resulting frequency range of estimation can capture as many users’ CFOs as possible. Numerical results reveal that our proposed algorithm can provide a flexible means to estimate CFO in a variety of low-resolution settings.}, number={2}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, D. and Bendlin, R. and Akoum, S. and Ghosh, A. and Heath, R.W.}, year={2020}, pages={1357–1372} }
 @article{myers_mezghani_heath_2019, title={FALP: Fast Beam Alignment in mmWave Systems with Low-Resolution Phase Shifters}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077049154&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2019.2941490}, abstractNote={Millimeter wave (mmWave) systems can enable high data rates if the link between the transmitting and receiving radios is configured properly. Fast configuration of mmWave links, however, is challenging due to the use of large antenna arrays and hardware constraints. For example, a large amount of training overhead is incurred by exhaustive search-based beam alignment in typical mmWave phased arrays. In this paper, we present a framework called FALP for Fast beam Alignment with Low-resolution Phase shifters. FALP uses an efficient set of antenna weight vectors to acquire channel measurements, and allows faster beam alignment when compared to exhaustive scan. The antenna weight vectors in FALP can be realized in ultra-low power phase shifters whose resolution can be as low as one-bit. From a compressed sensing (CS) perspective, the CS matrix designed in FALP satisfies the restricted isometry property and allows CS algorithms to exploit the fast Fourier transform. The proposed framework also establishes a new connection between channel acquisition in phased arrays and magnetic resonance imaging.}, number={12}, journal={IEEE Transactions on Communications}, author={Myers, N.J. and Mezghani, A. and Heath, R.W.}, year={2019}, pages={8739–8753} }
 @article{heath_2019, title={Feedback from the IEEE Signal Processing Magazine Board Meeting in 2019}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85068116006&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2909381}, abstractNote={I am writing this editorial just after ICASSP 2019 was held in Brighton, United Kingdom. During ICASSP, all IEEE Signal Processing Society (SPS) publications have their annual editorial board meetings. IEEE Signal Processing Magazine (SPM ) is no exception. In this column, I summarize some of the items that we discussed at the meeting and provide a general overview of the state of the magazine. This is also an appropriate time for me to reflect as my three-year term is half completed.}, number={4}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–4} }
 @article{heath_2019, title={Going Toward 6G [From the Editor]}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85065447935&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2899083}, abstractNote={T he cellular industry has just reached another milestone with the development of 5G wireless communication technology. While research in 5G is a core part of the Signal Processing for Communications and Networking (SPCOM) technical committee, it may also be found in several other technical areas. As this is also my core expertise, I have decided to discuss, in this editorial, the expected goals that 5G has and has not achieved and what might be coming in 6G cellular networks. Cellular communication systems are constantly evolving, with new releases every one to two years. Roughly every 10 years, one of the releases is branded as a new generation. Usually, the new generation standard has been designed to achieve some significant performance improvements. Two of the differentiating areas of 5G are illustrated in Figure 1. The “5G flower” is shown in Figure 1(a). It indicates the different performance objectives that might be achieved by a 5G link. Of course, they are conflicting and cannot be optimized simultaneously, e.g. low latency, high mobility, and high data rates may not be possible at the same time. Figure 1(b) shows some of the key industry verticals. Many different industries have made use of cellular communication in the past, but 5G also considered the requirements of devices associated to the different verticals. From an automobile to a smart meter or an industrial robot. A main motivation for looking at applications of cellular beyond wireless Internet access is alternative revenue sources. To meet the different requirements for new devices, changes are made in all aspects of the cellular system, including the Digital Object Identifier 10.1109/MSP.2019.2899083 Date of publication: 26 April 2019}, number={3}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–4} }
 @article{kountouris_popovski_hou_buzzi_müller_sesia_heath_2019, title={Guest Editorial Ultra-Reliable Low-Latency Communications in Wireless Networks}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063266158&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2019.2902262}, abstractNote={Ultra-high reliability and low latency have not been in the mainstream in most wireless networks. Mobile networks have been driven so far by human-centric communications, delay-tolerant content, and non-critical services. The main target have been to boosting data rate and increasing coverage, adopting a rather best-effort networking approach. As wireless connectivity starts to get the status of a commodity, there is an increasing focus on support of services that rely critically on wireless links and therefore the reliability of the wireless connections. Next generation wireless systems, mainly 5G and beyond, are designed to provide wireless connectivity for massive machine-type communications (mMTC) and to support ultra-reliable, low latency communication (URLLC) for mission-critical services. URLLC scenarios impose stringent requirements in terms of latency (ranging from 1 ms and below to few milliseconds end-to-end latency depending on the use cases) and reliability (higher than 99.9999%). This does not mean that there are interesting applications where reliability is of paramount importance, while latency can be in the order of seconds, as in e.g. certain remote healthcare applications. Nevertheless, the coupling of low-latency networking and reliable communication is mainly driven by the need to push the technology boundaries and address a plethora of socially useful services and business domains that could benefit greatly from it. Some of the most challenging use cases are factory automation and industrial control, automated driving/flying, haptic communications, and real-time remote healthcare. URLLC is also expected to revolutionize processes in the areas of smart cities, smart farming, smart grid, remote manufacturing, and algorithmic trading. Although the URLLC constraints and the nature of real-time mission-critical applications imply the predominance of short packets and low-rate transmissions, future evolution of URLLC may also consider rate requirements. The emergence of immersive services, such as augmented and virtual reality (AR/VR), high-definition entertainment and gaming, and consumer robotics, calls for real-time, high-fidelity, broadband networks operating at latencies of few milliseconds.}, number={4}, journal={IEEE Journal on Selected Areas in Communications}, author={Kountouris, M. and Popovski, P. and Hou, I.-H. and Buzzi, S. and Müller, A. and Sesia, S. and Heath, R.W.}, year={2019}, pages={701–704} }
 @article{heath_2019, title={IEEE Signal Processing Magazine and University Rankings [From the Editor]}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062696478&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2894485}, abstractNote={I recently had the privilege of reading a report by Prof. Domingo Docampo, of the University of Vigo, about research impact and research rankings. What I learned was surprising. I also had the pleasure of having several email exchanges with him regarding this editorial, and I’ve included some of his observations. One of the main rankings of universities worldwide is the Academic Ranking of World Universities (ARWU), sometimes called the ShanghaiRanking [1]. Unlike the U.S. News & World Report global rankings [2], which U.S. universities use most often, the ARWU ranks universities around the world based on a combination of five quantitative inputs and does not incorporate research reputation ranking indicators, which are derived from surveys, or teaching aspects. Based on discussions with international colleagues, I believe that the ARWU is the most commonly accepted ranking outside the United States. The ARWU provides rankings for different academic subjects. The most recent list of these subjects in engineering is presented in Table 1. The list of subjects in engineering includes the core engineering areas around which departments are built in the United States, such as mechanical, electrical and electronic, civil, biomedical, and chemical. There are also topics such as telecommunications engineering (more on this later), which find departments at universities outside the United States and is considered to be equivalent in most ways to electrical engineering in the United States. Other examples in this category include automation and control or marine/ocean engineering. A few subjects are more specialized and may be topics of research centers but not necessarily for academic units, such as nanoscience and nanotechnology or remote sensing. A striking observation is that there is no subject dedicated to signal processing. Five quantitative metrics are used to compute the ranking for a given university. PUB is the total number of “paper type” publications during a five-year period and is obtained from Clarivate Analytics. CNCI is a normalization of the papers from PUB based on the average citations in the appropriate subject area. TOP indexes the top journals in a given area that have been identified by an academic survey (like in electrical and electronic engineering) or by selecting from the InCites Journal Citation Report (JCR) top 20% journals according to their impact factors (e.g., telecommunications engineering). IC is the number of papers with collaborators from two different countr ies. The most recent rank ings, from 2018, were computed based on publications from 2012 to 2016. AWD is the number of people who have received what is considered the top prize in that field (for example, the IEEE Medal of Honor in electrical and electronic engineering). The data in each category are processed and the results are weighted together. More}, number={2}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–4} }
 @article{klautau_gonzalez-prelcic_heath_2019, title={LIDAR Data for Deep Learning-Based mmWave Beam-Selection}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85067956666&partnerID=MN8TOARS}, DOI={10.1109/LWC.2019.2899571}, abstractNote={Millimeter wave (mmWave) communication systems can leverage information from sensors to reduce the overhead associated with link configuration. Light detection and ranging (LIDAR) is one sensor widely used in autonomous driving for high resolution mapping and positioning. This letter shows how LIDAR data can be used for line-of-sight detection and to reduce the overhead in mmWave beam-selection. In the proposed distributed architecture, the base station broadcasts its position. The connected vehicle leverages its LIDAR data to suggest a set of beams selected via a deep convolutional neural network. Co-simulation of communications and LIDAR in a vehicle-to-infrastructure (V2I) scenario confirm that LIDAR can help configuring mmWave V2I links.}, number={3}, journal={IEEE Wireless Communications Letters}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Klautau, Aldebaro and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2019}, pages={909–912} }
 @article{ali_de carvalho_heath_2019, title={Linear Receivers in Non-Stationary Massive MIMO Channels with Visibility Regions}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85068006900&partnerID=MN8TOARS}, DOI={10.1109/LWC.2019.2898572}, abstractNote={In a massive MIMO system with large arrays, the channel becomes spatially non-stationary. We study the impact of spatial non-stationarity characterized by visibility regions (VRs) where the channel energy is significant on a portion of the array. Relying on a channel model based on VRs, we provide expressions of the signal-to-interference-plus-noise ratio (SINR) of conjugate beamforming (CB) and zero-forcing (ZF) precoders. We also provide an approximate deterministic equivalent of the SINR of ZF precoders. We identify favorable and unfavorable multi-user configurations of the VRs and compare the performance of both stationary and non-stationary channels through analysis and numerical simulations.}, number={3}, journal={IEEE Wireless Communications Letters}, author={Ali, A. and De Carvalho, E. and Heath, R.W.}, year={2019}, pages={885–888} }
 @inproceedings{saxena_mezghani_heath_andrews_2019, title={Linear Transmit Precoding with Optimized Dithering}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083304898&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9048877}, abstractNote={Implementation of 1-bit Digital-to-analog converters (DACs) at base station (BS) antennas reduces power consumption and hardware complexity for a downlink massive MIMO system while introducing non-linear impairments in the transmitted signal. Through low complexity quantized linear precoding at the BS it is possible to mitigate detrimental effects of these impairments on the received signal. For a small number of users, however, quantization noise components across BS antennas become highly correlated, deteriorating per-user performance severely. In this paper, we improve performance and reduce correlation among quantization noise components by adding colored dither to the signal before DAC. Using the Bussgang decomposition up to the third order harmonics, we estimate the closed form asymptotic expression for per-user signal to quantization, interference and noise ratio (SQINR) of the received signal as a function of the dither power. We evaluate optimum dither power by maximizing SQINR.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Saxena, A.K. and Mezghani, A. and Heath, R.W. and Andrews, J.G.}, year={2019}, pages={838–842} }
 @inproceedings{myers_heath_2019, title={Localized Random Sampling for Robust Compressive Beam Alignment}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85069004787&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2019.8683126}, abstractNote={Compressed sensing (CS)-based beam alignment is a promising solution for rapid link configuration in millimeter wave (mmWave) systems that use large arrays. Translating CS to practical mmWave radios, however, can be challenging due to carrier frequency offset (CFO). Standard sparse recovery techniques that use random sampling strategies to acquire channel measurements can fail even if there is a slight mismatch in carrier frequencies. In this paper, we show that restricting the randomness in compressive sampling to local sets can achieve robustness to structured errors due to CFO. The proposed approach requires fewer channel measurements than comparable algorithms and has the same complexity as standard CS.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Myers, N.J. and Heath, R.W.}, year={2019}, pages={4644–4648} }
 @inproceedings{takeda_klautau_mezghani_heath_2019, title={MIMO Channel Estimation with Non-Ideal ADCS: Deep Learning Versus GAMP}, volume={2019-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077709553&partnerID=MN8TOARS}, DOI={10.1109/MLSP.2019.8918799}, abstractNote={Channel estimation for massive MIMO using coarse quantizers is nontrivial due to severe nonlinear distortions caused by quantization and the large dimensional MIMO channel. The best solutions to this problem nowadays are based on the generalized approximate message passing (GAMP) and its variations. However, there are practical issues such as nonideal quantizers that may violate the assumptions in which GAMP algorithms rely. This motivates research on methods based on deep learning (DL), which provides a framework for designing new communication systems that embrace practical impairments. We explore DL applied to channel estimation of MIMO systems with low resolution analog-to-digital converters (ADCs). Assuming millimeter wave MIMO, the channel estimation results indicate that a single neural network trained in a range of practical conditions is more robust to ADC impairments than a GAMP variant. For example, for a fixed wireless scenario with channels obtained from raytracing, DL achieved a normalized mean-square error lower than GAMP’s by more than 5 dB.}, booktitle={IEEE International Workshop on Machine Learning for Signal Processing, MLSP}, author={Takeda, M.Y. and Klautau, A. and Mezghani, A. and Heath, R.W.}, year={2019} }
 @inproceedings{mezghani_heath_2019, title={MIMO beampattern and waveform design with low resolution DACs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073101894&partnerID=MN8TOARS}, DOI={10.1109/RADAR.2019.8835634}, abstractNote={Digital beamforming and waveform generation techniques in MIMO radar offer enormous advantages in terms of flexibility for signal design and performance compared to conventional radar systems based on analog implementations. To allow for such fully digital design with an efficient hardware complexity, we consider the use of low resolution digital-to-analog converters (DACs) while maintaining a separate radiofrequency chain per antenna. A sum of squared residuals (SSR) formulation for the beampattern and spectral shaping problem is solved based on the Generalized Approximate Message Passing (GAMP) algorithm. Numerical results demonstrate good performance in terms of spectral shaping as well as cross-correlation properties of the different probing waveforms even with just 2-bit resolution per antenna.}, booktitle={2019 IEEE Radar Conference, RadarConf 2019}, author={Mezghani, A. and Heath, R.W.}, year={2019} }
 @article{heath_2019, title={Making a Good Feature Article Submission [From the Editor]}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072219517&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2918010}, abstractNote={One of the main items of feedback during our recent IEEE Periodicals Review and Advisory Committee (PRAC) meeting was that IEEE Signal Processing Magazine (SPM) rejected too many feature article white papers as being “out of scope.” In this editorial, I attempt to outline the key features of a good feature article. I hope that you readers— as potential feature article authors—will also consider these tips when you prepare your submissions. I will start the description by providing some basic background. First and foremost, a feature article is a tutorial paper not associated with a special issue. In particular, it should not include new ideas or fundamental results, as might appear in IEEE Transactions on Signal Processing, for example. Any article that features new results is not in the scope of a feature article and will be rejected. A typical feature article is focused on a topic that is narrow enough to be adequately covered within the page and reference limits of the article (more on this later), with enough breadth to be inclusive of the perspectives from different authors. Good articles will provide an introduction to the area, provide a novel perspective on different technical directions, and, most importantly, will highlight directions for future work. The key relevant contributions of a feature article lie in the careful selection of references, thoughtful perspectives on them, and ideas for future work. Graphics should not be overlooked. Having insightful illustrations, and simulations that combine perspectives from different work, can be very useful for future readers. Feature articles focus on a hot topic or may provide a new perspective on an established topic. For example, an article that reviews fundamentals and highlights new applications of a tool would be a good fit for a feature article. Articles that focus purely on history or on teaching a mathematical tool may be a better fit for a column in the magazine. Submitting a feature article is a twostep process. The first step involves the submission of what is called a white paper. The paper is sent by Area Editor Matthew McKay for review to the Senior Editorial Board. Normally, about ten short reviews are received, which are used to indicate whether the full paper should be invited, if the white paper should be revised, or if a full paper is declined. Of course, being invited for a full paper is not a guarantee that the paper will be accepted eventually. The paper will be sent out to reviewers for detailed feedback, who are not, in general, on the editorial board. While it may seem like a hassle to prepare a white paper, it is actually a service for potential authors. The effort to prepare a white paper should be much lower than preparing the entire paper draft. The white paper stage is an opportunity for the authors to get early feedback, for example, that their article is not a good fit for the magazine. This avoids spending time on a tutorial paper that cannot be resubmitted to a different publication with different requirements. A white paper is usually five pages (or fewer) and must include several components. I enumerate each component and explain how this information is used in the following. ■ Proposed title: this is important to set the stage for the scope of the article. Good titles should not be too long, but they should not be too generic. Be sure to check the titles of related papers and be sure to differentiate your title. ■ Author list including biographies: the list is used to determine if the authors have expertise in the area. Good feature articles will be coauthored by different research groups. This helps to ensure the diverse perspective on different research lines as expected in tutorial and overview papers. ■ History, motivation, and significance of the topic: this is the place to make your case that a feature article will be of broad interest to the signal processing community. It is useful to cite some references in this section. It is also important to highlight differences with other tutorial, overview, or survey papers on related topics. ■ An outline of the proposed paper: here you provide a section and subsection outline of the final paper. It is useful to briefly mention the content of each section and also to list the papers that you plan to cite in this section. Making a Good Feature Article Submission}, number={5}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–4} }
 @article{myers_heath_2019, title={Message Passing-Based Joint CFO and Channel Estimation in mmWave Systems with One-Bit ADCs}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85067099773&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2909865}, abstractNote={Channel estimation at millimeter wave (mmWave) carrier frequencies is challenging when large antenna arrays are used. Prior work has leveraged the sparse nature of mmWave channels via compressed sensing-based algorithms for channel estimation. Most of these algorithms, though, assume perfect synchronization and are vulnerable to phase errors that arise due to carrier frequency offset and phase noise. Recently, sparsity-aware, non-coherent beamforming algorithms that are robust to phase errors were proposed for narrowband phased array systems with full resolution analog-to-digital converters. Such energy-based algorithms, however, are not robust to heavy quantization at the receiver. In this paper, we develop a joint carrier frequency offset and wideband channel estimation algorithm that is scalable across different hardware architectures. Our method exploits the sparse nature of mmWave channels in the angle-delay domain, in addition to the compressibility of the phase error vector. We formulate the joint estimation as a quantized sparse bilinear optimization problem and then use message passing for recovery. We also give an efficient implementation of a generalized bilinear message passing algorithm for the joint estimation in one-bit receivers. Simulation results show that our method is able to estimate the frequency offset and the channel compressibility, even in the presence of phase noise.}, number={6}, journal={IEEE Transactions on Wireless Communications}, author={Myers, N.J. and Heath, R.W.}, year={2019}, pages={3064–3077} }
 @article{wang_klautau_ribero_soong_heath_2019, title={MmWave Vehicular Beam Selection with Situational Awareness Using Machine Learning}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85069773768&partnerID=MN8TOARS}, DOI={10.1109/ACCESS.2019.2922064}, abstractNote={Establishing and tracking beams in millimeter-wave (mmWave) vehicular communication is a challenging task. Large antenna arrays and narrow beams introduce significant system overhead configuring the beams using exhaustive beam search. In this paper, we propose to learn the optimal beam pair index by exploiting the locations and types of the receiver vehicle and its neighboring vehicles (situational awareness), leveraging machine learning classification and past beam training data. We formulate the mmWave beam selection as a multi-class classification problem based on hand-crafted features that capture the situational awareness in different coordinates. We then provide a comprehensive comparison of the different classification models and various levels of situational awareness. Furthermore, we examine several practical issues in the implementation: localization is susceptible to inaccuracy; situational awareness at the base station (BS) can be outdated due to vehicle mobility and limited location reporting frequencies; the situational awareness may be incomplete since vehicles could be invisible to the BS if they are not connected. To demonstrate the scalability of the proposed beam selection solution in the large antenna array regime, we propose two solutions to recommend multiple beams and exploit an extra phase of beam sweeping among the recommended beams. The numerical results show that situational awareness-assisted beam selection using machine learning is able to provide beam prediction, with accuracy that increases with more complete knowledge of the environment.}, journal={IEEE Access}, author={Wang, Y. and Klautau, A. and Ribero, M. and Soong, A.C.K. and Heath, R.W.}, year={2019}, pages={87479–87493} }
 @article{yi_liu_deng_nallanathan_heath_2019, title={Modeling and Analysis of MmWave V2X Networks with Vehicular Platoon Systems}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078538865&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2019.2947928}, abstractNote={Due to the low traffic congestion, high fuel efficiency, and comfortable travel experience, vehicular platoon systems (VPSs) become one of the most promising applications in millimeter wave (mmWave) vehicular networks. In this paper, an effective spatial framework for mmWave vehicle-to-everything (V2X) networks with VPSs is proposed by utilizing stochastic geometry approaches. Base stations (BSs) are modeled by a Poisson point process and vehicles are distributed according to multiple type II Matérn hard-core processes. To characterize the blockage process caused by vehicles, a closed-form expression is deduced to distinguish line-of-sight (LOS) and non-LOS transmission. This expression demonstrates that LOS links are independent of horizontal communication distances. Several closed-form probability density functions of the communication distance between a reference platoon and its serving transmitter (other platoons or BSs) are derived for analyzing the generated path loss. After designing three practical user association techniques, tractable expressions for coverage probabilities are figured out. Our work theoretically shows that the maximum density of VPSs exists and large antenna scales benefit the networks’ coverage performance. The numerical results illustrate that platoons outperform individual vehicles in terms of road spectral efficiency and the considered system is LOS interference-limited.}, number={12}, journal={IEEE Journal on Selected Areas in Communications}, author={Yi, W. and Liu, Y. and Deng, Y. and Nallanathan, A. and Heath, R.W.}, year={2019}, pages={2851–2866} }
 @inproceedings{jurdi_andrews_heath_2019, title={On the Violation of Hard Deadlines in Networked Control Systems}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85070202263&partnerID=MN8TOARS}, DOI={10.1109/ICC.2019.8761678}, abstractNote={Many control applications demand stringent latency and reliability requirements that cannot be met by existing wireless technologies. Prior work has modeled a pilot-assisted, variable-rate communication procedure that describes the transmission of commands from a centralized controller to a number of agents (e.g. actuators) through a number of access points (APs). This procedure comprises two phases with pre-allocated time budgets per transmission cycle. There is a training phase where channel states between AP-agent pairs are estimated, and a downlink phase where the commands are sequentially transmitted at adapted rates before the start of a new cycle — a hard deadline. System reliability is compromised, though, when the agents fail to receive the controller's commands before the start of a subsequent cycle. In this paper, we calculate a closed-form expression for the probability of hard-deadline violation when there are two agents. We find upper and lower bounds on this probability when there are more agents. We observe that these bounds approximate the probability of hard-deadline violation which reflects the reliability of the system.}, booktitle={IEEE International Conference on Communications}, author={Jurdi, R. and Andrews, J.G. and Heath, R.W.}, year={2019} }
 @article{va_shimizu_bansal_heath_2019, title={Online Learning for Position-Aided Millimeter Wave Beam Training}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85064654092&partnerID=MN8TOARS}, DOI={10.1109/ACCESS.2019.2902372}, abstractNote={Accurate beam alignment is essential for the beam-based millimeter wave communications. The conventional beam sweeping solutions often have large overhead, which is unacceptable for mobile applications, such as a vehicle to everything. The learning-based solutions that leverage the sensor data (e.g., position) to identify the good beam directions are one approach to reduce the overhead. Most existing solutions, though, are supervised learning, where the training data are collected beforehand. In this paper, we use a multi-armed bandit framework to develop the online learning algorithms for beam pair selection and refinement. The beam pair selection algorithm learns coarse beam directions in some predefined beam codebook, e.g., in discrete angles, separated by the 3 dB beamwidths. The beam refinement fine-tunes the identified directions to match the peak of the power angular spectrum at that position. The beam pair selection uses the upper confidence bound with a newly proposed risk-aware feature, while the beam refinement uses a modified optimistic optimization algorithm. The proposed algorithms learn to recommend the good beam pairs quickly. When using  $16\times 16$  arrays at both transmitter and receiver, it can achieve, on average, 1-dB gain over the exhaustive search (over  $271\times 271$  beam pairs) on the unrefined codebook within 100 time steps with a training budget of only 30 beam pairs.}, journal={IEEE Access}, author={Va, V. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2019}, pages={30507–30526} }
 @article{venugopal_gonzalez-prelcic_heath_2019, title={Optimal Frequency-Flat Precoding for Frequency-Selective Millimeter Wave Channels}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077304736&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2932932}, abstractNote={The two key features of millimeter wave-(mmWave) based MIMO communication are the use of large antenna arrays at the transceivers and large bandwidth. The former complicates the design of optimal beamformers, while the latter makes the system frequency-selective and, thus, requires equalization. Conventionally, for wideband mmWave channels, choosing the precoders/combiners have involved frequency-selective designs that are based on channel state information. In this paper, we show that under some assumptions, semi-unitary frequency-flat precoding and combining are sufficient for low-scattering millimeter wave channels. To show this, we evaluate the conditions and practical settings under which the dominant subspaces of the frequency-domain channel matrices are similar. We model the frequency-dependence of uniform linear antenna arrays, which leads to what is known as beam-squint, for two different practical setting to analyze the optimality of frequency-flat beamforming designs under practical hardware impairments. For the cases when the optimality conditions hold, we propose novel techniques based on compressive subspace estimation to design the optimal frequency-flat, semi-unitary precoders and combiners. Simulation results show that the system achieves near-digital spectral efficiencies at very small implementation cost of beamformers and channel training overhead.}, number={11}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Venugopal, Kiran and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2019}, pages={5098–5112} }
 @article{heath_2019, title={Organizing a Special Issue of IEEE SPM [From the Editor]}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074524900&partnerID=MN8TOARS}, DOI={10.1109/MSP.2019.2938152}, abstractNote={In my September editorial [1], I outlined the important components of good feature articles in response to feedback from our recent IEEE Periodicals Review and Advisory Committee (PRAC) meeting. In this issue's editorial, I discuss the process of organizing a special issue (SI) for IEEE Signal Processing Magazine (SPM). The purpose is twofold. First, I want to encourage readers to become involved in organizing an SI. Second, I want provide more information about the decision-making process so that authors understand how they can have an article accepted.}, number={6}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–4} }
 @inproceedings{mollén_choi_larsson_heath_2019, title={Performance of linear receivers for wideband massive MIMO with one-bit ADCs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073575189&partnerID=MN8TOARS}, booktitle={WSA 2016 - 20th International ITG Workshop on Smart Antennas}, author={Mollén, C. and Choi, J. and Larsson, E.G. and Heath, R.W.}, year={2019}, pages={509–515} }
 @inproceedings{dias_klautau_gonzalez-prelcic_heath_2019, title={Position and LIDAR-Aided mmWave Beam Selection using Deep Learning}, volume={2019-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072321417&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2019.8815569}, abstractNote={Modern communication systems may benefit from the availability of sensor data leveraged by sophisticated machine learning algorithms. We recently described how LIDAR (light detection and ranging) on a vehicle can be used for line-of-sight detection and to reduce the overhead associated with link configuration in millimeter wave communication systems. LIDAR is used in autonomous driving for high resolution mapping and positioning. In this paper, we present new LIDAR-based features for machine learning and compare the previously proposed distributed architecture with two centralized schemes: using a single LIDAR located at the base station (BS) and fusing LIDAR data from neighboring vehicles at the BS. We also quantify the advantages of LIDAR-based solutions over solutions based on connected vehicles informing their positions. We use deep convolutional neural networks to process images composed of LIDAR data and/or positions. Using co-simulation of communications and LIDAR in a vehicle-to-infrastructure (V2I) scenario, we find that the distributed LIDAR-based architecture provides robust performance irrespective of car penetration rate, outperforming the single LIDAR at BS and position-based solutions. Under the simulated conditions, the benefits of a centralized data fusion over distributed processing are not significant, meaning that machine learning for line-of-sight detection and beam selection can be conveniently executed at vehicles equipped with LIDAR.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Dias, M. and Klautau, A. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2019} }
 @inproceedings{xia_heath_prelcic_2019, title={Robust analog precoding designs for millimeter wave MIMO transceivers}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073573477&partnerID=MN8TOARS}, booktitle={WSA 2016 - 20th International ITG Workshop on Smart Antennas}, author={Xia, P. and Heath, R.W. and Prelcic, N.G.}, year={2019}, pages={8–15} }
 @inproceedings{buckley_heath_2019, title={Selective OFDM transmission for simultaneous wireless information and power transfer}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85081965873&partnerID=MN8TOARS}, DOI={10.1109/GLOBECOM38437.2019.9013415}, abstractNote={Simultaneous wireless information and power transfer (SWIPT) augments a wireless device's capability to transmit data by adding a harvesting component to the standard information processing receiver. With SWIPT, both information processing and energy scavenging are combined to allow a SWIPT device to become its own recurrent power source. This paper proposes, analyzes, and compares the functionality of a novel selective orthogonal frequency-division multiplexing (OFDM) receiver architecture and evaluates the rate-energy (R-E) tradeoff of the system. Additionally, the feasibility of harvesting sufficient energy to power the OFDM processing portion of the receiver is examined. To facilitate the evaluation, Monte Carlo simulations are run presenting system level R-E tradeoff and feasibility performance. Selective OFDM transmission is found to provide improved energy harvesting and sustainability over conventional OFDM transmission.}, booktitle={2019 IEEE Global Communications Conference, GLOBECOM 2019 - Proceedings}, author={Buckley, R.F. and Heath, R.W.}, year={2019} }
 @inproceedings{kaleva_myers_tolli_heath_madhow_2019, title={Short Range 3D MIMO mmWave Channel Reconstruction via Geometry-aided AoA Estimation}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083312079&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9048890}, abstractNote={In some millimeter wave (mmWave) applications, such as wearables, the distance between the transceivers is relatively short. Further, the channel has significant angular spread in both azimuth and elevation domains even in line-of-sight (LoS). Under such conditions, hybrid mmWave architectures with multiple analog uniform planar arrays (UPAs) potentially allow spatial multiplexing even in LoS provided that the high rank structure of the channel is captured. The conventional far-field channel estimation methods are not generally suitable for these scenarios and perform poorly. We consider parametrized spatial channel estimation, where the known antenna array geometry is exploited to recover the angle-of-arrivals (AoAs) of the 3D multiple-input multiple-output (MIMO) channel. The channel is then reconstructed using these AoA estimates and the known geometry. We show that conventional maximum a posteriori (MAP) estimation of the channel parameters suffers from high computational complexity and may not be not applicable for low powered devices. To this end, we propose a lower complexity message passing algorithm for short range channel estimation. We show, by numerical examples, that the proposed technique achieves good performance with fewer pilot resources when compared to compressed sensing or antenna specific pilot based channel estimation.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Kaleva, J. and Myers, N.J. and Tolli, A. and Heath, R.W. and Madhow, U.}, year={2019}, pages={427–431} }
 @inproceedings{zhang_patel_shakkottai_heath_2019, title={Side-information-aided non-coherent beam alignment design for millimeter wave systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85069752514&partnerID=MN8TOARS}, DOI={10.1145/3323679.3326532}, abstractNote={Designing efficient and robust beam alignment strategies for millimeter wave (mmWave) systems is important for overcoming training overheads and practical hardware impairments. In this work, we leverage side information in the form of prior knowledge of the angular support of the propagation channel (direction information) to design a compressive sensing (CS) based beam alignment algorithm. Existing CS based channel estimation approaches assume perfect phase information of the measurements, which is not the case with the low-cost off-the-shelf mmWave phased arrays. Instead, we develop a two-stage algorithm where we use the magnitude of measurements (aka non-coherent measurements); using phase retrieval (PR) followed by sparse recovery, we estimate the channel gain across various (quantized) spatial angles. To validate the proposed algorithm, we develop a fully reconfigurable mmWave testbed with custom-made 2-bit phased arrays. We perform a careful calibration to the phased arrays, thus enabling generations of precise desired beam patterns. Our implementation and real experiments validate both the proposed algorithm and calibration process by demonstrating consistency between the experimental results and the theoretical analysis.}, booktitle={Proceedings of the International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc)}, author={Zhang, Y. and Patel, K. and Shakkottai, S. and Heath, R.W.}, year={2019}, pages={341–350} }
 @article{park_ali_gonzalez-prelcic_heath_2020, title={Spatial Channel Covariance Estimation for Hybrid Architectures Based on Tensor Decompositions}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85079821667&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2951135}, abstractNote={Spatial channel covariance information can replace full instantaneous channel state information for the analog precoder design in hybrid analog/digital architectures. Obtaining spatial channel covariance estimation, however, is challenging in the hybrid structure due to the use of fewer radio frequency (RF) chains than the number of antennas. In this paper, we propose a spatial channel covariance estimation method based on higher-order tensor decomposition for spatially sparse time-varying frequency-selective channels. The proposed method leverages the fact that the channel can be represented as a low-rank higher-order tensor. We also derive the Cramér-Rao lower bound on the estimation accuracy of the proposed method. Numerical results and theoretical analysis show that the proposed tensor-based approach achieves higher estimation accuracy in comparison with prior compressive-sensing-based approaches or conventional angle-of-arrival estimation approaches.}, number={2}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Park, Sungwoo and Ali, Anum and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2020}, pages={1084–1097} }
 @article{ali_gonzalez-prelcic_heath_2019, title={Spatial Covariance Estimation for Millimeter Wave Hybrid Systems Using Out-of-Band Information}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85076699819&partnerID=MN8TOARS}, DOI={10.1109/TWC.2019.2932404}, abstractNote={In high mobility applications of millimeter wave (mmWave) communications, e.g., vehicle-to-everything communication and next-generation cellular communication, frequent link configuration can be a source of significant overhead. We use the sub-6 GHz channel covariance as an out-of-band side information for mmWave link configuration. Assuming: 1) a fully digital architecture at sub-6 GHz and 2) a hybrid analog–digital architecture at mmWave, we propose an out-of-band covariance translation approach and an out-of-band aided compressed covariance estimation approach. For covariance translation, we estimate the parameters of sub-6 GHz covariance and use them in theoretical expressions of covariance matrices to predict the mmWave covariance. For out-of-band aided covariance estimation, we use weighted sparse signal recovery to incorporate out-of-band information in compressed covariance estimation. The out-of-band covariance translation eliminates the in-band training completely, whereas out-of-band aided covariance estimation relies on in-band as well as out-of-band training. We also analyze the loss in the signal-to-noise ratio due to an imperfect estimate of the covariance. The simulation results show that the proposed covariance estimation strategies can reduce the training overhead compared to the in-band only covariance estimation.}, number={12}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Ali, Anum and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2019}, pages={5471–5485} }
 @inproceedings{ribeiro_de almeida_myers_heath_2019, title={Tensor-based Estimation of mmWave MIMO Channels with Carrier Frequency Offset}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85068984247&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2019.8683496}, abstractNote={Millimeter wave multiple-input-multiple-output (MIMO) achieves the best performance when reliable channel state information is used to design the beams. Most channel estimation methods proposed in the literature, however, ignore practical hardware impairments such as carrier frequency offset (CFO) and may fail under such impairment. In this paper, we present a joint CFO and channel estimation method based on tensor modeling and compressed sensing. Simulation results indicate that the proposed method yields better channel recovery performance than the benchmark and that it is more robust to a small number of channel measurements.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Ribeiro, L.N. and De Almeida, A.L.F. and Myers, N.J. and Heath, R.W.}, year={2019}, pages={4155–4159} }
 @inproceedings{klautau_batista_gonzalez-prelcic_wang_heath_2018, title={5G MIMO data for machine learning: Application to beam-selection using deep learning}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85057288224&partnerID=MN8TOARS}, DOI={10.1109/ITA.2018.8503086}, abstractNote={The increasing complexity of configuring cellular networks suggests that machine learning (ML) can effectively improve 5G technologies. Deep learning has proven successful in ML tasks such as speech processing and computational vision, with a performance that scales with the amount of available data. The lack of large datasets inhibits the flourish of deep learning applications in wireless communications. This paper presents a methodology that combines a vehicle traffic simulator with a ray-tracing simulator, to generate channel realizations representing 5G scenarios with mobility of both transceivers and objects. The paper then describes a specific dataset for investigating beam-selection techniques on vehicle-to-infrastructure using millimeter waves. Experiments using deep learning in classification, regression and reinforcement learning problems illustrate the use of datasets generated with the proposed methodology.}, booktitle={2018 Information Theory and Applications Workshop, ITA 2018}, author={Klautau, A. and Batista, P. and Gonzalez-Prelcic, N. and Wang, Y. and Heath, R.W.}, year={2018} }
 @inproceedings{jeon_lee_hong_heath_2018, title={A Low Complexity ML Detection for Uplink Massive MIMO Systems with One-Bit ADCs}, volume={2018-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050125824&partnerID=MN8TOARS}, DOI={10.1109/VTCSpring.2018.8417580}, abstractNote={This paper presents a low complexity maximum likelihood detection (MLD) algorithm called one-bit-sphere- decoding for an uplink massive multiple-input multiple- output (MIMO) system with one-bit analog-to-digital converters (ADCs). The idea of the proposed algorithm is to estimate the transmitted symbol vector sent by uplink users (a codeword vector) by searching over a sphere, which contains a collection of codeword vectors close to the received signal vector at the base station in terms of a weighted Hamming distance. To reduce the computational complexity for the construction of the sphere, the proposed algorithm divides the received signal vector into multiple sub-vectors each with reduced dimension. Then, it generates multiple spheres in parallel, where each sphere is centered at the sub- vector and contains a list of sub-codeword vectors. Simulation results demonstrate that the proposed algorithm achieves near-MLD performance, while reducing the computational complexity compared to the existing MLD method.}, booktitle={IEEE Vehicular Technology Conference}, author={Jeon, Y.-S. and Lee, N. and Hong, S.-N. and Heath, R.W.}, year={2018}, pages={1–5} }
 @inproceedings{jurdi_gupta_andrews_heath_2018, title={A Model for Infrastructure Sharing in mmWave Cellular Networks}, volume={2018-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85051441124&partnerID=MN8TOARS}, DOI={10.1109/ICC.2018.8422673}, abstractNote={Competing cellular operators aggressively share infrastructure in many major US markets. If operators also share spectrum licenses, intra- cellular interference will become correlated with inter-cellular interference. We propose a mathematical framework to model a two-operator millimeter-wave (mmWave) cellular network with co- located base-stations (BSs). We then characterize the SINR distribution for an arbitrary network to understand the impact of varying the spatial correlation between the operators' networks. An interesting observation is that sharing spectrum and infrastructure yields a higher rate coverage probability for higher rate thresholds, but has a lower coverage for lower thresholds. This suggests that networks catering for low-rate, limited-QoS devices, are at a disadvantage when spectrum and infrastructure are shared.}, booktitle={IEEE International Conference on Communications}, author={Jurdi, R. and Gupta, A.K. and Andrews, J.G. and Heath, R.W.}, year={2018} }
 @article{wang_wong_jin_zheng_heath_2018, title={A New Look at Physical Layer Security, Caching, and Wireless Energy Harvesting for Heterogeneous Ultra-Dense Networks}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85048862157&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2018.1700439}, abstractNote={Heterogeneous ultra-dense networks enable ultra-high data rates and ultra-low latency through the use of dense sub-6 GHz and millimeter-wave small cells with different antenna configurations. Existing work has widely studied spectral and energy efficiency in such networks and shown that high spectral and energy efficiency can be achieved. This article investigates the benefits of heterogeneous ultra-dense network architecture from the perspectives of three promising technologies, physical layer security, caching, and wireless energy harvesting, and provides an enthusiastic outlook toward application of these technologies in heterogeneous ultra-dense networks. Based on the rationale of each technology, opportunities and challenges are identified to advance the research in this emerging network.}, number={6}, journal={IEEE Communications Magazine}, author={Wang, L. and Wong, K.-K. and Jin, S. and Zheng, G. and Heath, R.W.}, year={2018}, pages={49–55} }
 @inproceedings{a uav-based traffic monitoring system - invited paper_2018, volume={2018-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050967177&partnerID=MN8TOARS}, DOI={10.1109/VTCSpring.2018.8417546}, abstractNote={Traffic monitoring is important in urban areas. Traffic sensing solutions based on static cameras, however, do not offer a flexible, inexpensive solution for short-term traffic studies. To overcome the limitations of static solutions, we propose an aerial traffic monitoring system. It uses unmanned aerial vehicles (UAV) and onboard cameras to capture traffic video, which is sent and processed in the cloud. To validate the proposed system, we implemented a prototype with a quadcopter, an onboard camera with video and data processing algorithms, and a web application. The video processing module includes a vehicle detection stage based on the Haar cascade model and a frame-by-frame tracking stage. After experimental testing based on videos collected by the UAV, we conclude that the designed system can monitor traffic with high accuracy and flexibility.}, booktitle={IEEE Vehicular Technology Conference}, year={2018}, pages={1–5} }
 @article{algorithms for the construction of incoherent frames under various design constraints_2018, volume={152}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85049335757&partnerID=MN8TOARS}, DOI={10.1016/j.sigpro.2018.06.015}, abstractNote={Unit norm finite frames are generalizations of orthonormal bases with many applications in signal processing. An important property of a frame is its coherence, a measure of how close any two vectors of the frame are to each other. Low coherence frames are useful in compressed sensing applications. When used as measurement matrices, they successfully recover highly sparse solutions to linear inverse problems. This paper describes algorithms for the design of various low coherence frame types: real, complex, unital (constant magnitude) complex, sparse real and complex, nonnegative real and complex, and harmonic (selection of rows from Fourier matrices). The proposed methods are based on solving a sequence of convex optimization problems that update each vector of the frame. This update reduces the coherence with the other frame vectors, while other constraints on its entries are also imposed. Numerical experiments show the effectiveness of the methods compared to the Welch bound, as well as other competing algorithms, in compressed sensing applications.}, journal={Signal Processing}, year={2018}, pages={363–372} }
 @article{park_heath_2018, title={Analysis of Blockage Sensing by Radars in Random Cellular Networks}, volume={25}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053135080&partnerID=MN8TOARS}, DOI={10.1109/LSP.2018.2869279}, abstractNote={We characterize the detection probability of blockage sensing by radars deployed on towers in cellular networks. If the signal-to-interference ratio of the reflected pilot signal is larger than the predefined threshold and there is no other blockage between the radar and the corresponding blockage, the radar successfully detects the blockage. Modeling radar and blockage locations using stochastic geometry, we derive the detection probability as a function of the system parameters, chiefly the radar and blockage densities. Leveraging the obtained expression, we provide some guidelines for efficiently deploying radars to enhance the detection probability.}, number={11}, journal={IEEE Signal Processing Letters}, author={Park, J. and Heath, R.W.}, year={2018}, pages={1620–1624} }
 @inproceedings{usman_heath_shimizu_bansal_2018, title={Automotive radar using IEEE 802.11p signals}, volume={2018-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85049162079&partnerID=MN8TOARS}, DOI={10.1109/WCNC.2018.8377043}, abstractNote={In this paper, we develop a framework for using the dedicated short range communication (DSRC) waveform for the purposes of automotive radar. Our approach operates on the frequency domain channel estimates generated by the OFDM physical layer used in DSRC. We consider a two path channel model, with the first cluster corresponding to direct signal interference and the second cluster corresponding to the signal reflected from the target. The target ranging and direction of arrival information is encoded in the parameters of the reflected path. We estimate the parameters of the direct and reflected path using the least squares matching pursuit algorithm by exploiting their relative power difference. The performance of the algorithm is evaluated through numerical simulations assuming low power omnidirectional 5 dBi antennas, Swerling type 0 and type 3 target models, 10 MHz transmission bandwidth and different analog-to-digital quantization resolutions. Simulations results show submeter accuracy in location estimation for a significant range of target distances.}, booktitle={IEEE Wireless Communications and Networking Conference, WCNC}, author={Usman, K. and Heath, R.W. and Shimizu, T. and Bansal, G.}, year={2018}, pages={1–6} }
 @inproceedings{scarborough_venugopal_alkhateeb_heath_2018, title={Beamforming in Millimeter Wave Systems: Prototyping and Measurement Results}, volume={2018-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85064902438&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2018.8690621}, abstractNote={Demonstrating the feasibility of large antenna array beamforming is essential for realizing mmWave communication systems. This is due to the dependency of these systems on the large array beamforming gains to provide sufficient received signal power. In this paper, the design of a proof-of-concept prototype that demonstrates these gains in practice is explained in detail. We develop a mmWave system with digitally controlled analog front-end. The developed prototype uses 60 GHz phased arrays and universal software radio peripheral (USRP) controllers. The software interface of our design is easily reproducible and can be leveraged for future mmWave prototypes and demonstrations.}, booktitle={IEEE Vehicular Technology Conference}, author={Scarborough, C. and Venugopal, K. and Alkhateeb, A. and Heath, R.W.}, year={2018} }
 @article{gonzalez-coma_rodriguez-fernandez_gonzalez-prelcic_castedo_heath_2018, title={Channel Estimation and Hybrid Precoding for Frequency Selective Multiuser mmWave MIMO Systems}, volume={12}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044368836&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2018.2819130}, abstractNote={Configuring the hybrid precoders and combiners in a millimeter wave multiuser multiple-input multiple-output system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink. In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates the BS designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: First, a factorization of the purely digital solution; and second, an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to the state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.}, number={2}, journal={IEEE Journal on Selected Topics in Signal Processing}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Gonzalez-Coma, Jose P. and Rodriguez-Fernandez, Javier and Gonzalez-Prelcic, Nuria and Castedo, Luis and Heath, Robert W.}, year={2018}, pages={353–367} }
 @inproceedings{channel estimation for millimeter wave mimo systems in the presence of cfo uncertainties_2018, volume={2018-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85051428691&partnerID=MN8TOARS}, DOI={10.1109/ICC.2018.8422342}, abstractNote={Channel estimation at millimeter wave (mmWave) allows designing hybrid precoders and combiners to maximize performance metrics such as the spectral efficiency. A major source of difficulty when estimating the channel is the carrier frequency offset. In this paper, we propose the first joint compressive - Maximum Likelihood (ML) algorithm for joint carrier frequency offset and channel estimation at mmWave. Simulation results show that small estimation errors and near-optimal values of spectral efficiency can be achieved without incurring in significant overhead and/or computational complexity.}, booktitle={IEEE International Conference on Communications}, year={2018} }
 @article{shen_dai_shim_wang_heath_2018, title={Channel feedback based on AoD-adaptive subspace codebook in FDD massive MIMO systems}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85049070240&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2018.2849755}, abstractNote={Channel feedback is essential in frequency division duplexing (FDD) massive multiple-input multiple-output (MIMO) systems. Unfortunately, prior work on multiuser MIMO has shown that the feedback overhead scales linearly with the number of base station (BS) antennas, which is large in massive MIMO systems. To reduce the feedback overhead, we propose an angle-of-departure (AoD) adaptive subspace codebook for channel feedback in FDD massive MIMO systems. Our key insight is to leverage the observation that path AoDs vary more slowly than the path gains. Within the angle coherence time, by utilizing the constant AoD information, the proposed AoD-adaptive subspace codebook is able to quantize the channel vector in a more accurate way. From the performance analysis, we show that the feedback overhead of the proposed codebook only scales linearly with a small number of dominant (path) AoDs instead of the large number of BS antennas. Moreover, we compare the proposed quantized feedback technique using the AoD-adaptive subspace codebook with a comparable analog feedback method. Extensive simulations show that the proposed AoD-adaptive subspace codebook achieves good channel feedback quality, while requiring low overhead.}, number={11}, journal={IEEE Transactions on Communications}, author={Shen, W. and Dai, L. and Shim, B. and Wang, Z. and Heath, R.W.}, year={2018}, pages={5235–5248} }
 @article{eltayeb_al-naffouri_heath_2018, title={Compressive Sensing for Millimeter Wave Antenna Array Diagnosis}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040572931&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2018.2790403}, abstractNote={The radiation pattern of an antenna array depends on the excitation weights and the geometry of the array. Due to wind and atmospheric conditions, outdoor millimeter wave antenna elements are subject to full or partial blockages from a plethora of particles like dirt, salt, ice, and water droplets. Handheld devices are also subject to blockages from random finger placement and/or finger prints. These blockages cause absorption and scattering to the signal incident on the array, modify the array geometry, and distort the far-field radiation pattern of the array. This paper studies the effects of blockages on the far-field radiation pattern of linear arrays and proposes several array diagnosis techniques for millimeter wave antenna arrays. The proposed techniques jointly estimate the locations of the blocked antennas and the induced attenuation and phase-shifts given knowledge of the angles of arrival/departure. Numerical results show that the proposed techniques provide satisfactory results in terms of fault detection with reduced number of measurements (diagnosis time) provided that the number of blockages is small compared to the array size.}, number={6}, journal={IEEE Transactions on Communications}, author={Eltayeb, M.E. and Al-Naffouri, T.Y. and Heath, R.W.}, year={2018}, pages={2708–2721} }
 @article{han_huang_heath_2019, title={Connectivity and Blockage Effects in Millimeter-Wave Air-To-Everything Networks}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054348545&partnerID=MN8TOARS}, DOI={10.1109/LWC.2018.2873361}, abstractNote={Millimeter-wave (mmWave) offers high data rate and bandwidth for air-to-everything (A2X) communications including air-to-air, air-to-ground, and air-to-tower. MmWave communication in the A2X network is sensitive to buildings blockage effects. In this letter, we propose an analytical framework to define and characterise the connectivity for an aerial access point (AAP) by jointly using stochastic geometry and random shape theory. The buildings are modelled as a Boolean line-segment process with fixed height. The blocking area for an arbitrary building is derived and minimized by optimizing the altitude of AAP. A lower bound on the connectivity probability is derived as a function of the altitude of AAP and different parameters of users and buildings including their densities, sizes, and heights. This letter yields guidelines on practical mmWave A2X networks deployment.}, number={2}, journal={IEEE Wireless Communications Letters}, author={Han, K. and Huang, K. and Heath, R.W.}, year={2019}, pages={388–391} }
 @inproceedings{klautau_gonzalez-prelcic_mezghani_heath_2019, title={Detection and Channel Equalization with Deep Learning for Low Resolution MIMO Systems}, volume={2018-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062963806&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2018.8645551}, abstractNote={Deep learning (DL) provides a framework for designing new communication systems that embrace practical impairments. In this paper, we present an exploration of DL as applied to design the physical layer for MIMO systems with low resolution analog-to-digital converters. The application of DL is nontrivial thanks to the severe nonlinear distortion caused by quantization and the large dimensional MIMO channel. We investigate network architectures for channel estimation and detection. The channel estimation results indicate that the adopted DL architectures lead to good results in the large signal-to-noise ratio (SNR) regime, but are outperformed by state-of-the-art iterative message passing algorithms. For decoding, we adopted a multilabel classification architecture with implicit equalization and output size scaling linearly with the number of data symbols to be estimated. While feasible for high MIMO dimensions, the adopted DL architecture for decoding converged only for relatively small MIMO dimensions. A main conclusion of our paper is that DL still has potential but more efficient architectures are required, given the convergence problems associated with time-varying channels and 1-bit quantization.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Klautau, A. and Gonzalez-Prelcic, N. and Mezghani, A. and Heath, R.W.}, year={2019}, pages={1836–1840} }
 @article{heath_gonzalez-prelcic_2018, title={Erratum: Deep convolutional neural models for picture-quality prediction (IEEE Signal Processing Magazine (2017) 34: 6 (130-141) DOI: 10.1109/MSP.2017.2736018)}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053384113&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2858444}, abstractNote={Various puzzles, quizzes, games, humorous definitions, or mathematical that should engage the interest of readers.}, number={5}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W. and Gonzalez-Prelcic, N.}, year={2018}, pages={186} }
 @inproceedings{amiri_angjelichinoski_de carvalho_heath_2019, title={Extremely Large Aperture Massive MIMO: Low Complexity Receiver Architectures}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063448873&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2018.8644126}, abstractNote={This paper focuses on new communication paradigms arising in massive multiple-input-multiple-output systems where the antenna array at the base station is of extremely large dimension (xMaMIMO). Due to the extreme dimension of the array, xMaMIMO is characterized by spatial non-stationary field properties along the array; this calls for a multi-antenna transceiver design that is adapted to the array dimension but also its non-stationary properties. We address implementation aspects of xMaMIMO, with computational efficiency as our primary objective. To reduce the computational burden of centralized schemes, we distribute the processing into smaller, disjoint sub-arrays. Then, we consider several low-complexity data detection algorithms as candidates for uplink communication in crowded xMaMIMO systems. Drawing inspiration from coded random access, one of the main contributions of the paper is the design of low complexity scheme that exploits the non-stationary nature of xMaMIMO systems and where the data processing is decentralized. We evaluate the bit-error-rate performance of the transceivers in crowded xMaMIMO scenarios. The results confirm their practical potential.}, booktitle={2018 IEEE Globecom Workshops, GC Wkshps 2018 - Proceedings}, author={Amiri, A. and Angjelichinoski, M. and De Carvalho, E. and Heath, R.W.}, year={2019} }
 @article{park_lee_heath_2018, title={Feedback Design for Multi-Antenna K-Tier Heterogeneous Downlink Cellular Networks}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044866414&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2817193}, abstractNote={We characterize the ergodic spectral efficiency of a non-cooperative and a cooperative type of  $K$ -tier heterogeneous network with limited feedback. In the non-cooperative case, a multi-antenna base station (BS) serves a single-antenna user using maximum-ratio transmission based on limited feedback. In the cooperative case, a BS coordination set is formed by using dynamic clustering across the tiers, wherein the intra-cluster interference is mitigated by using multi cell zero-forcing also based on limited feedback. Modeling the network based on stochastic geometry, we derive analytical expressions for the ergodic spectral efficiency as a function of the system parameters. Leveraging the obtained expressions, we formulate feedback partition problems and obtain solutions to improve the ergodic spectral efficiency. Simulations show the spectral efficiency improvement by using the obtained feedback partitions. Our major findings are as follows: 1) in the non-cooperative case, the feedback is only useful in a particular tier if the mean interference is small enough; 2) in the cooperative case, allocating more feedback to stronger intra-cluster BSs is efficient; and 3) in both cases, the obtained solutions do not change depending on the instantaneous signal-to-interference ratio.}, number={6}, journal={IEEE Transactions on Wireless Communications}, author={Park, J. and Lee, N. and Heath, R.W.}, year={2018}, pages={3861–3876} }
 @article{rodriguez-fernandez_gonzalez-prelcic_venugopal_heath_2018, title={Frequency-Domain Compressive Channel Estimation for Frequency-Selective Hybrid Millimeter Wave MIMO Systems}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85042885110&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2804943}, abstractNote={Channel estimation is useful in millimeter wave (mm-wave) MIMO communication systems. Channel state information allows optimized designs of precoders and combiners under different metrics, such as mutual information or signal-to-interference noise ratio. At mm-wave, MIMO precoders and combiners are usually hybrid, since this architecture provides a means to trade-off power consumption and achievable rate. Channel estimation is challenging when using these architectures, however, since there is no direct access to the outputs of the different antenna elements in the array. The MIMO channel can only be observed through the analog combining network, which acts as a compression stage of the received signal. Most of the prior work on channel estimation for hybrid architectures assumes a frequency-flat mm-wave channel model. In this paper, we consider a frequency-selective mm-wave channel and propose compressed sensing-based strategies to estimate the channel in the frequency domain. We evaluate different algorithms and compute their complexity to expose tradeoffs in complexity overhead performance as compared with those of previous approaches.}, number={5}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Rodriguez-Fernandez, Javier and Gonzalez-Prelcic, Nuria and Venugopal, Kiran and Heath, Robert W.}, year={2018}, pages={2946–2960} }
 @inproceedings{zochmann_va_rupp_heath_2019, title={Geometric Tracking of Vehicular mmWave Channels to Enable Machine Learning of Onboard Sensors}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063436622&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2018.8644440}, abstractNote={Estimating time-selective millimeter wave wireless channels with directive antennas poses a challenging task. A feasible way of relaxing this channel estimation problem is to focus on the tracking of a few multipath components (MPCs). Aligning antenna beams to the tracked MPCs increases the channel coherence time by several orders of magnitude. We propose to track the MPCs geometrically. Our geometric trackers are based on algorithms known as Doppler-bearing tracking. We reformulate recent work on geometric-polar tracking into an efficient recursive version. If the relative position of the MPCs are known, other sensors on board of a vehicle, for example, lidar, radar, camera, will be capable of performing supervised learning based on their own observed data. Learning the relationship between sensor data and MPCs allows onboard sensors to participate in the channel tracking. Joint tracking from many onboard sensors possibly increases the reliability of the MPC tracking.}, booktitle={2018 IEEE Globecom Workshops, GC Wkshps 2018 - Proceedings}, author={Zochmann, E. and Va, V. and Rupp, M. and Heath, R.W.}, year={2019} }
 @article{heath_2018, title={GlobalSIP and beyond [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053256605&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2849042}, abstractNote={Presents the introductory editorial for this issue of the publication.}, number={5}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018}, pages={3–15} }
 @article{zhu_choi_cheng_xiao_heath_2018, title={High-resolution angle tracking for mobile wideband millimeter-wave systems with antenna array calibration}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85052707098&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2865759}, abstractNote={Millimeter-wave (mmWave) systems use directional beams to support high-rate data communications. Small misalignment between the transmit and receive beams (e.g., due to the mobility) can result in a significant drop of the received signal quality, especially in line-of-sight communication channels. In this paper, we propose and evaluate high-resolution angle tracking strategies for wideband mmWave systems with mobility. We custom design pairs of auxiliary beams as the tracking beams, and use them to capture the angle variations, toward which the steering directions of the data beams are adjusted. Different from conventional beam tracking designs, the proposed framework neither depends on the angle variation model nor requires an on-grid assumption. For practical implementation of the proposed methods, we examine the impact of the array calibration errors on the auxiliary beam pair design. Numerical results reveal that by employing the proposed methods, good angle tracking performance can be achieved under various antenna array configurations, channel models, and mobility conditions.}, number={11}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, D. and Choi, J. and Cheng, Q. and Xiao, W. and Heath, R.}, year={2018}, pages={7173–7189} }
 @article{heath_2018, title={Highlights from the IEEE SPM's Editorial Board Meeting [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85049521963&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2834778}, abstractNote={Presents information on Editorial board meetings for this issue of the publication.}, number={4}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018}, pages={3–4} }
 @inproceedings{gupta_andrews_heath_2018, title={Impact of correlation between link blockages on macro-diversity gains in mmWave Networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050262958&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2018.8403598}, abstractNote={Blocking objects between a transmitter and receiver cause the transition of wireless communication links from line-of-sight (LOS) to non-line-of-sight (NLOS) propagation. This reduces the received power, particularly at higher frequencies such as millimeter wave (mmWave). We consider a form of macro-diversity in a mmWave cellular network in which a mobile user attempts to connect to two base stations (BSs) simultaneously, to increase the probability of having at least one LOS link. We develop a framework for determining the LOS probability when taking into account the correlation between blockages: for example, a single blockage close to the device could block multiple BSs. We also show that the BS density must scale as the square of the blockage density to maintain a given level of LOS probability.}, booktitle={2018 IEEE International Conference on Communications Workshops, ICC Workshops 2018 - Proceedings}, author={Gupta, A.K. and Andrews, J.G. and Heath, R.W.}, year={2018}, pages={1–6} }
 @article{va_choi_shimizu_bansal_heath_2018, title={Impact of measurement noise on millimeter wave beam alignment using beam subsets}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045346051&partnerID=MN8TOARS}, DOI={10.1109/LWC.2018.2825326}, abstractNote={This letter derives bounds on performance degradation due to measurement noise in millimeter wave beam alignment methods that train a subset of beam pairs. The analysis applies the union bound and pairwise beam selection error for a wideband channel. While the analysis is oblivious to the method to select the subset of beam pairs, inverse fingerprinting beam alignment is used as a concrete numerical example. Performance evaluation results for different bandwidths show that a wideband system suffers less from measurement noise with the same training sequence duration.}, number={5}, journal={IEEE Wireless Communications Letters}, author={Va, V. and Choi, J. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2018}, pages={784–787} }
 @article{heath_2018, title={Introducing the New Editorial Team of IEEE Signaling Processing Magazine [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85046621767&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2809223}, abstractNote={Presents information on the new editorial team for this issue of the publication.}, number={3}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018}, pages={4–5} }
 @article{sun_wang_heath_schniter_2019, title={Joint Channel-Estimation/Decoding With Frequency-Selective Channels and Few-Bit ADCs}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85059009598&partnerID=MN8TOARS}, DOI={10.1109/TSP.2018.2887189}, abstractNote={We propose a fast and near-optimal approach to joint channel-estimation, equalization, and decoding of coded single-carrier (SC) transmissions over frequency-selective channels with few-bit analog-to-digital converters (ADCs). Our approach leverages parametric bilinear generalized approximate message passing to reduce the implementation complexity of joint channel estimation and (soft) symbol decoding to that of a few fast Fourier transforms. Furthermore, it learns and exploits sparsity in the channel impulse response. This paper is motivated by millimeter-wave systems with bandwidths on the order of Gsamples/sec, where few-bit ADCs, SC transmissions, and fast processing all lead to significant reductions in power consumption and implementation cost. We numerically demonstrate our approach using signals and channels generated according to the IEEE 802.11ad wireless local area network standard, in the case that the receiver uses analog beamforming and a single ADC.}, number={4}, journal={IEEE Transactions on Signal Processing}, author={Sun, P. and Wang, Z. and Heath, R.W. and Schniter, P.}, year={2019}, pages={899–914} }
 @article{mo_heath_2018, title={Limited Feedback in Single and Multi-User MIMO Systems with Finite-Bit ADCs}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85042879882&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2808955}, abstractNote={Communication systems with low-resolution analog-to-digital-converters (ADCs) can exploit channel state information at the transmitter and receiver. This paper presents codebook designs and performance analyses for limited feedback MIMO systems with finite-bit ADCs. A point-to-point single-user channel is firstly considered. When the received signal is sliced by 1-bit ADCs, the absolute phase at the receiver is important to align the phase of the received signals. A new codebook design for beamforming, which separately quantizes the channel direction and the residual phase, is therefore proposed. For the multi-bit case where the optimal transmission method is unknown, suboptimal Gaussian signaling and eigenvector beamforming is assumed to obtain a lower bound of the achievable rate. It is found that to limit the rate loss, more feedback bits are needed in the medium SNR regime than the low and high SNR regimes, which is quite different from the conventional infinite-bit ADC case. Second, a multi-user system where a multiple-antenna transmitter sends signals to multiple single-antenna receivers with finite-bit ADCs is considered. Based on the derived performance loss due to finite-bit ADCs and finite-bit CSI feedback, the number of bits per feedback should increase linearly with the ADC resolution in order to restrict the rate loss.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Mo, J. and Heath, R.W.}, year={2018}, pages={3284–3297} }
 @inproceedings{mazher_mezghani_heath_2019, title={Low Resolution Millimeter Wave Radar: Bounds and Performance}, volume={2018-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062943429&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2018.8645394}, abstractNote={Millimeter wave is widely used for consumer radar applications like driver assistance systems in automated vehicles and gesture recognition. To cope with the increased hardware complexity, higher costs and power consumption of wideband systems at millimeter wave frequencies, we propose a fully digital architecture with low resolution analog-to-digital converters (ADCs) on each radio-frequency chain. The effect of the low resolution ADCs on radar parameter estimation is characterized by the Cramér-Rao bound under the proposed hardware constraints. Prior work has shown that at low signal-to-noise ratio a radar system with 1-bit ADCs suffers a performance loss of 2 dB in parameter estimation compared to a system with ideal infinite resolution ADCs. In this paper, we design an analog preprocessing unit to introduce correlated noise in the system and show that the performance can be improved beyond the 2 dB limit. Numerical results demonstrate the potential of the proposed architecture to meet the requirements of high resolution sensing.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Mazher, K.U. and Mezghani, A. and Heath, R.W.}, year={2019}, pages={554–558} }
 @inproceedings{kumari_mazher_mezghani_heath_2018, title={Low Resolution Sampling for Joint Millimeter-Wave MIMO Communication-Radar}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053840681&partnerID=MN8TOARS}, DOI={10.1109/SSP.2018.8450774}, abstractNote={We propose the deployment of millimeter-wave MIMO for joint vehicular high speed communication and high resolution radar sensing. To cope with the significant hardware complexity, we consider the use of low resolution analog-to-digital converters (ADC) while maintaining a separate radio-frequency chain per antenna. The system performance is analyzed in terms of Cramér Rao lower bound and achievable data rate, and compared to the ideal case with infinite resolution ADCs. Additionally, we study the impact of quantization on the trade-off between these performance metrics. Numerical results demonstrate the potential of the concept to meet the challenging requirements of next-generation vehicles.}, booktitle={2018 IEEE Statistical Signal Processing Workshop, SSP 2018}, author={Kumari, P. and Mazher, K.U. and Mezghani, A. and Heath, R.W.}, year={2018}, pages={801–805} }
 @inproceedings{upadhya_vorobyov_heath_2018, title={Low-Overhead Receiver-Side Channel Tracking for Mmwave Mimo}, volume={2018-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054272712&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2018.8461320}, abstractNote={Millimeter wave (mmWave) multiple-input multiple-output (MIMO) transceivers employ narrow beams to obtain a large array-gain, rendering them sensitive to changes in the angles of arrival and departure of the paths. Since the singular vectors that span the channel subspace are used to design the precoder and combiner, we propose a method to track the receiver-side channel subspace during data transmission using a separate radio frequency (RF) chain dedicated for channel tracking. Under certain conditions on the transmit precoder, we show that the receiver-side channel subspace can be estimated during data transmission without knowing the structure of the precoder or the transmitted data. The performance of the proposed method is evaluated through simulations.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Upadhya, K. and Vorobyov, S.A. and Heath, R.W.}, year={2018}, pages={3859–3863} }
 @article{heath_2018, title={Making Papers, Code, and Data Accessible [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85056845862&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2869962}, abstractNote={Presents the introductory editorial for this issue of the publication.}, number={6}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018}, pages={3–4} }
 @inproceedings{shimizu_va_bansal_heath_2019, title={Millimeter wave V2X communications: Use cases and design considerations of beam management}, volume={2018-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85061801719&partnerID=MN8TOARS}, DOI={10.23919/APMC.2018.8617303}, abstractNote={Sharing perception sensor information (e.g., camera, radar, and LIDAR) among vehicles in proximity using vehicle-to-everything (V2X) communications provides non-line-of-sight (NLOS) information about the surrounding environment, improving the safety and traffic efficiency of cooperative automated driving. However, such application requires high data rates on the order of 10s to 100s of Mbps (or 1 Gbps), which cannot be supported by the existing V2X communication technologies such as IEEE 802.11p-based dedicated short-range communication (DSRC) in 5.9 GHz band. Millimeter wave (mmWave) communication has the potential to meet the high data rate demand thanks to its wide bandwidth. In this paper, we introduce potential use cases of mmWave V2X communications. Also, we discuss technical challenges and design considerations for mmWave V2X beam management. Particularly, we elaborate on how we can utilize low-frequency communication (e.g., 5.9 GHz DSRC), onboard sensors mounted at vehicles and infrastructures, and DSRC messages (e.g., vehicle position, speed, acceleration, and path prediction) to facilitate mmWave V2X beam management.}, booktitle={Asia-Pacific Microwave Conference Proceedings, APMC}, author={Shimizu, T. and Va, V. and Bansal, G. and Heath, R.W.}, year={2019}, pages={183–185} }
 @inproceedings{wang_narasimha_heath_2018, title={MmWave Beam Prediction with Situational Awareness: A Machine Learning Approach}, volume={2018-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053455479&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2018.8445969}, abstractNote={Millimeter-wave communication is a challenge in the highly mobile vehicular context. Traditional beam training is inadequate in satisfying low overheads and latency. In this paper, we propose to combine machine learning tools and situational awareness to learn the beam information (power, optimal beam index, etc) from past observations. We consider forms of situational awareness that are specific to the vehicular setting including the locations of the receiver and the surrounding vehicles. We leverage regression models to predict the received power with different beam power quantizations. The result shows that situational awareness can largely improve the prediction accuracy and the model can achieve throughput with little performance loss with almost zero overhead.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Wang, Y. and Narasimha, M. and Heath, R.W.}, year={2018} }
 @inproceedings{cuvelier_heath_2018, title={MmWave MU-MINO for aerial networks}, volume={2018-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85056719111&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2018.8491045}, abstractNote={Millimeter wave offers high bandwidth for air-to-air (A2A) communication. In this paper, we evaluate the rate performance of a multiuser MIMO (MU-MIMO) configuration where several aircraft communicate with a central hub. We consider a hybrid subarray architecture, single path channels, and realistic atmospheric attenuation effects. We propose a mathematical framework for the analysis of millimeter wave (mmWave) MU-MIMO networks. Via Monte Carlo simulation, we demonstrate that mmWave is a promising technology for delivering gigabit connectivity in next-generation aerial networks.}, booktitle={Proceedings of the International Symposium on Wireless Communication Systems}, author={Cuvelier, T. and Heath, R.W.}, year={2018} }
 @article{wang_venugopal_molisch_heath_2018, title={MmWave Vehicle-to-Infrastructure Communication: Analysis of Urban Microcellular Networks}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045630495&partnerID=MN8TOARS}, DOI={10.1109/TVT.2018.2827259}, abstractNote={Vehicle-to-infrastructure (V2I) communication may provide high data rates to vehicles via millimeter-wave (mmWave) microcellular networks. This paper uses stochastic geometry to analyze the coverage of urban mmWave microcellular networks. Prior work used a pathloss model with a line-of-sight probability function based on randomly oriented buildings, to determine whether a link was line-of-sight or non-line-of-sight. In this paper, we use a pathloss model inspired by measurements, which uses a Manhattan distance pathloss model and accounts for differences in pathloss exponents and losses when turning corners. In our model, streets are randomly located as a Manhattan poisson line process and the base stations (BSs) are distributed according to a Poisson point process. Our model is well suited for urban microcellular networks where the BSs are deployed at street level. Based on this new approach, we derive the coverage probability under certain BS association rules to obtain closed-form solutions without much complexity. In addition, we draw two main conclusions from our work. First, non-line-of-sight BSs are not a major benefit for association or source of interference most of the time. Second, there is an ultra-dense regime where deploying active BSs does not enhance coverage.}, number={8}, journal={IEEE Transactions on Vehicular Technology}, author={Wang, Y. and Venugopal, K. and Molisch, A.F. and Heath, R.W.}, year={2018}, pages={7086–7100} }
 @inproceedings{wang_klautau_ribero_narasimha_heath_2019, title={MmWave Vehicular Beam Training with Situational Awareness by Machine Learning}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062974348&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2018.8644288}, abstractNote={Configuring beams in millimeter-wave (mmWave) vehicular communication is a challenging task. Large antenna arrays and narrow beams are deployed at transceivers to exploit beamforming gain, which leads to significant system overhead if an exhaustive beam search is adopted. In this paper, we propose to learn the optimal beam pair index by exploiting the locations and sizes of the receiver and its neighboring vehicles (parts of the situational awareness for automated driving), leveraging machine learning tools with the past beam training records. MmWave beam selection is formulated as a classification problem based on situational awareness. We provide a comprehensive comparison of different classification models and various levels of situational awareness. Practical issues are considered in realistic implementations, including GPS inaccuracies, out-dated locations due to fixed location reporting frequencies and missing features with limited connected vehicles penetration rate. The result shows that we can achieve up to 86% of alignment probability with ideal assumptions.}, booktitle={2018 IEEE Globecom Workshops, GC Wkshps 2018 - Proceedings}, author={Wang, Y. and Klautau, A. and Ribero, M. and Narasimha, M. and Heath, R.W.}, year={2019} }
 @inproceedings{park_lee_heath_2018, title={Multi-cell coordination in K-tier heterogeneous downlink cellular networks: Dynamic clustering and feedback allocation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85048349959&partnerID=MN8TOARS}, DOI={10.23919/WIOPT.2018.8362870}, abstractNote={We characterize the ergodic spectral efficiency of a cooperative type of K-tier heterogeneous networks (HetNets) with limited feedback. Specifically, a base station (BS) coordination set is formed by using dynamic clustering across the tiers, wherein the intra-cluster interference is mitigated by using multi-cell zero-forcing based on limited feedback. Modeling the network based on stochastic geometry, we derive analytical expressions for the ergodic spectral efficiency as a function of the system parameters. Leveraging the obtained expression, we formulate a feedback allocation problem and obtain a solution to improve the ergodic spectral efficiency. Simulations show the spectral efficiency improvement by using the proposed feedback allocation. One major finding in the obtained solution is that allocating more feedback to stronger intra-cluster BSs is efficient.}, booktitle={2018 16th International Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks, WiOpt 2018}, author={Park, J. and Lee, N. and Heath, R.W.}, year={2018} }
 @article{zhang_huang_guo_yuan_heath_2019, title={Multibeam for joint communication and radar sensing using steerable analog antenna arrays}, volume={68}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85057882759&partnerID=MN8TOARS}, DOI={10.1109/TVT.2018.2883796}, abstractNote={Beamforming has a great potential for joint communication and radar sensing (JCAS), which is becoming a demanding feature on many emerging platforms, such as unmanned aerial vehicles and smart cars. Although beamforming has been extensively studied for communication and radar sensing respectively, its application in the joint system is not straightforward due to different beamforming requirements by communication and sensing. In this paper, we propose a novel multibeam framework using steerable analog antenna arrays, which allows seamless integration of communication and sensing. Different to conventional JCAS schemes that support JCAS using a single beam, our framework is based on the key innovation of multibeam technology: providing fixed subbeam for communication and packet-varying scanning subbeam for sensing, simultaneously from a single transmitting array. We provide a system architecture and protocols for the proposed framework, complying well with modern packet communication systems with multicarrier modulation. We also propose low-complexity and effective multibeam design and generation methods, which offer great flexibility in meeting different communication and sensing requirements. We further develop sensing parameter estimation algorithms using conventional digital Fourier transform and one-dimensional compressive sensing techniques, matching well with the multibeam framework. Simulation results are provided and validate the effectiveness of our proposed framework, beamforming design methods, and the sensing algorithms.}, number={1}, journal={IEEE Transactions on Vehicular Technology}, author={Zhang, J.A. and Huang, X. and Guo, Y.J. and Yuan, J. and Heath, R.W.}, year={2019}, pages={671–685} }
 @article{jeon_lee_hong_heath_2018, title={One-bit sphere decoding for uplink massive MIMO systems with one-bit ADCs}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045728511&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2827028}, abstractNote={This paper presents a low-complexity near-maximum-likelihood-detection (near-MLD) algorithm called one-bit sphere decoding for an uplink massive multiple-input multiple-output system with one-bit analog-to-digital converters. The idea of the proposed algorithm is to estimate the transmitted symbol vector sent by uplink users (a codeword vector) by searching over a sphere, which contains a collection of codeword vectors close to the received signal vector at the base station in terms of a weighted Hamming distance. To reduce the computational complexity for the construction of the sphere, the proposed algorithm divides the received signal vector into multiple subvectors each with a reduced dimension. Then, it generates multiple spheres in parallel, where each sphere is centered at the subvector and contains a list of subcodeword vectors. The detection performance of the proposed algorithm is also analyzed by characterizing the probability that the proposed algorithm performs worse than the MLD. The analysis shows how the dimension of each sphere and the size of the subcodeword list are related to the performance-complexity tradeoff achieved by the proposed algorithm. Simulation results demonstrate that the proposed algorithm achieves near-MLD performance, while reducing the computational complexity compared to the existing MLD method.}, number={7}, journal={IEEE Transactions on Wireless Communications}, author={Jeon, Y.-S. and Lee, N. and Hong, S.-N. and Heath, R.W.}, year={2018}, pages={4509–4521} }
 @article{khan_yazdan_heath_2018, title={Optimization of power transfer efficiency and energy efficiency for wireless-powered systems with massive MIMO}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85052696928&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2865727}, abstractNote={Massive MIMO is attractive for wireless information and energy transfer due to its ability to focus energy toward desired spatial locations. In this paper, the overall power transfer efficiency (PTE) and the energy efficiency (EE) of a wirelessly powered massive MIMO system are investigated, where a multi-antenna base-station (BS) uses wireless energy transfer to charge single-antenna energy harvesting users on the downlink. The users may exploit the harvested energy to transmit information to the BS on the uplink. The overall system performance is analyzed while accounting for the nonlinear nature of practical energy harvesters. First, for wireless energy transfer, the PTE is characterized using a scalable model for the BS circuit power consumption. The PTE-optimal values for the number of BS antennas and users are derived. Then, for wireless energy and information transfer, the EE performance is characterized. The EE-optimal BS transmit power is derived in terms of the key system parameters, such as the number of BS antennas and the number of users. As the number of antennas becomes large, increasing the transmit power improves the EE for moderate to large number of antennas. Simulation results suggest that it is energy efficient to operate the system in the massive MIMO regime.}, number={11}, journal={IEEE Transactions on Wireless Communications}, author={Khan, T.A. and Yazdan, A. and Heath, R.W.}, year={2018}, pages={7159–7172} }
 @article{mollen_larsson_gustavsson_eriksson_heath_2018, title={Out-of-Band Radiation from Large Antenna Arrays}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045883033&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2018.1601063}, abstractNote={The OOB radiation from large arrays with nonlinear hardware has a different radiation pattern than the beamformed in-band signal. This is the main difference between the OOB radiation from large arrays and from well-studied legacy systems. Beamforming might focus the OOB radiation in certain directions but also significantly reduce the total power that has to be transmitted. For cost and power-consumption reasons, large arrays might have to be built from low-complexity hardware without advanced precompensation for linearization, which increases the relative amount of OOB radiation. Given that large arrays will be used in future base stations, a correct understanding of the OOB radiation is crucial to specify appropriate linearity requirements for the hardware. We show that the OOB radiation from large arrays varies little between coherence times; it is isotropic in many cases; and when it is beamformed, it is directed toward the served user in a very narrow beam with an array gain equal to or less than that of the in-band signal. We draw the conclusion that, compared to legacy systems, less stringent linearity requirements can be used in many systems with large arrays by virtue of the lower transmit power needed}, number={4}, journal={IEEE Communications Magazine}, author={Mollen, C. and Larsson, E.G. and Gustavsson, U. and Eriksson, T. and Heath, R.W.}, year={2018}, pages={196–203} }
 @article{jurdi_khosravirad_viswanathan_andrews_heath_2019, title={Outage of Periodic Downlink Wireless Networks with Hard Deadlines}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85056357612&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2018.2879934}, abstractNote={We consider a downlink periodic wireless communications system, where multiple access points cooperatively transmit packets to a number of devices, e.g., actuators in an industrial control system. Each period consists of two phases: an uplink training phase and a downlink data transmission phase. Each actuator must successfully receive its unique packet within a single transmission phase; else, an outage is declared. Such an outage can be caused by two events: a transmission error due to transmission at a rate that the channel cannot actually support or time overflow, where the downlink data phase is too short, given the channel conditions to successfully communicate all the packets. We determine the closed-form expressions for the time overflow probability when there are just two field devices, as well as the transmission error probability for an arbitrary number of devices. In addition, we provide upper and lower bounds on the time overflow probability for an arbitrary number of devices. We propose a novel variable-rate transmission method that eliminates time overflow. Detailed system-level simulations are used to identify system design guidelines, such as the optimal amount of training time, as well as for benchmarking the proposed system design versus non-cooperative cellular, cooperative fixed-rate, and cooperative relaying.}, number={2}, journal={IEEE Transactions on Communications}, author={Jurdi, R. and Khosravirad, S.R. and Viswanathan, H. and Andrews, J.G. and Heath, R.W.}, year={2019}, pages={1238–1253} }
 @inproceedings{position-aided compressive channel estimation and tracking for millimeter wave multi-user mimo air-to-air communications_2018, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050276542&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2018.8403629}, abstractNote={We consider a multi-user (MU) millimeter wave (mmWave) network of aerial vehicles. One aircraft acts as an access point that communicates with multiple airborne users through mmWave communication links. We focus on the problem of estimating and tracking the multi-user mmWave frequency- selective MIMO channel, leveraging prior information about the position and trajectory of each user (aerial vehicle). We develop a channel estimation and tracking strategy based on compressed sensing tools which introduces a low overhead, even when operating in the high mobility aerial scenario. We show by simulation that, even with a reduced number of training symbols and in the low SNR regime, the new channel estimation and tracking algorithm provides channel state information (CSI) of enough quality to obtain and maintain near-optimal values of spectral efficiency.}, booktitle={2018 IEEE International Conference on Communications Workshops, ICC Workshops 2018 - Proceedings}, year={2018}, pages={1–6} }
 @article{heath_2018, title={Research Gems Found Digging with Industry [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85043754968&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2793165}, abstractNote={Presents the introductory editorial for this issue of the publication.}, number={2}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018} }
 @inproceedings{eltayeb_heath_2018, title={Securing mmWave vehicular communication links with multiple transmit antennas}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050269091&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2018.8403556}, abstractNote={This paper presents a low-complexity physical layer security technique for millimeter wave (mmWave) vehicular communication systems. The proposed technique exploits the large dimensional antenna arrays available in mmWave systems and the road reflected path to generate location dependent transmission. This results in coherent transmission to the legitimate receiver and interference that jams eavesdroppers with sensitive receivers along the same lane of travel. Theoretical and simulation results demonstrate the validity and effectiveness of the proposed technique and show that high secrecy throughput can be achieved when compared to directional array transmission techniques.}, booktitle={2018 IEEE International Conference on Communications Workshops, ICC Workshops 2018 - Proceedings}, author={Eltayeb, M.E. and Heath, R.W.}, year={2018}, pages={1–6} }
 @inproceedings{kumari_eltayeb_heath_2018, title={Sparsity-aware adaptive beamforming design for IEEE 802.11ad-based joint communication-radar}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85049956438&partnerID=MN8TOARS}, DOI={10.1109/RADAR.2018.8378684}, abstractNote={The IEEE 802.11ad millimeter-wave WiFi waveform can be simultaneously used for high range/velocity resolution radar and Gbps data rate communication in vehicular applications. The angular field of view (FoV) for radar sensing, however, is limited by the employed directional analog beam. In this paper, a sparsity-aware beamforming design is proposed that permits a trade-off between communication rate and radar detection/estimation performance in the angular domain. The proposed design uses random transmit antenna subsets to form a coherent beam towards a communication receiver while simultaneously perturbing the sidelobes of the resulting beam pattern. Sidelobe perturbation results in random grading lobes, which we exploit for compressed radar sensing. The system performance trade-off involved in the adaptive beamforming design is evaluated using a novel joint communication-radar rate metric. Numerical results demonstrate subset size optimization enables the detection of short-range radar targets with high probability and angular resolution in a wide FoV at the expense of a slight hit in the communication data rate.}, booktitle={2018 IEEE Radar Conference, RadarConf 2018}, author={Kumari, P. and Eltayeb, M.E. and Heath, R.W.}, year={2018}, pages={923–928} }
 @article{park_heath_2018, title={Spatial Channel Covariance Estimation for the Hybrid MIMO Architecture: A Compressive Sensing-Based Approach}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054689118&partnerID=MN8TOARS}, DOI={10.1109/TWC.2018.2873592}, abstractNote={Spatial channel covariance information can replace full knowledge of the entire channel matrix for designing analog precoders in hybrid multiple-input-multiple-output (MIMO) architecture. Spatial channel covariance estimation, however, is challenging for the hybrid MIMO architecture because the estimator operating at baseband can only obtain a lower dimensional pre-combined signal through fewer radio frequency chains than antennas. In this paper, we propose two approaches to covariance estimation based on compressive sensing techniques. One is to apply a time-varying sensing matrix, and the other is to exploit the prior knowledge that the covariance matrix is Hermitian. We present the rationale behind the two ideas and validate the superiority of the proposed methods by theoretical analysis and numerical simulations. We conclude the paper by extending the proposed algorithms from narrowband MIMO systems with a single receive antenna to wideband systems with multiple receive antennas.}, number={12}, journal={IEEE Transactions on Wireless Communications}, author={Park, S. and Heath, R.W.}, year={2018}, pages={8047–8062} }
 @inproceedings{myers_mezghani_heath_2019, title={Spatial Zadoff-Chu Modulation for Rapid Beam Alignment in mmWave Phased Arrays}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063481124&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2018.8644177}, abstractNote={Compressed sensing (CS) algorithms can be used to perform beam alignment in mmWave phased arrays with fewer channel measurements by exploiting the sparsity of mmWave channels. Unfortunately, several efficient CS matrices cannot be realized in phased arrays due to hardware constraints. In this work, we propose to use shifted Zadoff-Chu (ZC) sequences in the antenna domain to realize efficient CS matrices for channel estimation or beam alignment. We prove that the shifted ZC-based CS matrix satisfies the restricted isometry property with high probability. Furthermore, CS algorithms that use the proposed training can exploit the fast Fourier transform and are computationally efficient over those that use pseudo-random phase shifts. Using simulations, we show that the proposed CS matrix achieves better beam alignment performance than the commonly used random phased shift-based CS matrices.}, booktitle={2018 IEEE Globecom Workshops, GC Wkshps 2018 - Proceedings}, author={Myers, N.J. and Mezghani, A. and Heath, R.W.}, year={2019} }
 @inproceedings{park_ali_gonzalez-prelcic_heath_2019, title={Spatial channel covariance estimation for the hybrid architecture at a base station: A tensor-decomposition-based approach}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063094610&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2018.8646605}, abstractNote={Spatial channel covariance information can replace instantaneous full channel state information for designing hybrid analog/digital precoders. Estimating the spatial channel covariance is challenging due to the inherent limitation of the hybrid architecture, i.e., much fewer radio frequency (RF) chains than antennas. In this paper, we propose a spatial channel covariance estimation method for spatially sparse time-varying frequency-selective channels. The proposed method leverages the fact that the channel can be represented as a low-rank higher-order tensor. Numerical results demonstrate that the proposed approach achieves higher estimation accuracy in comparison with existing covariance estimation methods.}, booktitle={2018 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2018 - Proceedings}, author={Park, S. and Ali, A. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2019}, pages={1008–1012} }
 @article{myers_mezghani_heath_2019, title={Swift-Link: A Compressive Beam Alignment Algorithm for Practical mmWave Radios}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85058143591&partnerID=MN8TOARS}, DOI={10.1109/TSP.2018.2886168}, abstractNote={Next generation wireless networks will exploit the large amount of spectrum available at millimeter wave (mmWave) frequencies. The design of mmWave systems, however, is challenging due to the strict power, cost, and hardware constraints at higher bandwidths. To achieve a good SNR for communication, mmWave systems use large antenna arrays. Beamforming with highly directional beams is one way to use the antennas. As the channel changes over time, the beams that maximize the SNR have to be estimated quickly to reduce the training overhead. Prior research has exploited the observation that mmWave channels are sparse to perform compressed sensing (CS) based beam alignment with few channel measurements. Most of the existing CS-based algorithms, however, assume perfect synchronization and fail in the presence of carrier frequency offset (CFO). This paper presents Swift-Link, a fast beam alignment algorithm that is robust against the offset. Swift-Link includes a novel randomized beam training sequence that minimizes the beam alignment errors due to CFO and a low-complexity algorithm that corrects these errors. Even with strict hardware constraints, our algorithm uses fewer channel measurements than comparable CS algorithms and has analytical guarantees. Swift-Link requires a small output dynamic range at the analog-to-digital converter compared to beam-scanning techniques.}, number={4}, journal={IEEE Transactions on Signal Processing}, author={Myers, N.J. and Mezghani, A. and Heath, R.W.}, year={2019}, pages={1104–1119} }
 @article{heath_2018, title={Taking the Next Step for IEEE Signal Processing Magazine [From the Editor]}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040689597&partnerID=MN8TOARS}, DOI={10.1109/MSP.2017.2770481}, abstractNote={Presents the introductory editorial for this issue of the publication.}, number={1}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2018} }
 @inproceedings{mezghani_heath_2018, title={The information and wave-theoretic limits of analog beamforming}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85057251441&partnerID=MN8TOARS}, DOI={10.1109/ITA.2018.8503216}, abstractNote={The performance of broadband millimeter-wave (mmWave) RF architectures, is generally determined by mathematical concepts such as the Shannon capacity. These systems have also to obey physical laws such as the conservation of energy and the propagation laws. Taking the physical and hardware limitations into account is crucial for characterizing the actual performance of mm Wave systems under certain architecture such as analog beamforming. In this context, we consider a broadband frequency dependent array model that explicitly includes incremental time shifts instead of phase shifts between the individual antennas and incorporates a physically defined radiated power. As a consequence of this model, we present a novel joint approach for designing the optimal waveform and beamforming vector for analog beamforming. Our results show that, for sufficiently large array size, the achievable rate is mainly limited by the fundamental trade-off between the analog beamformina gain and siznal bandwidth.}, booktitle={2018 Information Theory and Applications Workshop, ITA 2018}, author={Mezghani, A. and Heath, R.W.}, year={2018} }
 @inproceedings{wang_narasimha_heath_2019, title={Towards Robustness: Machine Learning for MmWave V2X with Situational Awareness}, volume={2018-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062977914&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2018.8645370}, abstractNote={High mobility poses challenges to millimeter wave (mmWave) vehicular beam tracking. In this paper, we propose to recommend a set of candidate beam pairs using machine learning classification and ranking, based on the target vehicle’s situational awareness. We observe that the optimal beam directions can be captured by the location of the receiver and the sizes and locations of the neighboring vehicles. This information is a natural part of the situational awareness exploited by connected and automated vehicles. The infrastructure conducts the beam search within the recommended beam set with the receiver. To further reduce overhead, we leverage a new beam search mechanism, where the search can automatically terminate when the current beam measurement exceeds a specific target power. We set the target power as the predicted average power of the top two beams. The numerical results show that the beam search overhead can be reduced by 70% when seven beams are recommended, at a comparable beam alignment probability.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Wang, Y. and Narasimha, M. and Heath, R.W.}, year={2019}, pages={1577–1581} }
 @inproceedings{ali_jiang_patil_li_heath_2018, title={Vehicle-to-Vehicle Communication for Autonomous Vehicles: Safety and Maneuver Planning}, volume={2018-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85064951957&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2018.8690946}, abstractNote={Autonomous vehicles (AVs) have the potential to transform road transportation by improving safety and increasing traffic/fuel efficiency. Currently, AVs rely primarily on line-of-sight sensing technologies to gather information about their surroundings. Some of the surrounding objects (e.g., vehicles, infrastructure, and pedestrians), however, can potentially communicate with the AV to help it create a better digital map of the world around it. In this work, we quantify the gains vehicle-to-vehicle (V2V) communication can provide for AVs. Specifically, we focus on the safety and maneuver planning of the AVs. Our preliminary findings indicate that V2V can reduce a significant fraction of AV collisions. Further, V2V can considerably reduce the maneuver completion time.}, booktitle={IEEE Vehicular Technology Conference}, author={Ali, A. and Jiang, L. and Patil, S. and Li, J. and Heath, R.W.}, year={2018} }
 @article{heath_2019, title={Vehicular applications of signal processing}, volume={36}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85059508620&partnerID=MN8TOARS}, DOI={10.1109/MSP.2018.2880687}, abstractNote={S ignal processing is what’s behind many developments in vehicle automation. As of the writing of this editorial, I am preparing for a talk at the University of California, Los Angeles, and a distinguished symposium talk at IEEE GlobalSIP on “Signal Processing for Automated Driving.” To prepare, I have been reading beyond my core areas of expertise and have gained an appreciation for the wide range of signal processing opportunities in vehicular systems. Vehicles are becoming automated thanks, in part, to diverse kinds of sensors, not to mention communication modalities. Higher levels of automation remove control from the driver, eventually replacing the need for human drivers entirely. Radars and cameras are found in most vehicles with any level of automation, while lidar is used only at higher levels due to cost. Various types of communication, including 4G cellular, wireless local area networks, and dedicated short-range communication, are also found in different combinations. Signal processing is everywhere. In this editorial, I review some applications of signal processing in vehicular systems. I focus specifically on applications that are pertinent to automated driving (the so-called higher levels of automation). Many technologies also support driver assistance applications. In a future issue of IEEE Signal Processing Magazine (SPM), a call for papers will be announced for a special issue on autonomous driving. Radar is widely used at all levels of automation. Ultrasonic radars are implemented for short-range positioning, while millimeter-wave radars are applied during driving for ranging, velocity estimation, mapping, and localization. Most automated vehicle prototypes feature at least a forwardand backward-facing radar, but some have six or more radars. The advantages of radar over light-based sensing modalities are that it works equally well during either day or night, and it is able to penetrate fog and smog. Statistical signal processing is used extensively in radar. Current research challenges include designing hardware and algorithms for mass-produced millimeter-wave radars with large arrays, high bandwidth, and distortions due to radio-frequency components, as well as managing coexistence and interferences. SPM has an upcoming two-part special issue on “Advances in Radar Systems for Modern Civilian and Commercial Applications.” Lidar is used at the highest levels of automation. Automated vehicle prototypes often have a single larger lidar or two side-mounted lidar units. In principle, lidar is the light-based equivalent of radar, although, practically, the way the signals are generated and processed is vastly different. Lidar implements laser beams with mechanical scanning, while most radars have beams that are electronically steered. The system is used primarily to generate 3D point clouds that allow accurate mapping and object detection. Types of signal processing include image processing and machine learning. One of the most significant impediments to the wider deployment of lidar is the high cost of the mechanical solutions. Companies are currently working on solid-state versions where there will likely be many more signal processing applications to compensate for hardware imperfections. Cameras, making use of multiple frames at the visible-light level, are widely used sensors in vehicle automation. This development is not surprising, as human drivers primarily rely on their eyes. Most automated vehicle prototypes have several cameras, both forwardand rearfacing. Cameras are used in combination with machine-learning algorithms as a core part of computer vision. Generally, the cameras are used to mimic functions performed by the human driver. They have a particular application for object detection, especially bicycles and pedestrians, which are otherwise hard to see with radar or lidar. Different camera configurations are possible, including stereo and multiview. There are many research challenges related to exploiting cameras for automated driving, including better algorithms for object recognition, combining the views of multiple cameras, enhancing performance in low-light conditions, and the use of the infrared spectrum. Sensor fusion is a key signal processing application in automated driving. Each of the aforementioned sensors provides information about the shared environment from a different physical and spectral perspective. Sensor fusion is the}, number={1}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W.}, year={2019}, pages={3–6} }
 @inproceedings{kumari_heath_vorobyov_2018, title={Virtual Pulse Design for IEEE 802.11AD-Based Joint Communication-Radar}, volume={2018-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054285456&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2018.8461678}, abstractNote={The millimeter wave WLAN standard can be used for joint communication-radar by exploiting the waveform preamble as a radar pulse. The velocity estimation accuracy with this approach, however, is limited due to the short integration time. A physical increase in the radar pulse integration duration, however, leads to a decrease in the communication data rate. In this paper, a coprime-based pulse design approach for IEEE 802.11ad-based radar is proposed that uses only a few non-uniformly placed preambles to construct several virtual pulses for enhancing the velocity estimation accu-racy/resolution as compared to the conventional approach without sacrificing the communication data rate. The simulation results demonstrate that the coprime-based virtual pulse design improves the velocity estimation resolution by a factor of about 60x at a vehicle separation distance of 10m and by a factor of about 20x at a distance of 100 m, while simultaneously achieving 7 Gbps data rate.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Kumari, P. and Heath, R.W. and Vorobyov, S.A.}, year={2018}, pages={3315–3319} }
 @inproceedings{myers_heath_2017, title={A compressive channel estimation technique robust to synchronization impairments}, volume={2017-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044186831&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2017.8227747}, abstractNote={Initial access at millimeter wave frequencies is a challenging problem due to hardware non-idealities and low SNR measurements prior to beamforming. Prior work has exploited the observation that mmWave MIMO channels are sparse in the spatial angle domain and has used compressed sensing based algorithms for channel estimation. Most of them, however, ignore hardware impairments like carrier frequency offset and phase noise, and fail to perform well when such impairments are considered. In this paper, we develop a compressive channel estimation algorithm for narrowband mmWave systems, which is robust to such non idealities. We address this problem by constructing a tensor that models both the mmWave channel and CFO, and estimate the tensor while still exploiting the sparsity of the mmWave channel. Simulation results show that under the same settings, our method performs better than comparable algorithms that are robust to phase errors.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Myers, N.J. and Heath, R.W.}, year={2017}, pages={1–5} }
 @inproceedings{a compressive sensing-maximum likelihood approach for off-grid wideband channel estimation at mmwave_2018, volume={2017-December}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050297018&partnerID=MN8TOARS}, DOI={10.1109/CAMSAP.2017.8313157}, abstractNote={Obtaining accurate channel state information is crucial to configure the antenna arrays and the digital precoders and combiners in hybrid millimeter wave (mmWave) MIMO architectures. Most of prior work on channel estimation with hybrid MIMO architectures relies on the use of finite-resolution dictionaries to estimate angles of arrival (AoA) and angles of departure (AoD). When the AoAs or AoDs do not fall within the quantization grids used to generate these dictionaries, there is an unavoidable grid error in the estimation of the channel. In this paper, we propose a mixed compressed sensing-maximum likelihood algorithm that uses continuous dictionaries to estimate the channel. The quantization error due to using finite resolution dictionaries can be neglected with this approach, enhancing estimation performance without resorting to very large dictionaries. Simulation results show how the new algorithm outperforms approaches based on finite resolution dictionaries previously proposed for the estimation of mmWave channels.}, booktitle={2017 IEEE 7th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2017}, year={2018}, pages={1–5} }
 @inproceedings{rodriguez-fernandez_venugopal_gonzalez-prelcic_heath_2017, title={A frequency-domain approach to wideband channel estimation in millimeter wave systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028355759&partnerID=MN8TOARS}, DOI={10.1109/ICC.2017.7997047}, abstractNote={Channel estimation allows millimeter wave (mmWave) MIMO communication systems to design pre-coders and combiners under different objective functions. Hybrid MIMO architectures provide a good trade-off power consumption-performance at mmWave frequencies, but most of the prior work on channel estimation for these structures assumes a narrowband channel model. In this paper, we propose a sparse approach for frequency selective channel estimation for mmWave channels, assuming a hybrid architecture. Simulation results show that the estimation error is small and the computational complexity is kept low. Moreover, the algorithm requires less training overhead than competing approaches based on beam training.}, booktitle={IEEE International Conference on Communications}, author={Rodriguez-Fernandez, J. and Venugopal, K. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2017} }
 @inproceedings{hriba_valenti_venugopal_heath_2017, title={Accurately Accounting for Random Blockage in Device-to-Device mmWave Networks}, volume={2018-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85046476415&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2017.8254892}, abstractNote={Millimeter-wave systems are characterized by the use of highly directional antennas and the presence of blockages, which significantly alter the path-loss and small-scale fading parameters. The received power of each interferer depends on the direction it points and whether it is line-of- sight (LOS), non-LOS (i.e., partially blocked), or completely blocked. While interferers that are sufficiently far away will almost certainly be completely blocked, a finite number of interferers in close proximity will be subject to random partial blockages. Previous attempts to characterize mmWave networks have made the simplifying assumption that all interferers within some radius, called the LOS ball, are unblocked, while interferers beyond that radius are non-LOS. However, compared to simulation results, the LOS ball assumption tends to overestimate outage. In this paper, we present an accurate yet tractable analysis of finite mmWave networks that dispenses with the LOS ball assumption. In the analysis, each interferer has a distribution that is selected randomly from several possibilities, each representing different blockage and directivity states. First, the exact outage probability is found for a finite network with interferers in fixed locations. Then, the spatially averaged outage probability is found by averaging over the interferer locations. While the focus is on device-to-device networks, the analysis is general enough to find applications outside of the present mmWave framework.}, booktitle={2017 IEEE Global Communications Conference, GLOBECOM 2017 - Proceedings}, author={Hriba, E. and Valenti, M.C. and Venugopal, K. and Heath, R.W.}, year={2017}, pages={1–6} }
 @inproceedings{mollén_choi_larsson_heath_2017, title={Achievable uplink rates for massive MIMO with coarse quantization}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023751046&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7953406}, abstractNote={The high hardware complexity of a massive MIMO base station, which requires hundreds of radio chains, makes it challenging to build commercially. One way to reduce the hardware complexity and power consumption of the receiver is to lower the resolution of the analog-to-digital converters (ADCs). We derive an achievable rate for a massive MIMO system with arbitrary quantization and use this rate to show that ADCs with as low as 3 bits can be used without significant performance loss at spectral efficiencies around 3.5 bpcu per user, also under interference from stronger transmitters and with some imperfections in the automatic gain control.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Mollén, C. and Choi, J. and Larsson, E.G. and Heath, R.W.}, year={2017}, pages={6488–6492} }
 @inproceedings{jafari_park_heath_2017, title={Analysis of interference mitigation in mmWave communications}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028343388&partnerID=MN8TOARS}, DOI={10.1109/ICC.2017.7996403}, abstractNote={Millimeter wave (mmWave) cellular systems will enable gigabit-per-second data rates due to the large bandwidth available at mmWave frequencies. Thanks to the small wavelength corresponding to the mmWave frequencies, mmWave systems can benefit from exploiting large antenna arrays at both transmitter and receiver. Although highly directional beamforming has been envisioned to play a key role to realize sufficient link margin, it is also possible to use these large arrays in other ways. For example, a hybrid array architecture can be exploited to either cancel or null an interferer. In this paper we analyse the effect of interference cancellation in downlink mmWave communications. We exploit partial zero forcing (PZF) at the user in order to cancel the interference from a set of interfering base stations (BSs) and derive closed form expression for the probability of coverage. Simulation results show that as the density of base stations increases, interference mitigation through partial zero forcing enhances the probability of coverage implying the necessity of interference mitigation in dense mmWave networks.}, booktitle={IEEE International Conference on Communications}, author={Jafari, A.H. and Park, J. and Heath, R.W.}, year={2017} }
 @inproceedings{khan_heath_2017, title={Analyzing wireless power transfer in millimeter wave networks with human blockages}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85042357449&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2017.8170827}, abstractNote={Wireless power transfer may enable remotely powered operation of low-energy devices. The use of directional antenna arrays makes millimeter wave (mmWave) band a potential candidate for wireless power transfer. The mmWave signals, however, are prone to blockage due to human bodies. This paper presents a feasibility study of using mmWave for wireless power transfer in a large-scale network consisting of power beacons and energy harvesters. Using stochastic geometry, system performance is characterized while treating the network nodes as potential blockages to mmWave signals, thus coupling the network density and blockage density. For both mmWave and sub 6 GHz bands, analytical expressions are derived for a link-level metric (energy coverage probability) and a network-level metric (success probability) when the harvesters are served using time-division multiplexing. The former characterizes the survival function of the harvested energy, while the latter also takes the network load into account. Numerical simulations suggest that network densification helps improve the performance, despite an increase in the blockage density. For a given network density, deploying an optimal fraction of nodes as power beacons maximizes the number of successful energy harvesters per unit area. Finally, mmWave may potentially outperform lower frequency solutions despite blockages.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Khan, T.A. and Heath, R.W.}, year={2017}, pages={115–120} }
 @inproceedings{shen_dai_gui_wang_heath_adachi_2017, title={AoD-adaptive subspace codebook for channel feedback in FDD massive MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028352977&partnerID=MN8TOARS}, DOI={10.1109/ICC.2017.7997168}, abstractNote={Channel feedback is essential for frequency division duplex (FDD) massive multiple-input multiple-output (MIMO) systems to realize precoding and power allocation. Traditional codebooks for channel feedback, where the required number of feedback bits is proportional to the number of base station (BS) antennas, can not scale up with massive MIMO due to the large number of BS antennas. To solve this problem, in this paper, we propose an angle-of-departure (AoD) adaptive subspace codebook to reduce the codebook size and feedback overhead. Specifically, by leveraging the concept of angle coherence time, which implies that the path AoDs vary much slower than path gains, we propose an AoD-adaptive subspace codebook to quantize the channel vector in a more accurate way. We also provide performance analysis of the proposed AoD-adaptive subspace codebook, where we prove that the required number of feedback bits only scales linearly with the number of resolvable AoDs, which is much smaller than the number of BS antennas. This quantitative result is also verified by simulations.}, booktitle={IEEE International Conference on Communications}, author={Shen, W. and Dai, L. and Gui, G. and Wang, Z. and Heath, R.W. and Adachi, F.}, year={2017} }
 @article{zhu_choi_heath_2017, title={Auxiliary Beam Pair Enabled AoD and AoA Estimation in Closed-Loop Large-Scale Millimeter-Wave MIMO Systems}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029025977&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2702617}, abstractNote={Channel estimation is of critical importance in millimeter-wave (mmWave) multiple-input multiple-output (MIMO) systems. Due to the use of large antenna arrays, low-complexity mmWave specific channel estimation algorithms are required. In this paper, an auxiliary beam pair design is proposed to provide high-resolution estimates of the channel’s angle-of-departure (AoD) and angle-of-arrival (AoA) for mmWave MIMO systems. By performing an amplitude comparison with respect to each auxiliary beam pair, a set of ratio measures that characterize the channel’s AoD and AoA are obtained by the receiver. Either the best ratio measure or the estimated AoD is quantized and fed back to the transmitter via a feedback channel. The proposed technique can be incorporated into control channel design to minimize initial access delay. Though the design principles are derived assuming a high-power regime, evaluation under more realistic assumption shows that by employing the proposed method, good angle estimation performance is achieved under various signal-to-noise ratio levels and channel conditions.}, number={7}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, D. and Choi, J. and Heath, R.W.}, year={2017}, pages={4770–4785} }
 @inproceedings{wang_venugopal_molisch_heath_2017, title={Blockage and Coverage Analysis with MmWave Cross Street BSS Near Urban Intersections}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028341382&partnerID=MN8TOARS}, DOI={10.1109/ICC.2017.7996974}, abstractNote={Millimeter wave (mmWave) communication offers Gbps data transmission, which can support massive data sharing in vehicle-to-infrastructure (V2I) networks. In this paper, we analyze the blockage effects among different vehicles and coverage probability of a typical receiver, considering cross street BSs near urban intersections in a multi-lane mmWave vehicular network. First, a three-dimensional model of blockage among vehicles on different lanes is considered. Second, we compute the coverage probability considering the interference of cross street base stations. Incorporating the blockage effects, we derive an exact and semi closed-form expression of the cumulative distribution density (CDF) of the association link path gain. Then, a tight approximation of the coverage probability is computed. We provide numerical results to verify the accuracy of the analytic results. We demonstrate the effects of blockage and the cross street interference. Also, we compare coverage probability with different BSs intensities under various street settings. It is shown that in multi-lane V2I networks, blockage among vehicles is not significant. Also, deploying more BSs does not increase coverage probability efficiently in ultra-dense streets.}, booktitle={IEEE International Conference on Communications}, author={Wang, Y. and Venugopal, K. and Molisch, A.F. and Heath, R.W.}, year={2017} }
 @article{venugopal_alkhateeb_gonzalez prelcic_heath_2017, title={Channel Estimation for Hybrid Architecture-Based Wideband Millimeter Wave Systems}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023777565&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2017.2720856}, abstractNote={Hybrid analog and digital precoding allows millimeter wave (mmWave) systems to achieve both array and multiplexing gain. The design of the hybrid precoders and combiners, though, is usually based on the knowledge of the channel. Prior work on mmWave channel estimation with hybrid architectures focused on narrowband channels. Since mmWave systems will be wideband with frequency selectivity, it is vital to develop channel estimation solutions for hybrid architectures-based wideband mmWave systems. In this paper, we develop a sparse formulation and compressed sensing-based solutions for the wideband mmWave channel estimation problem for hybrid architectures. First, we leverage the sparse structure of the frequency-selective mmWave channels and formulate the channel estimation problem as a sparse recovery in both time and frequency domains. Then, we propose explicit channel estimation techniques for purely time or frequency domains and for combined time/frequency domains. Our solutions are suitable for both single carrier-frequency domain equalization and orthogonal frequency-division multiplexing systems. Simulation results show that the proposed solutions achieve good channel estimation quality, while requiring small training overhead. Leveraging the hybrid architecture at the transceivers gives further improvement in estimation error performance and achievable rates.}, number={9}, journal={IEEE Journal on Selected Areas in Communications}, author={Venugopal, K. and Alkhateeb, A. and Gonzalez Prelcic, N. and Heath, R.W.}, year={2017}, pages={1996–2009} }
 @article{mo_schniter_heath_2018, title={Channel Estimation in Broadband Millimeter Wave MIMO Systems with Few-Bit ADCs}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039765139&partnerID=MN8TOARS}, DOI={10.1109/TSP.2017.2781644}, abstractNote={We develop a broadband channel estimation algorithm for millimeter wave (mmWave) multiple input multiple output (MIMO) systems with few-bit analog-to-digital converters (ADCs). Our methodology exploits the joint sparsity of the mmWave MIMO channel in the angle and delay domains. We formulate the estimation problem as a noisy quantized compressed-sensing problem and solve it using efficient approximate message passing (AMP) algorithms. In particular, we model the angle-delay coefficients using a Bernoulli–Gaussian-mixture distribution with unknown parameters and use the expectation-maximization forms of the generalized AMP and vector AMP algorithms to simultaneously learn the distributional parameters and compute approximately minimum mean-squared error (MSE) estimates of the channel coefficients. We design a training sequence that allows fast, fast Fourier transform based implementation of these algorithms while minimizing peak-to-average power ratio at the transmitter, making our methods scale efficiently to large numbers of antenna elements and delays. We present the results of a detailed simulation study that compares our algorithms to several benchmarks. Our study investigates the effect of SNR, training length, training type, ADC resolution, and runtime on channel estimation MSE, mutual information, and achievable rate. It shows that, in a mmWave MIMO system, the methods we propose to exploit joint angle-delay sparsity allow 1-bit ADCs to perform comparably to infinite-bit ADCs at low SNR, and 4-bit ADCs to perform comparably to infinite-bit ADCs at medium SNR.}, number={5}, journal={IEEE Transactions on Signal Processing}, author={Mo, J. and Schniter, P. and Heath, R.W.}, year={2018}, pages={1141–1154} }
 @inproceedings{ali_heath_2017, title={Compressed beam-selection in millimeterwave systems with out-of-band partial support information}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023771213&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7952807}, abstractNote={Compressed beam-selection (CBS) exploits the limited scattering of the millimeter wave (mmWave) channel using compressed sensing and finds the best beam-pair with limited overhead. The CBS procedure can further benefit from the knowledge of some additional structure in the channel. As mmWave systems are envisioned to be deployed in conjunction with sub-6 GHz systems, we use the spatial information extracted at sub-6 GHz as out-of-band side information about the mmWave channel. In particular, we formulate beam-selection as a weighted sparse signal recovery problem, and obtain the weights using sub-6 GHz angular information. Furthermore, we formalize the notion of spatial congruence between sub-6 GHz and mmWave, and numerically evaluate the degree of spatial congruence necessary for the success of the proposed approach. The simulation results illustrate that the proposed approach reduces the training overhead of the CBS approach by 3×.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Ali, A. and Heath, R.W.}, year={2017}, pages={3499–3503} }
 @inproceedings{daniels_yeh_heath_2017, title={Cost-effective vehicular radar through minimally-modified IEEE 802.11 devices}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021445871&partnerID=MN8TOARS}, DOI={10.1109/RADAR.2017.7944288}, abstractNote={Increasing safety and automation in transportation systems has led to the proliferation of radar and IEEE 802.11 dedicated short range communication (DSRC) in vehicles. Conceivably, vehicles could be equipped with both options, as DSRC enables detection around corners through communications as a supplement to standard radar detection. While DSRC devices are cost-effective, vehicular radar devices are currently expensive. We suggest an alternative where DSRC is used for both communications and radar. This is accomplished with minimal modification to existing IEEE 802.11 devices via algorithm processing only on frequency-domain channel estimates. Simulations show that our solution can deliver acceptable vehicular radar performance with the 10–20 MHz of spectrum provided by IEEE 802.11, demonstrating the feasibility of substantial cost reduction in vehicular radar.}, booktitle={2017 IEEE Radar Conference, RadarConf 2017}, author={Daniels, R.C. and Yeh, E.R. and Heath, R.W.}, year={2017}, pages={0675–0680} }
 @inproceedings{nguyen_heath_2017, title={Delay and Doppler processing for multi-target detection with IEEE 802.11 OFDM signaling}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023747760&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7952790}, abstractNote={This paper investigates the processing of delay and Doppler information with IEEE 802.11p OFDM signaling for multi-target detection. We study the feasibility of extending IEEE 802.11p short-range communication (DSRC) in vehicles to automotive radio detection and ranging (radar) functionality. By exploiting the unique structure of 802.11p OFDM packets over multiple subcarriers and multiple time-slots, we apply the estimation of signal parameters via rotational invariance technique (ESPRIT) for concurrent multi-target detection and range/velocity estimation. Numerical results show sub-0.2m accuracy in range estimation and sub-0.02m/s accuracy in velocity estimation with high probability.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Nguyen, D.H.N. and Heath, R.W.}, year={2017}, pages={3414–3418} }
 @inproceedings{zhu_heath_2018, title={Directional timing synchronization in wideband millimeter wave cellular systems with low-resolution ADCs}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85051032888&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2017.8335131}, abstractNote={We develop a new beamforming strategy for millimeter-wave systems to improve the frame timing synchronization performance under low-resolution analog-to-digital converters. In the proposed method, identical synchronization sequences are sent across a cluster of simultaneously probed synchronization beams. The beam codewords in the beam cluster are selected to maximize the received synchronization signal-to-quantization-plus-noise ratio (SQNR). Numerical results reveal that the frame timing synchronization performance of the proposed method outperforms the existing approach due to the improvement in the received synchronization SQNR.}, booktitle={Conference Record of 51st Asilomar Conference on Signals, Systems and Computers, ACSSC 2017}, author={Zhu, D. and Heath, R.W.}, year={2018}, pages={37–41} }
 @article{park_alkhateeb_heath_2017, title={Dynamic Subarrays for Hybrid Precoding in Wideband mmWave MIMO Systems}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021728711&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2671869}, abstractNote={Hybrid analog/digital precoding architectures can address the tradeoff between achievable spectral efficiency and power consumption in large-scale MIMO systems. This makes them a promising candidate for millimeter wave systems, which deploy large antenna arrays at both the transmitter and the receiver to guarantee sufficient received signal power. Most prior work on hybrid precoding focused on narrowband channels and assumed fully connected hybrid architectures. Millimeter wave (mmWave) systems, though, are expected to be wideband with frequency selectivity. In this paper, a closed-form solution for fully connected OFDM-based hybrid analog/digital precoding is developed for frequency selective mmWave systems. This solution is then extended to partially connected but fixed architectures in which each RF chain is connected to a specific subset of the antennas. The derived solutions give insights into how the hybrid subarray structures should be designed. Based on this, a novel technique that dynamically constructs the hybrid subarrays knowing the long-term channel characteristics is developed. Simulation results show that the proposed hybrid precoding solutions achieve spectral efficiencies close to that obtained with fully digital architectures in wideband mmWave channels. Furthermore, the results indicate that the developed dynamic subarray solution outperforms the fixed hybrid subarray structures in various system and channel conditions.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Park, S. and Alkhateeb, A. and Heath, R.W.}, year={2017}, pages={2907–2920} }
 @inproceedings{park_park_yazdan_heath_2017, title={Dynamic bit selection in mixed-ADC cloud-RAN systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028340121&partnerID=MN8TOARS}, DOI={10.1109/ICC.2017.7997352}, abstractNote={We propose a new mixed-analog-to-digital convertor (mixed-ADC) architecture for cloud-RAN (C-RAN) systems. The RRH is equipped with a mixed-ADC pool that includes multiple ADC units with various resolutions. In this pool, the RRH selects the appropriate ADCs and connects the selected ADCs to each antenna to quantize the received signals, thereby each antenna can have a different resolution ADC. The quantized signals are sent to a centralized baseband unit (BBU) via a capacity limited fronthaul pipe. To maximize the spectral efficiency of the considered system in a single-user uplink phase, we formulate an optimization problem for ADC selection by exploiting an approximation of the generalized mutual information (GMI). Subsequently we propose a solution. The simulations show the improvement in the GMI by using the proposed ADC selection. Our major findings are: i) In a C-RAN with limited fronthaul capacity, the proposed mixed-ADC makes more efficient use of the fronthaul capacity. ii) In selecting ADCs, assigning a high resolution ADC to a strong channel is beneficial.}, booktitle={IEEE International Conference on Communications}, author={Park, J. and Park, S. and Yazdan, A. and Heath, R.W.}, year={2017} }
 @article{george_venugopal_lozano_heath_2017, title={Enclosed mmWave Wearable Networks: Feasibility and Performance}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85018189142&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2662681}, abstractNote={This paper investigates the feasibility of mmWave frequencies for personal networks of wireless wearable devices in enclosed settings (e.g., commuter trains, subways, airplanes, airports, or offices). At these frequencies, specular reflections off surfaces are expected to contribute to the capture of intended signal power and, simultaneously, to aggravate the interference at the receivers. Meanwhile, blockages by obstacles and people—including the individuals wearing the devices—are expected to shield receivers from interference. With the aid of stochastic geometry and random shape theory, we assess the interplay of surface reflections and blockages for dense deployments of wearable networks equipped with directional antenna arrays in relevant indoor settings.}, number={4}, journal={IEEE Transactions on Wireless Communications}, author={George, G. and Venugopal, K. and Lozano, A. and Heath, R.W.}, year={2017}, pages={2300–2313} }
 @inproceedings{khan_yazdan_maguire_heath_2017, title={Energy Efficiency of Wireless Information and Power Transfer with Massive MIMO}, volume={2017-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85025430205&partnerID=MN8TOARS}, DOI={10.1109/VTCSpring.2017.8108320}, abstractNote={Massive MIMO, a building block of future 5G systems, is attractive for wireless information and energy transfer. This is largely due to its ability to focus energy towards desired spatial locations. In this paper, the overall energy efficiency of a wirelessly powered massive MIMO system is investigated where a multi-antenna base-station uses wireless energy transfer to charge single- antenna energy harvesting users on the downlink. The users exploit the harvested energy to transmit information to the base-station on the uplink. Using a scalable model for the circuit power consumption at the base-station, the energy efficiency performance (measured in bits/joule) of the overall system is characterized. A closed-form expression is derived for the energy efficiency- optimal downlink transmit power in terms of the key system parameters such as the number of base- station antennas and the number of users. Simulation results suggest that it is energy efficient to operate the system in the massive MIMO regime. As the number of antennas becomes large, increasing the transmit power as well as serving more users help improve the energy efficiency for moderate to large number of antennas.}, booktitle={IEEE Vehicular Technology Conference}, author={Khan, T.A. and Yazdan, A. and Maguire, Y. and Heath, R.W.}, year={2017} }
 @article{yazdan_park_park_khan_heath_2017, title={Energy-efficient massive MIMO}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85025457456&partnerID=MN8TOARS}, DOI={10.1109/MMM.2017.2691422}, abstractNote={Increasing access to telecommunication services in rural parts of the world has the potential to alleviate the digital divide felt by the people of these regions. Recently, both public and private sector entities have shown more interest in tackling this problem on a global scale. In the United States, for example, the Federal Communications Commission (FCC) subsidizes large telecommunication companies to offer affordable services in unserved areas through the Connect America Fund. Facebook's Internet. org [1] and Google's Project Loon [2] are some examples of large-scale rural connectivity efforts in the private sector. Deployment of connectivity solutions in rural communities, however, faces many practical challenges. Lack of availability and access to reliable electricity sources is one of the major hindrances for rural connectivity, particularly in underdeveloped countries. As such, wireless connectivity solutions for such applications must focus on low-power hardware operations and high energy efficiency (EE).}, number={5}, journal={IEEE Microwave Magazine}, author={Yazdan, A. and Park, J. and Park, S. and Khan, T.A. and Heath, R.W.}, year={2017}, pages={18–30} }
 @article{eltayeb_choi_al-naffouri_heath_2017, title={Enhancing Secrecy with Multiantenna Transmission in Millimeter Wave Vehicular Communication Systems}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029950424&partnerID=MN8TOARS}, DOI={10.1109/TVT.2017.2681965}, abstractNote={Millimeter wave (mmWave) vehicular communication systems will provide an abundance of bandwidth for the exchange of raw sensor data and support driver-assisted and safety-related functionalities. Lack of secure communication links, however, may lead to abuses and attacks that jeopardize the efficiency of transportation systems and the physical safety of drivers. In this paper, we propose two physical layer (PHY) security techniques for vehicular mmWave communication systems. The first technique uses multiple antennas with a single radio-frequency (RF) chain to transmit information symbols to a target receiver and noise-like signals in nonreceiver directions. The second technique uses multiple antennas with a few RF chains to transmit information symbols to a target receiver and opportunistically inject artificial noise in controlled directions, thereby reducing interference in vehicular environments. Theoretical and numerical results show that the proposed techniques provide higher secrecy rate when compared to traditional PHY security techniques that require digital or more complex antenna architectures.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, author={Eltayeb, M.E. and Choi, J. and Al-Naffouri, T.Y. and Heath, R.W.}, year={2017}, pages={8139–8151} }
 @article{thornburg_heath_2018, title={Ergodic Rate of Millimeter Wave Ad Hoc Networks}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85035789216&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2772251}, abstractNote={In this paper, we use a stochastic geometry approach to quantify the ergodic rate of each user in an outdoor mm-wave ad hoc network. For a variety of use cases, it is reasonable to assume users will be clustered around a central point (e.g., WiFi hotspot or clusterhead) rather than uniformly distributed. Our results indicate that, in contrast to sub-6-GHz networks, clustered mm-wave ad hoc networks tolerate the increased interference because directional antenna arrays reduce the interference. For certain antenna array configurations and user densities, uncoordinated users within a cluster outperform TDMA. Additionally, we derive a scaling law for uniform mm-wave ad hoc networks and propose a heuristic scaling for clustered networks. The per user ergodic rate remains constant if mm-wave antenna arrays scale sub-linearly with the number of users for uniform networks or linearly in clustered networks as users are added to the cluster. Last, we compute expressions that quantify the loss in ergodic rate per user when alignment error occurs at the receiver and transmitter. Our results show that even relatively small errors in alignment can lead to significant ergodic rate reduction.}, number={2}, journal={IEEE Transactions on Wireless Communications}, author={Thornburg, A. and Heath, R.W.}, year={2018}, pages={914–926} }
 @article{garcía-naya_heath_kaltenberger_rupp_vía_2017, title={Experimental evaluation in wireless communications}, volume={2017}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016938095&partnerID=MN8TOARS}, DOI={10.1186/s13638-017-0842-2}, abstractNote={This editorial sums up relevant topics on the assessment of wireless communication systems covered by the especial issue entitled “Experimental Evaluation in Wireless Communications.” The topics include practical aspects on the implementation of distributed asynchronous non-linear kernel methods over wireless sensor networks; localization methods based on the exploitation of radio-frequency identification (RFID) wireless sensors and cellular networks or on sparsity approximations; channel sounding and assessment of broadband orthogonal frequency-division multiplexing (OFDM)-based wireless systems in high-speed vehicular communications; coexistence analysis of femtocell-based and outdoor-to-indoor systems; techniques for peak-to-average power ratio (PAPR) reduction; new solutions for baseband and radio frequency (RF) hardware impairments in full-duplex wireless systems; and, finally, suitability of interference alignment for broadband indoor wireless communications.}, number={1}, journal={Eurasip Journal on Wireless Communications and Networking}, author={García-Naya, J.A. and Heath, R. and Kaltenberger, F. and Rupp, M. and Vía, J.}, year={2017} }
 @inproceedings{rodriguez-fernandez_gonzalez-prelcic_venugopal_heath_2017, title={Exploiting Common Sparsity for Frequency-Domain Wideband Channel Estimation at mmWave}, volume={2018-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85046471582&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2017.8254834}, abstractNote={Hybrid architectures reduce power consumption for MIMO systems at millimeter wave (mmWave) with respect to all-digital solutions. Fast configuration of hybrid arrays is one of the main challenges to be solved to unlock the potential of mmWave in high mobility scenarios. Channel estimation is one of the means of obtaining the information needed to configure these arrays. Prior work on channel estimation at mmWave considers a frequency-flat narrowband channel model, although the the mmWave channel is intrinsically wideband and frequency-selective. In this paper, we design a frequency-domain algorithm that leverages the sparsity in the channel to estimate the frequency-selective mmWave channel. We evaluate this technique by simulation and study its complexity, comparing the trade-offs complexity-overhead-performance to those of previous approaches. We conclude that the proposed approach outperforms previous algorithms, providing a better performance at a lower computational cost.}, booktitle={2017 IEEE Global Communications Conference, GLOBECOM 2017 - Proceedings}, author={Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N. and Venugopal, K. and Heath, R.W.}, year={2017}, pages={1–6} }
 @article{pesyna_humphreys_heath_novlan_zhang_2017, title={Exploiting antenna motion for faster initialization of centimeter-accurate gnss positioning with low-cost antennas}, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029581711&partnerID=MN8TOARS}, DOI={10.1109/TAES.2017.2665221}, abstractNote={This paper investigates the effectiveness of multipath-decorrelating antenna motion in reducing the initialization time of global navigation satellite system (GNSS) receivers employing low-cost single-frequency antennas for carrier-phase differential GNSS (CDGNSS) positioning. Fast initialization times with low-cost antennas will encourage the expansion of CDGNSS into the mass market, bringing the benefits of globally referenced centimeter-accurate positioning to many consumer applications, such as augmented reality and autonomous vehicles, that have so far been hampered by the several-meter-level errors of traditional GNSS positioning. Poor multipath suppression common to low-cost antennas results in large and strongly time-correlated phase errors when a receiver is static. Such errors can result in the CDGNSS initialization time, the so-called time to ambiguity resolution (TAR), extending to hundreds of seconds—many times longer than for higher cost survey-grade antennas, which have substantially better multipath suppression. This paper demonstrates that TAR can be significantly reduced through antenna motion, particularly gentle wavelength-scale random antenna motion. Such motion acts to decrease the correlation time of the multipath-induced phase errors. A priori knowledge of the motion profile is shown to further reduce TAR, with the reduction shown to be more pronounced as the initialization scenario is more challenging.}, number={4}, journal={IEEE Transactions on Aerospace and Electronic Systems}, author={Pesyna, K.M. and Humphreys, T.E. and Heath, R.W. and Novlan, T.D. and Zhang, J.C.}, year={2017}, pages={1597–1613} }
 @article{park_park_yazdan_heath_2017, title={Exploiting spatial channel covariance for hybrid precoding in massive MIMO systems}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021707623&partnerID=MN8TOARS}, DOI={10.1109/TSP.2017.2701321}, abstractNote={We propose a new hybrid precoding technique for massive multi-input multi-output (MIMO) systems using spatial channel covariance matrices in the analog precoder design. Applying a regularized zero-forcing precoder for the baseband precoding matrix, we find an unconstrained analog precoder that maximizes signal-to-leakage-plus-noise ratio (SLNR) while ignoring analog phase shifter constraints. Subsequently, we develop a technique to design a constrained analog precoder that mimics the obtained unconstrained analog precoder under phase shifter constraints. The main idea is to adopt an additional baseband precoding matrix, which we call a compensation matrix. We analyze the SLNR loss due to the proposed hybrid precoding compared to fully digital precoding, and determine which factors have a significant impact on this loss. In the simulations, we show that if the channel is spatially correlated and the number of users is smaller than the number of RF chains, the SLNR loss becomes negligible compared to fully digital precoding. The main benefit of our method stems from the use of spatial channel matrices in such a way that not only is each user's desired signal considered, but also the inter-user interference is incorporated in the analog precoder design.}, number={14}, journal={IEEE Transactions on Signal Processing}, author={Park, S. and Park, J. and Yazdan, A. and Heath, R.W.}, year={2017}, pages={3818–3832} }
 @article{rico-alvarino_lopez-valcarce_mosquera_heath_2017, title={FER Estimation in a Memoryless BSC with Variable Frame Length and Unreliable ACK/NAK Feedback}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85020885402&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2686845}, abstractNote={We consider the problem of estimating the frame error rate (FER) of a given memoryless binary symmetric channel by observing the success or failure of transmitted packets. Whereas FER estimation is relatively straightforward if all observations correspond to packets with equal length, the problem becomes considerably more complex when this is not the case. We develop FER estimators when transmissions of different lengths are observed, together with the Cramer–Rao lower bound (CRLB). Although the main focus is on maximum likelihood (ML) estimation, we also obtain low-complexity schemes performing close to optimal in some scenarios. In a second stage, we consider the case in which FER estimation is performed at a node different from the receiver, and incorporate the impairment of unreliable observations by considering noisy ACK/NAK feedback links. The impact of unreliable feedback is analyzed by means of the corresponding CRLB. In this setting, the ML estimator is obtained by applying the expectation–maximization algorithm to jointly estimate the error probabilities of the data and feedback links. Simulation results illustrate the benefits of the proposed estimators.}, number={6}, journal={IEEE Transactions on Wireless Communications}, author={Rico-Alvarino, A. and Lopez-Valcarce, R. and Mosquera, C. and Heath, R.W.}, year={2017}, pages={3661–3673} }
 @inproceedings{lee_heath_2018, title={Fast link configuration for mmwave multiuser MIMO downlink using spatial AoD angular supports}, volume={2017-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045272485&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2017.8288372}, abstractNote={Wireless communication is now moving to automotive systems. In the near future, vehicles and road side units (RSUs) will be equipped with various sensors such as radars, LIDARs, cameras, and GPSs to generate enormous sensor information. To enhance the safety and efficiency, they will share this raw sensor information each other through Gbps data rates using millimeter-wave (mmWave) communications. To realize this vision, two unfavorable attributes of mmWave communications should be addressed: 1) Overcoming blockage and 2) fast link configuration. One viable approach to addressing these issues is to exploit RSU equipped with many sensors. At a higher position such as at the top of the RSU, LoS links between vehicles and the RSU are more likely to be guaranteed as well as it facilitates the extraction of useful information for the fast link configuration such as the distances or the angles between vehicles and the RSU using the sensors. In this paper, we assume that from the sensors (aside from the relevant technology) the angular support information for the LoS link of each vehicle is known to the RSU in prior and we propose a fast link configuration method using the angular support information.}, booktitle={IEEE Vehicular Technology Conference}, author={Lee, G. and Heath, R.W.}, year={2018}, pages={1–2} }
 @article{daniels_yeh_heath_2018, title={Forward Collision Vehicular Radar with IEEE 802.11: Feasibility Demonstration Through Measurements}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021291125&partnerID=MN8TOARS}, DOI={10.1109/TVT.2017.2758581}, abstractNote={Increasing safety and automation in transportation systems has led to the proliferation of radar and IEEE 802.11p-based dedicated short-range communication (DSRC) in vehicles. However, current implementations of vehicular radar devices are expensive, use a substantial amount of bandwidth, and are susceptible to multiple security risks. In this paper, we use the IEEE 802.11 orthogonal frequency-division multiplexing communications waveform, as found in IEEE 802.11a/g/p, to perform radar functions. In this paper, we present an approach that determines the mean-normalized channel energy from frequency-domain channel estimates and models it as a direct sinusoidal function of target range, enabling closest target range estimation. In addition, we propose an alternative to vehicular forward collision detection by extending IEEE 802.11 DSRC and WiFi technology to radar, extending the foundation of joint communications and radar frameworks. Furthermore, we perform an experimental demonstration near DSRC spectrum using IEEE 802.11 standard compliant software defined radios with potentially minimal modification through algorithm processing on frequency-domain channel estimates. The results of this paper show that our solution delivers sufficient accuracy and reliability for vehicular RADAR if we use the largest bandwidth available to IEEE 802.11p ( $20\,\text{MHz}$). This indicates significant potential for industrial devices with joint vehicular communications and radar capabilities.}, number={2}, journal={IEEE Transactions on Vehicular Technology}, author={Daniels, R.C. and Yeh, E.R. and Heath, R.W.}, year={2018}, pages={1404–1416} }
 @inproceedings{zhang_cantoni_huang_guo_heath_2017, title={Framework for an Innovative Perceptive Mobile Network Using Joint Communication and Sensing}, volume={2017-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040570043&partnerID=MN8TOARS}, DOI={10.1109/VTCSpring.2017.8108564}, abstractNote={In this paper, we develop a framework for an innovative perceptive mobile (i.e. cellular) network that integrates sensing with communication, and supports new applications widely in transportation, surveillance and environmental sensing. Three types of sensing methods implemented in the base-stations are proposed, using either uplink or downlink multiuser communication signals. The required changes to system hardware and major technical challenges are briefly discussed. We also demonstrate the feasibility of estimating sensing parameters via developing a compressive sensing based scheme and providing simulation results to validate its effectiveness.}, booktitle={IEEE Vehicular Technology Conference}, author={Zhang, J.A. and Cantoni, A. and Huang, X. and Guo, Y.J. and Heath, R.W.}, year={2017} }
 @inproceedings{rodriguez-fernandez_gonzaiez-prelcic_heath_2018, title={Frequency-domain wideband channel estimation and tracking for hybrid MIMO systems}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050274913&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2017.8335678}, abstractNote={Hybrid MIMO architectures are a popular alternative to all-digital solutions to operate at millimeter wave (mmWave) frequencies with large antenna arrays at a reduced power consumption. They are also relevant for massive MIMO systems at sub-6 GHz frequencies, since power consumption remains high due to the large number of antennas. Prior work on compressive estimation of the frequency selective mmWave or massive MIMO channel focuses on single shot channel estimation. In this paper, we propose an algorithm for channel tracking which updates the channel estimation in the previous frame reducing the training overhead required. Then, we develop an strategy to update the precoders and combiners from the output of the channel tracking stage. Simulation results with practical channel realizations extracted from QuaDRiGa, show the ability of the proposed algorithms to achieve spectral efficiency very close to that obtained with perfect channel knowledge.}, booktitle={Conference Record of 51st Asilomar Conference on Signals, Systems and Computers, ACSSC 2017}, author={Rodriguez-Fernandez, J. and Gonzaiez-Prelcic, N. and Heath, R.W.}, year={2018}, pages={1829–1833} }
 @article{mo_alkhateeb_abu-surra_heath_2017, title={Hybrid Architectures with Few-Bit ADC Receivers: Achievable Rates and Energy-Rate Tradeoffs}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85018176864&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2661749}, abstractNote={Hybrid analog/digital architectures and receivers with low-resolution analog-to-digital converters (ADCs) are two low power solutions for wireless systems with large antenna arrays, such as millimeter wave and massive multiple-input multiple-output systems. Most prior work represents two extreme cases in which either a small number of radio frequency (RF) chains with full-resolution ADCs, or low-resolution ADC with a number of RF chains equal to the number of antennas is assumed. In this paper, a generalized hybrid architecture with a small number of RF chains and a finite number of ADC bits is proposed. For this architecture, achievable rates with channel inversion and singular value decomposition-based transmission methods are derived. Results show that the achievable rate is comparable to that obtained by full-precision ADC receivers at low and medium SNRs. A trade-off between the achievable rate and power consumption for the different numbers of bits and RF chains is devised. This enables us to draw some conclusions on the number of ADC bits needed to maximize the system energy efficiency. Numerical simulations show that coarse ADC quantization is optimal under various system configurations. This means that hybrid combining with coarse quantization achieves better energy-rate trade-off compared with both hybrid combining with full-resolutions ADCs and 1-bit ADC combining.}, number={4}, journal={IEEE Transactions on Wireless Communications}, author={Mo, J. and Alkhateeb, A. and Abu-Surra, S. and Heath, R.W.}, year={2017}, pages={2274–2287} }
 @article{nguyen_le_le-ngoc_heath_2017, title={Hybrid MMSE Precoding and Combining Designs for mmWave Multiuser Systems}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85030720182&partnerID=MN8TOARS}, DOI={10.1109/ACCESS.2017.2754979}, abstractNote={Hybrid analog/digital precoding architectures are a low-complexity alternative for fully digital precoding in millimeter-wave (mmWave) MIMO wireless systems. This is motivated by the reduction in the number of radio frequency and mixed signal hardware components. Hybrid precoding involves a combination of analog and digital processing that enables both beamforming and spatial multiplexing gains in mmWave systems. This paper develops hybrid analog/digital precoding and combining designs for mmWave multiuser systems, based on the mean-squared error (MSE) criteria. In the first design with the analog combiners being determined at the users, the proposed hybrid minimum MSE (MMSE) precoder is realized by minimizing the sum-MSE of the data streams intended for the users. In the second design, both the hybrid precoder and combiners are jointly designed in an iterative manner to minimize a weighted sum-MSE cost function. By leveraging the sparse structure of mmWave channels, the MMSE precoding/combining design problems are then formulated as sparse reconstruction problems. An orthogonal matching pursuit-based algorithm is then developed to determine the MMSE precoder and combiners. Simulation results show the performance advantages of the proposed precoding/combining designs in various system settings.}, journal={IEEE Access}, author={Nguyen, D.H.N. and Le, L.B. and Le-Ngoc, T. and Heath, R.W.}, year={2017}, pages={19167–19181} }
 @inproceedings{park_park_heath_yazdan_2017, title={Hybrid precoding using long-term channel statistics for massive MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023742075&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7952797}, abstractNote={Hybrid analog/digital precoding in the downlink of multiuser massive MIMO systems can reduce the number of RF chains hence reducing total cost and improving power efficiency. Having few RF chains, however, makes it difficult for a base station to acquire instantaneous channel state information across all antennas. We develop a hybrid technique that uses only long-term (slowly changing) channel statistics in computing the analog precoding matrix. The proposed analog precoder is designed to maximize signal-to-leakage-plus-noise ratio (SLNR) when combined with a baseband precoder. We also propose a constrained precoder design that reduces the effect of a hardware constraint where the analog precoders are realized with phase shifters.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Park, S. and Park, J. and Heath, R.W. and Yazdan, A.}, year={2017}, pages={3449–3453} }
 @article{kumari_choi_gonzalez-prelcic_heath_2018, title={IEEE 802.11ad-Based Radar: An Approach to Joint Vehicular Communication-Radar System}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85035092280&partnerID=MN8TOARS}, DOI={10.1109/TVT.2017.2774762}, abstractNote={Millimeter-wave (mmWave) radar is widely used in vehicles for applications such as adaptive cruise control and collision avoidance. In this paper, we propose an IEEE 802.11ad-based radar for long-range radar (LRR) applications at the 60 GHz unlicensed band. We exploit the preamble of a single-carrier physical layer frame, which consists of Golay complementary sequences with good correlation properties that make it suitable for radar. This system enables a joint waveform for automotive radar and a potential mmWave vehicular communication system based on the mmWave consumer wireless local area network standard, allowing hardware reuse. To formulate an integrated framework of vehicle-to-vehicle communication and LRR, we make typical assumptions for LRR applications, incorporating the full duplex radar operation. This new feature is motivated by the recent development of systems with sufficient isolation and self-interference cancellation. We develop single- and multi-frame radar receiver algorithms for target detection as well as range and velocity estimation for both single- and multi-target scenarios. Our proposed radar processing algorithms leverage channel estimation and time–frequency synchronization techniques used in a conventional IEEE 802.11ad receiver with minimal modifications. Analysis and simulations show that in a single-target scenario, a gigabits-per-second data rate is achieved simultaneously with cm-level range accuracy and cm/s-level velocity accuracy. The target vehicle is detected with a high probability (above 99.99 $\%$) at a low false alarm rate of 10 $^{-6}$ for an equivalent isotropically radiated power of 40 dBm up to a vehicle separation distance of about 200 m. The proposed IEEE 802.11ad-based radar meets the minimum accuracy/resolution requirement of range and velocity estimates for LRR applications.}, number={4}, journal={IEEE Transactions on Vehicular Technology}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Kumari, Preeti and Choi, Junil and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2018}, pages={3012–3027} }
 @inproceedings{jurdi_andrews_parsons_heath_2018, title={Identifying coverage holes: Where to densify?}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050956773&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2017.8335588}, abstractNote={We develop a methodology to identify coverage holes in cellular markets, given demographic and socio-economic information of the population, detailed knowledge of the current macro-cellular infrastructure, and the spectrum capabilities of a cellular operator. Coverage holes are defined as areas where users contending for macro BS resources outnumber those who eventually get serviced with minimum acceptable average rates. We develop a method that begins with filtering out users who connect to the Internet through Wi-Fi or to their data networks through small cell BSs. Since the rate observed by a user is a function of their own SINR as well as that of other connected users, we generate an SINR heat map and use a fair scheduling algorithm that allows us to determine the effective load on a BS as a number of users. By comparing the cell load to the number of users with acceptable rates, we obtain a coverage map that identifies holes that should be targeted by further densification. We observe that more cells will be in major deficit ten years from now, with up to 25% of an operator's cells requiring more than four additional micro cell BSs to fill the gap. Finally, we give a list of extensions of this work.}, booktitle={Conference Record of 51st Asilomar Conference on Signals, Systems and Computers, ACSSC 2017}, author={Jurdi, R. and Andrews, J.G. and Parsons, D. and Heath, R.W.}, year={2018}, pages={1417–1421} }
 @article{alkhateeb_nam_rahman_zhang_heath_2017, title={Initial Beam Association in Millimeter Wave Cellular Systems: Analysis and Design Insights}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028703511&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2666806}, abstractNote={Enabling the high data rates of millimeter wave (mmWave) cellular systems requires deploying large antenna arrays at both the basestations and mobile users. Prior work on coverage and rate of mmWave cellular networks focused on the case when basestations and mobile beamforming vectors are predesigned for maximum beamforming gains. Designing beamforming/combining vectors, though, requires training, which may impact both the SINR coverage and rate of mmWave systems. This paper evaluates mmWave cellular network performance while accounting for the beam training/association overhead. First, a model for the initial beam association is developed based on beam sweeping and downlink control pilot reuse. To incorporate the impact of beam training, a new metric, called the effective reliable rate, is defined and adopted. Using stochastic geometry, the effective rate of mmWave cellular networks is derived for two special cases: near-orthogonal pilots and full pilot reuse. Analytical and simulation results provide insights into the answers of two important questions. First, what is the impact of beam association on mmWave network performance? Then, should orthogonal or reused pilots be employed? The results show that unless the employed beams are very wide, initial beam training with full pilot reuse is nearly as good as perfect beam alignment.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Alkhateeb, A. and Nam, Y.-H. and Rahman, M.S. and Zhang, J. and Heath, R.W.}, year={2017}, pages={2807–2821} }
 @article{va_choi_shimizu_bansal_heath_2018, title={Inverse multipath fingerprinting for millimeter wave V2I beam alignment}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040321961&partnerID=MN8TOARS}, DOI={10.1109/TVT.2017.2787627}, abstractNote={Efficient beam alignment is a crucial component in millimeter wave systems with analog beamforming, especially in fast-changing vehicular settings. This paper proposes to use the vehicle's position (e.g., available via GPS) to query a multipath fingerprint database, which provides prior knowledge of potential pointing directions for reliable beam alignment. The approach is the inverse of fingerprinting localization, where the measured multipath signature is compared to the fingerprint database to retrieve the most likely position. The power loss probability is introduced as a metric to quantify misalignment accuracy and is used for optimizing candidate beam selection. Two candidate beam selection methods are developed, where one is a heuristic while the other minimizes the misalignment probability. The proposed beam alignment is evaluated using realistic channels generated from a commercial ray-tracing simulator. Using the generated channels, an extensive investigation is provided, which includes the required measurement sample size to build an effective fingerprint, the impact of measurement noise, the sensitivity to changes in traffic density, and beam alignment overhead comparison with IEEE 802.11ad as the baseline. Using the concept of beam coherence time, which is the duration between two consecutive beam alignments, and parameters of IEEE 802.11ad, the overhead is compared in the mobility context. The results show that while the proposed approach provides increasing rates with larger antenna arrays, IEEE 802.11ad has decreasing rates due to the higher beam training overhead that eats up a large portion of the beam coherence time, which becomes shorter with increasing mobility.}, number={5}, journal={IEEE Transactions on Vehicular Technology}, author={Va, V. and Choi, J. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2018}, pages={4042–4058} }
 @inproceedings{myers_heath_2018, title={Joint CFO and channel estimation in millimeter wave systems with one-bit ADCs}, volume={2017-December}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050718401&partnerID=MN8TOARS}, DOI={10.1109/CAMSAP.2017.8313145}, abstractNote={We develop a method to jointly estimate the carrier frequency offset (CFO) and the narrowband channel in millimeter wave (mmWave) MIMO systems operating with one-bit analog-to-digital converters (ADCs). We assume perfect timing synchronization and transform the underlying CFO-channel optimization problem to a higher dimensional space using lifting techniques. Exploiting the sparse nature of mmWave MIMO channels in the angle domain, we perform the joint estimation by solving a noisy quantized compressed sensing problem of the lifted version, using generalized approximate message passing. Simulation results show that our method is able to recover both the channel and the CFO using one-bit measurements.}, booktitle={2017 IEEE 7th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2017}, author={Myers, N.J. and Heath, R.W.}, year={2018}, pages={1–5} }
 @inproceedings{zhang_cantoni_huang_guo_heath_2017, title={Joint Communications and Sensing Using Two Steerable Analog Antenna Arrays}, volume={2017-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040597770&partnerID=MN8TOARS}, DOI={10.1109/VTCSpring.2017.8108565}, abstractNote={Beam-steering has great potentials for joint communications and sensing, which is becoming a demanding feature on many emerging platforms such as unmanned aerial vehicles and smart cars. Although beam-steering has been extensively studied for communications and radar sensing respectively, its application in the joint system is not straightforward due to different beamforming requirements by communications and sensing. In this paper, we propose a low-cost system framework which allows seamless operation of communications and sensing, using two small- size steerable analog antenna arrays. We provide system architecture, high-level protocols, detailed signal model, novel beamforming design and advanced 1D compressive sensing algorithms for joint communications and sensing. We also provide preliminary simulation results which validate the effectiveness of the proposed technique in resolving closely located objects.}, booktitle={IEEE Vehicular Technology Conference}, author={Zhang, J.A. and Cantoni, A. and Huang, X. and Guo, Y.J. and Heath, R.W.}, year={2017} }
 @inproceedings{sun_wang_heath_schniter_2018, title={Joint channel-estimation/decoding with frequency-selective channels and few-bit ADCs}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050966042&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2017.8335677}, abstractNote={We propose a fast and near-optimal approach to joint channel-estimation, equalization, and decoding of coded single-carrier (SC) transmissions over frequency-selective channels with few-bit analog-to-digital converters (ADCs). Our approach leverages approximate message passing to reduce the implementation complexity of joint channel estimation and (soft) symbol decoding to that of a few fast Fourier transforms (FFTs). Furthermore, it learns and exploits sparsity in the channel impulse response. Our work is motivated by millimeter-wave systems with bandwidths on the order of Gsamples/sec, where few-bit ADCs, SC transmissions, and fast processing all lead to significant reductions in power consumption and implementation cost. We numerically demonstrate our approach using signals and channels generated according to the IEEE 802.11ad wireless LAN standard, in the case that the receiver uses analog beamforming and a single ADC.}, booktitle={Conference Record of 51st Asilomar Conference on Signals, Systems and Computers, ACSSC 2017}, author={Sun, P. and Wang, Z. and Heath, R.W. and Schniter, P.}, year={2018}, pages={1824–1828} }
 @article{gupta_andrews_heath_2018, title={Macrodiversity in Cellular Networks with Random Blockages}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85035762899&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2773058}, abstractNote={Blocking objects (blockages) between a transmitter and receiver cause wireless communication links to transition from line-of-sight (LOS) to non-LOS propagation, which can greatly reduce the received power, particularly at the higher frequencies such as millimeter wave. We consider a cellular network in which a mobile user attempts to connect to two or more base stations (BSs) simultaneously, to increase the probability of at least one LOS link, which is a form of macrodiversity. We develop a framework for determining the LOS probability as a function of the number of BSs, when taking into account the correlation between blockages: for example, a single blockage close to the device—including the user’s own body—could block multiple BSs. We consider the impact of the size of blocking objects on the system’s  $n$ th order LOS probability and show that macrodiversity gains are higher when the blocking objects are small. We also show that the BS density must scale as the square of the blockage density to maintain a given level of LOS probability.}, number={2}, journal={IEEE Transactions on Wireless Communications}, author={Gupta, A.K. and Andrews, J.G. and Heath, R.W.}, year={2018}, pages={996–1010} }
 @article{yoo_cotton_heath_chun_2017, title={Measurements of the 60 GHz UE to eNB Channel for Small Cell Deployments}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85018177041&partnerID=MN8TOARS}, DOI={10.1109/LWC.2017.2650225}, abstractNote={In this letter, we report the results of a series of experiments which were performed to examine the impact of terminal handling and movement upon the user equipment (UE) to evolved NodeB (eNB) communications channel at 60 GHz. Three key utilization scenarios, in which a user imitated making a voice call, sending a text message or simply carrying the device in a pocket, are investigated. Each of these three user cases were studied under line of sight (LOS) and non-LOS (NLOS) channel conditions when the user was mobile in a range of different indoor and outdoor small cell scenarios. It is shown that the mode of UE operation (i.e., how the device is handled) will be important for future 60 GHz cellular applications. In particular, for short-range UE to eNB links which are in true NLOS, body shadowing is the dominating factor. To allow our results to be readily incorporated into network simulations, we have characterized the channel by decomposing the received signal into its constituent path loss, shadowed and small-scale fading components. In particular, we have had good success modeling the shadowed fading using the gamma distribution, whereas the small-scale fading observed in the LOS and NLOS channels has been appropriately modeled using the Rice and Nakagami- $m$  distributions, respectively.}, number={2}, journal={IEEE Wireless Communications Letters}, author={Yoo, S.K. and Cotton, S.L. and Heath, R.W. and Chun, Y.J.}, year={2017}, pages={178–181} }
 @article{ali_gonzalez-prelcic_heath_2018, title={Millimeter Wave Beam-Selection Using Out-of-Band Spatial Information}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85035747217&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2773532}, abstractNote={Millimeter wave (mmWave) communication is one feasible solution for high data-rate applications like vehicular-to-everything communication and next generation cellular communication. Configuring mmWave links, which can be done through channel estimation or beam-selection, however, is a source of significant overhead. In this paper, we propose using spatial information extracted at sub-6 GHz to help establish the mmWave link. Assuming a fully digital architecture at sub-6 GHz; and an analog architecture at mmWave, we outline a strategy to extract spatial information from sub-6 GHz and its use in mmWave compressed beam-selection. Specifically, we formulate compressed beam-selection as a weighted sparse signal recovery problem, and obtain the weighting information from sub-6 GHz channels. In addition, we outline a structured precoder/combiner design to tailor the training to out-of-band information. We also extend the proposed out-of-band aided compressed beam-selection approach to leverage information from all active subcarriers at mmWave. To simulate multi-band frequency dependent channels, we review the prior work on frequency dependent channel behavior and outline a multi-frequency channel model. The simulation results for achievable rate show that out-of-band aided beam-selection can considerably reduce the training overhead of in-band only beam-selection.}, number={2}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Ali, Anum and Gonzalez-Prelcic, Nuria and Heath, Robert W.}, year={2018}, pages={1038–1052} }
 @article{gonzález-prelcic_ali_va_heath_2017, title={Millimeter-Wave Communication with Out-of-Band Information}, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040120876&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2017.1700207}, abstractNote={Configuring the antenna arrays is the main source of overhead in mmWave communication systems. In high mobility scenarios, the problem is exacerbated, as achieving the highest rates requires frequent link reconfiguration. One solution is to exploit spatial congruence between signals in different frequency bands and extract mmWave channel parameters with the aid of side information obtained in another band. In this article we propose the concept of out-of-band information aided mmWave communication. We analyze different strategies to leverage information derived from sensors or from other communication systems operating at sub-6 GHz bands to help configure the mmWave communication link. The overhead reductions that can be obtained when exploiting out-of-band information are characterized in a preliminary study. Finally, the challenges associated with using out-of-band signals as a source of side information at mmWave are analyzed in detail.}, number={12}, journal={IEEE Communications Magazine}, author={González-Prelcic, N. and Ali, A. and Va, V. and Heath, R.W.}, year={2017}, pages={140–146} }
 @article{alkhateeb_leus_heath_2017, title={Multi-Layer Precoding: A Potential Solution for Full-Dimensional Massive MIMO Systems}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021806319&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2716362}, abstractNote={Massive MIMO systems achieve high sum spectral efficiency by simultaneously serving large numbers of users. In time division duplexing systems, however, the reuse of uplink training pilots among cells results in channel estimation errors, which causes downlink inter-cell interference. Handling this interference is challenging due to the large channel dimensionality and the high complexity associated with implementing large precoding/combining matrices. In this paper, we propose multi-layer precoding to enable efficient and low-complexity operation in full-dimensional massive MIMO, where a large number of antennas are used in two dimensions. In multi-layer precoding, the precoding matrix of each base station is written as a product of a number of precoding matrices. Multi-layer precoding: 1) leverages the directional characteristics of large-scale MIMO channels to manage inter-cell interference with low channel knowledge requirements and 2) allows for an efficient implementation using hybrid analog/digital architectures. We present and analyze a specific multi-layer precoding design for full-dimensional massive MIMO systems. The asymptotic optimality of the proposed design is then proved for some special yet important channels. Numerical simulations verify the analytical results and illustrate the potential gains of multi-layer precoding compared with other multi-cell precoding solutions.}, number={9}, journal={IEEE Transactions on Wireless Communications}, author={Alkhateeb, A. and Leus, G. and Heath, R.W.}, year={2017}, pages={5810–5824} }
 @article{venugopal_gonzalez-prelcic_heath_2017, title={Optimality of Frequency Flat Precoding in Frequency Selective Millimeter Wave Channels}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028459401&partnerID=MN8TOARS}, DOI={10.1109/LWC.2017.2686854}, abstractNote={Millimeter wave (mmWave) MIMO communication is a key feature of next generation wireless systems. The selection of precoders and combiners for wideband mmWave channels has involved frequency selective designs based on channel state information. In this letter, we show that under some assumptions, the dominant subspaces of the frequency domain channel matrices are equivalent. This means that semi-unitary frequency flat precoding and combining are sufficient to achieve the maximum spectral efficiency when there is not too much scattering in the channel. It also motivates the use of techniques such as compressive subspace estimation as an alternative to estimating the full channel.}, number={3}, journal={IEEE Wireless Communications Letters}, author={Venugopal, K. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2017}, pages={330–333} }
 @article{park_park_yazdan_heath_2017, title={Optimization Of Mixed-ADC multi-antenna systems for cloud-RAN deployments}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023776596&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2017.2707099}, abstractNote={We propose a mixed analog-to-digital converter ADC (mixed-ADC) structure for a cloud-RAN system, where a single-antenna user terminal communicates with a multi-antenna remote radio head (RRH). In the proposed structure, the RRH is equipped with a mixed-ADC pool that includes multiple ADC units with various resolutions. In this pool, the RRH selects the appropriate ADCs and connects the selected ADCs to each antenna to quantize the received signals; thereby each antenna can have a different resolution ADC. The fronthaul capacity is limited, so that the sum of the bits produced in the selected ADCs is also limited. To maximize the spectral efficiency or the energy efficiency of such a system, we propose algorithms for ADC resolution selection based on an approximation of the generalized mutual information in the low signal-to-noise regime. In the proposed algorithms, we show that for spectral efficiency, using high-resolution ADC on the strong channels is beneficial. The results for energy efficiency maximization are similar, though the largest resolutions are reduced to save power. The simulations show that the proposed method provides significant performance improvement.}, number={9}, journal={IEEE Transactions on Communications}, author={Park, J. and Park, S. and Yazdan, A. and Heath, R.W.}, year={2017}, pages={3962–3975} }
 @inproceedings{kumari_nguyen_heath_2017, title={Performance trade-off in an adaptive IEEE 802.11AD waveform design for a joint automotive radar and communication system}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023740581&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7952964}, abstractNote={The IEEE 802.11ad waveform can be used for automotive radar by exploiting the Golay complementary sequences in the preamble of a frame. The performance of radar, however, is limited by the preamble structure. In this paper, we propose an adaptive preamble design that permits a trade-off between radar parameters' estimation accuracy and communication rate. To quantify this trade-off, we propose a minimum mean square error (MMSE) metric based on rate distortion theory. The simulation results demonstrate that by adapting the preamble structure, we can achieve decimeter-level range mean square error (MSE) per symbol duration and gigabit per second (Gbps) data rates simultaneously for a distance upto 280 m.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Kumari, P. and Nguyen, D.H.N. and Heath, R.W.}, year={2017}, pages={4281–4285} }
 @inproceedings{va_shimizu_bansal_heath_2018, title={Position-aided millimeter wave V2I beam alignment: A learning-to-rank approach}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045248876&partnerID=MN8TOARS}, DOI={10.1109/PIMRC.2017.8292679}, abstractNote={Millimeter wave (mmWave) could be a key technology to support high data rate demands for automated vehicles. MmWave needs array gain for the best performance, but this requires correctly pointing the beam, known as beam alignment. Dynamic blockages make beam alignment challenging in the vehicular setting. This paper proposes to leverage a vehicle's position along with past beam measurements to rank desirable pointing directions that can reduce the required beam training to a small set of pointing directions. The ranking is conducted using a learning-to-rank approach, which is a popular machine learning method used in recommender systems. The learning uses a kernel based model, and a new metric for evaluating ranked lists of pointing directions tailored to beam alignment is proposed. The proposed method provides a scalable framework for exploiting context information.}, booktitle={IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC}, author={Va, V. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2018}, pages={1–5} }
 @article{zhu_wang_wong_heath_2017, title={Secure Communications in Millimeter Wave Ad Hoc Networks}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028734205&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2676087}, abstractNote={Wireless networks with directional antennas, like millimeter wave (mmWave) networks, have enhanced security. For a large-scale mmWave ad hoc network in which eavesdroppers are randomly located, however, eavesdroppers can still intercept the confidential messages, since they may reside in the signal beam. This paper explores the potential of physical layer security in mmWave ad hoc networks. Specifically, we characterize the impact of mmWave channel characteristics, random blockages, and antenna gains on the secrecy performance. For the special case of uniform linear array (ULA), a tractable approach is proposed to evaluate the average achievable secrecy rate. We also characterize the impact of artificial noise in such networks. Our results reveal that in the low transmit power regime, the use of low mmWave frequency achieves better secrecy performance, and when increasing transmit power, a transition from low mmWave frequency to high mmWave frequency is demanded for obtaining a higher secrecy rate. More antennas at the transmitting nodes are needed to decrease the antenna gain obtained by the eavesdroppers when using ULA. Eavesdroppers can intercept more information by using a wide beam pattern. Furthermore, the use of artificial noise may be ineffective for enhancing the secrecy rate.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, Y. and Wang, L. and Wong, K.-K. and Heath, R.W.}, year={2017}, pages={3205–3217} }
 @article{yeh_bhat_heath_choi_prelcic_2017, title={Security in automotive radar and vehicular networks}, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021274268&partnerID=MN8TOARS}, number={5}, journal={Microwave Journal}, author={Yeh, E.R. and Bhat, C.R. and Heath, R.W. and Choi, J. and Prelcic, N.G.}, year={2017}, pages={148–164} }
 @inproceedings{venugopal_alkhateeb_heath_prelcic_2017, title={Time-domain channel estimation for wideband millimeter wave systems with hybrid architecture}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023776479&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2017.7953407}, abstractNote={Millimeter wave (mmWave) systems will likely employ large antennas at both the transmitter and receiver for directional beamforming. Hybrid analog/digital MIMO architectures have been proposed previously for leveraging both array gain and multiplexing gain, while reducing the power consumption in analog-to-digital converters. Channel knowledge is needed to design the hybrid precoders/combiners, which is difficult to obtain due to the large antenna arrays and the frequency selective nature of the channel. In this paper, we propose a sparse recovery based time-domain channel estimation technique for hybrid architecture based frequency selective mmWave systems. The proposed compressed sensing channel estimation algorithm is shown to provide good estimation error performance, while requiring small training overhead. The simulation results show that using multiple RF chains at the receiver and the transmitter further reduces the training overhead.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Venugopal, K. and Alkhateeb, A. and Heath, R.W. and Prelcic, N.G.}, year={2017}, pages={6493–6497} }
 @inproceedings{upadhya_heath_vorobyov_2018, title={Tracking abruptly changing channels in mmWave systems using overlaid data and training}, volume={2017-December}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050739709&partnerID=MN8TOARS}, DOI={10.1109/CAMSAP.2017.8313188}, abstractNote={Millimeter-wave (mmWave) multiple-input multiple-output (MIMO) links are sensitive to abrupt changes in the channel due to blockage and node mobility. We propose to estimate the channel by overlaying pilot and data transmissions. The data transmission is performed over the signal subspace of the channel matrix, while the training, for estimating the parameters of newly appearing paths, is performed over the null-space of the channel matrix. A sparse Bayesian learning-based approach is employed for jointly estimating the channel and data at the receiver. Simulations are used to validate the performance of the proposed method in abruptly changing channel scenarios.}, booktitle={2017 IEEE 7th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2017}, author={Upadhya, K. and Heath, R.W. and Vorobyov, S.A.}, year={2018}, pages={1–5} }
 @article{zhu_choi_heath_2017, title={Two-dimensional AoD and AoA acquisition for wideband millimeter-wave systems with dual-polarized MIMO}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85030686481&partnerID=MN8TOARS}, DOI={10.1109/TWC.2017.2754369}, abstractNote={In this paper, a novel two-dimensional super-resolution angle-of-departure (AoD) and angle-of-arrival (AoA) estimation technique is proposed for wideband millimeter-wave multiple-input multiple-output systems with dual-polarized antenna elements. The key ingredient of the proposed method is the custom designed beam pairs, from which there exists an invertible function of the AoD/AoA. A new multi-layer reference signal structure is developed for the proposed method to facilitate angle estimation for wideband channels with dual-polarized antenna elements. To reduce feedback in closed-loop frequency division duplexing systems, a novel differential feedback strategy is proposed to feedback the estimated angle pairs. Numerical results demonstrate that good azimuth/elevation AoD and AoA estimation performance can be achieved under different levels of signal-to-noise ratio, channel conditions, and antenna array configurations.}, number={12}, journal={IEEE Transactions on Wireless Communications}, author={Zhu, D. and Choi, J. and Heath, R.W.}, year={2017}, pages={7890–7905} }
 @article{sakaguchi_haustein_barbarossa_strinati_clemente_destino_pärssinen_kim_chung_kim_et al._2017, title={Where, when, and how mmwave is used in 5G and beyond}, volume={E100.C}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85042054829&partnerID=MN8TOARS}, DOI={10.1587/transele.E100.C.790}, abstractNote={Wireless engineers and business planners commonly raise the question on where, when, and how millimeter-wave (mmWave) will be used in 5G and beyond. Since the next generation network is not just a new radio access standard, but instead an integration of networks for vertical markets with diverse applications, answers to the question depend on scenarios and use cases to be deployed. This paper gives four 5G mmWave deployment examples and describes in chronological order the scenarios and use cases of their probable deployment, including expected system architectures and hardware prototypes. The paper starts with 28 GHz outdoor backhauling for fixed wireless access and moving hotspots, which will be demonstrated at the PyeongChang winter Olympic games in 2018. The second deployment example is a 60 GHz unlicensed indoor access system at the Tokyo-Narita airport, which is combined with Mobile Edge Computing (MEC) to enable ultra-high speed content download with low latency. The third example is mmWave mesh network to be used as a micro Radio Access Network ({\mu}-RAN), for cost-effective backhauling of small-cell Base Stations (BSs) in dense urban scenarios. The last example is mmWave based Vehicular-to-Vehicular (V2V) and Vehicular-to-Everything (V2X) communications system, which enables automated driving by exchanging High Definition (HD) dynamic map information between cars and Roadside Units (RSUs). For 5G and beyond, mmWave and MEC will play important roles for a diverse set of applications that require both ultra-high data rate and low latency communications.}, number={10}, journal={IEICE Transactions on Electronics}, author={Sakaguchi, K. and Haustein, T. and Barbarossa, S. and Strinati, E.C. and Clemente, A. and Destino, G. and Pärssinen, A. and Kim, I. and Chung, H. and Kim, J. and et al.}, year={2017}, pages={790–808} }
 @article{khan_heath_2017, title={Wireless Power Transfer in Millimeter Wave Tactical Networks}, volume={24}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021796822&partnerID=MN8TOARS}, DOI={10.1109/LSP.2017.2715324}, abstractNote={Wireless power transfer may enable remotely powered operation of low-energy devices in challenging environments such as tactical networks. Millimeter wave (mmWave) is a possible candidate for wireless power transfer, thanks to the use of directional antenna arrays. This letter presents a feasibility study of using mmWave for wireless power transfer in a large-scale network consisting of power beacons and energy harvesters. Using stochastic geometry, system performance is characterized while treating the network nodes as potential blockages to mmWave signals. A link-level metric (energy coverage probability) and a network-level metric (overall success probability) are considered. The former captures whether a typical energy transfer link provides the requisite energy, while the latter also takes the network load into account. Numerical simulations suggest that network densification helps improve the performance, despite an increase in the blockage density. For a given density, deploying an optimal fraction of nodes as power beacons maximizes the number of successful energy harvesters. Finally, mmWave outperforms lower frequency solutions despite blockages.}, number={9}, journal={IEEE Signal Processing Letters}, author={Khan, T.A. and Heath, R.W.}, year={2017}, pages={1284–1287} }
 @article{wang_wong_heath_yuan_2017, title={Wireless Powered Dense Cellular Networks: How Many Small Cells Do We Need?}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85023754129&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2017.2720858}, abstractNote={This paper focuses on wireless powered 5G dense cellular networks, where base station (BS) delivers energy to user equipment (UE) via the microwave radiation in sub-6 GHz or millimeter wave (mmWave) frequency, and UE uses the harvested energy for uplink information transmission. By addressing the impacts of employing different numbers of antennas and bandwidths at lower and higher frequencies, we evaluate the amount of harvested energy and throughput in such networks. Based on the derived results, we obtain the required small cell density to achieve an expected level of harvested energy or throughput. Also, we obtain that when the ratio of the number of sub-6-GHz BSs to that of the mmWave BSs is lower than a given threshold, UE harvests more energy from an mmWave BS than a sub-6-GHz BS. We find how many mmWave small cells are needed to perform better than the sub-6-GHz small cells from the perspectives of harvested energy and throughput. Our results reveal that the amount of harvested energy from the mmWave tier can be comparable to the sub-6-GHz counterpart in the dense scenarios. For the same tier scale, mmWave tier can achieve higher throughput. Furthermore, the throughput gap between different mmWave frequencies increases with the mmWave BS density.}, number={9}, journal={IEEE Journal on Selected Areas in Communications}, author={Wang, L. and Wong, K.-K. and Heath, R.W. and Yuan, J.}, year={2017}, pages={2010–2024} }
 @article{khan_heath_popovski_2017, title={Wirelessly powered communication networks with short packets}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028511146&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2017.2742512}, abstractNote={Wirelessly powered communications will entail short packets due to naturally small payloads, low-latency requirements, and/or insufficient energy resources to support longer transmissions. In this paper, a wireless-powered communication system is investigated, where an energy harvesting transmitter, charged by power beacons via wireless energy transfer, attempts to communicate with a receiver over a noisy channel. Under a save-then-transmit protocol, the system performance is characterized using metrics, such as the energy supply probability at the transmitter, and the achievable rate at the receiver for the case of short packets. The analytical treatment is provided for two cases: a three-node setup with a single power beacon and a large-scale network with multiple power beacons. Leveraging finite-length information theory, tractable analytical expressions are derived for the considered metrics in terms of the harvest blocklength, the transmit blocklength, the harvested power, the transmit power, and the network density. The analysis provides several useful design guidelines. Though using a small transmit power or a small transmit blocklength helps avoid energy outages, the consequently smaller signal-to-noise ratio or the fewer coding opportunities may cause a data decoding error. Scaling laws are derived to capture this inherent tradeoff between the harvest and transmit blocklengths. Numerical results reveal that power control is essential for improving the achievable rate of the considered system. The asymptotically optimal transmit power yields nearly optimal performance in the finite blocklength regime.}, number={12}, journal={IEEE Transactions on Communications}, author={Khan, T.A. and Heath, R.W. and Popovski, P.}, year={2017}, pages={5529–5543} }
 @article{khan_orlik_kim_heath_sawa_2017, title={A Stochastic Geometry Analysis of Large-Scale Cooperative Wireless Networks Powered by Energy Harvesting}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029487969&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2623314}, abstractNote={Energy harvesting is an emerging technology for enabling green, sustainable, and autonomous wireless networks. In this paper, a large-scale wireless network with energy harvesting transmitters is considered, where a group of transmitters forms a cluster to cooperatively serve a desired receiver amid interference and noise. To characterize the link-level performance, closed-form expressions are derived for the transmission success probability at a receiver in terms of key parameters such as node densities, energy harvesting parameters, channel parameters, and cluster size, for a given cluster geometry. The analysis is further extended to characterize a network-level performance metric, capturing the tradeoff between link quality and the fraction of receivers served. Numerical simulations validate the accuracy of the analytical model. Several useful insights are provided. For example, while more cooperation helps improve the link-level performance, the network-level performance might degrade with the cluster size. Numerical results show that a small cluster size (typically 3 or smaller) optimizes the network-level performance. Furthermore, substantial performance can be extracted with a relatively small energy buffer. Moreover, the utility of having a large energy buffer increases with the energy harvesting rate as well as with the cluster size in sufficiently dense networks.}, number={8}, journal={IEEE Transactions on Communications}, author={Khan, T.A. and Orlik, P.V. and Kim, K.J. and Heath, R.W. and Sawa, K.}, year={2017}, pages={3343–3358} }
 @inproceedings{park_andrews_heath_lee_2016, title={A lower bound on the optimum feedback rate for downlink multi-antenna cellular networks}, volume={2016-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84985910995&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2016.7541569}, abstractNote={We consider a multi-antenna downlink cellular network using either single-user maximal ratio transmission (MRT) or multi-user zero-forcing (ZF) transmission. The locations of the base stations are modeled by a Poisson point process to allow the inter-cell interference to be tractably analyzed. A tight lower bound on the optimum number of feedback bits maximizing the net spectral efficiency is derived, whereby the cost of feedback sent via uplink is subtracted from the corresponding gain in downlink spectral efficiency. When using MRT, the optimum number of feedback bits is shown to scale linearly with the number of antennas, and logarithmically with the channel coherence time. With ZF, the optimum amount of feedback scales the same as with MRT, but additionally also increases linearly with the pathloss exponent.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Park, J. and Andrews, J.G. and Heath, R.W. and Lee, N.}, year={2016}, pages={1601–1605} }
 @inproceedings{park_heath_2016, title={Adaptive feedback partitions in dynamic zero-forcing beamforming based on stochastic geometry}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015957570&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2016.7848830}, abstractNote={In this paper, we analyze the performance of dynamic zero-forcing beamforming with limited feedback and propose adaptive feedback partition strategies. Assuming a cellular network model based on stochastic geometry, we derive analytical expressions for the complementary cumulative distribution function (CCDF) of the instantaneous signal-to-interference ratio (SIR) and the ergodic spectral efficiency as functions of the relevant system parameters: the cluster size, the number of feedback bits, the intra-cluster geometry, and the path-loss exponent. By leveraging these expressions, we propose two feedback partition strategies, each of which is applied for known intra-cluster geometry and unknown intra- cluster geometry, respectively. In the simulations, we verify the accurateness of our analysis and demonstrate the proposed strategies by comparing to the equal feedback partition. Our results show that by adaptively partitioning the feedback bits, the intra- cluster interference is efficiently managed whether the intra-cluster geometry is known or not.}, booktitle={2016 IEEE Globecom Workshops, GC Wkshps 2016 - Proceedings}, author={Park, J. and Heath, R.W.}, year={2016} }
 @article{lee_heath_2016, title={Advanced interference management technique: Potentials and limitations}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84977107061&partnerID=MN8TOARS}, DOI={10.1109/MWC.2016.7498072}, abstractNote={Interference management has the potential to improve spectrum efficiency in current and next generation wireless systems (e.g., 3GPP LTE and IEEE 802.11). Recently, new paradigms for interference management have emerged to tackle interference in a general class of wireless networks: interference shaping and interference exploitation. Interference shaping is a technique that creates a particular linear combination of interference signals when transmitters propagate signals to minimize the aggregated interference effect at each receiver. Interference exploitation is a method that harnesses interference in decoding as side information to improve data rates. Both approaches offer better performance in interference-limited communication regimes than traditionally thought possible. This article provides a high-level overview of several different interference shaping and exploitation techniques for single-hop, multihop, and multi-way network architectures with graphical illustrations. The article concludes with a discussion of practical challenges associated with adopting sophisticated interference management strategies in the future.}, number={3}, journal={IEEE Wireless Communications}, author={Lee, N. and Heath, R.W.}, year={2016}, pages={30–38} }
 @article{heath_gonzalez-prelcic_rangan_roh_sayeed_2016, title={An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84966430691&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2016.2523924}, abstractNote={Communication at millimeter wave (mmWave) frequencies is defining a new era of wireless communication. The mmWave band offers higher bandwidth communication channels versus those presently used in commercial wireless systems. The applications of mmWave are immense: wireless local and personal area networks in the unlicensed band, 5G cellular systems, not to mention vehicular area networks, ad hoc networks, and wearables. Signal processing is critical for enabling the next generation of mmWave communication. Due to the use of large antenna arrays at the transmitter and receiver, combined with radio frequency and mixed signal power constraints, new multiple-input multiple-output (MIMO) communication signal processing techniques are needed. Because of the wide bandwidths, low complexity transceiver algorithms become important. There are opportunities to exploit techniques like compressed sensing for channel estimation and beamforming. This article provides an overview of signal processing challenges in mmWave wireless systems, with an emphasis on those faced by using MIMO communication at higher carrier frequencies.}, number={3}, journal={IEEE Journal on Selected Topics in Signal Processing}, author={Heath, R.W. and Gonzalez-Prelcic, N. and Rangan, S. and Roh, W. and Sayeed, A.M.}, year={2016}, pages={436–453} }
 @article{taranetz_heath_rupp_2016, title={Analysis of Urban Two-Tier Heterogeneous Mobile Networks With Small Cell Partitioning}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994515475&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2595559}, abstractNote={In this paper, we investigate the performance of typical indoor users in urban two-tier heterogeneous mobile networks with indoor-deployed small cell base stations (BSs) and outdoor BSs. The urban topology is modeled by a Boolean scheme of blockages, which is tied together with the deployment of outdoor and indoor BSs as well as the signal propagation characteristics. Each building is assumed to be served by an indoor small cell BS with a certain probability, which is denoted as occupation probability. Indoor and outdoor environments are partitioned by walls with a certain penetration loss. We separately account for outdoor BSs in line of sight (LOS) and non-line of sight (NLOS), as well as small cell BSs in neighboring and non-neighboring buildings. We derive expressions for the signal to interference ratio (SIR)-coverage probability and the average spectral efficiency of a typical indoor user. Numerical evaluations are performed with 3GPP compliant macro- and pico-BS scenarios. Our results exhibit that the indoor area coverage, which is a function of the building density and size, significantly affects the impact of the target building's wall penetration loss. The analysis is simplified by various assumptions, which are verified by extensive Monte Carlo simulations.}, number={10}, journal={IEEE Transactions on Wireless Communications}, author={Taranetz, M. and Heath, R.W. and Rupp, M.}, year={2016}, pages={7044–7057} }
 @inproceedings{bai_heath_2017, title={Analysis of beam sweep channel estimation in MmWave massive MIMO networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85019228614&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2016.7905915}, abstractNote={Millimeter wave (mmWave) is a promising carrier frequency to implement massive MIMO networks. Due to the limitation on the number of RF chains and the channel sparsity in clusters, mmWave massive MIMO networks may apply a beam sweep method to estimate the channel in the angular domain. We propose a stochastic geometry framework to study the performance of beam sweep in the uplink. Based on the framework, we compute an approximate upper bound for the success probability of beam sweep. Numerical results verify the analysis, and show a non-monotonic trend between the beam width used in the beam sweep and the success probability.}, booktitle={2016 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2016 - Proceedings}, author={Bai, T. and Heath, R.W.}, year={2017}, pages={615–619} }
 @inproceedings{wang_venugopal_molisch_heath_2016, title={Analysis of urban millimeter wave microcellular networks}, volume={0}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85017017293&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2016.7880906}, abstractNote={Millimeter wave (mmWave) networks are sensitive to blockages due to buildings in urban areas. This is critical for vehicle-to-infrastructure networks which are cellular networks designed to support emerging vehicular applications. Motivated by measurement and ray tracing results in urban microcells, instead of characterizing the pathloss by Euclidean distance, we calculate it by the weighted sum of segment length along the propagation path, i.e., Manhattan distance, and a certain corner loss at the intersections along the path. We analyze network performance by modeling the urban microcell network by a Manhattan Poisson line process. Our results show significant differences between Manhattan and Euclidean distance- based pathloss models. Assuming the receiver is associated with the base station (BS) with the smallest pathloss, we derive closed-form expression of the distribution of the associated link pathloss. We obtain the coverage probability and reveal the impacts of interference from the LOS and NLOS BSs. It is shown that in this scenario the interference from a NLOS parallel street is negligible.}, booktitle={IEEE Vehicular Technology Conference}, author={Wang, Y. and Venugopal, K. and Molisch, A.F. and Heath, R.W.}, year={2016} }
 @article{mungara_zhang_lozano_heath_2016, title={Analytical characterization of ITLinQ: Channel allocation for device-to-device communication networks}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969497670&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2523987}, abstractNote={Device-to-device (D2D) communication allows serving local wireless traffic bypassing the system's infrastructure. One way to control the interference in D2D networks is to carefully channelize transmissions. This paper presents an analytical characterization of ITLinQ, one of the principal D2D channelization schemes proposed to date. Recognizing that it captures well the spatial characteristics of D2D networks, a stochastic geometry setting is utilized for this analysis. The derived expressions enable gleaning insights on how ITLinQ avoids situations of excessive interference, and they facilitate optimizing the controllable parameters of ITLinQ so as to maximize the system spectral efficiency (bits/s/Hz per unit area). With the parameters thus optimized, the ultimate performance of ITLinQ can be evaluated with respect to other D2D channel allocation schemes. In particular, performance evaluation comparisons with the FlashLinQ scheme are provided, and the gains with respect to an unchannelized network are quantified.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Mungara, R.K. and Zhang, X. and Lozano, A. and Heath, R.W.}, year={2016}, pages={3603–3615} }
 @article{bai_heath_2016, title={Analyzing Uplink SINR and Rate in Massive MIMO Systems Using Stochastic Geometry}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84998679036&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2591007}, abstractNote={This paper proposes a stochastic geometry framework to analyze the signal-to-noise-and-interference ratio (SINR) and rate performance in a large-scale uplink massive multiple-input and multiple-output (MIMO) network. Based on the model, expressions are derived for spatial average SINR distributions over user and base station distributions with maximum ratio combining (MRC) and zero-forcing (ZF) receivers. We show that, using massive MIMO, the uplink SINR in certain urban marcocell scenarios is limited by interference. In the interference-limited regime, the results reveal that for MRC receivers, a superlinear (polynomial) scaling law between the number of base station antennas and scheduled users per cell preserves the uplink signal-to-interference ratio (SIR) distribution, while a linear scaling applies to ZF receivers. ZF receivers are shown to outperform MRC receivers in the SIR coverage, and the performance gap is quantified in terms of the difference in the number of antennas to achieve the same SIR distribution. Numerical results verify the analysis. It is found that the optimal compensation fraction in fractional power control to optimize rate is generally different for MRC and ZF receivers. Besides, simulations show that the scaling results derived from the proposed framework apply to the networks, where base stations are distributed according to a hexagonal lattice.}, number={11}, journal={IEEE Transactions on Communications}, author={Bai, T. and Heath, R.W.}, year={2016}, pages={4592–4606} }
 @inproceedings{rusu_gonzalez-prelcic_heath_2016, title={Array thinning for antenna selection in millimeter wave MIMO systems}, volume={2016-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973370319&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2016.7472311}, abstractNote={This paper addresses the problem of designing thinned arrays with minimized side lobe levels for antenna selection in millimeter wave MIMO systems. We propose a new optimization solution based on compressed sensing techniques and convex optimization relaxation which we show to be a heuristic that solves the original binary optimization problem of side lobe level minimization. We compare the proposed method with other approaches from the literature like simulated annealing and genetic algorithms showing the superiority of the method in terms of performance, running time and ease of parameter tuning. The simulation results cover a wide range of dimensions and situations.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Rusu, C. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2016}, pages={3416–3420} }
 @inproceedings{zhu_choi_heath_2016, title={Auxiliary beam pair design in mmWave cellular systems with hybrid precoding and limited feedback}, volume={2016-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973392513&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2016.7472306}, abstractNote={Auxiliary beam pairs are proposed in millimeter-wave cellular systems for closed-loop hybrid precoding. Pairs of custom designed analog beams are formed to help acquire channel information. It is shown via simulations that auxiliary beam pairs have lower complexity and better achievable rates with a moderate amount of feedback, compared to conventional beam training methods.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Zhu, D. and Choi, J. and Heath, R.W.}, year={2016}, pages={3391–3395} }
 @inproceedings{zhu_choi_heath_2016, title={Auxiliary beam pair enabled AoD and AoA estimation in mmWave FD-MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015362357&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7841616}, abstractNote={In this paper, auxiliary beam pair design is developed to provide high-resolution estimates of channel's elevation/azimuth angle-of-departure (AoD) and angle-of-arrival (AoA) for millimeter- wave full-dimension MIMO (FD-MIMO) systems. Pairs of custom designed analog beams are formed at both the transmitter and receiver to help acquire channel information. It is shown via simulation results that by employing the proposed method, promising elevation/azimuth AoD and AoA estimation performance can be achieved under various signal- to-noise ratio levels and channel conditions.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Zhu, D. and Choi, J. and Heath, R.W.}, year={2016} }
 @inproceedings{va_shimizu_bansal_heath_2016, title={Beam design for beam switching based millimeter wave vehicle-to-infrastructure communications}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84981334716&partnerID=MN8TOARS}, DOI={10.1109/ICC.2016.7511414}, abstractNote={Beam alignment is a source of overhead in mobile millimeter wave communication systems due to the need for frequent repointing. Beam switching architectures can reduce the amount of repointing required by leveraging position prediction. This paper presents an optimization of beam design in terms of rate. We consider a non-congested two-lane highway scenario where road side units are installed on lighting poles. Under this scenario, line-of-sight to the road side unit is very likely and vehicle speed does not vary much. We formulate and solve numerically using a gradient descent method for an optimal beam design to maximize the data rate for non-overlap beams. The result shows close performance to the equal coverage beam design. We study the effect of the overlap on the average rate and outage and compare the equal coverage with the equal beamwidth design. Numerical examples show that the equal coverage design can achieve up to 1.5× the rate of the equal beamwidth design confirming the importance of the choice of beam design.}, booktitle={2016 IEEE International Conference on Communications, ICC 2016}, author={Va, V. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2016} }
 @inproceedings{va_vikalo_heath_2017, title={Beam tracking for mobile millimeter wave communication systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85019217328&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2016.7905941}, abstractNote={Millimeter wave (mmWave) is an attractive option for high data rate applications. Enabling mmWave communications requires appropriate beamforming, which is conventionally realized by a lengthy beam training process. Such beam training will be a challenge for applying mmWave to mobile environments. As a solution, a beam tracking method requiring to train only one beam pair to track a path in the analog beamforming architecture is developed. Considering its low complexity which is suitable for mobile settings, the extended Kalman filter is chosen as the tracking filter. Several effects impacting the performance of the proposed tracking algorithm, such as the signal-to-noise ratio (SNR) and array size, are investigated. It is found that at the same SNR, narrower beams, which are more sensitive to angular changes, can provide more accurate estimate. Too narrow beams, however, degrade tracking performance because beam misalignment could happen during the measurement. Finally, a comparison to prior work is given where it is shown that our approach is more suitable for fast-changing environments thanks to the low measurement overhead.}, booktitle={2016 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2016 - Proceedings}, author={Va, V. and Vikalo, H. and Heath, R.W.}, year={2017}, pages={743–747} }
 @inproceedings{gupta_andrews_heath_2017, title={Can operators simply share millimeter wave spectrum licenses?}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988582759&partnerID=MN8TOARS}, DOI={10.1109/ITA.2016.7888158}, abstractNote={Because of their often noise-limited behavior, millimeter wave systems may be able to share spectrum licenses without any coordination. We establish the theoretical feasibility of uncoordinated sharing by considering a downlink system containing multiple mmWave cellular providers. We compute the downlink rate distribution, and compare that against systems with exclusive licenses. We show that shared licenses can use a smaller bandwidth to achieve the same per-user median rate as providers with an exclusive spectrum license. We also show that both total interference and available bandwidth increase with the size of the spectrum sharing coalition, which implies that the optimal amount of spectrum sharing depends on the target rate.}, booktitle={2016 Information Theory and Applications Workshop, ITA 2016}, author={Gupta, A.K. and Andrews, J.G. and Heath, R.W.}, year={2017} }
 @inproceedings{thornburg_heath_2016, title={Capacity and coverage in clustered LOS mmWave Ad Hoc networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015364520&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7841648}, abstractNote={Recent advancements in manufacturing and favorable regulatory positions make millimeter wave (mmWave) a target for next-generation wireless networks. For a variety of use-cases, it is reasonable to assume users will be clustered around a central point (e.g. WiFi hotspot) rather than uniformly located. In this paper, we develop analytic expressions for the capacity of clustered mmWave ad hoc networks under two channel access assumptions: uncoordinated channel access (UCA) and TDMA. We show that with directional antenna arrays UCA can outperform TDMA in terms of per cluster capacity if the cluster density is not too large. We define and develop a method for characterizing the balance between the intra-cluster coverage with the inter-cluster coverage. For small cluster sizes, the required inter-cluster distance severely impacts the inter-cluster communication rate.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Thornburg, A. and Heath, R.W.}, year={2016} }
 @inproceedings{thornburg_heath_daniels_2016, title={Capacity and scaling laws of dense mmWave and interference alignment ad hoc networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85011911905&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2016.7795346}, abstractNote={Ad hoc networks continue to be severely interference limited. One approach to increase capacity is to go to millimeter wave (mmWave). Higher bandwidth but also directional antennas to reduce interference. Another approach is to stay at conventional frequencies but use techniques like interference alignment (IA). In this paper, we derive the scaling laws for dense mmWave ad hoc networks with directional beamforming and ad hoc networks with IA. We also give an exact expression for the capacity of mmWave ad hoc networks with random dipole distance. Our results indicate that the number of antennas with uncoordinated beamforming must scale sub-linearly with the user density while the number of users coordinating with IA must scale super-linearly.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Thornburg, A. and Heath, R.W. and Daniels, R.}, year={2016}, pages={319–324} }
 @inproceedings{rodriguez-fernandez_gonzalez-prelcic_heath_2017, title={Channel estimation in mixed hybrid-low resolution MIMO architectures for mmWave communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016267906&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2016.7869150}, abstractNote={Mixed hybrid-low resolution MIMO architectures reduce both the number of ADCs and their resolution to reduce power consumption. In such a system, the received signal suffers from two compression stages. One is due to the analog processing stage and the reduced number of RF chains, while the other is a result of signal quantization by the ADC converters. The compression steps both complicate the estimation of the MIMO channel. In this paper we propose a compressive channel estimator that exploits the sparse structure of the millimeter wave channel to compensate for the loss of information inherent to the proposed architecture.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2017}, pages={768–773} }
 @article{sim_park_chae_heath_2016, title={Compressed channel feedback for correlated massive MIMO}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963928973&partnerID=MN8TOARS}, DOI={10.1109/JCN.2016.000012}, abstractNote={Massive multiple-input multiple-output (MIMO) is a promising approach for cellular communication due to its energy efficiency and high achievable data rate. These advantages, however, can be realized only when channel state information (CSI) is available at the transmitter. Since there are many antennas, CSI is too large to feed back without compression. To compress CSI, prior work has applied compressive sensing (CS) techniques and the fact that CSI can be sparsified. The adopted sparsifying bases fail, however, to reflect the spatial correlation and channel conditions or to be feasible in practice. In this paper, we propose a new sparsifying basis that reflects the long-term characteristics of the channel, and needs no change as long as the spatial correlation model does not change. We propose a new reconstruction algorithm for CS, and also suggest dimensionality reduction as a compression method. To feed back compressed CSI in practice, we propose a new codebook for the compressed channel quantization assuming no other-cell interference. Numerical results confirm that the proposed channel feedback mechanisms show better performance in point-to-point (single-user) and point-to-multi-point (multi-user) scenarios.}, number={1}, journal={Journal of Communications and Networks}, author={Sim, M.S. and Park, J. and Chae, C.-B. and Heath, R.W.}, year={2016}, pages={95–104} }
 @inproceedings{gonzalez-prelcic_truongt_rusu_heath_2016, title={Compressive channel estimation in FDD multi-cell massive MIMO systems with arbitrary arrays}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015931359&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2016.7848864}, abstractNote={Efficient downlink channel state information acquisition at the base station is crucial to achieve the potential gains of FDD massive MIMO systems. Conventional approaches for channel estimation require a training and feedback overhead which scales with the number of base station antennas, which make them unsuitable for large scale FDD MIMO systems. Alternative strategies exploiting the sparsity in the massive MIMO channel rely on additional assumptions on a shared common support between downlink channels, a specific array geometry at the base station (BS) and user equipments (UE), or a given modulation scheme. In this paper we propose a general approach for downlink channel estimation in FDD massive MIMO systems which leverages the individual sparsity in the different downlink channels without any other additional assumption. The designed precoding strategy allows channel estimation at the UE without knowledge of the BS array geometry. Simulation results show the accuracy of the estimations and the significant overhead reduction over conventional approaches.}, booktitle={2016 IEEE Globecom Workshops, GC Wkshps 2016 - Proceedings}, author={Gonzalez-Prelcic, N. and Truongt, K.T. and Rusu, C. and Heath, R.W.}, year={2016} }
 @inproceedings{eltayeb_al-naffouri_heath_2016, title={Compressive sensing for blockage detection in vehicular millimeter wave antenna arrays}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015389511&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7841677}, abstractNote={The radiation pattern of an antenna array depends on the excitation weights and the geometry of the array. Due to mobility, some vehicular antenna elements might be subjected to full or partial blockages from a plethora of particles like dirt, salt, ice, and water droplets. These particles cause absorption and scattering to the signal incident on the array, and as a result, change the array geometry. This distorts the radiation pattern of the array mostly with an increase in the sidelobe level and decrease in gain. In this paper, we propose a blockage detection technique for millimeter wave vehicular antenna arrays that jointly estimates the locations of the blocked antennas and the attenuation and phase-shifts that result from the suspended particles. The proposed technique does not require the antenna array to be physically removed from the vehicle and permits real-time array diagnosis. Numerical results show that the proposed technique provides satisfactory results in terms of block detection with low detection time provided that the number of blockages is small compared to the array size.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Eltayeb, M.E. and Al-Naffouri, T.Y. and Heath, R.W.}, year={2016} }
 @article{venugopal_valenti_heath_2016, title={Device-to-Device Millimeter Wave Communications: Interference, Coverage, Rate, and Finite Topologies}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84987786123&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2580510}, abstractNote={Emerging applications involving device-to-device communication among wearable electronics require gigabits per second throughput, which can be achieved by utilizing millimeter-wave (mmWave) frequency bands. When many such communicating devices are indoors in close proximity, such as in a train, car, or airplane cabin, interference can be a serious impairment. This paper uses stochastic geometry to analyze the performance of mmWave networks with a finite number of interferers in a finite network region. Prior work considered either lower carrier frequencies with different antenna and channel assumptions, or a network with an infinite spatial extent. In this paper, human users not only carry potentially interfering devices, but also act to block interfering signals. Using a sequence of simplifying assumptions, accurate expressions for coverage and rate are developed that capture the effects of key antenna characteristics, such as directivity and gain, and are a function of the finite area and number of users. The assumptions are validated through a combination of analysis and simulation. The main conclusions are that mmWave frequencies can provide gigabits per second throughput even with omni-directional transceiver antennas, and larger, more directive antenna arrays give better system performance.}, number={9}, journal={IEEE Transactions on Wireless Communications}, author={Venugopal, K. and Valenti, M.C. and Heath, R.W.}, year={2016}, pages={6175–6188} }
 @inproceedings{park_alkhateeb_heath_2017, title={Dynamic subarray architecture for wideband hybrid precoding in millimeter wave massive MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85019249817&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2016.7905912}, abstractNote={Hybrid precoding architectures can address the trade-off between achievable spectral efficiency and power consumption in large-scale MIMO systems. Most of the prior work on hybrid precoding focused on narrowband channels and assumed fully-connected hybrid architectures. In this paper, a closed-form solution for OFDM-based wideband hybrid precoding is developed for both fully-connected and subarray architectures in frequency selective channels. The closed form solution provides useful insights into how hybrid subarray structures should be designed. Based on that, we propose a novel technique that dynamically constructs the hybrid subarrays based on the long-term channel characteristics.}, booktitle={2016 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2016 - Proceedings}, author={Park, S. and Alkhateeb, A. and Heath, R.W.}, year={2017}, pages={600–604} }
 @article{gao_dai_han_chih-lin_heath_2016, title={Energy-Efficient Hybrid Analog and Digital Precoding for MmWave MIMO Systems with Large Antenna Arrays}, volume={34}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84970029046&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2016.2549418}, abstractNote={Millimeter wave (mmWave) MIMO will likely use hybrid analog and digital precoding, which uses a small number of RF chains to reduce the energy consumption associated with mixed signal components like analog-to-digital components not to mention baseband processing complexity. However, most hybrid precoding techniques consider a fully connected architecture requiring a large number of phase shifters, which is also energy-intensive. In this paper, we focus on the more energy-efficient hybrid precoding with subconnected architecture, and propose a successive interference cancelation (SIC)-based hybrid precoding with near-optimal performance and low complexity. Inspired by the idea of SIC for multiuser signal detection, we first propose to decompose the total achievable rate optimization problem with nonconvex constraints into a series of simple subrate optimization problems, each of which only considers one subantenna array. Then, we prove that maximizing the achievable subrate of each subantenna array is equivalent to simply seeking a precoding vector sufficiently close (in terms of Euclidean distance) to the unconstrained optimal solution. Finally, we propose a low-complexity algorithm to realize SIC-based hybrid precoding, which can avoid the need for the singular value decomposition (SVD) and matrix inversion. Complexity evaluation shows that the complexity of SIC-based hybrid precoding is only about 10% as complex as that of the recently proposed spatially sparse precoding in typical mmWave MIMO systems. Simulation results verify that SIC-based hybrid precoding is near-optimal and enjoys higher energy efficiency than the spatially sparse precoding and the fully digital precoding.}, number={4}, journal={IEEE Journal on Selected Areas in Communications}, author={Gao, X. and Dai, L. and Han, S. and Chih-Lin, I. and Heath, R.W.}, year={2016}, pages={998–1009} }
 @inproceedings{estimating millimeter wave channels using out-of-band measurements_2017, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85005990286&partnerID=MN8TOARS}, DOI={10.1109/ITA.2016.7888146}, abstractNote={Channel estimation and beam training can be a source of significant overhead in establishing millimeter wave (mmWave) communication links, especially in high mobility applications like connected vehicles. In this paper, we highlight the opportunities and challenges associated with leveraging channel state information acquired at a lower frequency as a form of side information on a higher frequency channel. We focus on the relationship between spatial correlation matrices of sub-6 GHz and mmWave channels. We provide a transform that can be used to relate the spatial correlation matrix derived at one frequency to another much different frequency. We derive an expression for the excess mean squared error and use it to evaluate the performance experienced by using the transformed correlation in mmWave channel estimation.}, booktitle={2016 Information Theory and Applications Workshop, ITA 2016}, year={2017} }
 @article{rusu_gonzalez-prelcic_heath_2016, title={Fast Orthonormal Sparsifying Transforms Based on Householder Reflectors}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994017836&partnerID=MN8TOARS}, DOI={10.1109/TSP.2016.2612168}, abstractNote={Dictionary learning is the task of determining a data-dependent transform that yields a sparse representation of some observed data. The dictionary learning problem is nonconvex, and usually solved via computationally complex iterative algorithms. Furthermore, the resulting transforms obtained generally lack structure that permits their fast application to data. To address this issue, this paper develops a framework for learning orthonormal dictionaries, which are built from products of a few Householder reflectors. Two algorithms are proposed to learn the reflector coefficients: one that considers a sequential update of the reflectors and one with a simultaneous update of all reflectors that imposes an additional internal orthogonal constraint. The proposed methods have low computational complexity and are shown to converge to local minimum points, which can be described in terms of the spectral properties of the matrices involved. The resulting dictionaries balance between the computational complexity and the quality of the sparse representations by controlling the number of householder reflectors in their product. Simulations of the proposed algorithms are shown in the image processing setting, where well-known fast transforms are available for comparisons. The proposed algorithms have favorable reconstruction error and the advantage of a fast implementation relative to the classical, unstructured dictionaries.}, number={24}, journal={IEEE Transactions on Signal Processing}, author={Rusu, C. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2016}, pages={6589–6599} }
 @article{alkhateeb_heath_2016, title={Frequency Selective Hybrid Precoding for Limited Feedback Millimeter Wave Systems}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84971350949&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2549517}, abstractNote={Hybrid analog/digital precoding offers a compromise between hardware complexity and system performance in millimeter wave (mmWave) systems. This type of precoding allows mmWave systems to leverage large antenna array gains that are necessary for sufficient link margin, while permitting low cost and power consumption hardware. Most prior work has focused on hybrid precoding for narrow-band mmWave systems, with perfect or estimated channel knowledge at the transmitter. MmWave systems, however, will likely operate on wideband channels with frequency selectivity. Therefore, this paper considers wideband mmWave systems with a limited feedback channel between the transmitter and receiver. First, the optimal hybrid precoding design for a given RF codebook is derived. This provides a benchmark for any other heuristic algorithm and gives useful insights into codebook designs. Second, efficient hybrid analog/digital codebooks are developed for spatial multiplexing in wideband mmWave systems. Finally, a low-complexity yet near-optimal greedy frequency selective hybrid precoding algorithm is proposed based on Gram-Schmidt orthogonalization. Simulation results show that the developed hybrid codebooks and precoder designs achieve very-good performance compared with the unconstrained solutions while requiring much less complexity.}, number={5}, journal={IEEE Transactions on Communications}, author={Alkhateeb, A. and Heath, R.W.}, year={2016}, pages={1801–1818} }
 @inproceedings{gonzález-coma_gonzález-prelcic_castedo_heath_2017, title={Frequency selective multiuser hybrid precoding for mmWave systems with imperfect channel knowledge}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016296236&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2016.7869044}, abstractNote={In this work we design the precoders/combiners for a multiuser mmWave system considering a frequency selective channel model. We assume that the available channel information is imperfect, and only the estimates of the autocorrelation of the received signal, and the cross-correlation with the transmitted signal are available. Using this knowledge, the precoder and combiners are designed via an iterative MMSE algorithm. The solutions are decomposed into the frequency-flat analog and frequency-selective digital precoders/combiners, which obtain a close performance to the all-digital solution in terms of achievable rate as shown in the simulations.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={González-Coma, J.P. and González-Prelcic, N. and Castedo, L. and Heath, R.W.}, year={2017}, pages={291–295} }
 @article{gupta_alkhateeb_andrews_heath_2016, title={Gains of Restricted Secondary Licensing in Millimeter Wave Cellular Systems}, volume={34}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84998865419&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2016.2615218}, abstractNote={Sharing the spectrum among multiple operators seems promising in millimeter wave (mmWave) systems. One explanation is the highly directional transmission in mmWave, which reduces the interference caused by one network on the other networks sharing the same resources. In this paper, we model a mmWave cellular system, where an operator that primarily owns an exclusive-use license of a certain band can sell a restricted secondary license of the same band to another operator. This secondary network has a restriction on the maximum interference it can cause to the original network. Using stochastic geometry, we derive expressions for the coverage and the rate of both networks, and establish the feasibility of secondary licensing in licensed mmWave bands. To explain economic tradeoffs, we consider a revenue-pricing model for both operators in the presence of a central licensing authority. Our results show that the original operator and central network authority can benefit from secondary licensing when the maximum interference threshold is properly adjusted. This means that the original operator and central licensing authority have an incentive to permit a secondary network to restrictively share the spectrum. Our results also illustrate that the spectrum sharing gains increase with narrow beams and when the network densifies.}, number={11}, journal={IEEE Journal on Selected Areas in Communications}, author={Gupta, A.K. and Alkhateeb, A. and Andrews, J.G. and Heath, R.W.}, year={2016}, pages={2935–2950} }
 @inproceedings{alkhateeb_heath_2016, title={Gram Schmidt based greedy hybrid precoding for frequency selective millimeter wave MIMO systems}, volume={2016-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84971223221&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2016.7472307}, abstractNote={Hybrid analog/digital precoding allows millimeter wave MIMO systems to leverage large antenna array gains while permitting low cost and power consumption hardware. Most prior work has focused on hybrid precoding for narrow-band mmWave systems. MmWave systems, however, will likely operate on wideband channels with frequency selectivity. Therefore, this paper considers frequency selective hybrid precoding with RF beamforming vectors taken from a quantized codebook. For this system, a low-complexity yet near-optimal greedy algorithm is developed for the design of the hybrid analog/digital precoders. The proposed algorithm greedily selects the RF beamforming vectors using Gram-Schmidt orthogonalization. Simulation results show that the developed precoding design algorithm achieves very good performance compared with the unconstrained solutions while requiring less complexity.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Alkhateeb, A. and Heath, R.W.}, year={2016}, pages={3396–3400} }
 @article{mendez-rial_rusu_gonzalez-prelcic_alkhateeb_heath_2016, title={Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches?}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84979849127&partnerID=MN8TOARS}, DOI={10.1109/ACCESS.2015.2514261}, abstractNote={Hybrid analog/digital multiple-input multiple-output architectures were recently proposed as an alternative for fully digital-precoding in millimeter wave wireless communication systems. This is motivated by the possible reduction in the number of RF chains and analog-to-digital converters. In these architectures, the analog processing network is usually based on variable phase shifters. In this paper, we propose hybrid architectures based on switching networks to reduce the complexity and the power consumption of the structures based on phase shifters. We define a power consumption model and use it to evaluate the energy efficiency of both structures. To estimate the complete MIMO channel, we propose an open-loop compressive channel estimation technique that is independent of the hardware used in the analog processing stage. We analyze the performance of the new estimation algorithm for hybrid architectures based on phase shifters and switches. Using the estimate, we develop two algorithms for the design of the hybrid combiner based on switches and analyze the achieved spectral efficiency. Finally, we study the tradeoffs between power consumption, hardware complexity, and spectral efficiency for hybrid architectures based on phase shifting networks and switching networks. Numerical results show that architectures based on switches obtain equal or better channel estimation performance to that obtained using phase shifters, while reducing hardware complexity and power consumption. For equal power consumption, all the hybrid architectures provide similar spectral efficiencies.}, journal={IEEE Access}, author={Mendez-Rial, R. and Rusu, C. and Gonzalez-Prelcic, N. and Alkhateeb, A. and Heath, R.W.}, year={2016}, pages={247–267} }
 @inproceedings{lopez-valcarce_gonzalez-prelcic_rusu_heath_2016, title={Hybrid precoders and combiners for mmwave MIMO systems with per-antenna power constraints}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015398495&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7841881}, abstractNote={This paper considers the design of hybrid precoders and combiners for mmWave MIMO systems with per-antenna power constraints and the additional limitations introduced by the phase-shifting network in the analog processing stage. Previous hybrid designs were obtained using a total power constraint, but in practical implementations per-antenna constraints are more realistic, specially at mmWave, given the large number of power amplifiers used in the transmit array. Assuming perfect channel knowledge, we obtain first an approximation to the alldigital solution for the precoder and the combiner given the per-antenna constraints. Then, we develop a new method for the design of the hybrid precoder and combiner which attempts to match such all- digital approximation. Simulation results show the effectiveness of the proposed approach, which performs close to the all-digital solution.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Lopez-Valcarce, R. and Gonzalez-Prelcic, N. and Rusu, C. and Heath, R.W.}, year={2016} }
 @article{heath_gonzalez-prelcic_rangan_roh_sayeed_2016, title={Introduction to the Special Issue on Signal Processing for Millimeter Wave Wireless Communications}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964287519&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2016.2535519}, abstractNote={The eleven papers in this special section focus signal processing for millimeter wave (mmWave) wireless communications. These mmWave frequencies are defining a new era of wireless communication. The mmWave band relieves spectral gridlock at lower frequencies by offering much higher bandwidth communication channels than presently used in commercial wireless systems. The next generation of wireless local area networks is exploiting the mmWave unlicensed band at 60 GHz to provide multigigabit-per-second data rates. There is also growing interest in using mmWave licensed spectrum for 5G cellular systems. MmWave communication could also provide important benefits in other application scenarios like wearable networks, vehicular communications, or autonomous robots. The potential for mmWave is immense. Signal processing is critical for enabling the next generation of mmWave communication. Because of the wide bandwidth, overall complexity and mixed signal power consumption are significant concerns. The papers in this section bring together contributions from researchers and practitioners in the area of signal processing for wireless communications with an emphasis on communication at millimeter wave frequencies. I}, number={3}, journal={IEEE Journal on Selected Topics in Signal Processing}, author={Heath, R.W. and Gonzalez-Prelcic, N. and Rangan, S. and Roh, W. and Sayeed, A.}, year={2016}, pages={433–435} }
 @article{heath_honig_nagata_parkvall_soong_2016, title={LTE-Advanced Pro: Part 3 [Guest Editorial]}, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84979300176&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2016.7509378}, abstractNote={The mobile Internet that will be enabled by LTE-Advanced Pro and the way in which we interact with it will change significantly within the next few years. From a usage point of view, it is anticipated that by 2020, each person, globally speaking, will consume on average as much as 5 GB of data each month, in addition to the traffic generated by 20-30 billion connected things. More significantly, not only will the mobile network be significantly faster, but even more applications will become possible. The video experience will be improved globally; mobile virtual reality will be available; networked vehicles and perhaps self-driving cars will roam our streets; the Internet of Things will enrich and make our lives more productive; and a mobile cloud will allow access to our data anytime, anywhere.}, number={7}, journal={IEEE Communications Magazine}, author={Heath, R.W. and Honig, M. and Nagata, S. and Parkvall, S. and Soong, A.C.K.}, year={2016}, pages={52–53} }
 @article{heath_honig_nagata_parkvall_soong_2016, title={LTE-advanced pro: Part 1}, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975248765&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2016.7470938}, abstractNote={Long Term Evolution (LTE) has become the most successful mobile wireless broadband technology, serving over one billion users as of the beginning of 2016. However, looking at the penetration rate, LTE serves only 14.5 percent of the current 7.3 billion mobile subscriptions. Consequently, there is still significant room for LTE to grow as a mobile technology; it will be serving users for a long time to come.}, number={5}, journal={IEEE Communications Magazine}, author={Heath, R.W. and Honig, M. and Nagata, S. and Parkvall, S. and Soong, A.C.K.}, year={2016}, pages={74–75} }
 @article{heath_honig_nagata_parkvall_soong_2016, title={LTE-advanced pro: Part 2 [Guest Editorial]}, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976568118&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2016.7497760}, abstractNote={Our society is undergoing an unprecedented transformation as more and more devices are being interconnected over the wide area network. This will profoundly change our productivity and human interaction. The Internet of Things (IoT) has been one of the most successful growth segments in cellular-based applications in recent years, with an annual growth rate in the range of 30 percent. It is anticipated that the ratio of connected things to people will rise sharply over the next 5-10 years, to around 7:1 or even higher, which means that there would be 50 billion or even more connected things. The GSM Association believes the number could grow to 24 billion by 2020 [1], while Gartner forecasts that number to be 35 billion [2]. Applications include smart building, smart metering, smart city, ehealth, smart environment, consumer electronics, and telematics/vehicle to everything (V2X). Consequently, this part of the Feature Topic will focus on technologies for Long Term Evolution (LTE) that will enhance the support of IoT.}, number={6}, journal={IEEE Communications Magazine}, author={Heath, R.W. and Honig, M. and Nagata, S. and Parkvall, S. and Soong, A.C.K.}, year={2016}, pages={12–13} }
 @inproceedings{mo_heath_2017, title={Limited feedback in MISO systems with finite-bit ADCs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016296200&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2016.7869085}, abstractNote={We analyze limited feedback in systems where a multiple-antenna transmitter sends signals to single-antenna receivers with finite-bit ADCs. If channel state information (CSI) is not available with high resolution at the transmitter and the precoding is not well designed, the inter-user interference is a big decoding challenge for receivers with low-resolution quantization. In this paper, we derive achievable rates with finite-bit ADCs and finite-bit CSI feedback. The performance loss compared to the case with perfect CSI is then analyzed. The results show that the number of bits per feedback should increase linearly with the ADC resolution to restrict the loss.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Mo, J. and Heath, R.W.}, year={2017}, pages={479–483} }
 @inproceedings{venugopal_heath_2016, title={Location based performance model for indoor mmWave wearable communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84981313842&partnerID=MN8TOARS}, DOI={10.1109/ICC.2016.7511279}, abstractNote={Simultaneous use of high-end wearable wireless devices like smart glasses is challenging in a dense indoor environment due to the high nature of interference. In this scenario, the millimeter wave (mmWave) band offers promising potential for achieving gigabits per second throughput. Here we propose a novel system model for analyzing system performance of mmWave based communication among wearables. The proposed model accounts for the non-isotropy of the indoor environment and the effects of reflections that are predominant for indoor mmWave signals. The effect of human body blockages are modeled and the system performance is shown to hugely vary depending on the user location, body orientation and the density of the network. Closed form expressions for spatially averaged signal to interference plus noise ratio distribution are also derived as a function of the location and orientation of a reference user.}, booktitle={2016 IEEE International Conference on Communications, ICC 2016}, author={Venugopal, K. and Heath, R.W.}, year={2016} }
 @article{park_heath_2016, title={Low Complexity Antenna Selection for Low Target Rate Users in Dense Cloud Radio Access Networks}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988028212&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2577580}, abstractNote={We propose a low complexity antenna selection algorithm for low target rate users in cloud radio access networks. The algorithm consists of two phases. In the first phase, each remote radio head (RRH) determines whether to be included in a candidate set by using a predefined selection threshold. In the second phase, RRHs are randomly selected within the candidate set made in the first phase. To analyze the performance of the proposed algorithm, we model RRHs' and users' locations by a homogeneous Poisson point process, whereby the signal-to-interference ratio (SIR) complementary cumulative distribution function is derived. By approximating the derived expression, an approximate optimum selection threshold that maximizes the SIR coverage probability is obtained. Using the obtained threshold, we characterize the performance of the algorithm in an asymptotic regime, where the RRH density goes to infinity. The obtained threshold is then modified depending on various algorithm options. A distinguishable feature of the proposed algorithm is that the algorithm complexity keeps constant independent to the RRH density, so that a user is able to connect to a network without heavy computation at baseband units.}, number={9}, journal={IEEE Transactions on Wireless Communications}, author={Park, J. and Heath, R.W.}, year={2016}, pages={6022–6032} }
 @article{rusu_mendez-rial_gonzalez-prelcic_heath_2016, title={Low Complexity Hybrid Precoding Strategies for Millimeter Wave Communication Systems}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85006789642&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2614495}, abstractNote={Millimeter communication systems use large antenna arrays to provide good average received power and to take advantage of multi-stream MIMO communication. Unfortunately, due to power consumption in the analog front-end, it is impractical to perform beamforming and fully digital precoding at baseband. Hybrid precoding/combining architectures have been proposed to overcome this limitation. The hybrid structure splits the MIMO processing between the digital and analog domains, while keeping the performance close to that of the fully digital solution. In this paper, we introduce and analyze several algorithms that efficiently design hybrid precoders and combiners starting from the known optimum digital precoder/combiner, which can be computed when perfect channel state information is available. We propose several low complexity solutions which provide different trade-offs between performance and complexity. We show that the proposed iterative solutions perform better in terms of spectral efficiency and/or are faster than previous methods in the literature. All of them provide designs which perform close to the known optimal digital solution. Finally, we study the effects of quantizing the analog component of the hybrid design and show that even with coarse quantization, the average rate performance is good.}, number={12}, journal={IEEE Transactions on Wireless Communications}, author={Rusu, C. and Mendez-Rial, R. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2016}, pages={8380–8393} }
 @article{alkhateeb_nam_zhang_heath_2016, title={Massive MIMO Combining with Switches}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976547107&partnerID=MN8TOARS}, DOI={10.1109/LWC.2016.2522963}, abstractNote={Massive multiple-input multiple-output (MIMO) is expected to play a central role in future wireless systems. The deployment of large antenna arrays at the base station and the mobile users offers multiplexing and beamforming gains that boost system spectral efficiency. Unfortunately, the high cost and power consumption of components like analog-to-digital converters makes assigning an RF chain per antenna and applying typical fully digital precoding/combining solutions difficult. In this letter, a novel architecture for massive MIMO receivers, consisting of arrays of switches and constant (non-tunable) phase shifters, is proposed. This architecture applies a quasi-coherent combining in the RF domain to reduce the number of required RF chains. An algorithm that designs the RF combining for this architecture is developed and analyzed. Results show that the proposed massive MIMO combining model can achieve a comparable performance to the fully digital receiver architecture in single-user and multiuser massive MIMO setups.}, number={3}, journal={IEEE Wireless Communications Letters}, author={Alkhateeb, A. and Nam, Y.-H. and Zhang, J.C. and Heath, R.W.}, year={2016}, pages={232–235} }
 @article{khan_alkhateeb_heath_2016, title={Millimeter Wave Energy Harvesting}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84987942342&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2577582}, abstractNote={The millimeter wave (mmWave) band, a prime candidate for 5G cellular networks, seems attractive for wireless energy harvesting since it will feature large antenna arrays and extremely dense base station (BS) deployments. The viability of mmWave for energy harvesting though is unclear, due to the differences in propagation characteristics, such as extreme sensitivity to building blockages. This paper considers a scenario where low-power devices extract energy and/or information from the mmWave signals. Using stochastic geometry, analytical expressions are derived for the energy coverage probability, the average harvested power, and the overall (energy-and-information) coverage probability at a typical wireless-powered device in terms of the BS density, the antenna geometry parameters, and the channel parameters. Numerical results reveal several network and device level design insights. At the BSs, optimizing the antenna geometry parameters, such as beamwidth, can maximize the network-wide energy coverage for a given user population. At the device level, the performance can be substantially improved by optimally splitting the received signal for energy and information extraction, and by deploying multi-antenna arrays. For the latter, an efficient low-power multi-antenna mmWave receiver architecture is proposed for simultaneous energy and information transfer. Overall, simulation results suggest that mmWave energy harvesting generally outperforms lower frequency solutions.}, number={9}, journal={IEEE Transactions on Wireless Communications}, author={Khan, T.A. and Alkhateeb, A. and Heath, R.W.}, year={2016}, pages={6048–6062} }
 @article{venugopal_heath_2016, title={Millimeter Wave Networked Wearables in Dense Indoor Environments}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84979844539&partnerID=MN8TOARS}, DOI={10.1109/ACCESS.2016.2542478}, abstractNote={Supporting high data rate wireless connectivity among wearable devices in a dense indoor environment is challenging. This is primarily due to bandwidth scarcity when many users operate multiple devices simultaneously. The millimeter-wave (mmWave) band has the potential to address this bottleneck, thanks to more spectrum and less interference because of signal blockage at these frequencies. In this paper, we explain the potential and challenges associated with using mmWave for wearable networks. To provide a means for concrete analysis, we present a system model that admits easy analysis of dense, indoor mmWave wearable networks. We evaluate the performance of the system while considering the unique propagation features at mmWave frequencies, such as human body blockages and reflections from walls. One conclusion is that the non-isotropy of the surroundings relative to a reference user causes variations in system performance depending on the user location, body orientation, and density of the network. The impact of using antenna arrays is quantified through analytic closed-form expressions that incorporate antenna gain and directivity. It is shown that using directional antennas, positioning the transceiver devices appropriately, and orienting the human user body in certain directions depending on the user location result in gigabits-per-second achievable ergodic rates for mmWave wearable networks.}, journal={IEEE Access}, author={Venugopal, K. and Heath, R.W.}, year={2016}, pages={1205–1221} }
 @inproceedings{heath_2016, title={Millimeter Wave: The Future of Commercial Wireless Systems}, volume={2016-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85006054025&partnerID=MN8TOARS}, DOI={10.1109/CSICS.2016.7751056}, abstractNote={Millimeter wave (mmWave) is coming to fifth generation (5G) cellular, wireless local area networks (WLANs), and wireless personal area networks (WPANs). The benefits of using millimeter wave carriers arise from the potential for larger bandwidths (hundreds of megahertz to gigahertz) compared to lower carrier frequencies. Differences in the propagation, challenges introduced by hardware constraints, and computational issues related to the high data rates have reinvigorated research in wireless communications. This paper, which accompanies a keynote talk with the same name, provides a roadmap to the author's research results in the evolving area of mmWave for consumers.}, booktitle={Technical Digest - IEEE Compound Semiconductor Integrated Circuit Symposium, CSIC}, author={Heath, R.W.}, year={2016} }
 @article{va_shimizu_bansal_heath_2016, title={Millimeter wave vehicular communications: A survey}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976271761&partnerID=MN8TOARS}, DOI={10.1561/1300000054}, abstractNote={Future vehicles will require massive sensing capability. Leveraging only onboard sensors, though, is challenging in crowded environments where the sensing field-of-view is obstructed. One potential solution is to share sensor data among the vehicles and infrastructure. This has the benefits of providing vehicles with an enhanced field-of-view and also additional redundancy to provide more reliability in the sensor data. A main challenge in sharing sensor data is providing the high data rates required to exchange raw sensor data. The large spectral channels at millimeter wave mmWave frequencies provide a means of achieving much higher data rates. This monograph provides an overview of mmWave vehicular communication with an emphasis on results on channel measurements, the physical PHY layer, and the medium access control MAC layer. The main objective is to summarize key findings in each area, with special attention paid to identifying important topics of future research. In addition to surveying existing work, some new simulation results are also presented to give insights on the effect of directionality and blockage, which are the two distinguishing features of mmWave vehicular channels. A main conclusion of this monograph is that given the renewed interest in high rate vehicle connectivity, many challenges remain in the design of a mmWave vehicular network.}, number={1}, journal={Foundations and Trends in Networking}, author={Va, V. and Shimizu, T. and Bansal, G. and Heath, R.W.}, year={2016}, pages={1–116} }
 @article{millimeter-wave vehicular communication to support massive automotive sensing_2016, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85012964565&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2016.1600071CM}, abstractNote={As driving becomes more automated, vehicles are being equipped with more sensors generating even higher data rates. Radars are used for object detection, visual cameras as virtual mirrors, and LIDARs for generating high resolution depth associated range maps, all to enhance the safety and efficiency of driving. Connected vehicles can use wireless communication to exchange sensor data, allowing them to enlarge their sensing range and improve automated driving functions. Unfortunately, conventional technologies, such as DSRC and 4G cellular communication, do not support the gigabit-per-second data rates that would be required for raw sensor data exchange between vehicles. This article makes the case that mmWave communication is the only viable approach for high bandwidth connected vehicles. The motivations and challenges associated with using mmWave for vehicle-to-vehicle and vehicle-to-infrastructure applications are highlighted. A high-level solution to one key challenge - the overhead of mmWave beam training - is proposed. The critical feature of this solution is to leverage information derived from the sensors or DSRC as side information for the mmWave communication link configuration. Examples and simulation results show that the beam alignment overhead can be reduced by using position information obtained from DSRC.}, number={12}, journal={IEEE Communications Magazine}, year={2016}, pages={160–167} }
 @article{pi_choi_heath_2016, title={Millimeter-wave gigabit broadband evolution toward 5G: Fixed access and backhaul}, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964267462&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2016.7452278}, abstractNote={As wireless communication evolves toward 5G, both fixed broadband and mobile broadband will play a crucial part in providing the gigabit-per-second infrastructure for a connected society. This article proposes an MGB system as the solution to two critical problems in this evolution: last-mile access for fixed broadband and small cell backhaul for mobile broadband. The key idea is to use spectrum that is already available in the millimeter-wave bands for fixed wireless access with optimized dynamic beamforming and massive MIMO infrastructure to achieve high capacity with wide area coverage. This article explains the MGB concept and describes potential array architectures for realizing the system. Simulations demonstrate that with 500 MHz of bandwidth (at 39 GHz band) and 28 dBm transmission power (55 dBm EIRP), it is possible to provide more than 11 Gb/s backhaul capacity for 96 small cells within a 1 km radius.}, number={4}, journal={IEEE Communications Magazine}, author={Pi, Z. and Choi, J. and Heath, R.}, year={2016}, pages={138–144} }
 @article{andrews_bai_kulkarni_alkhateeb_gupta_heath_2017, title={Modeling and Analyzing Millimeter Wave Cellular Systems}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85009959384&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2618794}, abstractNote={We provide a comprehensive overview of mathematical models and analytical techniques for millimeter wave (mmWave) cellular systems. The two fundamental physical differences from conventional sub-6-GHz cellular systems are: 1) vulnerability to blocking and 2) the need for significant directionality at the transmitter and/or receiver, which is achieved through the use of large antenna arrays of small individual elements. We overview and compare models for both of these factors, and present a baseline analytical approach based on stochastic geometry that allows the computation of the statistical distributions of the downlink signal-to-interference-plus-noise ratio (SINR) and also the per link data rate, which depends on the SINR as well as the average load. There are many implications of the models and analysis: 1) mmWave systems are significantly more noise-limited than at sub-6 GHz for most parameter configurations; 2) initial access is much more difficult in mmWave; 3) self-backhauling is more viable than in sub-6-GHz systems, which makes ultra-dense deployments more viable, but this leads to increasingly interference-limited behavior; and 4) in sharp contrast to sub-6-GHz systems cellular operators can mutually benefit by sharing their spectrum licenses despite the uncontrolled interference that results from doing so. We conclude by outlining several important extensions of the baseline model, many of which are promising avenues for future research.}, number={1}, journal={IEEE Transactions on Communications}, author={Andrews, J.G. and Bai, T. and Kulkarni, M.N. and Alkhateeb, A. and Gupta, A.K. and Heath, R.W.}, year={2017}, pages={403–430} }
 @article{park_heath_2016, title={Multiple-Antenna Transmission with Limited Feedback in Device-to-Device Networks}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963830357&partnerID=MN8TOARS}, DOI={10.1109/LWC.2016.2517652}, abstractNote={We consider an overlay device-to-device network, where each multiple-antenna transmitter sends data to a designated single-antenna receiver by using maximum ratio transmission based on limited feedback, and their locations are modeled by a homogeneous Poisson point process. We obtain a lower bound on the optimum number of feedback bits that maximizes net spectral efficiency, capturing the overall system gain of using limited feedback. Our main finding is that the optimum number of feedback bits scales linearly with the number of transmit antennas and logarithmically with the channel coherence time and the inverse of the node density.}, number={2}, journal={IEEE Wireless Communications Letters}, author={Park, J. and Heath, R.W.}, year={2016}, pages={200–203} }
 @article{choi_mo_heath_2016, title={Near Maximum-Likelihood Detector and Channel Estimator for Uplink Multiuser Massive MIMO Systems with One-Bit ADCs}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84970021748&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2545666}, abstractNote={In massive multiple-input multiple-output (MIMO) systems, it may not be power efficient to have a pair of high-resolution analog-to-digital converters (ADCs) for each antenna element. In this paper, a near maximum likelihood (nML) detector for uplink multiuser massive MIMO systems is proposed where each antenna is connected to a pair of one-bit ADCs, i.e., one for each real and imaginary component of the baseband signal. The exhaustive search over all the possible transmitted vectors required in the original maximum likelihood (ML) detection problem is relaxed to formulate an ML estimation problem. Then, the ML estimation problem is converted into a convex optimization problem which can be efficiently solved. Using the solution, the base station can perform simple symbol-by-symbol detection for the transmitted signals from multiple users. To further improve detection performance, we also develop a two-stage nML detector that exploits the structures of both the original ML and the proposed (one-stage) nML detectors. Numerical results show that the proposed nML detectors are efficient enough to simultaneously support multiple uplink users adopting higher-order constellations, e.g., 16 quadrature amplitude modulation. Since our detectors exploit the channel state information as part of the detection, an ML channel estimation technique with one-bit ADCs that shares the same structure with our proposed nML detector is also developed. The proposed detectors and channel estimator provide a complete low power solution for the uplink of a massive MIMO system.}, number={5}, journal={IEEE Transactions on Communications}, author={Choi, J. and Mo, J. and Heath, R.W.}, year={2016}, pages={2005–2018} }
 @article{gupta_andrews_heath_2016, title={On the Feasibility of Sharing Spectrum Licenses in mmWave Cellular Systems}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988556595&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2590467}, abstractNote={The highly directional and adaptive antennas used in mmWave communication open up the possibility of uncoordinated sharing of spectrum licenses between commercial cellular operators. There are several advantages to sharing including a reduction in license costs and an increase in spectrum utilization. In this paper, we establish the theoretical feasibility of spectrum license sharing among mmWave cellular operators. We consider a heterogeneous multi-operator system containing multiple independent cellular networks, each owned by an operator. We then compute the signal-to-interference-and-noise ratio and rate distribution for downlink mobile users of each network. Using the analysis, we compare the systems with fully shared licenses and exclusive licenses for different access rules and explore the trade-offs between system performance and spectrum cost. We show that sharing spectrum licenses increases the per-user rate when antennas have narrow beams and is also favored when there is a low density of users. We also consider a multi-operator system where BSs of all the networks are co-located to show that the simultaneous sharing of spectrum and infrastructure is also feasible. We show that all networks can share licenses with less bandwidth and still achieve the same per-user median rate as if they each had an exclusive license to spectrum with more bandwidth.}, number={9}, journal={IEEE Transactions on Communications}, author={Gupta, A.K. and Andrews, J.G. and Heath, R.W.}, year={2016}, pages={3981–3995} }
 @article{park_lee_andrews_heath_2016, title={On the optimal feedback rate in interference-limited multi-antenna cellular systems}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84982235347&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2569089}, abstractNote={We consider a downlink cellular network where multi-antenna base stations (BSs) transmit data to single-antenna users by using one of two linear precoding methods with limited feedback: 1) maximum ratio transmission (MRT) for serving a single user or 2) zero forcing (ZF) for serving multiple users. The BS and user locations are drawn from a Poisson point process, allowing expressions for the signal-to-interference coverage probability and the ergodic spectral efficiency to be derived as a function of system parameters, such as the number of BS antennas and feedback bits, and the pathloss exponent. We find a tight lower bound on the optimum number of feedback bits to maximize the net spectral efficiency, which captures the overall system gain by considering both of downlink and uplink spectral efficiency using limited feedback. Our main finding is that, when using MRT, the optimum number of feedback bits scales linearly with the number of antennas, and logarithmically with the channel coherence time. When using ZF, the feedback scales in the same ways as MRT, but also linearly with the pathloss exponent. The derived results provide system-level insights into the preferred channel codebook size by averaging the effects of short-term fading and long-term pathloss.}, number={8}, journal={IEEE Transactions on Wireless Communications}, author={Park, J. and Lee, N. and Andrews, J.G. and Heath, R.W.}, year={2016}, pages={5748–5762} }
 @inproceedings{eltayeb_choi_al-naffouri_heath_2016, title={On the security of millimeter wave vehicular communication systems using random antenna subsets}, volume={0}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016965929&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2016.7881128}, abstractNote={Millimeter wave (mmWave) vehicular communication systems have the potential to improve traffic efficiency and safety. Lack of secure communication links, however, may lead to a formidable set of abuses and attacks. To secure communication links, a physical layer precoding technique for mmWave vehicular communication systems is proposed in this paper. The proposed technique exploits the large dimensional antenna arrays available at mmWave systems to produce direction dependent transmission. This results in coherent transmission to the legitimate receiver and artificial noise that jams eavesdroppers with sensitive receivers. Theoretical and numerical results demonstrate the validity and effectiveness of the proposed technique and show that the proposed technique provides high secrecy throughput when compared to conventional array and switched array transmission techniques.}, booktitle={IEEE Vehicular Technology Conference}, author={Eltayeb, M.E. and Choi, J. and Al-Naffouri, T.Y. and Heath, R.W.}, year={2016} }
 @inproceedings{khan_heath_popovski_2016, title={On wirelessly powered communications with short packets}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015921400&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2016.7848984}, abstractNote={Wireless-powered communications will entail short packets due to naturally small payloads, low-latency requirements and/or insufficient energy resources to support longer transmissions. In this paper, a wireless-powered communication system is investigated where an energy harvesting transmitter, charged by a power beacon via wireless energy transfer, attempts to communicate with a receiver over a noisy channel. Leveraging the framework of finite-length information theory, the system performance is analyzed using metrics such as the energy supply probability at the transmitter, and the achievable rate at the receiver. The analysis yields useful insights into the system behavior in terms of key parameters such as the harvest blocklength, the transmit blocklength, the average harvested power and the transmit power. Closed-form expressions are derived for the asymptotically optimal transmit power. Numerical results suggest that power control is essential for improving the achievable rate of the system in the finite blocklength regime.}, booktitle={2016 IEEE Globecom Workshops, GC Wkshps 2016 - Proceedings}, author={Khan, T.A. and Heath, R.W. and Popovski, P.}, year={2016} }
 @inproceedings{mollen_choi_larsson_heath_2016, title={One-bit ADCs in wideband massive MIMO systems with OFDM transmission}, volume={2016-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973338746&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2016.7472305}, abstractNote={We investigate the performance of wideband massive MIMO base stations that use one-bit ADCs for quantizing the uplink signal. Our main result is to show that the many taps of the frequency-selective channel make linear combiners asymptotically consistent and the quantization noise additive and Gaussian, which simplifies signal processing and enables the straightforward use of OFDM. We also find that single-carrier systems and OFDM systems are affected in the same way by one-bit quantizers in wideband systems because the distribution of the quantization noise becomes the same in both systems as the number of channel taps grows.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Mollen, C. and Choi, J. and Larsson, E.G. and Heath, R.W.}, year={2016}, pages={3386–3390} }
 @article{park_park_yazdan_heath_2017, title={Optimal User Loading in Massive MIMO Systems with Regularized Zero Forcing Precoding}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85013439655&partnerID=MN8TOARS}, DOI={10.1109/LWC.2016.2638914}, abstractNote={We consider a downlink massive multiple-input multiple output system employing regularized zero-forcing precoding. We derive the asymptotic signal-to-leakage-plus-noise ratio (SLNR) as both the number of antennas and the number of users go to infinity at a fixed ratio. Focusing on spatially uncorrelated channels with homogeneous large scale fading gains, we show that the SLNR is asymptotically equal to signal-to-interference-plus-noise ratio, which allows us to optimize the user loading for spectral efficiency. The results show that the optimal user loading varies depending on the channel signal-to-noise ratio (SNR). As the SNR increases, the optimal user loading decreases at low SNR, but increases at high SNR.}, number={1}, journal={IEEE Wireless Communications Letters}, author={Park, S. and Park, J. and Yazdan, A. and Heath, R.W.}, year={2017}, pages={118–121} }
 @article{thornburg_bai_heath_2016, title={Performance Analysis of Outdoor mmWave Ad Hoc Networks}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84979085527&partnerID=MN8TOARS}, DOI={10.1109/TSP.2016.2551690}, abstractNote={Ad hoc networks provide an on-demand, infrastructure-free means to communicate between soldiers in war zones, aid workers in disaster areas, or consumers in device-to-device (D2D) applications. Unfortunately, ad hoc networks are limited by interference due to nearby transmissions. Millimeter-wave (mmWave) devices offer several potential advantages for ad hoc networks, including reduced interference due to directional antennas and building blockages, not to mention huge bandwidth channels for large data rates. This paper uses a stochastic geometry approach to characterize the one-way and two-way signal-to-interference ratio distribution of a mmWave ad hoc network with directional antennas, random blockages, and ALOHA channel access. The interference-to-noise ratio shows that a fundamental limitation of an ad hoc network, interference, may still be an issue. The performance of mmWave ad hoc networks is bounded by the transmission capacity and area spectral efficiency. The results show that mmWave networks can support much higher densities and larger spectral efficiencies, even in the presence of blockage, compared with lower frequency communication for certain link distances. Due to the increased bandwidth, the rate coverage of mmWave can be much greater than lower frequency devices.}, number={15}, journal={IEEE Transactions on Signal Processing}, author={Thornburg, A. and Bai, T. and Heath, R.W.}, year={2016}, pages={4065–4079} }
 @inproceedings{va_heath_2016, title={Performance analysis of beam sweeping in millimeter wave assuming noise and imperfect antenna patterns}, volume={0}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016951796&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2016.7881150}, abstractNote={Beam alignment is an important component of millimeter wave communication systems, which use analog beam steering.We analyze the performance of beam alignment methods based on the hierarchical beam sweeping that are adopted in IEEE 802.11ad and IEEE 802.15.3c. Previous work studies misalignment probability due to thermal noise or antenna gain fluctuation alone. At the early stage of the alignment process, however, both effects are important. We incorporate both effects and derive probability of misalignment not only with respect to the best path but also to secondary ones. Our analysis can capture the misalignment severity in terms of power loss with respect to the perfect alignment case. Numerical examples show that line-of-sight channels are more constrained by the thermal noise, while the non- line-of-sight channels are more constrained by the antenna gain fluctuations.}, booktitle={IEEE Vehicular Technology Conference}, author={Va, V. and Heath, R.W.}, year={2016} }
 @inproceedings{zhu_wang_wong_heath_2016, title={Physical layer security in large-scale millimeter wave ad hoc networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015434297&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7842143}, abstractNote={Wireless networks with directional antennas, like millimeter wave (mmWave) networks, have enhanced security. For a large scale mmWave ad hoc network in which eavesdroppers are randomly located, however, eavesdroppers can still intercept the confidential messages, since they may reside in the signal beam. This paper explores the potential of physical layer security in the mmWave ad hoc networks. Specifically, we characterize the impact of mmWave channel characteristics and large antenna arrays on the secrecy performance. We also characterize the impact of artificial noise in this networks. Our results reveal that in the low transmit power regime, the use of low mmWave frequency achieves better secrecy performance, when increasing transmit power, a transition from low mmWave frequency to high mmWave frequency is demanded for obtaining more secrecy rate. Eavesdroppers can intercept more information by using wide beam pattern. Furthermore, the use of artificial noise may be unable to enhance the secrecy rate for the case of low node density.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Zhu, Y. and Wang, L. and Wong, K.-K. and Heath, R.W.}, year={2016} }
 @article{properties of real and complex etfs and their application to the design of low coherence frames_2016, volume={508}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84978219426&partnerID=MN8TOARS}, DOI={10.1016/j.laa.2016.06.038}, abstractNote={Certain properties of an equiangular tight frame are contained in the sign (or phase) of its corresponding Gram matrix. In this paper we convey new properties for Gram matrices of equiangular tight frames and use them to construct new frames with low average and worst case coherences. Based on these results, we also provide properties of the synthesis matrix of equiangular tight frames.}, journal={Linear Algebra and Its Applications}, year={2016}, pages={81–90} }
 @inproceedings{gonzález-prelcic_méndez-rial_heath_2017, title={Radar aided beam alignment in MmWave V2I communications supporting antenna diversity}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85005942795&partnerID=MN8TOARS}, DOI={10.1109/ITA.2016.7888145}, abstractNote={Millimeter wave (mmWave) communication is the only viable approach for high bandwidth connected vehicles exchanging raw sensor data. A main challenge for mmWave in connected vehicles, is that it requires frequent link reconfiguration in mobile environments, which is a source of high overhead. In this paper we introduce the concept of radar aided mmWave vehicular communication. Side information derived from radar mounted on the infrastructure operating in a given mmWave band is used to adapt the beams of the vehicular communication system operating in another millimeter wave band. We propose a set of algorithms to perform the beam alignment task in a vehicle-to-infrastructure (V2I) scenario, from extracting information from the radar signal to configuring the beams that illuminate the different antennas in the vehicle. Simulation results confirm that radar can be a useful source of side information that helps configure the mmWave V2I link.}, booktitle={2016 Information Theory and Applications Workshop, ITA 2016}, author={González-Prelcic, N. and Méndez-Rial, R. and Heath, R.W.}, year={2017} }
 @inproceedings{gupta_kulkarni_visotsky_vook_ghosh_andrews_heath_2016, title={Rate analysis and feasibility of dynamic TDD in 5G cellular systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84981336710&partnerID=MN8TOARS}, DOI={10.1109/ICC.2016.7511412}, abstractNote={In conventional applications of time division duplex (TDD) in cellular systems, the time resource split between uplink (UL) and downlink (DL) is fixed across all base stations (BSs) in the network. This leads to under utilization of BS resources when there is a mismatch between the expected and experienced UL/DL traffic in a given cell. A dynamic split that varies in each cell is desirable, but is challenging due to the high interference experienced by UL receivers in one cell from DL transmissions in adjacent cells. This paper analyzes the performance of UL users in dynamic TDD enabled next generation cellular networks using a stochastic geometry framework. The analysis highlights the trade-off between spectral efficiency and resource utilization for dynamic TDD. With appropriate interference mitigation, dynamic TDD offers a significant gain in data rates as compared to static TDD, which is higher when the BSs are lightly loaded and/or the fraction of UL users is low.}, booktitle={2016 IEEE International Conference on Communications, ICC 2016}, author={Gupta, A.K. and Kulkarni, M.N. and Visotsky, E. and Vook, F.W. and Ghosh, A. and Andrews, J.G. and Heath, R.W.}, year={2016} }
 @inproceedings{gupta_alkhateeb_andrews_heath_2016, title={Restricted secondary licensing for mmWave cellular: How much gain can be obtained?}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84998627254&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2016.7842227}, abstractNote={Sharing the spectrum among multiple operators seems promising in millimeter wave (mmWave) systems. One explanation is the highly directional transmission in mmWave, which reduces the interference caused by one network on the other networks sharing the same resources. In this paper, we model a mmWave cellular system where an operator that primarily owns an exclusive-use license of a certain band can sell a restricted secondary license of the same band to another operator. This secondary network has a restriction on the maximum interference it can cause to the original network. Using stochastic geometry, we derive expressions for the coverage and rate of both networks, and establish the feasibility of secondary licensing in licensed mmWave bands. Results show that the restricted secondary operator can achieve good coverage with a small impact on the original operator. Our results also illustrate that the spectrum sharing gains increase with narrow beams and when the network densifies.}, booktitle={2016 IEEE Global Communications Conference, GLOBECOM 2016 - Proceedings}, author={Gupta, A.K. and Alkhateeb, A. and Andrews, J.G. and Heath, R.W.}, year={2016} }
 @article{xia_heath_gonzalez-prelcic_2016, title={Robust Analog Precoding Designs for Millimeter Wave MIMO Transceivers with Frequency and Time Division Duplexing}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85027405552&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2016.2604312}, abstractNote={Millimeter wave (mmWave) communication provides high data rates thanks to large arrays at the transmitter and receiver, coupled with large bandwidth channels. Exploiting the arrays is challenging due to the need to configure precoding at the transmitter based on the large frequency selective channel. In this paper, we exploit the power iteration principle and propose a robust analog precoding training algorithm that can be applied in both frequency division duplex transmission systems and time division duplex transmission systems with or without RF calibration. We further analyze the convergence of the proposed algorithm and show how it converges to the singular value decomposition optimality exponentially. We propose null space projection on top of the power iteration to form multiple orthogonal beams at the transmitter and receiver. Strongest tap selection with proper energy pruning is used to collect as much precoding gain as possible from a frequency selective fading channel. The exponential effective SINR mapping performance is evaluated and demonstrates that the overall approach works smoothly. Numerical simulation results demonstrate algorithm robustness and the algorithm works not only for the simplified mmWave directional channels, but also for more general rich scattering channels.}, number={11}, journal={IEEE Transactions on Communications}, author={Xia, P. and Heath, R.W. and Gonzalez-Prelcic, N.}, year={2016}, pages={4622–4634} }
 @inproceedings{park_heath_2017, title={Spatial channel covariance estimation for mmWave hybrid MIMO architecture}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85016260743&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2016.7869611}, abstractNote={Spatial channel covariance information can replace full channel state information for designing analog precoders in millimeter wave (mmWave) hybrid MIMO systems. Hybrid MIMO architectures, however, make it challenging to estimate the spatial channel covariance matrix because the estimator in baseband can only see the low-dimensional projections of the original channel. In this paper, we propose two key ideas for developing the covariance estimation techniques based on compressive sensing techniques. One is to use the Hermitian property of the covariance matrix, and the other is to use a time-varying analog combining matrix to effectively extend the measurement size.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Park, S. and Heath, R.W.}, year={2017}, pages={1424–1428} }
 @article{lee_baccelli_heath_2016, title={Spectral efficiency scaling laws in dense random wireless networks with multiple receive antennas}, volume={62}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84961987981&partnerID=MN8TOARS}, DOI={10.1109/TIT.2016.2519387}, abstractNote={This paper considers large random wireless networks, where transmit-and-receive node pairs communicate within a certain range while sharing a common spectrum. By modeling the spatial locations of nodes as a Poisson point process, analytical expressions for the ergodic spectral efficiency of a typical node pair are derived as a function of the channel state information available at a receiver (CSIR) in terms of relevant system parameters: the density of communication links, the number of receive antennas, the path loss exponent, and the operating signal-to-noise ratio. One key finding is that when the receiver only exploits CSIR for the direct link, the sum spectral efficiency increases linearly with the density, provided the number of receive antennas increases as a certain superlinear function of the density. When each receiver exploits CSIR for a set of dominant interfering links in addition to that of the direct link, the sum spectral efficiency increases linearly with both the density and the path loss exponent if the number of antennas is a linear function of the density. This observation demonstrates that having CSIR for dominant interfering links provides an order gain in the scaling law. It is also shown that this linear scaling holds for direct CSIR when incorporating the effect of the receive antenna correlation, provided that the rank of the spatial correlation matrix scales superlinearly with the density. These scaling laws are derived from integral representations of the distribution of the signal to interference and noise ratio, which are of independent interest and which in turn derived from stochastic geometry and more precisely from the theory of shot noise fields. Simulation results back the scaling laws and the integral representations.}, number={3}, journal={IEEE Transactions on Information Theory}, author={Lee, N. and Baccelli, F. and Heath, R.W.}, year={2016}, pages={1344–1359} }
 @article{va_choi_heath_2017, title={The Impact of Beamwidth on Temporal Channel Variation in Vehicular Channels and Its Implications}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028347986&partnerID=MN8TOARS}, DOI={10.1109/TVT.2016.2622164}, abstractNote={Millimeter wave (mmWave) has great potential in realizing high data rates, thanks to the large spectral channels. It is considered as a key technology for fifth-generation (5G) wireless networks and is already used in wireless LAN (e.g., IEEE 802.11ad). Using mmWave for vehicular communications, however, is often viewed with some skepticism due to a misconception that the Doppler spread would become too large at these high frequencies. This is not necessarily true when directional beams are employed. In this paper, closed-form expressions relating the channel coherence time and beamwidth are derived. Unlike prior work that assumed perfect beam pointing, the pointing error due to the receiver motion is incorporated to show that there exists a nonzero optimal beamwidth that maximizes the coherence time. We define a novel concept of beam coherence time, which is an effective measure of beam alignment frequency. Using the derived correlation function, the channel coherence time, and the beam coherence time, an overall performance metric considering both the channel time variation and the beam alignment overhead is derived. Using this metric, it is shown that beam realignment in every beam coherence time performs better than beam realignment in every channel coherence time.}, number={6}, journal={IEEE Transactions on Vehicular Technology}, author={Va, V. and Choi, J. and Heath, R.W.}, year={2017}, pages={5014–5029} }
 @inproceedings{rusu_gonzalez-prelcic_heath_2016, title={The use of unit norm tight measurement matrices for one-bit compressed sensing}, volume={2016-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973364868&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2016.7472437}, abstractNote={In this paper we analyze the mean squared error (MSE) for one-bit compressed sensing schemes based on measurement matrices that correspond to unit norm tight frames. We show that, as in the unquantized case, sensing with unit norm tight frames improves the MSE in the reconstruction of sparse vectors from one-bit measurements using l1 and thresholding algorithms. From our analytical and experimental results we conclude that when implementing one-bit compressed sensing schemes with fixed measurement matrices unit norm tight frames are the measurements of choice.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Rusu, C. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2016}, pages={4044–4048} }
 @article{li_jin_gao_heath_2016, title={Three-dimensional beamforming for large-scale FD-MIMO systems exploiting statistical channel state information}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85012263773&partnerID=MN8TOARS}, DOI={10.1109/TVT.2016.2519141}, abstractNote={In this paper, we provide a practical downlink transmission approach for single-cell full-dimension multiple-input-multiple-output (FD-MIMO) systems with large-scale uniform planar antenna array at the base station (BS). Under the assumption of only statistical channel state information (CSI) at the BS, we derive the optimal beamforming vector of each user by maximizing a lower bound on the average signal-to-leakage-plus-noise ratio. Some main guidelines for user scheduling are also derived under this criterion. Based on these results, a low-complexity 3-D beamforming space-division multiple-access (SDMA) transmission algorithm exploiting only the statistical CSI of each user is proposed. Furthermore, the ergodic sum rate of the proposed algorithm is analyzed, and a closed-form expression, as well as a low signal-to-noise ratio (SNR) approximation and a high-SNR approximation, are derived. Simulation results show that the proposed algorithm performs well in terms of achievable sum rate, and the analytical results on its achievable sum rate are validated.}, number={11}, journal={IEEE Transactions on Vehicular Technology}, author={Li, X. and Jin, S. and Gao, X. and Heath, R.W.}, year={2016}, pages={8992–9005} }
 @article{mollen_choi_larsson_heath_2017, title={Uplink Performance of Wideband Massive MIMO with One-Bit ADCs}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85009814512&partnerID=MN8TOARS}, DOI={10.1109/TWC.2016.2619343}, abstractNote={Analog-to-digital converters (ADCs) stand for a significant part of the total power consumption in a massive multiple-input multiple-output (MIMO) base station. One-bit ADCs are one way to reduce power consumption. This paper presents an analysis of the spectral efficiency of single-carrier and orthogonal-frequency-division-multiplexing (OFDM) transmission in massive MIMO systems that use one-bit ADCs. A closed-form achievable rate, i.e., a lower bound on capacity, is derived for a wideband system with a large number of channel taps that employ low-complexity linear channel estimation and symbol detection. Quantization results in two types of error in the symbol detection. The circularly symmetric error becomes Gaussian in massive MIMO and vanishes as the number of antennas grows. The amplitude distortion, which severely degrades the performance of OFDM, is caused by variations between symbol durations in received interference energy. As the number of channel taps grows, the amplitude distortion vanishes and OFDM has the same performance as single-carrier transmission. A main conclusion of this paper is that wideband massive MIMO systems work well with one-bit ADCs.}, number={1}, journal={IEEE Transactions on Wireless Communications}, author={Mollen, C. and Choi, J. and Larsson, E.G. and Heath, R.W.}, year={2017}, pages={87–100} }
 @article{motro_chu_choi_lavieri_pinjari_bhat_ghosh_heath_2016, title={Vehicular ad-hoc network simulations of overtaking maneuvers on two-lane rural highways}, volume={72}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988661186&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2016.09.006}, abstractNote={The objective of this paper is to evaluate the effectiveness of a dedicated short-range communication (DSRC)-based wireless vehicle-to-vehicle (V2V) communication system, called the overtaking assistant, devised for improving safety during overtaking (also referred to as passing) maneuvers on two-lane rural highways. Specifically, the paper examines the influence of vehicular kinematics (vehicle speeds, accelerations and distances), driver behavior (drivers’ perception/reaction time and overtaking rate), and DSRC characteristics (power settings, communication range, packet errors, sensor errors, and estimation inaccuracy) on the effectiveness of DSRC systems in predicting unsafe overtaking maneuvers. To this end, the paper utilizes a microscopic traffic simulator called VEhicles In Network Simulation (VEINS) that supports the simulation of wireless communication protocols in Vehicular Ad-hoc NEtworks (VANETs). 18,000 overtaking maneuvers – with roughly 10,000 collision maneuvers – were simulated to consider heterogeneity in vehicular kinematics, driver behavior, and DSRC performance. The overtaking assistant predicts whether a collision will occur and warns the driver before the maneuver begins. A descriptive analysis followed by a multivariate analysis (using binary discrete outcome models) of the simulated data reveals that the majority of collisions that could not be detected were due to the vehicles being out of communication range for the communication power settings used in the simulation. Packet errors, or failed communications, at a rate of up to 50% did not have a significant influence on the ability to detect collisions. These results suggest that the most important step in paving the way toward advanced driver assistance systems for rural highway overtaking maneuvers is to broaden the communication range of DSRC devices.}, journal={Transportation Research Part C: Emerging Technologies}, author={Motro, M. and Chu, A. and Choi, J. and Lavieri, P.S. and Pinjari, A.R. and Bhat, C.R. and Ghosh, J. and Heath, R.W.}, year={2016}, pages={60–76} }
 @article{pesyna_heath_humphreys_2015, title={Accuracy in the palm of your hand: Centimeter positioning with a smartphone-quality GNSS antenna}, volume={26}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964573502&partnerID=MN8TOARS}, number={2}, journal={GPS World}, author={Pesyna, K.M. and Heath, R.W. and Humphreys, T.E.}, year={2015}, pages={16–31} }
 @inproceedings{alkhateeb_heath_leus_2015, title={Achievable rates of multi-user millimeter wave systems with hybrid precoding}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84947718364&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2015.7247346}, abstractNote={Millimeter wave (mmWave) systems will likely employ large antenna arrays at both the transmitters and receivers. A natural application of antenna arrays is simultaneous transmission to multiple users, which requires multi-user precoding at the transmitter. Hardware constraints, however, make it difficult to apply conventional lower frequency MIMO precoding techniques at mmWave. This paper proposes and analyzes a low complexity hybrid analog/digital precoding algorithm for downlink multi-user mmWave systems. Hybrid precoding involves a combination of analog and digital processing that is motivated by the requirement to reduce the power consumption of the complete radio frequency and mixed signal hardware. The proposed algorithm configures hybrid precoders at the transmitter and analog combiners at multiple receivers with a small training and feedback overhead. For this algorithm, we derive a lower bound on the achievable rate for the case of single-path channels, show its asymptotic optimality at large numbers of antennas, and make useful insights for more general cases. Simulation results show that the proposed algorithm offers higher sum rates compared with analog-only beamforming, and approaches the performance of the unconstrained digital precoding solutions.}, booktitle={2015 IEEE International Conference on Communication Workshop, ICCW 2015}, author={Alkhateeb, A. and Heath, R.W. and Leus, G.}, year={2015}, pages={1232–1237} }
 @article{seifi_heath_coldrey_svensson_2016, title={Adaptive Multicell 3-D Beamforming in Multiantenna Cellular Networks}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84982290694&partnerID=MN8TOARS}, DOI={10.1109/TVT.2015.2478843}, abstractNote={We consider a cellular network with multiantenna base stations (BSs) and single-antenna users, multicell cooperation, imperfect channel state information (CSI), and directional antennas, each with a vertically adjustable beam. We investigate the impact of the elevation angle of the BS antenna pattern, which is denoted as tilt, on the performance of the considered network when employing either a traditional single-cell transmission or a fully cooperative multicell transmission. Using the results of this investigation, we propose a novel hybrid multicell cooperation technique in which the intercell interference is controlled via either cooperative beamforming in the horizontal plane or coordinated beamforming in the vertical plane of the wireless channel, which is denoted as adaptive multicell 3-D beamforming. The main idea is to divide the coverage area into two disjoint vertical regions and adapt the multicell cooperation strategy at the BSs when serving each region. A fair scheduler is used to share the time slots between the vertical regions. It is shown that the proposed technique can achieve performance comparable with that of a fully cooperative transmission but with significantly lower complexity and signaling requirements. To facilitate computationally efficient simulation and design space exploration, accurate approximations of the user ergodic rate are proposed for different transmission strategies under imperfect CSI.}, number={8}, journal={IEEE Transactions on Vehicular Technology}, author={Seifi, N. and Heath, R.W. and Coldrey, M. and Svensson, T.}, year={2016}, pages={6217–6231} }
 @inproceedings{adaptive hybrid precoding and combining in mmwave multiuser mimo systems based on compressed covariance estimation_2015, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963928899&partnerID=MN8TOARS}, DOI={10.1109/CAMSAP.2015.7383774}, abstractNote={In this paper we propose an adaptive multi user (MU) single-cell hybrid precoding strategy that iteratively designs the precoders/combiners exploiting the reciprocity of time division duplex (TDD) millimeter wave systems. The minimum mean square error (MMSE) criterion is considered to design the combiners, which relies on second order statistics of the channel. The covariance of the received signal at the antenna array is estimated online from compressed measurements, leveraging the sparse nature of mmWave channels. The proposed method avoids the explicit estimation of the channel matrix associated to each user, reducing the training overhead, and achieving sum spectral efficiencies comparable to the ones obtained with block diagonalization.}, booktitle={2015 IEEE 6th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2015}, year={2015}, pages={213–216} }
 @inproceedings{an attack on antenna subset modulation for millimeter wave communication_2015, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946070681&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2015.7178504}, abstractNote={Antenna subset modulation (ASM) is a physical layer security technique that is well suited for millimeter wave communication systems. The key idea is to vary the radiation pattern at the symbol rate by selecting one from a subset of patterns with a similar main lobe and different side lobes. This paper shows that ASM is not robust to an eavesdropper that makes multiple simultaneous measurements at multiple angles. The measurements are combined and used to formulate an estimation problem to undo the effects of the side lobe randomization. Simulations show the performance of the estimation algorithms and how the eavesdropper can effectively recover the information if the signal-to-noise ratio exceeds a certain threshold. Using fewer active radio frequency chains makes it harder for the attacker to recover the transmit symbol, at the expense of more grating lobes.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2015}, pages={2914–2918} }
 @inproceedings{venugopal_valenti_heath_2016, title={Analysis of millimeter wave networked wearables in crowded environments}, volume={2016-February}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969900761&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2015.7421261}, abstractNote={The millimeter wave (mmWave) band has the potential to provide high throughput among wearable devices. When mmWave wearable networks are used in crowded environments, such as on a bus or train, antenna directivity and orientation hold the key to achieving Gbps rates. Previous work using stochastic geometry often assumes an infinite number of interfering nodes drawn from a Poisson Point Process (PPP). Since indoor wearable networks will be isolated due to walls, a network with a finite number of nodes may be a more suitable model. In this paper, we characterize the significant sources of interference and develop closed-form expressions for the spatially averaged performance of a typical user's wearable communication link. The effect of human body blockage on the mmWave signals and the role of network density are investigated to show that an increase in interferer density reduces the mean number of significant interferers.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Venugopal, K. and Valenti, M.C. and Heath, R.W.}, year={2016}, pages={872–876} }
 @article{zhang_mckay_zhou_heath_2015, title={Artificial-noise-aided secure multi-antenna transmission with limited feedback}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84929330256&partnerID=MN8TOARS}, DOI={10.1109/TWC.2015.2391261}, abstractNote={We present an optimized secure multi-antenna transmission approach based on artificial-noise-aided beamforming, with limited feedback from a desired single-antenna receiver. To deal with beamformer quantization errors as well as unknown eavesdropper channel characteristics, our approach is aimed at maximizing throughput under dual performance constraints-a connection outage constraint on the desired communication channel and a secrecy outage constraint to guard against eavesdropping. We propose an adaptive transmission strategy that judiciously selects the wiretap coding parameters, as well as the power allocation between the artificial noise and the information signal. This optimized solution reveals several important differences with respect to solutions designed previously under the assumption of perfect feedback. We also investigate the problem of how to most efficiently utilize the feedback bits. The simulation results indicate that a good design strategy is to use approximately 20% of these bits to quantize the channel gain information, with the remainder to quantize the channel direction, and this allocation is largely insensitive to the secrecy outage constraint imposed. In addition, we find that 8 feedback bits per transmit antenna is sufficient to achieve approximately 90% of the throughput attainable with perfect feedback.}, number={5}, journal={IEEE Transactions on Wireless Communications}, author={Zhang, X. and McKay, M.R. and Zhou, X. and Heath, R.W.}, year={2015}, pages={2742–2754} }
 @inproceedings{bai_heath_2015, title={Asymptotic SINR for millimeter wave massive MIMO cellular networks}, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953374573&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2015.7227112}, abstractNote={Thanks to the small wavelength at millimeter wave (mmWave) frequency, it is promising to combine massive multiple-input and multiple-output (MIMO) with mmWave. MmWave massive MIMO will differ from the conventional massive MIMO, due to the differences in propagation and hardware constraints. This paper proposes a stochastic geometry framework for evaluating the performance in large-scale mmWave massive MIMO networks. Based on the system model, analytical expressions are provided for the asymptotic signal-to-interference-plus-noise ratio (SINR) distributions in both uplink and downlink, when the number of base station antennas goes to infinity. Numerical results indicate a fast convergence in the SINR distribution to its asymptotic equivalence in dense mmWave networks. A comparison with conventional massive MIMO shows that mmWave massive MIMO achieves a higher cell throughput with sufficiently dense deployments.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Bai, T. and Heath, R.W.}, year={2015}, pages={620–624} }
 @inproceedings{mendez-rial_gonzalez-prelcic_heath_2015, title={Augmented covariance estimation with a cyclic approach in DOA}, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946045711&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2015.7178478}, abstractNote={High resolution direction-of-arrival (DOA) estimation is an important problem in many array signal processing applications. This paper proposes an augmented covariance estimator for DOA estimation. The new method exploits the periodicity of the covariance lags when the DOAs are assumed on a discrete grid with a certain resolution. Then, it achieves twice the resolution of typical methods such as the direct augmentable approach or forward backward spatial smoothing. When the sources are not on the discrete grid, an interpolated array manifold technique is proposed to mitigate the grid mismatch error.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Mendez-Rial, R. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2015}, pages={2784–2788} }
 @inproceedings{park_lee_heath_2015, title={Base station cluster patterns for semi-static multi-cell cooperation in irregular network topologies}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963988180&partnerID=MN8TOARS}, DOI={10.1109/EUSIPCO.2015.7362823}, abstractNote={This paper proposes a clustering strategy for semi-static mul-ticell cooperation. Semi-static multicell cooperation exploits multiple predefined base station (BS) cluster patterns for improving cell-edge user throughput. The proposed clustering guarantees that every user communicates with their two closest BSs, so that users are protected from the dominant interferer. The key idea of the proposed clustering is to use the 2nd-order Voronoi region to form BS clusters. Each of the formed BS clusters is mapped into a particular cluster pattern by exploiting the edge-coloring in graph theory. Through simulations, the performance is compared to that of other conventional strategies. Our major finding is that the proposed clustering provides performance gains for cell-edge users compared to that of the conventional strategies.}, booktitle={2015 23rd European Signal Processing Conference, EUSIPCO 2015}, author={Park, J. and Lee, N. and Heath, R.W.}, year={2015}, pages={2441–2445} }
 @inproceedings{va_heath_2016, title={Basic relationship between channel coherence time and beamwidth in Vehicular Channels}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964414077&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2015.7390852}, abstractNote={The availability of large bandwidth at millimeter wave (mmWave) frequencies makes mmWave an ideal candidate to realize multi-Gbps data rates. MmWave has been applied to indoor applications (e.g. IEEE 802.11ad), and it is also being considered for cellular. There is not much work, however, in applying mmWave to vehicular channels. This is likely due to the misconception that the Doppler spread will be too high at small mmWave wavelengths, which is true only for omnidirectional communication. In this work, closed form approximate expressions relating the coherence time and beamwidth are derived taking directional communication into account. The result shows that the coherence time increases at least proportional to the inverse of the beamwidth. The derivation also takes the beam misalignment due to receiver movement into account and it can be shown that the optimal beamwidth is non-zero in contrast to models that assume perfect beam pointing.}, booktitle={2015 IEEE 82nd Vehicular Technology Conference, VTC Fall 2015 - Proceedings}, author={Va, V. and Heath, R.W.}, year={2016} }
 @inproceedings{va_zhang_heath_2016, title={Beam switching for millimeter wave communication to support high speed trains}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964562298&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2015.7390855}, abstractNote={This paper considers a millimeter wave (mmWave) system that enables multi-Gbps wireless service for high speed trains. MmWave systems require proper beam alignment to achieve good performance making it difficult to apply to high speed trains due to the need for frequent realignment. In this paper, using the IEEE 802.11ad system parameters, we first show that the channel coherence time will not be enough for communication with wide beams, and thus conventional approaches based on beam sweeping become inefficient if not impossible. Then we consider a beam switching approach that leverages the position information from the train control system for efficient beam alignment. Using this network architecture, we investigate the optimal choice of beamwidth and show that a properly optimized system can achieve multi-Gbps throughput.}, booktitle={2015 IEEE 82nd Vehicular Technology Conference, VTC Fall 2015 - Proceedings}, author={Va, V. and Zhang, X. and Heath, R.W.}, year={2016} }
 @article{mo_heath_2015, title={Capacity Analysis of One-Bit Quantized MIMO Systems with Transmitter Channel State Information}, volume={63}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84959378939&partnerID=MN8TOARS}, DOI={10.1109/TSP.2015.2455527}, abstractNote={With bandwidths on the order of a gigahertz in emerging wireless systems, high-resolution analog-to-digital convertors (ADCs) become a power consumption bottleneck. One solution is to employ low resolution one-bit ADCs. In this paper, we analyze the flat fading multiple-input multiple-output (MIMO) channel with one-bit ADCs. Channel state information is assumed to be known at both the transmitter and receiver. For the multiple-input single-output channel, we derive the exact channel capacity. For the single-input multiple-output and MIMO channel, the capacity at infinite signal-to-noise ratio (SNR) is found. We also derive upper bound at finite SNR, which is tight when the channel has full row rank. In addition, we propose an efficient method to design the input symbols to approach the capacity achieving solution. We incorporate millimeter wave channel characteristics and find the bounds on the infinite SNR capacity. The results show how the number of paths and number of receive antennas impact the capacity.}, number={20}, journal={IEEE Transactions on Signal Processing}, author={Mo, J. and Heath, R.W.}, year={2015}, pages={5498–5512} }
 @inproceedings{mendez-rial_rusu_alkhateeb_gonzalez-prelcic_heath_2015, title={Channel estimation and hybrid combining for mmWave: Phase shifters or switches?}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84961836201&partnerID=MN8TOARS}, DOI={10.1109/ITA.2015.7308971}, abstractNote={Precoding/combining and large antenna arrays are essential in millimeter wave (mmWave) systems. In traditional MIMO systems, precoding/combining is usually done digitally at baseband with one radio frequency (RF) chain and one analog-to-digital converter (ADC) per antenna. The high cost and power consumption of RF chains and ADCs at mmWave frequencies make an all-digital processing approach prohibitive. When only a limited number of RF chains is available, hybrid architectures that split the precoding/combining processing into the analog and digital domains are attractive. A previously proposed hybrid solution employs phase shifters and mixers in the RF precoding/combining stage. It obtains near optimal spectral efficiencies with a reduced number of RF channels. In this paper we propose a different hybrid architecture, which simplifies the hardware at the receiver by replacing the phase shifters with switches. We present a new approach for compressed sensing based channel estimation for the hybrid architectures. Given the channel estimate, we propose a novel algorithm that jointly designs the antenna subsets selected and the baseband combining. Using power consumption calculations and achievable rates, we compare the performance of hybrid combining with antenna switching and phase shifting, showing that antenna selection is preferred in a range of operating conditions.}, booktitle={2015 Information Theory and Applications Workshop, ITA 2015 - Conference Proceedings}, author={Mendez-Rial, R. and Rusu, C. and Alkhateeb, A. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2015}, pages={90–97} }
 @inproceedings{alkhateeby_leusz_heath_2015, title={Compressed sensing based multi-user millimeter wave systems: How many measurements are needed?}, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946012070&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2015.7178503}, abstractNote={Millimeter wave (mmWave) systems will likely employ directional beamforming with large antenna arrays at both the transmitters and receivers. Acquiring channel knowledge to design these beamformers, however, is challenging due to the large antenna arrays and small signal-to-noise ratio before beamforming. In this paper, we propose and evaluate a downlink system operation for multi-user mmWave systems based on compressed sensing channel estimation and conjugate analog beamforming. Adopting the achievable sum-rate as a performance metric, we show how many compressed sensing measurements are needed to approach the perfect channel knowledge performance. The results illustrate that the proposed algorithm requires an order of magnitude less training overhead compared with traditional lower-frequency solutions, while employing mmWave-suitable hardware. They also show that the number of measurements need to be optimized to handle the trade-off between the channel estimate quality and the training overhead.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Alkhateeby, A. and Leusz, G. and Heath, R.W.}, year={2015}, pages={2909–2913} }
 @article{park_lee_heath_2016, title={Cooperative base station coloring for pair-wise multi-cell coordination}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84958213974&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2015.2495355}, abstractNote={This paper proposes a method for designing base station (BS) clusters and cluster patterns for pair-wise BS coordination. The key idea is that each BS cluster is formed by using the second-order Voronoi region, and the BS clusters are assigned to a specific cluster pattern by using edge-coloring for a graph drawn by Delaunay triangulation. The main advantage of the proposed method is that the BS selection conflict problem is prevented, while users are guaranteed to communicate with their two closest BSs in any irregular BS topology. With the proposed coordination method, analytical expressions for the rate distribution and the ergodic spectral efficiency are derived as a function of relevant system parameters in a fixed irregular network model. In a random network model with a homogeneous Poisson point process, a lower bound on the ergodic spectral efficiency is characterized. Through system level simulations, the performance of the proposed method is compared with that of conventional coordination methods: dynamic clustering and static clustering. Our major finding is that, when users are dense enough in a network, the proposed method provides the same level of coordination benefit with dynamic clustering to edge users.}, number={1}, journal={IEEE Transactions on Communications}, author={Park, J. and Lee, N. and Heath, R.W.}, year={2016}, pages={402–415} }
 @inproceedings{mendez-rial_rusu_gonzalez-prelcic_heath_2015, title={Dictionary-free hybrid precoders and combiners for mmWave MIMO systems}, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953407083&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2015.7227018}, abstractNote={The high cost and power consumption of the radio frequency chain and data converters at mmWave frequencies introduce hardware limitations into the design of MIMO precoders and combiners. MmWave hybrid precoding overcomes this limitation by dividing the spatial signal processing between the radio frequency and baseband domains. Analog networks of phase shifters have been proposed to implement the radio frequency precoders, since they achieve a good compromise between complexity and performance. In this paper, we propose a low complexity hybrid precoding design for the architecture based on phase shifters. The new method is a greedy algorithm based on the orthogonal matching pursuit algorithm, but replacing the costly correlation operations over a dictionary with the element-wise normalization of the first singular vector of the residual. The main advantage is that the design avoids any assumption on the antenna array geometry. Additionally, numerical results show the superiority of the proposed method in terms of achievable spectral efficiency over other previous solutions.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Mendez-Rial, R. and Rusu, C. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2015}, pages={151–155} }
 @inproceedings{khan_alkhateeb_heath_2015, title={Energy coverage in millimeter wave energy harvesting networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84971264181&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2015.7414219}, abstractNote={Wireless energy harvesting in millimeter wave (mmWave) cellular networks is attractive, thanks to the large antenna arrays and the anticipated dense deployment of these systems. The signal propagation at mmWave frequencies, however, shows peculiar propagation characteristics such as extreme sensitivity to building blockages. This work analyzes the energy harvesting performance at receivers powered by a mmWave cellular network. Leveraging tools from stochastic geometry, analytical expressions are derived to characterize the energy coverage probability at a typical receiver in terms of the cellular network density, the antenna geometry parameters, and the channel parameters. Results show that there typically exists an optimum transmit antenna beamwidth that maximizes the network-wide energy coverage probability for many operating scenarios. Simulation results further suggest that mmWave energy harvesting could provide a substantial performance boost compared to lower frequency solutions.}, booktitle={2015 IEEE Globecom Workshops, GC Wkshps 2015 - Proceedings}, author={Khan, T.A. and Alkhateeb, A. and Heath, R.W.}, year={2015} }
 @inproceedings{thornburg_heath_2015, title={Ergodic capacity in mmWave ad hoc network with imperfect beam alignment}, volume={2015-December}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84959310159&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2015.7357653}, abstractNote={Millimeter wave (mmWave) technology has several technical hurdles to overcome before becoming an integral next-generation wireless technology. One such hurdle is the alignment of the steerable mmWave antenna array. This paper quantifies both the ergodic capacity of a mmWave ad hoc network with perfect alignment and the loss in ergodic capacity when error is introduced. First, we leverage stochastic geometry to evaluate the ergodic capacity (bits/sec/Hz/area) in the network. Next, we compare two antenna models that are commonly used: sectored and Gaussian. Lastly, we derive expressions that quantify the loss in ergodic capacity per user when alignment error occurs at the receiver and transmitter. Our results show that even relatively small errors in alignment can lead to significant ergodic capacity reduction.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Thornburg, A. and Heath, R.W.}, year={2015}, pages={1479–1484} }
 @inproceedings{truong_nikopour_heath_2016, title={FDD massive MIMO with analog csi feedback}, volume={2016-February}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969838828&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2015.7421139}, abstractNote={Massive multiple-input multiple-output (MIMO) is a breakthrough in cellular networking, through using a large number of antennas to serve simultaneously many users. Most prior work on massive MIMO assumes time-division duplex (TDD) operation to take advantage of channel reciprocity. Typical cellular systems, however, make use of spectrum dedicated for frequency-division duplex (FDD) operation. In this paper, we consider an FDD massive MIMO network where the base stations obtain downlink channel estimates via analog feedback from the users. Assuming an uncorrelated channel model, we derive closed-form expressions of the downlink achievable signal-to-interference-plus-noise ratio (SINR) and the corresponding rates. These expressions provide a means for studying rate with and without feedback overhead penalties. A surprising result is that depending on system parameters like the Doppler shift, it is desirable to shut off a subset of antennas at the base stations to maximize achievable downlink sum-rates.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Truong, K.T. and Nikopour, H. and Heath, R.W.}, year={2016}, pages={322–327} }
 @article{lee_dimakis_heath_2015, title={Index Coding With Coded Side-Information}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84924856040&partnerID=MN8TOARS}, DOI={10.1109/LCOMM.2015.2388477}, abstractNote={This letter investigates a new class of index coding problems. One sender broadcasts packets to multiple users, each desiring a subset, by exploiting prior knowledge of linear combinations of packets. We refer to this class of problems as index coding with coded side-information. Our aim is to characterize the minimum index code length that the sender needs to transmit to simultaneously satisfy all user requests. We show that the optimal binary vector index code length is equal to the minimum rank (minrank) of a matrix whose elements consist of the sets of desired packet indices and side-information encoding matrices. This is the natural extension of matrix minrank in the presence of coded side information. Using the derived expression, we propose a greedy randomized algorithm to minimize the rank of the derived matrix.}, number={3}, journal={IEEE Communications Letters}, author={Lee, N. and Dimakis, A.G. and Heath, R.W.}, year={2015}, pages={319–322} }
 @inproceedings{venugopal_valenti_heath_2015, title={Interference in finite-sized highly dense millimeter wave networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84961855527&partnerID=MN8TOARS}, DOI={10.1109/ITA.2015.7308984}, abstractNote={The potential of millimeter wave (mmWave) frequencies for device-to-device communication among wearable electronics is enormous for applications requiring Gbps through-put. In a dense usage scenario such as inside a train car or airplane cabin, many devices may be present within close proximity where interference is significant. Previous work that models interference in wireless networks has leveraged stochastic geometry and often assumes an infinite number of interferers. In the indoor wearables setting, a finite network may be a more realistic assumption. This paper analyzes mmWave networks with a finite number of interferers that are located in fixed positions. The paper considers the effect of blockages, which are primarily caused by the human bodies present in the operating environment. Expressions for coverage and rate are developed, which capture the effects of key antenna characteristics such as directivity and gain.}, booktitle={2015 Information Theory and Applications Workshop, ITA 2015 - Conference Proceedings}, author={Venugopal, K. and Valenti, M.C. and Heath, R.W.}, year={2015}, pages={175–180} }
 @inproceedings{thornburg_bai_heath_2015, title={Interference statistics in a random mmWave ad hoc network}, volume={2015-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946070491&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2015.7178502}, abstractNote={Wireless communication at millimeter wave (mmWave) frequencies is attractive for cellular, local area, and ad hoc networks due to the potential for channels with large bandwidths. As a byproduct of directional beamforming and propagation differences, some studies have claimed that mmWave networks will be noise rather than interference limited. This paper presents a derivation of the instantaneous interference-to-noise ratio (INR) distribution of a mmWave ad hoc network. Random network model of transmitters represented by a Poisson point process with a narrowband channel model is used to derive an approximation of the INR distribution. The analysis shows that the shape of the INR distribution is determined largely by the line-of-sight interferers, which depends on the overall network density and building blockage. A main conclusion drawn is that even with highly directional beamforming, interference can only sometimes be neglected in an ad hoc network. With a reasonable choice of system parameters, the interference is nearly always stronger than the noise power in dense networks.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Thornburg, A. and Bai, T. and Heath, R.W.}, year={2015}, pages={2904–2908} }
 @inproceedings{investigating the ieee 802.11ad standard for millimeter wave automotive radar_2016, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964470839&partnerID=MN8TOARS}, DOI={10.1109/VTCFall.2015.7390996}, abstractNote={Millimeter wave (mmWave) technology is widely used for automotive radar applications, like adaptive cruise control and obstacle detection. Unlike conventional radar waveforms which are usually propriety, this paper explores the use of a consumer wireless local area network (WLAN) waveform in the 60GHz unlicensed mmWave band for automotive radar applications. In particular, this paper develops a joint framework of long range automotive radar (LRR) and vehicle-to-vehicle communication (V2V) at 60 GHz by exploiting the special data-aided structure (repeated Golay complimentary sequences) of an IEEE 802.11ad single carrier physical layer (SCPHY) frame. This framework leverages the signal processing algorithms used in the typical WLAN receiver for time and frequency synchronization to perform radar parameter estimation. The initial simulation results show that it is possible to achieve the desired range accuracy of 0.1 m with a very high probability of detection (above 99%) using the preamble of a SCPHY frame. Furthermore, the velocity estimation algorithm achieves the desired accuracy of 0.1 m/s at high SNR using the preamble and pilot words of only a single frame.}, booktitle={2015 IEEE 82nd Vehicular Technology Conference, VTC Fall 2015 - Proceedings}, year={2016} }
 @article{alkhateeb_leus_heath_2015, title={Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84959441538&partnerID=MN8TOARS}, DOI={10.1109/TWC.2015.2455980}, abstractNote={Antenna arrays will be an important ingredient in millimeter-wave (mmWave) cellular systems. A natural application of antenna arrays is simultaneous transmission to multiple users. Unfortunately, the hardware constraints in mmWave systems make it difficult to apply conventional lower frequency multiuser MIMO precoding techniques at mmWave. This paper develops low-complexity hybrid analog/digital precoding for downlink multiuser mmWave systems. Hybrid precoding involves a combination of analog and digital processing that is inspired by the power consumption of complete radio frequency and mixed signal hardware. The proposed algorithm configures hybrid precoders at the transmitter and analog combiners at multiple receivers with a small training and feedback overhead. The performance of the proposed algorithm is analyzed in the large dimensional regime and in single-path channels. When the analog and digital precoding vectors are selected from quantized codebooks, the rate loss due to the joint quantization is characterized, and insights are given into the performance of hybrid precoding compared with analog-only beamforming solutions. Analytical and simulation results show that the proposed techniques offer higher sum rates compared with analog-only beamforming solutions, and approach the performance of the unconstrained digital beamforming with relatively small codebooks.}, number={11}, journal={IEEE Transactions on Wireless Communications}, author={Alkhateeb, A. and Leus, G. and Heath, R.W.}, year={2015}, pages={6481–6494} }
 @inproceedings{mo_heath_2016, title={Limited feedback in multiple-antenna systems with one-bit quantization}, volume={2016-February}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969781312&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2015.7421380}, abstractNote={Communication systems with low-resolution analog- to-digital-converters (ADCs) can exploit channel state information at the transmitter (CSIT) and receiver. This paper presents initial results on codebook design and performance analysis for limited feedback systems with one-bit ADCs. Different from the high-resolution case, the absolute phase at the receiver is important to align the phase of the received signals when the received signal is sliced by one-bit ADCs. A new codebook design for the beamforming case is proposed that separately quantizes the channel direction and the residual phase.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Mo, J. and Heath, R.W.}, year={2016}, pages={1432–1436} }
 @article{abdel khalek_caramanis_heath_2015, title={Loss Visibility Optimized Real-Time Video Transmission over MIMO Systems}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84960857916&partnerID=MN8TOARS}, DOI={10.1109/TMM.2015.2468196}, abstractNote={The structured nature of video data motivates introducing video-aware decisions that make use of this structure for improved video transmission over wireless networks . In this paper, we introduce an architecture for real-time video transmission over multiple-input multiple-output (MIMO) wireless communication systems using loss visibility side information . We quantify the perceptual importance of a packet through the packet loss visibility and use the loss visibility distribution to provide a notion of relative packet importance . To jointly achieve high video quality and low latency, we define the optimization objective function as the throughput weighted by the loss visibility of each packet, a proxy for the total perceptual value of successful packets per unit of time. We solve the problem of mapping video packets to MIMO subchannels and adapting per-stream rates to maximize the proposed objective . We show that the solution enables jointly reaping gains in terms of improved video quality and lower latency. Optimized packet-stream mapping enables transmission of more relevant packets over more reliable streams while unequal modulation opportunistically increases the transmission rate on the stronger streams to enable low latency delivery of high priority packets. Tested on H.264-encoded video sequences, for a 4 ×4 MIMO system with three spatial streams, the proposed architecture achieves 8 dB power reduction for the same video quality and supports 2.4× higher throughput due to unequal modulation. Furthermore, the gains are achieved at the expense of few bits of cross-layer overhead rather than a complex cross-layer design.}, number={10}, journal={IEEE Transactions on Multimedia}, author={Abdel Khalek, A. and Caramanis, C. and Heath, R.W.}, year={2015}, pages={1802–1817} }
 @inproceedings{rusu_mendez-rial_gonzalez-prelcicy_heath_2015, title={Low complexity hybrid sparse precoding and combining in millimeter wave MIMO systems}, volume={2015-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953433844&partnerID=MN8TOARS}, DOI={10.1109/ICC.2015.7248509}, abstractNote={Millimeter wave (mmWave) multiple-input multipleoutput (MIMO) communication with large antenna arrays has been proposed to enable gigabit per second communication for next generation cellular systems and local area networks. A key difference relative to lower frequency solutions is that in mmWave systems, precoding/combining can not be performed entirely at digital baseband, due to the high cost and power consumption of some components of the radio frequency (RF) chain. In this paper we develop a low complexity algorithm for finding hybrid precoders that split the precoding/combining process between the analog and digital domains. Our approach exploits sparsity in the received signal to formulate the design of the precoder/combiners as a compressed sensing optimization problem. We use the properties of the matrix containing the array response vectors to find first an orthonormal analog precoder, since sparse approximation algorithms applied to orthonormal sensing matrices are based on simple computations of correlations. Then, we propose to perform a local search to refine the analog precoder and compute the baseband precoder. We present numerical results demonstrate substantial improvements in complexity while maintaining good spectral efficiency.}, booktitle={IEEE International Conference on Communications}, author={Rusu, C. and Mendez-Rial, R. and Gonzalez-Prelcicy, N. and Heath, R.W.}, year={2015}, pages={1340–1345} }
 @inproceedings{rusu_gonzalez-prelcic_heath_2016, title={Low resolution adaptive compressed sensing for mmWave MIMO receivers}, volume={2016-February}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969786219&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2015.7421318}, abstractNote={Due to the high bandwidths and carrier frequencies to be used at millimeter wave (mmWave), high resolution and high sampling rate analog-to-digital converters (ADCs) with their corresponding interfacing cards become power hungry devices in the reception chain. One solution is to employ low resolution ADCs in the receiver. Acquiring channel state information is challenging in low-resolution systems. In this paper, we design an adaptive compressed sensing strategy to recover the sparse mmWave channel. The simulation results show that the new adaptive strategy outperforms conventional compressed sensing schemes based on fixed measurement matrices, while requiring a lower number of training pilots.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Rusu, C. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2016}, pages={1138–1143} }
 @article{li_heath_linehan_butler_2015, title={Metrocell antennas: The positive impact of a narrow vertical beamwidth and electrical downtilt}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84941027855&partnerID=MN8TOARS}, DOI={10.1109/MVT.2015.2410993}, abstractNote={This article discusses the positive impact that metrocell antennas with a narrow vertical beamwidth and electrical downtilt can have on heterogeneous cellular networks. Using a model of random cell placement based on a Poisson distribution, along with an innovative three-dimensional (3-D) building model that quantifies blockage due to shadowing, it is demonstrated that the network spectral efficiency and average user through-put both increase as the vertical beamwidth decreases and downtilt is applied to metrocell transmission. Moreover, the network becomes more energy efficient. Importantly, these additional gains in network performance can be achieved without any cooperation or exchange of information between macrocell base stations (BSs) and metrocells.}, number={3}, journal={IEEE Vehicular Technology Magazine}, author={Li, X. and Heath, R.W. and Linehan, K. and Butler, R.}, year={2015}, pages={51–59} }
 @inproceedings{thornburg_bai_heath_2015, title={MmWave ad hoc network coverage and capacity}, volume={2015-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953712917&partnerID=MN8TOARS}, DOI={10.1109/ICC.2015.7248504}, abstractNote={Ad hoc networks consistently under perform compared to cellular networks. Protocols either create residual interference that leads to poor signal-to-interference-plus-noise ratios or try to coordinate transmissions leading to fewer transmission opportunities. Communication with millimeter-wave (mmWave) devices offers the potential for higher bandwidth communication channels and reduced interference in ad hoc networks. This paper uses a stochastic geometry approach to characterize the coverage probability of a mmWave ad hoc network with directional antennas and random building blockages. The coverage probability in the presence of noise and both line-of-sight and non-line-of-sight interference is analyzed and used to derive the transmission capacity. Performance of mmWave is then analyzed in terms of area spectral efficiency and rate coverage. The results show that mmWave networks support larger densities, higher area spectral efficiencies, and better rate coverage compared to traditional, lower-frequency ad hoc networks.}, booktitle={IEEE International Conference on Communications}, author={Thornburg, A. and Bai, T. and Heath, R.W.}, year={2015}, pages={1310–1315} }
 @inproceedings{choi_heath_2015, title={Near maximum-likelihood detector with one-bit ADCs for multiuser massive MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963800557&partnerID=MN8TOARS}, DOI={10.1109/CAMSAP.2015.7383820}, abstractNote={In massive multiple-input multiple-output (MIMO) systems, deploying a high-resolution analog-to-digital converter (ADC) for each antenna element may not be power and cost efficient. Assuming each antenna is connected to a pair of one-bit ADCs, i.e., one for each real and imaginary component of the received signal, a near maximum likelihood (nML) detector for uplink multiuser massive MIMO systems is proposed in this paper. The proposed nML detector is based on a standard convex optimization technique and becomes optimal when the number of receive antennas at the base station goes to infinity. Simulation results show that the proposed nML detector can simultaneously support multiple users using higher-order constellations, e.g., 16 quadrature amplitude modulation, and performs much better than a simple zeroforcing (ZF)-type detector.}, booktitle={2015 IEEE 6th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2015}, author={Choi, J. and Heath, R.W.}, year={2015}, pages={397–400} }
 @inproceedings{eltayeb_alkhateeb_heath_al-naffouri_2016, title={Opportunistic beam training with hybrid analog/digital codebooks for mmWave systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964780385&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2015.7418208}, abstractNote={Millimeter wave (mmWave) communication is one solution to provide more spectrum than available at lower carrier frequencies. To provide sufficient link budget, mmWave systems will use beamforming with large antenna arrays at both the transmitter and receiver. Training these large arrays using conventional approaches taken at lower carrier frequencies, however, results in high overhead. In this paper, we propose a beam training algorithm that efficiently designs the beamforming vectors with low training overhead. Exploiting mmWave channel reciprocity, the proposed algorithm relaxes the need for an explicit feedback channel, and opportunistically terminates the training process when a desired quality of service is achieved. To construct the training beamforming vectors, a new multi-resolution codebook is developed for hybrid analog/digital architectures. Simulation results show that the proposed algorithm achieves a comparable rate to that obtained by exhaustive search solutions while requiring lower training overhead when compared to prior work.}, booktitle={2015 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2015}, author={Eltayeb, M.E. and Alkhateeb, A. and Heath, R.W. and Al-Naffouri, T.Y.}, year={2016}, pages={315–319} }
 @article{khan_orlik_kim_heath_2015, title={Performance Analysis of Cooperative Wireless Networks with Unreliable Backhaul Links}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939423495&partnerID=MN8TOARS}, DOI={10.1109/LCOMM.2015.2442991}, abstractNote={A cooperative wireless network, where a cluster of K single-antenna transmitters jointly serve a single-antenna receiver, is considered. Each transmitter is connected to the control unit (CU) via independent but unreliable backhaul links. The CU sends a common message to each transmitter over backhaul links, which upon successful reception, jointly transmit this message to the intended receiver. To facilitate analysis, a general expression is derived for the complementary cumulative distribution function of a sum of K independent random variables, where each random variable is a product of an exponential and a Bernoulli random variable. This result is applied to find a simple closed-form expression that characterizes the system outage performance as a function of network parameters and node geometry. The analytical model is validated using numerical simulations. As an application, the derived expression is also used for investigating the impact of backhaul assignment on the system performance.}, number={8}, journal={IEEE Communications Letters}, author={Khan, T.A. and Orlik, P. and Kim, K.J. and Heath, R.W.}, year={2015}, pages={1386–1389} }
 @inproceedings{park_lee_heath_2015, title={Performance analysis of pair-wise dynamic multi-user joint transmission}, volume={2015-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953715868&partnerID=MN8TOARS}, DOI={10.1109/ICC.2015.7248946}, abstractNote={This paper characterizes the performance of multiuser joint transmission (MU-JT) with pair-wise dynamic base station (BS) clustering. For analyzing the performance of such BS cooperation method, a tractable model is presented by means of stochastic geometry. Using tools of stochastic geometry, a tight lower bound of the instantaneous signal-to-interference ratio (SIR) distribution is derived in a closed form in terms of relevant system parameters: the path-loss exponent and the topologies of users in the BS cooperative region. Our key finding is that the pair-wise dynamic BS cooperation through MU-JT provides a better rate coverage performance than that of single-user joint transmission (SU-JT) over the entire range of the rate threshold when each user is close enough to the associated BS. Through simulations, the exactness of the derived analytical expression is verified.}, booktitle={IEEE International Conference on Communications}, author={Park, J. and Lee, N. and Heath, R.W.}, year={2015}, pages={3981–3986} }
 @inproceedings{mungara_zhang_lozano_heath_2015, title={Performance evaluation of ITLinQ and FlashLinQ for overlaid device-to-device communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84947721844&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2015.7247246}, abstractNote={We present a performance evaluation of ITLinQ and FlashLinQ, the two most popular schemes proposed to date to channelize D2D transmissions, i.e., to parse transmissions into noninterfering sets to be allocated to separate channels. Recognizing that it captures well the spatial characteristics of D2D networks, a stochastic geometry setting is utilized for this evaluation with the parameters of either scheme optimized in order to maximize the system spectral efficiency (bits/s/Hz per unit area). Although independently formulated and seemingly based on different principles, both schemes are found to exercise similar mechanisms to avoid situations of excessive interference, yielding substantial improvements with respect to unchannelized networks.}, booktitle={2015 IEEE International Conference on Communication Workshop, ICCW 2015}, author={Mungara, R.K. and Zhang, X. and Lozano, A. and Heath, R.W.}, year={2015}, pages={596–601} }
 @inproceedings{shin_han_lee_lee_heath_2015, title={Retrospective interference alignment for two-cell uplink MIMO cellular networks with delayed CSIT}, volume={2015-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953749958&partnerID=MN8TOARS}, DOI={10.1109/ICC.2015.7248973}, abstractNote={In this paper, we propose a new retrospective interference alignment for two-cell multiple-input multiple-output (MIMO) interfering multiple access channels (IMAC) with the delayed channel state information at the transmitters (CSIT). It is shown that having delayed CSIT can strictly increase the sum-DoF compared to the case of no CSIT. The key idea is to align multiple interfering signals from adjacent cells onto a small dimensional subspace over time by fully exploiting the previously received signals as side information with outdated CSIT in a distributed manner. Remarkably, we show that the retrospective interference alignment can achieve the optimal sum-DoF in the context of two-cell two-user scenario by providing a new outer bound.}, booktitle={IEEE International Conference on Communications}, author={Shin, W. and Han, Y. and Lee, J. and Lee, N. and Heath, R.W.}, year={2015}, pages={4144–4150} }
 @inproceedings{lin_heath_andrews_2015, title={Spectral efficiency of massive MIMO systems with D2D underlay}, volume={2015-September}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953741766&partnerID=MN8TOARS}, DOI={10.1109/ICC.2015.7249006}, abstractNote={This paper studies the interplay between massive MIMO and device-to-device (D2D) networking in a single cell setting, where cellular uplink resources are shared by D2D. The spatial positions of underlaid D2D transmitters are modeled by a Poisson point process. All the transmissions (both cellular and D2D) are SIMO (i.e., single-input multiple-output) with the base station (BS) having a very large antenna array. Assuming perfect channel state information at the receivers, we study cellular and D2D spectral efficiency. In the asymptotic regime where the number of BS antennas goes to infinity, we find that the received signal-to-interference-plus-noise ratio (SINR) of any cellular user increases unboundedly and the effects of noise, fast fading, and the interfering signals from the other co-channel cellular users and the infinite D2D transmitters vanish completely. In the non-asymptotic regime, we derive simple analytical lower bounds for both cellular and D2D spectral efficiency, which allow for efficient numerical evaluation.}, booktitle={IEEE International Conference on Communications}, author={Lin, X. and Heath, R.W. and Andrews, J.G.}, year={2015}, pages={4345–4350} }
 @article{lin_heath_andrews_2015, title={The interplay between massive MIMO and underlaid D2D networking}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84933059825&partnerID=MN8TOARS}, DOI={10.1109/TWC.2015.2404435}, abstractNote={In a device-to-device (D2D) underlaid cellular network, the uplink spectrum is reused by the D2D transmissions, causing mutual interference with the ongoing cellular transmissions. Massive MIMO is appealing in such a context as the base station's (BS's) large antenna array can nearly null the D2D-to-BS interference. The multi-user transmission in massive MIMO, however, may lead to increased cellular-to-D2D interference. This paper studies the interplay between massive MIMO and underlaid D2D networking in a multi-cell setting. We investigate cellular and D2D spectral efficiencies under both perfect and imperfect channel state information (CSI) at the receivers that employ partial zero-forcing. Compared to the case without D2D, there is a loss in cellular spectral efficiency due to D2D underlay. With perfect CSI, the loss can be completely overcome if the number of canceled D2D interfering signals is scaled with the number of BS antennas at an arbitrarily slow rate. With imperfect CSI, in addition to pilot contamination, a new asymptotic effect termed underlay contamination arises. In the non-asymptotic regime, simple analytical lower bounds are derived for both the cellular and D2D spectral efficiencies.}, number={6}, journal={IEEE Transactions on Wireless Communications}, author={Lin, X. and Heath, R.W. and Andrews, J.G.}, year={2015}, pages={3337–3351} }
 @article{a phase-reconstruction technique for low-power centimeter-accurate mobile positioning_2014, volume={62}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84900000892&partnerID=MN8TOARS}, DOI={10.1109/TSP.2014.2311967}, abstractNote={A carrier phase reconstruction technique is presented as an enabler for low-power centimeter-accurate mobile positioning. Reliable carrier phase reconstruction permits the duty cycling of a Global Navigation Satellite System (GNSS) receiver whose outputs are used for precise carrier-phase differential GNSS (CDGNSS) positioning. Existing CDGNSS techniques are power intensive because they require continuous tracking of each GNSS signal's carrier phase. By contrast, the less-precise code-ranging technique that is commonly used in mobile devices for 3-to-10-meter-accurate positioning allows for aggressive measurement duty-cycling, which enables low-power implementations. The technique proposed in this paper relaxes the CDGNSS continuous phase tracking requirement by solving a mixed real and integer estimation problem to reconstruct a continuous carrier phase time history from intermittent phase measurement intervals each having an ambiguous initial phase. Theoretical bounds on the probability of successful phase reconstruction, corroborated by Monte-Carlo-type simulation, are used to investigate the sensitivity of the proposed technique to various system parameters, including the time period between successive phase measurement intervals, the duration of each interval, the carrier-to-noise ratio, and the line-of-sight acceleration uncertainty. A demonstration on real data indicates that coupling a GNSS receiver with a consumer-grade inertial measurement unit enables reliable phase reconstruction with phase measurement duty cycles as low as 5%.}, number={10}, journal={IEEE Transactions on Signal Processing}, year={2014}, pages={2595–2610} }
 @inproceedings{khan_zhang_heath_2015, title={A stochastic geometry approach to analyzing cellular networks with semi-static clustering}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964887314&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417282}, abstractNote={Static base-station clustering allows clustered transmitters to jointly serve a group of users and thus eliminate the intra-cluster interference. The network performance is then bottlenecked by the cluster-edge users. Semi-static clustering can help improve the performance along the cluster edges by time-sharing between different clustering patterns. We propose a simple clustering and user scheduling algorithm to gauge the performance gain of semi-static clustering. Under a stochastic geometry framework, we derive analytical expressions for the coverage and rate of a user at a given location. As the cluster size goes to infinity, we show that the outage probability of semi-static clustering decays at the same order as that of static clustering. Thus, in the asymptotic regime, the performance gain provided by semi- static clustering can be characterized by a linear factor. Numerical results demonstrate the gain of semi-static clustering in the non-asymptotic regime.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Khan, T.A. and Zhang, X. and Heath, R.W.}, year={2015} }
 @inproceedings{rusu_mendez-rial_gonzalez-prelcic_heath_2015, title={Adaptive one-bit compressive sensing with application to low-precision receivers at mmWave}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964910034&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417853}, abstractNote={Multiple input multiple output (MIMO) systems employing large antenna arrays are the basic architecture for millimeter wave (mmWave) systems. Due to the higher bandwidths to be used at mmWave, the corresponding sampling rates of high-resolution analog-to-digital converters (ADCs) are also very high, so that ADCs become the most power hungry devices in the reception chain. One solution is to employ low resolution, i.e. one-bit, ADCs. We develop an adaptive one-bit compressed sensing scheme that can be used at low-resolution mmWave receivers for channel estimation. The simulation results show that the adaptive one-bit compressed sensing scheme outperforms the fixed one in the context of mmWave channel estimation.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Rusu, C. and Mendez-Rial, R. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2015} }
 @inproceedings{chen_zhu_de veciana_bovik_heath_2014, title={Adaptive video transmission with subjective quality constraints}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84931070578&partnerID=MN8TOARS}, DOI={10.1109/ICIP.2014.7025501}, abstractNote={We conducted a subjective study wherein we found that viewers' Quality of Experience (QoE) was strongly correlated with the empirical cumulative distribution function (eCDF) of the predicted video quality. Based on this observation, we propose a rate-adaptation algorithm that can incorporate QoE constraints on the empirical cumulative quality distribution per user. Simulation results show that the proposed technique can reduce network resource consumption by 29% over conventional average-quality maximized rate-adaptation algorithms.}, booktitle={2014 IEEE International Conference on Image Processing, ICIP 2014}, author={Chen, C. and Zhu, X. and De Veciana, G. and Bovik, A.C. and Heath, R.W.}, year={2014}, pages={2477–2481} }
 @inproceedings{choi_lee_love_kim_heath_2015, title={Advanced limited feedback designs for FD-MIMO using uniform planar arrays}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964886837&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417281}, abstractNote={Massive multiple-input multiple-output (MIMO) with uniform planar arrays (UPAs), which is often referred to as full-dimension (FD) MIMO, is being strongly considered for future wireless communication standards. FD-MIMO can control transmit and receive beams in both the horizontal and vertical domains and fully exploit the large number of antennas of massive MIMO. It is widely accepted that Kronecker-product codebooks using a discrete Fourier transform (DFT) structure are suitable to quantize the downlink channel at the user for FD-MIMO systems relying on frequency division duplexing (FDD). In this paper, we numerically study the characteristics of FD-MIMO channels using a three-dimensional (3D) channel model that captures realistic channel properties. Based on the study, we identify a limitation of conventional Kronecker-product codebooks and propose advanced channel quantization techniques that can further improve channel quantization quality.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Choi, J. and Lee, K. and Love, D.J. and Kim, T. and Heath, R.W.}, year={2015} }
 @inproceedings{zhang_baccelli_heath_2015, title={An indoor correlated shadowing model}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964810170&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417557}, abstractNote={A Manhattan Poisson line process divides the plane into an infinite number of rectangular rooms with walls extending infinitely along the axes. When the path loss is dominated by the penetration through each of the walls, a Poisson field of transmitters creates a heavy tailed interference at a randomly picked room, whose distribution is tractable in the Laplace domain. Interference correlation at different rooms is explicitly available. This model gives the first tractable mathematical abstraction to indoor physical environments where wireless signals are shadowed by (common) walls. Applying the analytical results leads to a formula for success probabilities of a transmission attempt between two given rooms.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Zhang, X. and Baccelli, F. and Heath, R.W.}, year={2015} }
 @article{bai_vaze_heath_2014, title={Analysis of blockage effects on urban cellular networks}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907221038&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2331971}, abstractNote={Large-scale blockages such as buildings affect the performance of urban cellular networks, especially at higher frequencies. Unfortunately, such blockage effects are either neglected or characterized by oversimplified models in the analysis of cellular networks. Leveraging concepts from random shape theory, this paper proposes a mathematical framework to model random blockages and analyze their impact on cellular network performance. Random buildings are modeled as a process of rectangles with random sizes and orientations whose centers form a Poisson point process on the plane. The distribution of the number of blockages in a link is proven to be a Poisson random variable with parameter dependent on the length of the link. Our analysis shows that the probability that a link is not intersected by any blockages decays exponentially with the link length. A path loss model that incorporates the blockage effects is also proposed, which matches experimental trends observed in prior work. The model is applied to analyze the performance of cellular networks in urban areas with the presence of buildings, in terms of connectivity, coverage probability, and average rate. Our results show that the base station density should scale superlinearly with the blockage density to maintain the network connectivity. Our analyses also show that while buildings may block the desired signal, they may still have a positive impact on the SIR coverage probability and achievable rate since they can block significantly more interference.}, number={9}, journal={IEEE Transactions on Wireless Communications}, author={Bai, T. and Vaze, R. and Heath, R.W.}, year={2014}, pages={5070–5083} }
 @inproceedings{bai_heath_2015, title={Analysis of self-body blocking effects in millimeter wave cellular networks}, volume={2015-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84940531598&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2014.7094804}, abstractNote={The millimeter wave (mmWave) spectrum is a strong candidate carrier frequency for access channels in fifth generation cellular networks. Unfortunately, the human body heavily attenuates mmWave signals. This paper evaluates the impact of self-body blocking (the blocking of the direct link by the handset user's body) in mmWave cellular networks using a stochastic geometric network model. A mathematical model for self-body blocking is proposed, allowing for position changes, and used to compute signal-to-interference-plus-noise ratio (SINR) distributions with two possible user association rules. The results characterize how self-body blocking impacts the SINR coverage. Even including self-body blocking, mmWave cellular systems can still outperform convention systems at lower frequencies, in terms of the achievable rate, due to the larger bandwidth.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Bai, T. and Heath, R.W.}, year={2015}, pages={1921–1925} }
 @inproceedings{taranetz_bai_heath_rupp_2014, title={Analysis of small cell partitioning in urban two-tier heterogeneous cellular networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84911979737&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2014.6933451}, abstractNote={This paper presents a system model that enables the analysis of indoor downlink performance in urban two-tier heterogeneous cellular networks. The urban building topology is modeled as a process of randomly distributed circles. Each building is deployed with an indoor small cell with a certain occupation probability. Macro base stations are sited outdoors. Their signals experience distance-dependent shadowing due to the blockage of the buildings. Given a typical building at the origin, expressions for the coverage probability with- and without small cell occupation are derived. The analysis of the asymmetric interference field as observed by a typical indoor user is simplified by approximations. Their accuracy is verified by Monte Carlo simulations. Our results show that isolation by wall partitioning can enhance indoor rate, and that the improvement is more sensitive to building density, rather than penetration loss per building.}, booktitle={2014 11th International Symposium on Wireless Communications Systems, ISWCS 2014 - Proceedings}, author={Taranetz, M. and Bai, T. and Heath, R.W. and Rupp, M.}, year={2014}, pages={739–743} }
 @inproceedings{artificial-noise-aided secure multi-antenna transmission in slow fading channels with limited feedback_2014, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905222854&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2014.6854346}, abstractNote={We study secure multi-antenna transmission with limited feedback from the intended receiver and no feedback from the malicious eavesdropper. Our system uses the artificial-noise-aided beamforming approach to enhance secrecy, considering slow fading channels with outage constraints on the reliability performance of legitimate communication and the secrecy performance against eavesdropping. Our analytical results provide conditions on the minimum number of feedback bits and the minimum strength of the intended channel for making secure transmission possible. We observe that strengthening the secrecy outage constraint puts higher requirements on the number of feedback bits and the strength of the intended channel. To maximize the achievable secrecy rate, the optimal transmit power allocation between the information signal and the artificial noise is also derived in closed form.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2014}, pages={3968–3972} }
 @inproceedings{bai_heath_2014, title={Asymptotic coverage and rate in massive MIMO networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84936154872&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2014.7032188}, abstractNote={Massive multiple-input multiple-output (MIMO) is a transmission technique for cellular systems that uses many antennas to support not-as-many users. In this paper, we analyze its performance in large-scale cellular networks with Poisson distributed base station locations. Our analysis incorporates the spatial correlation due to small-scale fading at base station antennas. Numerical results show that, though limited by pilot contamination, massive MIMO is still promising to provide high cell throughput by serving large number of users simultaneously.}, booktitle={2014 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2014}, author={Bai, T. and Heath, R.W.}, year={2014}, pages={602–606} }
 @inproceedings{pesyna_heath_humphreys_2014, title={Centimeter positioning with a smartphone-Quality GNSS antenna}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939449408&partnerID=MN8TOARS}, booktitle={27th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GNSS 2014}, author={Pesyna, K.M. and Heath, R.W. and Humphreys, T.E.}, year={2014}, pages={1568–1577} }
 @article{alkhateeb_el ayach_leus_heath_2014, title={Channel estimation and hybrid precoding for millimeter wave cellular systems}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907210437&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2014.2334278}, abstractNote={Millimeter wave (mmWave) cellular systems will enable gigabit-per-second data rates thanks to the large bandwidth available at mmWave frequencies. To realize sufficient link margin, mmWave systems will employ directional beamforming with large antenna arrays at both the transmitter and receiver. Due to the high cost and power consumption of gigasample mixed-signal devices, mmWave precoding will likely be divided among the analog and digital domains. The large number of antennas and the presence of analog beamforming requires the development of mmWave-specific channel estimation and precoding algorithms. This paper develops an adaptive algorithm to estimate the mmWave channel parameters that exploits the poor scattering nature of the channel. To enable the efficient operation of this algorithm, a novel hierarchical multi-resolution codebook is designed to construct training beamforming vectors with different beamwidths. For single-path channels, an upper bound on the estimation error probability using the proposed algorithm is derived, and some insights into the efficient allocation of the training power among the adaptive stages of the algorithm are obtained. The adaptive channel estimation algorithm is then extended to the multi-path case relying on the sparse nature of the channel. Using the estimated channel, this paper proposes a new hybrid analog/digital precoding algorithm that overcomes the hardware constraints on the analog-only beamforming, and approaches the performance of digital solutions. Simulation results show that the proposed low-complexity channel estimation algorithm achieves comparable precoding gains compared to exhaustive channel training algorithms. The results illustrate that the proposed channel estimation and precoding algorithms can approach the coverage probability achieved by perfect channel knowledge even in the presence of interference.}, number={5}, journal={IEEE Journal on Selected Topics in Signal Processing}, author={Alkhateeb, A. and El Ayach, O. and Leus, G. and Heath, R.W.}, year={2014}, pages={831–846} }
 @inproceedings{mo_schniter_prelcic_heath_2015, title={Channel estimation in millimeter wave MIMO systems with one-bit quantization}, volume={2015-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84940470523&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2014.7094595}, abstractNote={We develop channel estimation agorithms for millimeter wave (mmWave) multiple input multiple output (MIMO) systems with one-bit analog-to-digital converters (ADCs). Since the mmWave MIMO channel is sparse due to the propagation characteristics, the estimation problem is formulated as a one-bit compressed sensing problem. We propose a modified EM algorithm that exploits sparsity and has better performance than the conventional EM algorithm. We also present a second solution using the generalized approximate message passing (GAMP) algorithm to solve this optimization problem. The simulation results show that GAMP can reduce mean squared error in the important low and medium SNR regions.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Mo, J. and Schniter, P. and Prelcic, N.G. and Heath, R.W.}, year={2015}, pages={957–961} }
 @article{seifi_zhang_heath_svensson_coldrey_2014, title={Coordinated 3D beamforming for interference management in cellular networks}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907970881&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2349981}, abstractNote={We consider downlink transmission in a cellular network consisting of multi-antenna base stations (BSs), single-antenna mobile users, directional antennas each with a vertically adjustable beam, and control signaling delays in feedback and backhaul links. We propose a novel transmission technique in which intercell interference management is performed via coordinating the beamforming jointly in the horizontal and vertical planes of the wireless channel, denoted as coordinated 3D beamforming. In the horizontal plane, we focus on intercell interference cancelation (ICIC) and investigate its performance when the provided channel state information (CSI) is impaired due to delay/mobility. It is demonstrated that the superiority of ICIC over conventional intracell maximum ratio transmission is highly dependent on the CSI accuracy. In the vertical plane, we consider intercell interference control via coordinatively adapting the elevation angle of the BS antenna pattern, denoted as tilt, to the locations of the scheduled users. It is shown that with perfect CSI interference management should be performed in the horizontal plane using ICIC, while at high delay/mobility it should be done in the vertical plane via coordinated tilt adaptation. For intermediate values of delay/mobility, joint interference management in both planes is required.}, number={10}, journal={IEEE Transactions on Wireless Communications}, author={Seifi, N. and Zhang, J. and Heath, R.W. and Svensson, T. and Coldrey, M.}, year={2014}, pages={5396–5410} }
 @inproceedings{coordinated beamforming with dynamic clustering: a stochastic geometry approach_2014, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907000439&partnerID=MN8TOARS}, DOI={10.1109/ICC.2014.6883644}, abstractNote={This paper characterizes the performance of coordinated beamforming with dynamic clustering. A downlink cellular model based on stochastic geometry is put forth to analyze the performance of such base station (BS) coordination strategy. Analytical expressions for the complementary cumulative distribution function (CCDF) of the instantaneous signal-to-interference ratio (SIR) are derived in terms of relevant system parameters, chiefly the number of coordinated BSs, the number of antennas, and the path-loss exponent. Utilizing this CCDF, with pilot overheads further incorporated into the analysis, we formulate the computation of the number of coordinated BSs that maximizes the spectral efficiency for a given fading coherence. The results make precise the intuition that the slower the fading, the more beneficial the coordination. Simulation results verify the exactness of the SIR distribution derived for stochastic geometries, which is further compared with the corresponding distribution for a deterministic grid model.}, booktitle={2014 IEEE International Conference on Communications, ICC 2014}, year={2014}, pages={2165–2170} }
 @article{bai_alkhateeb_heath_2014, title={Coverage and capacity of millimeter-wave cellular networks}, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907217677&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2014.6894455}, abstractNote={The millimeter-wave (mmWave) band offers the potential for high-bandwidth communication channels in cellular networks. It is not clear, however, whether both high data rates and coverage in terms of signal-to-noise-plus-interference ratio can be achieved in interference-limited mmWave cellular networks due to the differences in propagation conditions and antenna topologies. This article shows that dense mmWave networks can achieve both higher data rates and comparable coverage relative to conventional microwave networks. Sum rate gains can be achieved using more advanced beamforming techniques that allow multiuser transmission. The insights are derived using a new theoretical network model that incorporates key characteristics of mmWave networks.}, number={9}, journal={IEEE Communications Magazine}, author={Bai, T. and Alkhateeb, A. and Heath, R.}, year={2014}, pages={70–77} }
 @article{bai_heath_2015, title={Coverage and rate analysis for millimeter-wave cellular networks}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922879239&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2364267}, abstractNote={Millimeter wave (mmWave) holds promise as a carrier frequency for fifth generation cellular networks. Because mmWave signals are sensitive to blockage, prior models for cellular networks operated in the ultra high frequency (UHF) band do not apply to analyze mmWave cellular networks directly. Leveraging concepts from stochastic geometry, this paper proposes a general framework to evaluate the coverage and rate performance in mmWave cellular networks. Using a distance-dependent line-of-site (LOS) probability function, the locations of the LOS and non-LOS base stations are modeled as two independent non-homogeneous Poisson point processes, to which different path loss laws are applied. Based on the proposed framework, expressions for the signal-to-noise-and-interference ratio (SINR) and rate coverage probability are derived. The mmWave coverage and rate performance are examined as a function of the antenna geometry and base station density. The case of dense networks is further analyzed by applying a simplified system model, in which the LOS region of a user is approximated as a fixed LOS ball. The results show that dense mmWave networks can achieve comparable coverage and much higher data rates than conventional UHF cellular systems, despite the presence of blockages. The results suggest that the cell size to achieve the optimal SINR scales with the average size of the area that is LOS to a user.}, number={2}, journal={IEEE Transactions on Wireless Communications}, author={Bai, T. and Heath, R.W.}, year={2015}, pages={1100–1114} }
 @inproceedings{cross-polarization rf precoding to mitigate mobile misorientation and polarization leakage_2014, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84906827815&partnerID=MN8TOARS}, DOI={10.1109/CCNC.2014.6866630}, abstractNote={Dual-polarized antennas provide a mechanism to achieve polarization diversity or multiplex non-interfering data streams over-the-air. Practical realization of dual-polarized transmission, however, must overcome challenges such as mobile misorientation with respect to the base station as well as non-ideal polarization isolation. With the advent of mmWave communications, dual-polarized large scale antenna arrays can be realized inexpensively. In this paper, we propose a beamforming algorithm that jointly designs the horizontal and vertical beamformers and combiners to maximize spectral efficiency in the presence of mobile misorientation and polarization leakage. First, we show that dual-polarized antenna arrays introduce channel structure that enables mobile orientation estimation without explicit channel knowledge. We derive the maximum likelihood estimate of the azimuth and elevation mobile rotation. Given these estimates, we design the horizontal and vertical beamformer and combiner with practical RF hardware constraints. We generalize the beamforming algorithm to enable multiplexing multiple data streams across different channel paths in the horizontal and vertical domain. Results show that mobile orientation estimation using dual-polarized antenna arrays achieves over 90% accuracy even below 0 dB. Using a realistic clustered channel model, spectral efficiency gains of the order of 5 dB are demonstrated over dual-polarized antennas with independent horizontal/vertical beam steering as well as single polarized antenna arrays.}, booktitle={2014 IEEE 11th Consumer Communications and Networking Conference, CCNC 2014}, year={2014}, pages={581–586} }
 @article{khalek_caramanis_heath_2015, title={Delay-constrained video transmission: Quality-driven resource allocation and scheduling}, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84921810289&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2014.2332304}, abstractNote={Real-time video demands quality-of-service (QoS) guarantees such as delay bounds for end-user satisfaction. Furthermore, the tolerable delay varies depending on the use case such as live streaming or two-way video conferencing. Due to the inherently stochastic nature of wireless fading channels, deterministic delay bounds are difficult to guarantee. Instead, we propose providing statistical delay guarantees using the concept of effective capacity. We consider a multiuser setup whereby different users have (possibly different) delay QoS constraints. We derive the resource allocation policy that maximizes the sum video quality and applies to any quality metric with concave rate-quality mapping. We show that the optimal operating point per user is such that the rate-distortion slope is the inverse of the supported video source rate per unit bandwidth, a key metric we refer to as the source spectral efficiency. We extend the resource allocation policy to capture video quality-driven adaptive user-subcarrier assignment in wideband channels as well as capture the impact of adaptive modulation and coding. We also solve the alternative problem of fairness-based resource allocation whereby the objective is to maximize the minimum video quality across users. Finally, we derive user admission and scheduling policies that enable selecting a maximal user subset such that all selected users can meet their statistical delay requirement. Results show that video users with differentiated QoS requirements can achieve similar video quality with vastly different resource requirements. Thus, QoS-aware scheduling and resource allocation enable supporting significantly more users under the same resource constraints.}, number={1}, journal={IEEE Journal on Selected Topics in Signal Processing}, author={Khalek, A.A. and Caramanis, C. and Heath, R.W.}, year={2015}, pages={60–75} }
 @article{lee_gerstlauer_heath_2015, title={Distributed real-time implementation of interference alignment with analog feedback}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939520224&partnerID=MN8TOARS}, DOI={10.1109/TVT.2014.2357391}, abstractNote={Interference alignment (IA) is a precoding technique that aligns interfering signals at receivers. It is known that IA achieves the maximum degrees of freedom over an interference channel under ideal assumptions. The real-world performance of IA depends on a range of practical issues, such as imperfect synchronization, channel estimation, and feedback. Practical issues have been studied in simulations and prototypes, but fully distributed operation of IA network nodes has not been considered. In this paper, we present the first investigation of real-time IA performance on a fully distributed 2  $\times$ 2 multiple-input–multiple-output (MIMO) prototype system with three physically independent user pairs. Over-the-air algorithms for time and frequency synchronization, as well as analog feedback, are studied and implemented. Sum rates are illustrated as a function of complexity and accuracy of different alignment, synchronization, and feedback algorithms. Corresponding tradeoffs are evaluated using an iterative IA method, the injection of residual frequency offset into synchronization, and analog versus quantization-based limited feedback approaches. We demonstrate that, while considering all possible error sources in estimation, synchronization, and feedback, the theoretical multiplexing gain of IA can be reached in practical systems with a constant sum rate loss that remains within 5 bits/Hz/s compared with an ideal simulation.}, number={8}, journal={IEEE Transactions on Vehicular Technology}, author={Lee, S. and Gerstlauer, A. and Heath, R.W.}, year={2015}, pages={3513–3525} }
 @article{lee_tandon_heath_2015, title={Distributed space-time interference alignment with moderately delayed CSIT}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922905910&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2363475}, abstractNote={This paper proposes an interference alignment method with distributed and delayed channel state information at the transmitter (CSIT) for a class of interference networks. The core idea of the proposed method is to align interference signals over time at the unintended receivers in a distributed manner. With the proposed method, achievable tradeoffs between the sum of degrees of freedom (sum-DoF) and feedback delay of CSI are characterized in both the X-channel and three-user interference channel to reveal the impact on how the CSI feedback delay affects the sum-DoF of the interference networks. A major implication of derived results is that distributed and moderately delayed CSIT is useful to strictly improve the sum-DoF over the case of no CSI at the transmitter in a certain class of interference networks. For a class of X-channels, the results show how to optimally use distributed and moderately delayed CSIT to yield the same sum-DoF as instantaneous and global CSIT. Furthermore, leveraging the proposed transmission method and the known outer bound results, the sum-capacity of the two-user X-channel with a particular set of channel coefficients is characterized within a constant number of bits.}, number={2}, journal={IEEE Transactions on Wireless Communications}, author={Lee, N. and Tandon, R. and Heath, R.W.}, year={2015}, pages={1048–1059} }
 @article{tafazolli_palazzo_payarõ_heath_2015, title={Enabling 5G: Energy and spectrally efficient communication systems}, volume={26}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84928885792&partnerID=MN8TOARS}, DOI={10.1002/ett.2917}, abstractNote={Current estimates predict that, by 2020, there will be 50 billion devices connected to Internet. As a result of the generated data traffic, the network load will be increased by three orders of magnitude compared to what operators are experiencing today. Such unprecedented burden of data delivery poses, essentially, two challenges that will need to be addressed by 5G technology. First, the coexistence of this unbelievably high number of devices requires an efficient approach to the spectrum use to overcome the, otherwise unavoidable, “spectrum crunch”. Second, the energy consumption needed to sustain such pervasive network infrastructure and wireless devices must be kept at a minimum to achieve environmental sustainability through Green communications. Such stringent requirements on the use of spectrum and energy open a wide research area for future 5G communication technology, that can impinge on a plethora of aspects related to communication systems: from very specific optimization designs at component level to largescale network-wide aspects. In this sense, viable solutions will impact both future commercial activities as well as standardization approaches. In light of the above, the main purpose of this Special Issue is to present some of the most promising and novel approaches in analyzing, designing and optimizing energy and spectrum constrained 5G communication systems focusing on the physical, link, and network layer aspects.}, number={1}, journal={Transactions on Emerging Telecommunications Technologies}, author={Tafazolli, R. and Palazzo, S. and Payarõ, M. and Heath, R.W.}, year={2015}, pages={1–2} }
 @inproceedings{rico-alvarino_heath_mosquera_2014, title={FER prediction with variable codeword length}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905234138&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2014.6853935}, abstractNote={Frame error rate (FER) prediction in wireless communication systems is an important tool with applications to system level simulations and link adaptation, among others. Although in realistic communication scenarios it is expected to have codewords of different lengths, previous work on FER prediction marginally treated the dependency of the FER on the codeword length. In this paper, we present a method to estimate the FER using codewords of different length. We derive a low complexity FER estimator for frames of different length transmitted over a binary symmetric channel of unknown error probability. We extend this technique to coded systems by the use of effective SNR FER predictors. The proposed estimation scheme is shown to outperform other simpler estimation methods.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Rico-Alvarino, A. and Heath, R.W. and Mosquera, C.}, year={2014}, pages={1931–1935} }
 @article{five disruptive technology directions for 5g_2014, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84896689961&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2014.6736746}, abstractNote={New research directions will lead to fundamental changes in the design of future fifth generation (5G) cellular networks. This article describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter wave, massive MIMO, smarter devices, and native support for machine-to-machine communications. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.}, number={2}, journal={IEEE Communications Magazine}, year={2014}, pages={74–80} }
 @inproceedings{park_heath_2015, title={Frequency selective hybrid precoding in millimeter wave OFDMA systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964880233&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417763}, abstractNote={Hybrid precoding, a combination of analog and digital precoding, is an attempt to reach a compromise between complexity and performance. By exploiting more than one RF chain, hybrid precoding enables a millimeter wave (mmWave) system to take advantage of both spatial multiplexing and beamforming gain. A major challenge with hybrid precoding is its configuration in wideband systems. The reason is that the analog beamforming weights are the same across the entire band. In this paper, we propose a frequency selective hybrid precoding technique for a mmWave orthogonal frequency division multiple access system. We propose an algorithm to jointly optimize the wideband analog beamformers and the per- subcarrier digital precoders. We compare an upper bound with our solution and show that our optimized approach achieves good performance in terms of beamforming gain in the sense that it minimizes the loss caused by allocating multiple users to different subcarriers with a limited number of radio frequency chains.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Park, S. and Heath, R.W.}, year={2015} }
 @inproceedings{mo_heath_2014, title={High SNR capacity of millimeter wave MIMO systems with one-bit quantization}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901218443&partnerID=MN8TOARS}, DOI={10.1109/ITA.2014.6804238}, abstractNote={Millimeter wave (mmWave) is a viable technology for future cellular systems. With bandwidths on the order of a gigahertz, high-resolution analog-to-digital converters (ADCs) become a power consumption bottleneck. One solution is to employ very low resolution one-bit ADCs. This paper analyzes the flat fading multiple-input multiple-output (MIMO) channel with one-bit ADC. Bounds on the high signal-to-noise ratio (SNR) capacity are derived for the single-input multiple-output (SIMO) channel and the general MIMO channel. The results show how the number of paths, number of transmit antennas, and number of receive antennas impact the capacity at high SNR.}, booktitle={2014 Information Theory and Applications Workshop, ITA 2014 - Conference Proceedings}, author={Mo, J. and Heath, R.W.}, year={2014} }
 @inproceedings{li_bai_heath_2014, title={Impact of 3D base station antenna in random heterogeneous cellular networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84912073837&partnerID=MN8TOARS}, DOI={10.1109/WCNC.2014.6952680}, abstractNote={In this paper, we study the influence of 3-dimensional (3D) base station (BS) antenna patterns on the of downlink random heterogeneous cellular networks. To assess the impact of downtilt and vertical beamwidth, we propose a mathematical framework based on stochastic geometry (to model BS locations) and random shape theory (to model building geometry). Using this framework, we study the impact of different antenna parameters at the pico cell on the coverage and area spectral efficiency of the network. Through simulations we find that narrowing the vertical beamwidth of the pico cell must be accompanied by downtilt to result in improved coverage and capacity. We also find that high building density leads to improved network performance, motivating the deployment of heterogeneous networks in dense urban areas.}, booktitle={IEEE Wireless Communications and Networking Conference, WCNC}, author={Li, X. and Bai, T. and Heath, R.W.}, year={2014}, pages={2254–2259} }
 @inproceedings{seifi_heath_coldrey_svensson_2014, title={Joint transmission mode and tilt adaptation in coordinated small-cell networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84906751855&partnerID=MN8TOARS}, DOI={10.1109/ICCW.2014.6881264}, abstractNote={We study downlink transmission in a wireless network consisting of small-sized cells, denoted as a small-cell network (SCN). We consider multi-antenna base stations (BSs) and single-antenna users, BS coordination, 3D antenna patterns, and fair scheduling. We investigate the impact of the elevation angle of the BS antenna pattern, denoted as tilt, on the performance of the considered SCN when employing either a conventional uncoordinated transmission mode or a fully coordinated transmission mode. Using the results of this investigation, we propose a novel hybrid-mode transmission technique that can achieve a performance comparable to that of a fully coordinated transmission but with a significantly lower complexity and signaling requirement. The main idea is to divide the coverage area into two so-called vertical regions and jointly adapt the transmission mode and the tilt at the BSs when serving each region. A fair scheduler is used to share the time-slots between the vertical regions and among the users in each region.}, booktitle={2014 IEEE International Conference on Communications Workshops, ICC 2014}, author={Seifi, N. and Heath, R.W. and Coldrey, M. and Svensson, T.}, year={2014}, pages={598–603} }
 @article{learning-based adaptive transmission for limited feedback multiuser mimo-ofdm_2014, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904320083&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2314104}, abstractNote={Performing link adaptation in a multiantenna and multiuser system is challenging because of the coupling between precoding, user selection, spatial mode selection and use of limited feedback about the channel. The problem is exacerbated by the difficulty of selecting the proper modulation and coding scheme when using orthogonal frequency division multiplexing (OFDM). This paper presents a data-driven approach to link adaptation for multiuser multiple input mulitple output (MIMO) OFDM systems. A machine learning classifier is used to select the modulation and coding scheme, taking as input the SNR values in the different subcarriers and spatial streams. A new approximation is developed to estimate the unknown interuser interference due to the use of limited feedback. This approximation allows to obtain SNR information at the transmitter with a minimum communication overhead. A greedy algorithm is used to perform spatial mode and user selection with affordable complexity, without resorting to an exhaustive search. The proposed adaptation is studied in the context of the IEEE 802.11ac standard, and is shown to schedule users and adjust the transmission parameters to the channel conditions as well as to the rate of the feedback channel.}, number={7}, journal={IEEE Transactions on Wireless Communications}, year={2014}, pages={3806–3820} }
 @article{sun_rappaport_heath_nix_rangan_2014, title={MIMO for millimeter-wave wireless communications: Beamforming, spatial multiplexing, or both?}, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84919497267&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2014.6979962}, abstractNote={The use of mmWave frequencies for wireless communications offers channel bandwidths far greater than previously available, while enabling dozens or even hundreds of antenna elements to be used at the user equipment, base stations, and access points. To date, MIMO techniques, such as spatial multiplexing, beamforming, and diversity, have been widely deployed in lower-frequency systems such as IEEE 802.11n/ac (wireless local area networks) and 3GPP LTE 4G cellphone standards. Given the tiny wavelengths associated with mmWave, coupled with differences in the propagation and antennas used, it is unclear how well spatial multiplexing with multiple streams will be suited to future mmWave mobile communications. This tutorial explores the fundamental issues involved in selecting the best communications approaches for mmWave frequencies, and provides insights, challenges, and appropriate uses of each MIMO technique based on early knowledge of the mmWave propagation environment.}, number={12}, journal={IEEE Communications Magazine}, author={Sun, S. and Rappaport, T.S. and Heath, R.W. and Nix, A. and Rangan, S.}, year={2014}, pages={110–121} }
 @article{mimo precoding and combining solutions for millimeter-wave systems_2014, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84919770810&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2014.6979963}, abstractNote={Millimeter-wave communication is one way to alleviate the spectrum gridlock at lower frequencies while simultaneously providing high-bandwidth communication channels. MmWave makes use of MIMO through large antenna arrays at both the base station and the mobile station to provide sufficient received signal power. This article explains how beamforming and precoding are different in MIMO mmWave systems than in their lower-frequency counterparts, due to different hardware constraints and channel characteristics. Two potential architectures are reviewed: hybrid analog/digital precoding/combining and combining with low-resolution analog- to-digital converters. The potential gains and design challenges for these strategies are discussed, and future research directions are highlighted.}, number={12}, journal={IEEE Communications Magazine}, year={2014}, pages={122–131} }
 @inproceedings{millimeter wave cellular channel models for system evaluation_2014, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84899531031&partnerID=MN8TOARS}, DOI={10.1109/ICCNC.2014.6785327}, abstractNote={The huge amount of (potentially) available spectrum makes millimeter wave (mmWave) a promising candidate for fifth generation cellular networks. Unfortunately, differences in the propagation environment as a function of frequency make it hard to make comparisons between systems operating at mmWave and microwave frequencies. This paper presents a simple channel model for evaluating system level performance in mmWave cellular networks. The model uses insights from measurement results that show mmWave is sensitive to blockages revealing very different path loss characteristics between line-of-sight (LOS) and non-line-of-sight (NLOS) links. The conventional path loss model with a single log-distance path loss function and a shadowing term is replaced with a stochastic path loss model with a distance-dependent LOS probability and two different path loss functions to account for LOS and NLOS links. The proposed model is used to compare microwave and mmWave networks in simulations. It is observed that mmWave networks can provide comparable coverage probability with a dense deployment, leading to much higher data rates thanks to the large bandwidth available in the mmWave spectrum.}, booktitle={2014 International Conference on Computing, Networking and Communications, ICNC 2014}, year={2014}, pages={178–182} }
 @inproceedings{wang_elkashlan_heath_di renzo_wong_2015, title={Millimeter wave power transfer and information transmission}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964904801&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417676}, abstractNote={Compared to the existing lower frequency wireless power transfer, millimeter wave (mmWave) power transfer takes advantage of the high-dimensional multi-antenna and narrow beam transmission. In this paper we introduce wireless power transfer for mmWave cellular networks. Here, we consider users with large energy storage that are recharged by the mmWave base stations prior to uplink information transmission, and analyze the average harvested energy and average achievable rate. Numerical results corroborate our analysis and show that the serving base station plays a dominant role in wireless power transfer, and the contribution of the interference power from the interfering base stations is negligible, even when the interfering base stations are dense. By examining the average achievable rate in the uplink, when increasing the base station density, a transition from a noise-limited regime to an interference-limited regime is observed.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Wang, L. and Elkashlan, M. and Heath, R.W. and Di Renzo, M. and Wong, K.-K.}, year={2015} }
 @article{chen_choi_de veciana_caramanis_heath_bovik_2014, title={Modeling the time-Varying subjective quality of HTTP video streams with rate adaptations}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84898677961&partnerID=MN8TOARS}, DOI={10.1109/TIP.2014.2312613}, abstractNote={Newly developed hypertext transfer protocol (HTTP)-based video streaming technologies enable flexible rate-adaptation under varying channel conditions. Accurately predicting the users' quality of experience (QoE) for rate-adaptive HTTP video streams is thus critical to achieve efficiency. An important aspect of understanding and modeling QoE is predicting the up-to-the-moment subjective quality of a video as it is played, which is difficult due to hysteresis effects and nonlinearities in human behavioral responses. This paper presents a Hammerstein-Wiener model for predicting the time-varying subjective quality (TVSQ) of rate-adaptive videos. To collect data for model parameterization and validation, a database of longer duration videos with time-varying distortions was built and the TVSQs of the videos were measured in a large-scale subjective study. The proposed method is able to reliably predict the TVSQ of rate adaptive videos. Since the Hammerstein-Wiener model has a very simple structure, the proposed method is suitable for online TVSQ prediction in HTTP-based streaming.}, number={5}, journal={IEEE Transactions on Image Processing}, author={Chen, C. and Choi, L.K. and De Veciana, G. and Caramanis, C. and Heath, R.W. and Bovik, A.C.}, year={2014}, pages={2206–2221} }
 @inproceedings{alkhateeb_leus_heath_2015, title={Multi-layer precoding for full-dimensional massive MIMO systems}, volume={2015-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84940528813&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2014.7094563}, abstractNote={Full-dimensional massive multiple-input multiple-output (MIMO) systems boost sum spectral efficiency by offering orders of magnitude increase in multiplexing gains. In time division duplexing systems, however, the reuse of the uplink training pilots among cells results in channel estimation errors, which lead to downlink inter-cell interference, especially for cell-edge users, even with large numbers of antennas. Handling this interference with conventional network MIMO techniques is challenging due to the high channel dimensionality. Further, large antenna precoding and combining implementation is associated with high hardware complexity. In this paper, we propose multi-layer precoding to enable efficient and low complexity full-dimensional MIMO operation. Multi-layer precoding (i) leverages the directional characteristics of large-scale MIMO channels to manage inter-cell interference with low channel requirements, and (ii) allows for an efficient implementation using low-complexity hybrid analog/digital architectures. We present and evaluate a specific multi-layer precoding design for full-dimensional MIMO systems. Simulation results show the potential gains of multi-layer precoding compared with traditional pilot-contaminated massive MIMO setups despite the low channel knowledge requirements and the low-complexity implementation.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Alkhateeb, A. and Leus, G. and Heath, R.W.}, year={2015}, pages={815–819} }
 @inproceedings{mungara_zhang_lozano_heath_2015, title={On the spatial spectral efficiency of ITLinQ}, volume={2015-April}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84940483413&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2014.7094779}, abstractNote={Device-to-device (D2D) communication allows serving local wireless traffic bypassing the system's infrastructure. The interference in D2D can be controlled by carefully allocating users to orthogonal channels. This paper analytically characterizes the spectral efficiency of the ITLinQ channelization technique. The analysis relies on a stochastic geometry formulation, which enables obtaining compact expressions while opening the door to an optimization of ITLinQ's parameters.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Mungara, R.K. and Zhang, X. and Lozano, A. and Heath, R.W.}, year={2015}, pages={1806–1810} }
 @inproceedings{truong_lozano_heath_2014, title={Optimal training in continuous flat-fading massive MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84916918363&partnerID=MN8TOARS}, booktitle={20th European Wireless Conference, EW 2014}, author={Truong, K.T. and Lozano, A. and Heath, R.W.}, year={2014}, pages={219–224} }
 @inproceedings{park_daniels_heath_2015, title={Optimizing the target error rate for link adaptation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964851012&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417770}, abstractNote={Wireless link adaptation configures the physical layer of the transmitter in the presence of dynamic channel conditions to maximize link performance under error rate reliability constraints. Typically, error rate reliability targets are defined externally and remain fixed, regardless of the physical layer configuration. In this paper, we allow link adaptation to define its own error rate reliability target.We derive a closed form expression for the optimal target block error rate (BLER) that maximizes the throughput with hybrid automatic repeat request (HARQ). This expression shows that the optimal BLER target is inversely proportional to the SINR. We conduct LTE simulations with link adaptation that exploits the optimal BLER target to reinforce the utility of our derived BLER target expression and to showcase that a performance improvements of up to 30% in throughput can be obtained through dynamic BLER targets for link adaptation.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Park, S. and Daniels, R.C. and Heath, R.W.}, year={2015} }
 @article{lee_lin_andrews_heath_2015, title={Power control for D2D underlaid cellular networks: Modeling, algorithms, and analysis}, volume={33}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922808929&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2014.2369612}, abstractNote={This paper proposes a random network model for a device-to-device (D2D) underlaid cellular system using stochastic geometry and develops centralized and distributed power control algorithms. The goal of centralized power control is twofold: ensure that the cellular users have sufficient coverage probability by limiting the interference created by underlaid D2D users, while scheduling as many D2D links as possible. For the distributed power control method, the optimal on-off power control strategy is proposed, which maximizes the sum rate of the D2D links. Expressions are derived for the coverage probabilities of cellular, D2D links, and the sum rate of the D2D links in terms of the density of D2D links and the path-loss exponent. The analysis reveals the impact of key system parameters on the network performance. For example, the bottleneck of D2D underlaid cellular networks is the cross-tier interference between D2D links and the cellular user, not the D2D intratier interference when the density of D2D links is sparse. Simulation results verify the exactness of the derived coverage probabilities and the sum rate of D2D links.}, number={1}, journal={IEEE Journal on Selected Areas in Communications}, author={Lee, N. and Lin, X. and Andrews, J.G. and Heath, R.W.}, year={2015}, pages={1–13} }
 @article{chen_zhu_de veciana_bovik_heath_2015, title={Rate adaptation and admission control for video transmission with subjective quality constraints}, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84921790863&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2014.2337277}, abstractNote={Adapting video data rate during streaming can effectively reduce the risk of playback interruptions caused by channel throughput fluctuations. The variations in rate, however, also introduce video quality fluctuations and thus potentially affects viewers' Quality of Experience (QoE). We show how the QoE of video users can be improved by rate adaptation and admission control. We conducted a subjective study wherein we found that viewers' QoE was strongly correlated with the empirical cumulative distribution function (eCDF) of the predicted video quality. Based on this observation, we propose a rate-adaptation algorithm that can incorporate QoE constraints on the empirical cumulative quality distribution per user. We then propose a threshold-based admission control policy to block users whose empirical cumulative quality distribution is not likely to satisfy their QoE constraint. We further devise an online adaptation algorithm to automatically optimize the threshold. Extensive simulation results show that the proposed scheme can reduce network resource consumption by 40% over conventional average-quality maximized rate-adaptation algorithms.}, number={1}, journal={IEEE Journal on Selected Topics in Signal Processing}, author={Chen, C. and Zhu, X. and De Veciana, G. and Bovik, A.C. and Heath, R.W.}, year={2015}, pages={22–36} }
 @inproceedings{wang_elkashlan_duong_heath_2014, title={Secure communication in cellular networks: The benefits of millimeter wave mobile broadband}, volume={2014-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84932640840&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2014.6941328}, abstractNote={This paper proposes millimeter wave (mmWave) mobile broadband for achieving secure communication in downlink cellular network. Analog beamforming with phase shifters is adopted for the mmWave transmission. The secrecy throughput is analyzed based on two different transmission modes, namely delay-tolerant transmission and delay-limited transmission. The impact of large antenna arrays at the mmWave frequencies on the secrecy throughput is examined. Numerical results corroborate our analysis and show that mmWave systems can enable significant secrecy improvement. Moreover, it is indicated that with large antenna arrays, multi-gigabit per second secure link at the mmWave frequencies can be reached in the delay-tolerant transmission mode and the adverse effect of secrecy outage vanishes in the delay-limited transmission mode.}, number={October}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Wang, L. and Elkashlan, M. and Duong, T.Q. and Heath, R.W.}, year={2014}, pages={115–119} }
 @article{heath_laus_quek_talwar_zhou_2014, title={Signal processing for the 5G revolution}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84908236177&partnerID=MN8TOARS}, DOI={10.1109/MSP.2014.2345430}, abstractNote={The articles in this special section provide an overview of recent advances in signal processing for communication with an emphasis on signal processing techniques that will be relevant for 5G cellular systems. It covers a wide range of topics including modulation, beamforming, cross-layer optimization based on different performance metrics, location-aware communication, cloud computing, and cloud radio access networks. The articles provide a diverse perspective on the potential challenges in 5G cellular systems.}, number={6}, journal={IEEE Signal Processing Magazine}, author={Heath, R.W. and Laus, G. and Quek, T.Q.S. and Talwar, S. and Zhou, P.}, year={2014}, pages={12–13} }
 @inproceedings{alkhateeb_el ayach_leus_heath_2014, title={Single-sided adaptive estimation of multi-path millimeter wave channels}, volume={2014-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84932611215&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2014.6941330}, abstractNote={Millimeter wave (mmWave) cellular systems will enable ultra high data rates by communicating over the large bandwidth available in mmWave frequencies. To overcome the channel propagation characteristics in this frequency band, large antenna arrays need to be deployed at both the base station and mobile users. While these large arrays provide sufficient beamforming gains to meet the required link margins, they make it challenging to estimate the mmWave channel. In this paper, we propose a mmWave channel estimation algorithm that exploits the sparse nature of the channel and leverages tools from adaptive compressed sensing to efficiently estimate the channel with a small training overhead. The proposed algorithm considers practical hardware constraints on the training beamforming design, and does not require the availability of a feedback channel between the base station and the mobile user. Simulation results indicate that comparable precoding gains can be achieved by the proposed channel estimation algorithm relative to the case when perfect channel knowledge exists.}, number={October}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Alkhateeb, A. and El Ayach, O. and Leus, G. and Heath, R.W.}, year={2014}, pages={125–129} }
 @article{lee_heath_2015, title={Space-Time Physical-Layer Network Coding}, volume={33}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84924978325&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2014.2384351}, abstractNote={A space-time physical-layer network coding (ST-PNC) method is presented for information exchange among multiple users over fully connected multiway relay networks. The method involves two steps: 1) side-information learning and 2) space-time relay transmission. In the first step, different sets of users are scheduled to send signals over networks, and the remaining users and relays overhear the transmitted signals, thereby learning the interference patterns. In the second step, multiple relays cooperatively send out linear combinations of signals received in the previous phase using space-time precoding so that all users efficiently exploit their side information in the form of 1) what they sent and 2) what they overheard in decoding. This coding concept is illustrated through two simple network examples. It is shown that ST-PNC improves the sum of degrees of freedom (sum-DoF) of the network compared to existing interference management methods. With ST-PNC, the sum-DoF of a general multiway relay network without channel knowledge at the users is characterized in terms of relevant system parameters, chiefly the number of users, the number of relays, and the number of antennas at relays. A major implication of the derived results is that efficiently harnessing both transmitted and overheard signals as side information brings significant performance improvements to fully connected multiway relay networks.}, number={2}, journal={IEEE Journal on Selected Areas in Communications}, author={Lee, N. and Heath, R.W.}, year={2015}, pages={323–336} }
 @inproceedings{lee_heath_2014, title={Space-time physical-layer network coding: Harnessing interference in multi-way communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84924365569&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7037030}, abstractNote={In this paper, a space-time physical-layer network coding method is proposed in fully-connected multi-way relay networks. The proposed method includes two essential steps: side-information learning; and space-time relay transmission. In the phase of side-information learning, sets of users are scheduled to transmit messages over networks and the remaining users and relays overhear the transmitted messages to learn the interference patterns. In the phase of space-time relay transmission, multiple relays cooperatively transmit linear combinations of received messages in the previous phase using space-time precoding so that all users simultaneously harness their side-information in decoding. With this transmission technique, it is shown that the sum of degrees of freedom of the network is improved compared to existing interference management methods. Our key finding is that efficiently exploiting both transmitted and overheard messages as side-information brings substantial performance gains to fully-connected multi-way relay networks.}, booktitle={2014 IEEE Global Communications Conference, GLOBECOM 2014}, author={Lee, N. and Heath, R.W.}, year={2014}, pages={1559–1564} }
 @article{ayach_rajagopal_abu-surra_pi_heath_2014, title={Spatially sparse precoding in millimeter wave MIMO systems}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897915272&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.011714.130846}, abstractNote={Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications and all cellular systems. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.}, number={3}, journal={IEEE Transactions on Wireless Communications}, author={Ayach, O.E. and Rajagopal, S. and Abu-Surra, S. and Pi, Z. and Heath, R.W.}, year={2014}, pages={1499–1513} }
 @article{lee_morales-jimenez_lozano_heath_2015, title={Spectral efficiency of dynamic coordinated beamforming: A stochastic geometry approach}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84921323055&partnerID=MN8TOARS}, DOI={10.1109/TWC.2014.2337305}, abstractNote={This paper characterizes the performance of coordinated beamforming with dynamic clustering. A downlink model based on stochastic geometry is put forth to analyze the performance of such a base station (BS) coordination strategy. Analytical expressions for the complementary cumulative distribution function (CCDF) of the instantaneous signal-to-interference ratio (SIR) are derived in terms of relevant system parameters, chiefly the number of BSs forming the coordination clusters, the number of antennas per BS, and the pathloss exponent. Utilizing this CCDF, with pilot overheads further incorporated into the analysis, we formulate the optimization of the BS coordination clusters for a given fading coherence. Our results indicate that: 1) coordinated beamforming is most beneficial to users that are in the outer part of their cells yet in the inner part of their coordination cluster and that 2) the optimal cluster cardinality for the typical user is small and it scales with the fading coherence. Simulation results verify the exactness of the SIR distributions derived for stochastic geometries, which are further compared with the corresponding distributions for deterministic grid networks.}, number={1}, journal={IEEE Transactions on Wireless Communications}, author={Lee, N. and Morales-Jimenez, D. and Lozano, A. and Heath, R.W.}, year={2015}, pages={230–241} }
 @inproceedings{park_heath_2015, title={Threshold-based antenna selection algorithm for dense cloud radio access networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964887790&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417757}, abstractNote={In this paper, we propose a threshold-based antenna selection algorithm for uplink dense cloud radio access networks (C- RANs), where a baseband processor unit (BBU) cloud is separately placed from densely deployed remote radio heads (RRHs). The proposed algorithm consists of two phases. In the first phase, each RRH determines whether to be included or not in a candidate set by using a predefined selection threshold. In the second phase, a RRH is randomly selected within the candidate set made in the first phase. By modeling the network with a homogeneous Poisson point process (PPP), the signal- to-interference ratio (SIR) complementary cumulative distribu- tion function (CCDF) is derived when applying the proposed algorithm. Exploiting the derived expression, an approximate optimum selection threshold that maximizes the SIR coverage performance is obtained in terms of relevant system parameters, chiefly the SIR target, the pathloss exponent, and the RRH and user densities. Simulations demonstrate the performance of the obtained selection threshold. The key advantage of the proposed algorithm is its simplicity. Since the complexity caused by selecting a RRH is reasonable irrespective of the density of the RRH while guaranteeing the certain performance, the proposed algorithm is easily applied in a C-RAN where the RRHs are densely deployed.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Park, J. and Heath, R.W.}, year={2015} }
 @inproceedings{bai_heath_2015, title={Uplink massive MIMO SIR analysis: how do antennas scale with users?}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964872728&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7417688}, abstractNote={Massive multiple-input multiple-output (MIMO) is a potential physical layer technology for 5G cellular networks. This paper leverages stochastic geometry to derive the uplink signal-to- interference (SIR) distribution in massive MIMO networks. Based on the derived expressions, a scaling law between the number of base station antennas and scheduled users per cell is provided to preserve the uplink SIR distribution, where the impacts of correlation in small-scale fading and power control in the form of fractional path loss compensation are taken account. Numerical results verify the analysis, and show that fractional power control with a compensation fraction of 0.5 is nearly optimal for the average achievable rate in certain cases.}, booktitle={2015 IEEE Global Communications Conference, GLOBECOM 2015}, author={Bai, T. and Heath, R.W.}, year={2015} }
 @article{heath_peters_wang_zhang_2013, title={A current perspective on distributed antenna systems for the downlink of cellular systems}, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876279935&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2013.6495775}, abstractNote={Providing uniformly high capacity in cellular systems is challenging due to fading, path loss, and interference. A partial solution to this problem is the deployment of distributed antenna systems, where transmission points are distributed throughout the cell using coax cable or fiber, instead of being centrally located on a single tower. This article reviews how distributed antenna systems are evolving to provide higher performance on the downlink in cellular systems. Research trends in distributed antennas for the downlink of cellular systems are described along with current progress on their integration into commercial wireless cellular standards. A key observation is that distributed antenna systems are tightly integrated into the cellular architecture, and incorporate physical layer technologies like MIMO communication and multiuser MIMO to provide higher data rates.}, number={4}, journal={IEEE Communications Magazine}, author={Heath, R. and Peters, S. and Wang, Y. and Zhang, J.}, year={2013}, pages={161–167} }
 @article{truong_heath_2013, title={A distributed algorithm using interference pricing for relay interference channels}, volume={2013}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84879567396&partnerID=MN8TOARS}, DOI={10.1186/1687-6180-2013-26}, abstractNote={Relays in cellular systems are sensitive to interference. A good relay design will transmit in a way that avoids excess out-of-cell interference. This article proposes a two-step algorithm for relay design for the relay interference channel, which models a relay-based cellular system. The transmitters and relays are equipped with multiple antennas while the receivers are equipped with a single antenna. In the first step, we propose to apply existing single-hop strategies to design the transmission parameters of the transmitters. In the second step, we propose to modify the interference pricing approach to design the relays. Interference pricing is used to provide the relays with information on how interference impacts the end-to-end achievable rates. A new method is proposed to compute interference prices via an approximation of the end-to-end achievable rate to integrate information about the relationship of the parameters in the two hops to alleviate mismatch between the rates on two hops experienced by the direct application of prior algorithms, which are designed specifically for the single-hop interference channel. Simulations show that the proposed algorithm outperforms the other designs, including the naive approach of applying the single-hop interference pricing strategies on two hops.}, number={1}, journal={Eurasip Journal on Advances in Signal Processing}, author={Truong, K.T. and Heath, R.W.}, year={2013} }
 @inproceedings{a dynamic system model of time-varying subjective quality of video streams over http_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890462158&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2013.6638329}, abstractNote={Newly developed HTTP-based video streaming technology enables flexible rate-adaptation in varying channel conditions. The users' Quality of Experience (QoE) of rate-adaptive HTTP video streams, however, is not well understood. Therefore, designing QoE-optimized rate-adaptive video streaming algorithms remains a challenging task. An important aspect of understanding and modeling QoE is to be able to predict the up-to-the-moment subjective quality of video as it is played. We propose a dynamic system model to predict the time-varying subjective quality (TVSQ) of rate-adaptive videos that is transported over HTTP. For this purpose, we built a video database and measured TVSQ via a subjective study. A dynamic system model is developed using the database and the measured human data. We show that the proposed model can effectively predict the TVSQ of rate-adaptive videos in an online manner, which is necessary to be able to conduct QoE-optimized online rate-adaptation for HTTP-based video streaming.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2013}, pages={3602–3606} }
 @article{chen_heath_bovik_de veciana_2013, title={A markov decision model for adaptive scheduling of stored scalable videos}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84878813576&partnerID=MN8TOARS}, DOI={10.1109/TCSVT.2013.2254896}, abstractNote={We propose two scheduling algorithms that seek to optimize the quality of scalably coded videos that have been stored at a video server before transmission. The first scheduling algorithm is derived from a Markov decision process (MDP) formulation developed here. We model the dynamics of the channel as a Markov chain and reduce the problem of dynamic video scheduling to a tractable Markov decision problem over a finite-state space. Based on the MDP formulation, a near-optimal scheduling policy is computed that minimizes the mean square error. Using insights taken from the development of the optimal MDP-based scheduling policy, the second proposed scheduling algorithm is an online scheduling method that only requires easily measurable knowledge of the channel dynamics, and is thus viable in practice. Simulation results show that the performance of both scheduling algorithms is close to a performance upper bound also derived in this paper.}, number={6}, journal={IEEE Transactions on Circuits and Systems for Video Technology}, author={Chen, C. and Heath, R.W. and Bovik, A.C. and De Veciana, G.}, year={2013}, pages={1081–1095} }
 @article{schwarz_heath_rupp_2013, title={Adaptive quantization on a grassmann-manifold for limited feedback beamforming systems}, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84883013264&partnerID=MN8TOARS}, DOI={10.1109/TSP.2013.2270466}, abstractNote={In this paper we examine delay limited adaptive quantization on the Grassmann manifold of 1-dimensional subspaces in n-dimensional space. Due to strict delay limits, vector quantization over multiple time instants cannot be applied to exploit the temporal correlation of the source signal. Instead, a vector predictive quantizer is proposed that combines prediction and differential quantization algorithms to achieve an efficient quantization of the correlated Grassmannian source. The proposed predictor is based on adaptive finite impulse response filters to adjust to the temporal statistics of the source signal. It is shown that the prediction error in the tangent space associated with the Grassmann manifold behaves approximately Gaussian, provided its norm is sufficiently small. The proposed quantization algorithm is applied to channel state information quantization in multi-user beamforming wireless communication systems. Large throughput gains are demonstrated in comparison to memoryless quantization, due to reduced multi-user interference.}, number={18}, journal={IEEE Transactions on Signal Processing}, author={Schwarz, S. and Heath, R.W. and Rupp, M.}, year={2013}, pages={4450–4462} }
 @inproceedings{schwarz_heath_rupp_2013, title={Adaptive quantization on the Grassmann-manifold for limited feedback multi-user MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890520684&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2013.6638617}, abstractNote={We propose an adaptive quantization algorithm for subspace tracking on the Grassmann-manifold of p-dimensional subspaces in the n-dimensional Euclidean space. This quantization problem arises naturally in limited feedback based wireless communication systems, which apply precoding for interference cancellation and alignment. The proposed algorithm exploits the differential geometry associated with the Grassmann-manifold for efficient differential and predictive quantization. The algorithm is applied to channel state information quantization in a multi-user block-diagonalization based wireless communication system, demonstrating large throughput gains compared to memoryless quantization.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Schwarz, S. and Heath, R.W. and Rupp, M.}, year={2013}, pages={5021–5025} }
 @inproceedings{valliappan_heath_lozano_2013, title={Antenna Subset Modulation for secure millimeter-wave wireless communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84902985644&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2013.6825166}, abstractNote={The small carrier wavelength at millimeter-wave (mm-Wave) frequencies enables featuring a large number of co-located antennas. This paper exploits the potential of large antenna arrays to develop a low-complexity directional modulation technique, Antenna Subset Modulation (ASM), for point-to-point secure wireless communication. The main idea in ASM is to modulate the radiation pattern at the symbol rate by driving only a subset of antennas in the array. This results in a directional radiation pattern that projects a sharply defined constellation in the desired direction and expanded further randomized constellation in other directions. Two techniques for implementing ASM are proposed. The first technique selects an antenna subset randomly for every symbol. While randomly switching antenna subsets does not affect the symbol modulation for a desired receiver along the main direction, it effectively randomizes the amplitude and phase of the received symbol for an eavesdropper along a sidelobe. Using a simplified statistical model, an expression for the average uncoded symbol error rate (SER) is derived as a function of the observation angle. To overcome the problem of large sidelobes in random antenna subset switching, the second technique uses an optimized antenna subset selection procedure based on simulated annealing to achieve superior performance compared with random selection. Numerical comparisons of the SER performance and secrecy capacity of the proposed techniques against those of conventional array transmission are presented to highlight the potential of ASM.}, booktitle={2013 IEEE Globecom Workshops, GC Wkshps 2013}, author={Valliappan, N. and Heath, R.W. and Lozano, A.}, year={2013}, pages={1258–1263} }
 @article{antenna subset modulation for secure millimeter-wave wireless communication_2013, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84883296165&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2013.061013.120459}, abstractNote={This paper exploits the potential of large antenna arrays at millimeter-wave (mm-Wave) frequencies to develop a low-complexity directional modulation technique, Antenna Subset Modulation (ASM), for point-to-point secure wireless communication. The main idea in ASM is to modulate the radiation pattern at the symbol rate by driving only a subset of antennas in the array. This results in a directional radiation pattern that projects a sharply defined constellation in the desired direction and randomizes the constellation in other directions. To implement ASM, a simple antenna selection technique that selects an antenna subset randomly for every symbol is proposed. While randomly switching antenna subsets does not affect the symbol modulation for a desired receiver along the main direction, it effectively randomizes the amplitude and phase of the received symbol for an eavesdropper along a sidelobe. Statistical analysis and numerical examples are presented to highlight the superior performance and secrecy capacity achieved by ASM.}, number={8}, journal={IEEE Transactions on Communications}, year={2013}, pages={3231–3245} }
 @inproceedings{lee_heath_morales-jimenez_lozano_2013, title={Base station cooperation with dynamic clustering in super-dense cloud-RAN}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84902986739&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2013.6825084}, abstractNote={In this paper, a downlink cellular model based on stochastic geometry is put forth in order to analyze the performance of base station (BS) cooperation with dynamic clustering in highly dense BS deployments. Utilizing the proposed model, a closed-form expression for the complementary cumulative distribution function of the instantaneous signal-to-interference ratio (SIR) is derived in terms of relevant system parameters, chiefly the number of cooperating base stations and the path-loss exponent. From the derived SIR distribution, some of the benefits of cooperation can be characterized. Through simulation, the derived SIR distribution is verified and further compared with the corresponding distribution for a deterministic grid model.}, booktitle={2013 IEEE Globecom Workshops, GC Wkshps 2013}, author={Lee, N. and Heath, R.W. and Morales-Jimenez, D. and Lozano, A.}, year={2013}, pages={784–788} }
 @article{cascaded orthogonal space-time block codes for wireless multi-hop relay networks_2013, volume={2013}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84878055233&partnerID=MN8TOARS}, DOI={10.1186/1687-1499-2013-113}, abstractNote={Distributed space–time block coding is a diversity technique to mitigate the effects of fading in multi-hop wireless networks, where multiple relay stages are used by a source to communicate with its destination. This article proposes a new distributed space–time block code called the cascaded orthogonal space–time block code (COSTBC) for the case where the source and destination are equipped with multiple antennas and each relay stage has one or more multiple antenna relays. Each relay stage is assumed to have receive channel state information (CSI) for all the channels from the source and all relays from previous stages to itself, while the destination is assumed to have receive CSI for all the channels. To construct the COSTBC, multiple orthogonal space–time block codes (OSTBCs) are used in cascade by the source and each relay stages. In the COSTBC, each relay stage separates the constellation symbols of the OSTBC sent by the preceding relay stage using its CSI, and then transmits another OSTBC to the next relay stage. COSTBCs are shown to achieve the maximum diversity gain in a multi-hop wireless network with linear decoding complexity thanks to the connection to OSTBCs. Several explicit constructions of COSTBCs are also provided, and their performance is simulated in different relay configurations.}, number={1}, journal={Eurasip Journal on Wireless Communications and Networking}, year={2013} }
 @article{cooperative algorithms for mimo amplify-and-forward relay networks_2013, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873922560&partnerID=MN8TOARS}, DOI={10.1109/TSP.2012.2236832}, abstractNote={Interference is a common impairment in wireless communication systems. Multi-hop relay networks use a set of intermediate nodes called relays to facilitate communication between multiple transmitters and multiple receivers through multiple hops. Relay based communication is especially sensitive to interference because the interference impacts both the received signal at the relay, and the received signal at the destination. Interference alignment is a signaling technique that provides high multiplexing gain in the interference channel. In this paper, inspired by an algorithmic approach for interference alignment, three cooperative algorithms are proposed to find suboptimal solutions for end-to-end sum-rate maximization problem in a multiple-antenna amplify-and-forward (AF) relay interference channel. The first algorithm aims at minimizing the sum power of enhanced noise from the relays and interference at the receivers. The second and third algorithms aim at minimizing matrix-weighted sum mean square errors with either equality or inequality power constraints to utilize a connection between mean square error and mutual information. The resulting iterative algorithms are convergent to points that we conjecture to be stationary points of the corresponding problems. Simulations show that the proposed algorithms achieve higher end-to-end sum-rates and multiplexing gains that existing strategies for AF relays, decode-and-forward relays, and direct transmission. The first algorithm outperforms the other algorithms at high signal-to-noise ratio (SNR) but performs worse than them at low SNR. Thanks to power control, the third algorithm outperforms the second algorithm at the cost of additional overhead.}, number={5}, journal={IEEE Transactions on Signal Processing}, year={2013}, pages={1272–1287} }
 @inproceedings{coverage analysis for millimeter wave cellular networks with blockage effects_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897678368&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2013.6736994}, abstractNote={Millimeter wave (mmWave) is promising for the fifth generation cellular systems. The sensitivity of mmWave signals to blockages, such as buildings in cities, however, makes the network performance hard to predict. Using concepts from stochastic geometry and random shape theory, this paper proposes an analytical framework to incorporate the blockage effects and evaluate the performance of mmWave cellular networks, in terms of coverage probability and achievable rate. Leveraging prior work on a blockage model, a stochastic characterization of the regions covered by line-of-sight (LOS) and non-LOS links is obtained, which allows different path loss laws to be applied to the LOS and non-LOS links, respectively. Based on the proposed framework, analytical expressions for the mmWave downlink coverage probability are derived, and then the network performance is examined. Numerical results show that millimeter wave (mmWave) networks can provide comparable coverage probability and much higher data rates than microwave networks.}, booktitle={2013 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2013 - Proceedings}, year={2013}, pages={727–730} }
 @inproceedings{wu_mckay_heath_2013, title={Coverage and area spectral efficiency in downlink random cellular networks with channel estimation error}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890544304&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2013.6638492}, abstractNote={We investigate the impact of channel estimation on the performance of downlink random cellular networks. First, we derive a new closed-form expression for the coverage probability under certain practical conditions. We show that the coverage probability is dependent on the user and base station (BS) densities solely through their ratio for arbitrary pilot-training length. Next, we derive the optimal pilot-training length that maximizes the area spectral efficiency (ASE) in several asymptotic regimes, and capture the dependence of this optimal length on the ratio between the user and BS densities. The ASE loss due to training is shown to be less significant in small cell networks with a larger base station density.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Wu, Y. and McKay, M.R. and Heath, R.W.}, year={2013}, pages={4404–4408} }
 @inproceedings{coverage in dense millimeter wave cellular networks_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901248233&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2013.6810669}, abstractNote={The millimeter wave (mmWave) spectrum holds promise for future cellular systems. MmWave cellular deployments are likely to be dense to achieve good coverage. This paper proposes a simplified approach to evaluate the coverage probability in dense mmWave networks. By simplifying a framework that incorporates blockages, analytical expressions are derived to compute the coverage probability. The results show that the average cell size should scale with the average size of the line-of-sight region to maintain the same coverage probability in a dense network, and that the optimum base station density should be dense enough but not too dense.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2013}, pages={2062–2066} }
 @article{lee_heath_2013, title={Degrees of freedom for the two-cell two-hop MIMO interference channel: Interference-free relay transmission and spectrally efficient relaying protocol}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876771480&partnerID=MN8TOARS}, DOI={10.1109/TIT.2013.2237753}, abstractNote={This paper considers the two-cell two-hop multiple-input-multiple-output (MIMO) interference channel, where two source groups consisting of multiple users with a single antenna wish to communicate with two multiantenna destinations by sharing two multiantenna relays. For such a channel, an inner bound on the degrees of freedom is derived for different channel knowledge assumptions and relay operations. Assuming global channel knowledge at the relays and full-duplex relay operation, it is shown that two cascaded interfering links can be decomposed into two independent parallel relay channels while sharing the spectrum. The key to showing this result is a novel amplify-and-forward interference-free relay transmission method, which performs interference-shaping during reception and interference neutralization during transmission. Assuming that the relays have global channel knowledge only for the first hop, a spectrally efficient relaying protocol is proposed that overcomes the loss due to half-duplex relaying constraint. The proposed protocol improves performance compared to a trivial time-division-multiple access method for the two-cell two-hop MIMO interference channel.}, number={5}, journal={IEEE Transactions on Information Theory}, author={Lee, N. and Heath, R.W.}, year={2013}, pages={2882–2896} }
 @inproceedings{lee_heath_2013, title={Degrees of freedom of completely-connected multi-way interference networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890357702&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2013.6620491}, abstractNote={This paper considers a fully-connected interference network with a relay in which multiple users equipped with a single antenna want to exchange multiple unicast messages with other users in the network by sharing the relay equipped with multiple antennas. For such a network, the optimal degrees of freedom (DoF) are derived by providing both converse and achievability. Further, considering single-antenna relays in the three-user fully-connected interference network, it is shown that three distributed relays with a single antenna is sufficient to achieve the optimal DoF. A major implication of the derived DoF results is that a relay with multiple antennas or distributed relays employing a single antenna increases the capacity scaling law of the multi-user interference network when multiple directional information flows are considered, even if the networks are fully-connected and all nodes operate in half-duplex. These results verify the intuition that the relay is useful in increasing DoF for the multi-way the interference network.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Lee, N. and Heath, R.W.}, year={2013}, pages={1571–1575} }
 @article{truong_heath_2013, title={Effects of channel aging in massive MIMO systems}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84885409411&partnerID=MN8TOARS}, DOI={10.1109/JCN.2013.000065}, abstractNote={Multiple-input multiple-output (MIMO) communication may provide high spectral efficiency through the deployment of a very large number of antenna elements at the base stations. The gains from massive MIMO communication come from the use of multiuser MIMO on the uplink and downlink, but with a large excess of antennas at the base station compared to the number of served users. Initial work on massive MIMO did not fully address several practical issues associated with its deployment. This paper considers the impact of channel aging on the performance of massive MIMO systems. The effects of channel variation are characterized as a function of different system parameters assuming a simple model for the channel time variations at the transmitter. Channel prediction is proposed to overcome channel aging effects. The analytical results on aging show how capacity is lost due to time variation in the channel. Numerical results in a multiceli network show that massive MIMO works even with some channel variation and that channel prediction could partially overcome channel aging effects.}, number={4}, journal={Journal of Communications and Networks}, author={Truong, K.T. and Heath, R.W.}, year={2013}, pages={338–351} }
 @article{lozano_heath_andrews_2013, title={Fundamental limits of cooperation}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84880148105&partnerID=MN8TOARS}, DOI={10.1109/TIT.2013.2253153}, abstractNote={Cooperation is viewed as a key ingredient for interference management in wireless networks. This paper shows that cooperation has fundamental limitations. First, it is established that in systems that rely on pilot-assisted channel estimation, the spectral efficiency is upper-bounded by a quantity that does not depend on the transmit powers; in this framework, cooperation is possible only within clusters of limited size, which are subject to out-of-cluster interference whose power scales with that of the in-cluster signals. Second, an upper bound is also shown to exist if the cooperation extends to an entire (large) system operating as a single cluster; here, pilot-assisted transmission is necessarily transcended. Altogether, it is concluded that cooperation cannot in general change an interference-limited network to a noise-limited one. Consequently, the existing literature that routinely assumes that the high-power spectral efficiency scales with the log-scale transmit power provides only a partial characterization. The complete characterization proposed in this paper subdivides the high-power regime into a degree-of-freedom regime, where the scaling with the log-scale transmit power holds approximately, and a saturation regime, where the spectral efficiency hits a ceiling that is independent of the power. Using a cellular system as an example, it is demonstrated that the spectral efficiency saturates at power levels of operational relevance.}, number={9}, journal={IEEE Transactions on Information Theory}, author={Lozano, A. and Heath, R.W. and Andrews, J.G.}, year={2013}, pages={5213–5226} }
 @inproceedings{general chairs' welcome_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897674585&partnerID=MN8TOARS}, DOI={10.1109/GlobalSIP.2013.6736790}, abstractNote={On behalf of the IEEE Global Conference on Signal and Information Processing (GlobalSIP) Organizing Committee, we would like to cordially welcome you to Austin, Texas. Austin is known as the Live Music Capital of the World. Indeed, Austin offers many opportunities to experience music just a stones throw away from the convention center, but it also has much to offer residents and visitors. Austin is also the capital of Texas and is home to the University of Texas at Austin, leading technology companies, and a number of emerging startups in diverse areas from signal processing, communications, semiconductors, and gaming. Several annual events are held in Austin including the Formula 1 US Grand Prix, the Austin City Limits Music Festival (ACL), and the South-by-Southwest music, film, and interactive conference and festival (SXSW also called “Southy-By”).}, booktitle={2013 IEEE Global Conference on Signal and Information Processing, GlobalSIP 2013 - Proceedings}, year={2013} }
 @inproceedings{hf mimo nvis measurements with co-located dipoles for future tactical communications_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897699725&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2013.210}, abstractNote={Multiple antennas in transceivers can increase system spectral efficiency, reduce transmit power, enable robustness to interference, and increase overall reliability through multiple-input multiple-output processing (MIMO). Consequently, high frequency (HF) networks, which feature extreme spectrum scarcity and unreliability, are prime for MIMO exploitation. Unfortunately, the desired antenna spacing for MIMO is proportional to the wavelength (tens of meters at HF). One promising approach is to utilize two antennas in a single antenna footprint through cross-polarization. This configuration has not yet been fully validated for MIMO at HF. In this paper, we demonstrate MIMO capabilities in near vertical incidence sky-wave (NVIS) propagation through a measurement campaign. This paper shows that MIMO is a game changer for HF NVIS with up to 2.27 times higher data rate, up to 9 times less transmit power, and more than 3 times fewer link failures. This paper also provides critical channel metrics for baseband designers of future MIMO HF protocols as demonstrated in our companion paper.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2013}, pages={1250–1255} }
 @inproceedings{ayach_heath_2013, title={Interference alignment - Recent results and future directions}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876778132&partnerID=MN8TOARS}, DOI={10.1109/RWS.2013.6486689}, abstractNote={Interference alignment (IA) is a cooperative transmission strategy that can mitigate the effects of interference in distributed wireless networks. As its name suggests, the idea of alignment is to structure interference such that multiple interfering signals overlap at the receiver. The alignment property enables efficient interference cancellation. This article provides a brief review of the concept of interference alignment, with a focus on linear precoding-based IA strategies for the multiple-input multiple-output (MIMO) interference channel, and surveys some of the authors' recent results on the subject, with a special focus on making IA more practical. The key concepts discussed range from algorithmic improvements to early IA solutions, to managing the overhead associated with user cooperation. The article highlights some active areas, and future directions, in IA research.}, booktitle={IEEE Radio and Wireless Symposium, RWS}, author={Ayach, O.E. and Heath, R.W.}, year={2013}, pages={205–207} }
 @article{akoum_heath_2013, title={Interference coordination: Random clustering and adaptive limited feedback}, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897583701&partnerID=MN8TOARS}, DOI={10.1109/TSP.2013.2238933}, abstractNote={Interference coordination improves data rates and reduces outages in cellular networks. Accurately evaluating the gains of coordination, however, is contingent upon using a network topology that models realistic cellular deployments. In this paper, we model the base stations locations as a Poisson point process to provide a better analytical assessment of the performance of coordination. Since interference coordination is only feasible within clusters of limited size, we consider a random clustering process where cluster stations are located according to a random point process and groups of base stations associated with the same cluster coordinate. We assume channel knowledge is exchanged among coordinating base stations, and we analyze the performance of interference coordination when channel knowledge at the transmitters is either perfect or acquired through limited feedback. We apply intercell interference nulling (ICIN) to coordinate interference inside the clusters. The feasibility of ICIN depends on the number of antennas at the base stations. Using tools from stochastic geometry, we derive the probability of coverage and the average rate for a typical mobile user. We show that the average cluster size can be optimized as a function of the number of antennas to maximize the gains of ICIN. To minimize the mean loss in rate due to limited feedback, we propose an adaptive feedback allocation strategy at the mobile users. We show that adapting the bit allocation as a function of the signals' strength increases the achievable rate with limited feedback, compared to equal bit partitioning. Finally, we illustrate how this analysis can help solve network design problems such as identifying regions where coordination provides gains based on average cluster size, number of antennas, and number of feedback bits.}, number={7}, journal={IEEE Transactions on Signal Processing}, author={Akoum, S. and Heath, R.W.}, year={2013}, pages={1822–1834} }
 @inproceedings{learning based link adaptation in multiuser mimo-ofdm_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901343679&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, year={2013} }
 @inproceedings{link adaptation in mimo-ofdm with practical impairments_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901272256&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2013.6810579}, abstractNote={Performing link adaptation in multiple-input-multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems is challenging due to the problem of mapping the channel state information to a frame error rate (FER) value. This difficulty comes from the spatial and frequential selectivity of the channel, which makes the different symbols in a codeword to observe different signal to noise ratio values. Moreover, practical impairments like non-Gaussian noise, different frame lengths, or channel nonlinearities can severely affect the adaptation procedure in real scenarios. In this paper we study different FER prediction techniques, which can be classified in parametric, non-parametric and semi-parametric. We evaluate the performance of the FER predictors under practical impairments, and compare the achieved throughput when used in conjunction with link adaptation algorithms.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2013}, pages={1649–1653} }
 @article{bai_heath_2013, title={Location-specific coverage in heterogeneous networks}, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84880807471&partnerID=MN8TOARS}, DOI={10.1109/LSP.2013.2272400}, abstractNote={Cellular networks are taking on a more heterogeneous character with the introduction of low power nodes. Besides the aggregate performance over the entire network, it is also of interest to evaluate the coverage probability conditioning on a specific user location in a cell. This letter proposes a method to compute the location-specific coverage probability inside the inscribing ball of a weighted Voronoi cell in heterogeneous networks. Simulations show that the proposed closed-form approximation accurately characterizes the coverage probability at locations of interest inside the inscribing ball. Moreover the expression can also be applied to evaluate the performance at locations outside the inscribing ball with minor loss.}, number={9}, journal={IEEE Signal Processing Letters}, author={Bai, T. and Heath, R.W.}, year={2013}, pages={873–876} }
 @article{nosrat-makouei_ganti_andrews_heath_2013, title={MIMO interference alignment in random access networks}, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84891739623&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2013.111213.120518}, abstractNote={In this paper, we analyze a multiple-input multiple-output (MIMO) interference channel where nodes are randomly distributed on a plane as a spatial Poisson cluster point process. A Poisson cluster point process consists of clusters with fixed number of points randomly distributed as with the cluster centers distributed randomly on the plane. The nodes in each cluster use interference alignment (IA) to suppress intra-cluster interference but unlike most work on IA, we do not neglect inter-cluster interference. We also connect the accuracy of channel state information to the distance between the nodes, i.e., the quality of CSI degrades with increasing distance. Accounting for the training and feedback overhead, we derive the transmission capacity of this MIMO IA ad hoc network and then compare it to open-loop (interference-blind) spatial multiplexing. Finally, we present exemplary system setups where spatial multiplexing outperforms IA due to the imperfect channel state information or the non-aligned inter-cluster interference.}, number={12}, journal={IEEE Transactions on Communications}, author={Nosrat-Makouei, B. and Ganti, R.K. and Andrews, J.G. and Heath, R.W.}, year={2013}, pages={5042–5055} }
 @article{modeling heterogeneous network interference using poisson point processes_2013, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84880831887&partnerID=MN8TOARS}, DOI={10.1109/TSP.2013.2262679}, abstractNote={Cellular systems are becoming more heterogeneous with the introduction of low power nodes including femtocells, relays, and distributed antennas. Unfortunately, the resulting interference environment is also becoming more complicated, making evaluation of different communication strategies challenging in both analysis and simulation. Leveraging recent applications of stochastic geometry to analyze cellular systems, this paper proposes to analyze downlink performance in a fixed-size cell, which is inscribed within a weighted Voronoi cell in a Poisson field of interferers. A nearest out-of-cell interferer, out-of-cell interferers outside a guard region, and cross-tier interferers are included in the interference calculations. Bounding the interference power as a function of distance from the cell center, the total interference is characterized through its Laplace transform. An equivalent marked process is proposed for the out-of-cell interference under additional assumptions. To facilitate simplified calculations, the interference distribution is approximated using the Gamma distribution with second order moment matching. The Gamma approximation simplifies calculation of the success probability and average rate, incorporates small-scale and large-scale fading, and works with co-tier and cross-tier interference. Simulations show that the proposed model provides a flexible way to characterize outage probability and rate as a function of the distance to the cell edge.}, number={16}, journal={IEEE Transactions on Signal Processing}, year={2013}, pages={4114–4126} }
 @inproceedings{multi-user real-time wireless video with perceptual constraints_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901269861&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2013.6810480}, abstractNote={The current military and civil ground based surveillance and reconnaissance is characterized by a large number of sensor assets, feeding resource constrained analysis nodes, and constrained by extremely limited transport bandwidth. The complexity of such systems is captured by the Networks of Systems (NoS) concept which use tools from wireless engineering and control theory. In this paper, the role of perceptual quality constraints on video-based NoS is investigated. An algorithm to determine the feasible set of nodes and optimal packet schedule via a Markov decision process, given their perceptual constraints and channel qualities is proposed. The H.264/SVC codec is used for compression, while the visual information fidelity pixel-domain (VIFP) full-reference image quality assessment model is used to measure perceptual quality. The algorithm is implemented in custom C;; software with over-the-air transmissions using 802.11 WLAN hardware. Real-time encoding is done for over-the-air transmission of the video. The results indicate that gains of 10-30% can be realized for a three node system.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2013}, pages={1183–1187} }
 @article{hwang_chae_lee_heath_2013, title={Multicell cooperative systems with multiple receive antennas}, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84874992843&partnerID=MN8TOARS}, DOI={10.1109/MWC.2013.6472199}, abstractNote={Multicell cooperation may play an important role in achieving high performance in cellular systems. Multicell cooperation with a single receive antenna at the mobile station has been widely investigated. Applying cooperation with multiple receive antennas, allowed in several emerging wireless standards, has met with some challenges. This is primarily because the transmit precoding/beamforming vector and receive processing have to be jointly optimized in multiple cells to combat out-of-cell interference. In this article, we first discuss the role of the receive antennas in a multicell environment, and then review recently proposed multicell cooperative algorithms and receive antenna techniques for different interference statistics. Finally, we highlight recent work on the fundamental limits of cooperation and the possibility of overcoming such limits by using multiple receive antennas in multicell cooperative networks.}, number={1}, journal={IEEE Wireless Communications}, author={Hwang, I. and Chae, C.-B. and Lee, J. and Heath, R.}, year={2013}, pages={50–58} }
 @inproceedings{el ayach_heath_rajagopal_pi_2013, title={Multimode precoding in millimeter wave MIMO transmitters with multiple antenna sub-arrays}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904102799&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2013.6831611}, abstractNote={Millimeter wave (mmWave) systems must use beamforming to overcome the heavy attenuation at mmWave frequencies and establish high-quality communication links with reasonably high spectral efficiency. When received signal power is sufficiently large and the propagation channel is sufficiently rich, beamforming with multiple data streams, known as precoding, could further increase data rates in mmWave systems. The high cost of digital devices in mmWave systems, however, implies that precoding is predominantly done in the analog domain, making mmWave precoding significantly more constrained than traditional multiple-input multiple-output (MIMO) solutions. In this paper, we propose an iterative precoding algorithm for a practical mmWave transmitter architecture in which all precoding is done in the analog domain. In addition to precoding, the proposed algorithm allows the mmWave system to adapt the rank of its transmission in response to varying propagation conditions. We present numerical results showing that the proposed multimode precoding algorithm allows systems to achieve large data rates, in some cases approaching channel capacity.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, author={El Ayach, O. and Heath, R.W. and Rajagopal, S. and Pi, Z.}, year={2013}, pages={3476–3480} }
 @inproceedings{precision limits of low-energy gnss receivers_2013, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84898958688&partnerID=MN8TOARS}, booktitle={26th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GNSS 2013}, year={2013}, pages={2828–2834} }
 @article{bhagavatula_heath_2013, title={Predictive vector quantization for multicell cooperation with delayed limited feedback}, volume={12}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84880145400&partnerID=MN8TOARS}, DOI={10.1109/TWC.2013.040413.112037}, abstractNote={Base station cooperation can use knowledge of the users' channel state information (CSI) at the transmitters to manage co-channel interference. A reasonable way to provide CSI to the base stations is through a finite rate limited feedback channel. Existing multicell limited feedback techniques require a large amount of feedback, which incurs an overhead penalty on the uplink. In this paper, a new feedback approach based on predictive vector quantization (PVQ) is proposed to reduce feedback requirements in multicell systems and provide high resolution CSI at base stations by exploiting temporal correlation in the channels. Transmitter and receiver structures are proposed to implement predictive limited feedback accounting for delay, for signals on the Grassmann manifold. Simulations show that the proposed PVQ framework yields higher sum-rates than memoryless quantization approaches for multicell limited feedback, in a cooperative system using intercell interference nulling.}, number={6}, journal={IEEE Transactions on Wireless Communications}, author={Bhagavatula, R. and Heath, R.W.}, year={2013}, pages={2588–2597} }
 @article{single-user mimo versus multi-user mimo in distributed antenna systems with limited feedback_2013, volume={2013}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84886693758&partnerID=MN8TOARS}, DOI={10.1186/1687-6180-2013-54}, abstractNote={This article investigates the performance of cellular networks employing distributed antennas in addition to the central antennas of the base station. Distributed antennas are likely to be implemented using remote radio units, which is enabled by a low latency and high bandwidth dedicated link to the base station. This facilitates coherent transmission from potentially all available antennas at the same time. Such distributed antenna system (DAS) is an effective way to deal with path loss and large-scale fading in cellular systems. DAS can apply precoding across multiple transmission points to implement single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) transmission. The throughput performance of various SU-MIMO and MU-MIMO transmission strategies is investigated in this article, employing a Long-Term evolution (LTE) standard compliant simulation framework. The previously theoretically established cell-capacity improvement of MU-MIMO in comparison to SU-MIMO in DASs is confirmed under the practical constraints imposed by the LTE standard, even under the assumption of imperfect channel state information (CSI) at the base station. Because practical systems will use quantized feedback, the performance of different CSI feedback algorithms for DASs is investigated. It is shown that significant gains in the CSI quantization accuracy and in the throughput of especially MU-MIMO systems can be achieved with relatively simple quantization codebook constructions that exploit the available temporal correlation and channel gain differences.}, number={1}, journal={Eurasip Journal on Advances in Signal Processing}, year={2013} }
 @article{lee_heath_2014, title={Space-time interference alignment and degree-of-freedom regions for the MISO broadcast channel with periodic CSI feedback}, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84891541346&partnerID=MN8TOARS}, DOI={10.1109/TIT.2013.2285216}, abstractNote={This paper characterizes the degree-of-freedom (DoF) regions for the multiuser vector broadcast channel with periodic channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-time interference alignment is proposed, which exploits both the current and past CSI jointly. Using the proposed alignment technique, an inner bound of the sum-DoF region is characterized as a function of a normalized CSI feedback frequency, which measures CSI feedback speed compared to the speed of user's channel variations. One consequence of the result is that the achievable sum-DoF gain is improved significantly when a user sends back both current and outdated CSI compared to the case where the user sends back current CSI only. Then, a tradeoff between CSI feedback delay and the sum-DoF gain is characterized for the multiuser vector broadcast channel in terms of a normalized CSI feedback delay that measures CSI obsoleteness compared to channel coherence time. A crucial insight is that it is possible to achieve the optimal DoF gain if the feedback delay is less than a derived fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligible.}, number={1}, journal={IEEE Transactions on Information Theory}, author={Lee, N. and Heath, R.W.}, year={2014}, pages={515–528} }
 @article{el ayach_peters_heath_2013, title={The practical challenges of interference alignment}, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84874978120&partnerID=MN8TOARS}, DOI={10.1109/MWC.2013.6472197}, abstractNote={Interference alignment is a revolutionary wireless transmission strategy that reduces the impact of interference. The idea of interference alignment is to coordinate multiple transmitters so that their mutual interference aligns at the receivers, facilitating simple interference cancellation techniques. Since interference alignment's inception, researchers have investigated its performance and proposed several improvements. Research efforts have been primarily focused on verifying interference alignment's ability to achieve the maximum degrees of freedom (an approximation of sum capacity), developing algorithms for determining alignment solutions, and designing transmission strategies that relax the need for perfect alignment but yield better performance. This article provides an overview of the concept of interference alignment as well as an assessment of practical issues including performance in realistic propagation environments, the role of channel state information at the transmitter, and the practicality of interference alignment in large networks.}, number={1}, journal={IEEE Wireless Communications}, author={El Ayach, O. and Peters, S. and Heath, R.W.}, year={2013}, pages={35–42} }
 @inproceedings{the viability of distributed antennas for massive mimo systems_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901278795&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2013.6810508}, abstractNote={Massive multiple-input multiple-output (MIMO) is a breakthrough communication technique for providing high spectral efficiency. The idea is to deploy a very large number of antennas at each base station and to use multiuser MIMO transmission to serve a smaller number of users. In this paper, the viability of using distributed antennas for massive MIMO on the uplink is investigated for a particular spatial correlation channel model. Both maximal ratio combining (MRC) and minimum mean squared error (MMSE) beamforming are found to provide higher performance in several distributed settings. An algorithm for remote radio head selection is proposed that allows MRC to approach the performance achieved by the MMSE beamforming solution yet retaining its lower complexity.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2013}, pages={1318–1323} }
 @inproceedings{video quality-maximizing resource allocation and scheduling with statistical delay guarantees_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904125372&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2013.6831324}, abstractNote={Real-time video demands quality-of-service (QoS) guarantees such as delay bounds for end-user satisfaction. Due to the stochastic nature of wireless fading channels, deterministic delay bounds are prohibitively difficult to guarantee. Instead, this paper proposes providing statistical delay guarantees using the concept of effective capacity. A multiuser setup is considered whereby different users have (possibly different) delay QoS constraints. The resource allocation policy that maximizes the sum video quality is derived and has a useful intuitive interpretation: The optimal operating point per user is such that the rate-distortion slope is the inverse of the supported video source rate per unit bandwidth, termed source spectral efficiency. Scheduling policies are also proposed to select a maximal user subset such that all selected users can meet their statistical delay requirement. Results show that QoS-aware scheduling and resource allocation enable supporting significantly more users under the same resource constraints.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2013}, pages={1736–1740} }
 @article{a cross-layer design for perceptual optimization of h.264/svc with unequal error protection_2012, volume={30}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84864623707&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2012.120802}, abstractNote={Delivering high perceptual quality video over wireless channels is challenging due to the changing channel quality and the variations in the importance of one source packet to the next for the end-user's perceptual experience. Leveraging perceptual metrics in concert with link adaptation to maximize perceptual quality and satisfy real-time delay constraints is largely unexplored. We introduce an APP/MAC/PHY cross-layer architecture that enables optimizing perceptual quality for delay-constrained scalable video transmission. We propose an online QoS-to-QoE mapping technique to quantify the loss visibility of packets from each video layer using the ACK history and perceptual metrics. At the PHY layer, we develop a link adaptation technique that uses the QoS-to-QoE mapping to provide perceptually-optimized unequal error protection per layer according to packet loss visibility. At the APP layer, the source rate is adapted by selecting the set of temporal and quality layers to be transmitted based on the channel statistics, source rates, and playback buffer state. The proposed cross-layer optimization framework allows the channel to adapt at a faster time scale than the video codec. Furthermore, it provides a tradeoff between playback buffer occupancy and perceptual quality. We show that the proposed architecture prevents playback buffer starvation, provides immunity against short-term channel fluctuations, regulates the buffer size, and achieves a 30% increase in video capacity versus throughput-optimal link adaptation.}, number={7}, journal={IEEE Journal on Selected Areas in Communications}, year={2012}, pages={1157–1171} }
 @inproceedings{lee_heath_2012, title={CSI feedback delay and degrees of freedom gain trade-off for the MISO interference channel}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876257405&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2012.6489357}, abstractNote={Channel state information at the transmitter (CSIT) plays a significant role in managing interference for the multiple-input-single-output (MISO) interference channel. Because of channel state information (CSI) feedback delay and time-variation in the wireless channel, perfect CSIT may be difficult to obtain, which results in the system performance degradation. In this paper, a CSI feedback delay-DoF gain trade-off is characterized for the MISO interference by using space-time interference alignment that exploits current and outdated CSI jointly. From this trade-off characterization, insight is provided the interplay between CSI feedback delay and DoF gain in the MISO interference channel.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Lee, N. and Heath, R.W.}, year={2012}, pages={1851–1854} }
 @inproceedings{akoum_el ayach_heath_2012, title={Coverage and capacity in mmWave cellular systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876261829&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2012.6489099}, abstractNote={Millimeter wave (mmWave) communication has recently been proposed for use in commercial cellular systems as a solution to the microwave spectrum gridlock. MmWave spectrum is (potentially) available around the globe and recent hardware advances make mass market deployments feasible. In this paper, we study the coverage and capacity of mmWave cellular systems with a special focus on their key differentiating factors such as the limited scattering nature of mmWave channels, and the use of RF beamforming strategies such as beam steering to provide highly directional transmission with limited hardware complexity.We show that, in general, coverage in mmWave systems can rival or even exceed coverage in microwave systems assuming that the link budgets promised by existing mmWave system designs are in fact achieved. This comparable coverage translates into a superior average rate performance for mmWave systems as a result of the larger bandwidth available for transmission.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Akoum, S. and El Ayach, O. and Heath, R.W.}, year={2012}, pages={688–692} }
 @inproceedings{akoum_chen_debbah_heath_2012, title={Data sharing coordination and blind interference alignment for cellular networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84877671225&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2012.6503789}, abstractNote={We consider coordination in a multi-user multiple input single output cellular system. In contrast with existing base station cooperation methods that rely on sharing CSI with or without user data to manage interference, we propose to share user data only. We consider a system where blind interference alignment (BIA) is applied to serve multiple users in each cell. We apply interference coordination through data sharing to mitigate other-cell interference at the cell-edge users. While BIA mitigates intra-cell interference in MU-MISO systems, it does not address the problem of inter-cell interference. We apply interference coordination through data sharing to mitigate inter-cell interference at the cell-edge users. We propose a new cooperative BIA scheme that takes into account the users whose data is being shared between adjacent base stations. We derive the achievable sum rate with interference mitigation and we compare it to achievable rates with the original BIA strategy. Numerical results show that the achievable sum rate of the cell-edge users with data sharing decreases with increasing number of served users in each cell and increasing number of antennas at the base stations.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, author={Akoum, S. and Chen, C.S. and Debbah, M. and Heath, R.W.}, year={2012}, pages={4273–4277} }
 @article{el ayach_heath_2012, title={Grassmannian differential limited feedback for interference alignment}, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870493187&partnerID=MN8TOARS}, DOI={10.1109/TSP.2012.2218238}, abstractNote={Channel state information (CSI) in the interference channel can be used to reduce the dimension of received interference and helps achieve the channel's maximum multiplexing gain through what is known as interference alignment (IA). Most interference alignment algorithms require knowledge of all the interfering channels to compute the alignment precoders. CSI, considered available at the receivers, can be shared with the transmitters via limited feedback. When IA is done by coding over frequency extensions in a single antenna system, the required CSI lies on the Grassmannian manifold and its structure can be exploited in feedback. Unfortunately, the number of channels to be shared grows with the square of the number of users, creating too much overhead with conventional feedback methods. This paper proposes Grassmannian differential feedback to reduce feedback overhead by exploiting both the channel's temporal correlation and Grassmannian structure. The performance of the proposed algorithm is characterized both analytically and numerically as a function of channel length, mobility, and the number of feedback bits. The main conclusions are that the proposed feedback strategy allows IA to perform well over a wide range of Doppler spreads, and to approach perfect CSI performance in slowly varying channels. Numerical results highlight the trade-off between the frequency of feedback and the accuracy of individual feedback updates.}, number={12}, journal={IEEE Transactions on Signal Processing}, author={El Ayach, O. and Heath, R.W.}, year={2012}, pages={6481–6494} }
 @inproceedings{massey_starr_lee_lee_gerstlauer_heath_2012, title={Implementation of a real-time wireless interference alignment network}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876213212&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2012.6488968}, abstractNote={Interference alignment (IA) is a cooperative transmission technique for the interference channel. This paper describes two testbeds that implement real-time Multiple-input multiple-output (MIMO) IA for a network with three 2-antenna user pairs using software defined radio techniques: a PC-based testbed for rapid prototyping of potential IA protocols and an embedded testbed for evaluating IA under real-world computational constraints. The IA implementations rely on a wired backbone to share global channel state information (CSI) and a shared clock for frequency and timing synchronization. The testbeds are used to demonstrate the viability of IA, and to compare its robustness with several alternative transmission strategies, such as 2 × 2 MIMO TDMA, in terms of sum-rates. Results show that we are able to successfully achieve over-the-air IA in our three-user 2×2 MIMO testbed. The paper highlights key challenges with the practical realization of IA that are encountered while developing the testbed and identifies areas for future research.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Massey, J.W. and Starr, J. and Lee, S. and Lee, D. and Gerstlauer, A. and Heath, R.W.}, year={2012}, pages={104–108} }
 @inproceedings{interference alignment with limited feedback for two-cell interfering mimo-mac_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84871287378&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2012.6328431}, abstractNote={In this paper, we consider a two-cell interfering multiple-input multiple-output multiple access channel (MIMO-MAC). We first investigate the multiplexing gain of such channel when users have perfect channel state information at the transmitter (CSIT) by exploiting an interference alignment scheme. In addition, we propose a quantized transmit beamforming vector feedback method for the interference alignment in the limited feedback system. On the basis of the proposed feedback framework, we analyze the rate gap loss and it is shown that in order to keep the same multiplexing gain with the case of perfect CSIT, the number of feedback bits per receiver should be increased linearly with the SNR in dB scale. Throughout the simulation results, it is shown that the sum-rate performance coincides with the derived results.}, booktitle={Proceedings of the International Symposium on Wireless Communication Systems}, year={2012}, pages={566–570} }
 @article{interference aware-coordinated beamforming in a multi-cell system_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867897247&partnerID=MN8TOARS}, DOI={10.1109/TWC.2012.081312.112119}, abstractNote={In this paper, we propose jointly optimized linear transceiver algorithms called interference aware-coordinated beamforming (IA-CBF) for a two-cell system where each base station is equipped with multiple transmit antennas. To generalize IA-CBF to more than two-cell scenarios, a new beam-switching mechanism combined with IA-CBF is proposed. For a two-cell system, we derive a minimum-mean-square-error-type IA-CBF algorithm based on a lower bound on the achievable sum rate. We propose optimal (under an assumption of zero other-cell interference) and suboptimal transmit/receive beamforming vectors through zero-forcing IA-CBF algorithms. We also investigate the optimality of the proposed IA-CBF algorithms with respect to the number of receive antennas. Numerical results confirm that the proposed system with two transmit/receive antennas achieves the full degrees of freedom (a.k.a. multiplexing gain) of the two-cell multiple-input multiple-output channel while showing a better sum rate performance than competitive solutions such as non-cooperative eigen-beamforming and interference nulling. A three-dimensional ray tracing tool is also used to evaluate the proposed multi-cell IA-CBF algorithm.}, number={10}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={3692–3703} }
 @inproceedings{interference leakage minimization for convolutive mimo interference channels_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867602257&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2012.6288506}, abstractNote={An alternating optimization algorithm was recently proposed for the K-user multiple-input multiple-output (MIMO) interference channel. For flat-fading channels and feasible problems, this algorithm successfully aligns the interfering signals exploiting the spatial dimensions. In this paper, we consider the case in which all pairwise MIMO channels are frequency-selective (convolutive), and the users transmit broadband signals using a single-carrier scheme. Unlike the flat-fading case, for frequency-selective channels it is necessary to add a spectral mask in the frequency response of the precoders and decoders to avoid trivial solutions. We show in the paper that each step of the alternating minimization algorithm can be reformulated as a convex optimization problem in which the autocorrelation function of the precoders or decoders is obtained. Upon convergence, a final spectral factorization stage must be applied to obtain the precoders and decoders from their autocorrelation functions. Simulation results are provided to illustrate the performance of the proposed algorithm.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2012}, pages={2829–2832} }
 @inproceedings{interference-free relay transmission without csi-s for the mimo two-hop interference channel_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863895761&partnerID=MN8TOARS}, booktitle={2012 19th International Conference on Systems, Signals and Image Processing, IWSSIP 2012}, year={2012}, pages={217–221} }
 @article{introduction to the issue on signal processing in heterogeneous networks for future broadband wireless systems_2012, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861199727&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2012.2190541}, abstractNote={The six papers in this special issue focus on signal processing in heterogeneous networks for future broadband wireless systems.}, number={3}, journal={IEEE Journal on Selected Topics in Signal Processing}, year={2012}, pages={213–215} }
 @article{joint transmit precoding for the relay interference broadcast channel_2013, volume={62}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84879584011&partnerID=MN8TOARS}, DOI={10.1109/TVT.2012.2228888}, abstractNote={Relays in cellular systems are interference limited. The highest end-to-end sum rates are achieved when the relays are jointly optimized with the transmit strategy. Unfortunately, interference couples the links together, making joint optimization challenging. Further, the end-to-end multihop performance is sensitive to rate mismatch when some links have a dominant first link, whereas others have a dominant second link. This paper proposes an algorithm for designing the linear transmit precoders at the transmitters and relays of the relay interference broadcast channel, which is a generic model for relay-based cellular systems, to maximize the end-to-end sum rates. First, the relays are designed to maximize the second-hop sum rates. Next, approximate end-to-end rates that depend on the time-sharing fraction and the second-hop rates are used to formulate a sum-utility maximization problem to design the transmitters. This problem is solved by iteratively minimizing the weighted sum of mean square errors (MSEs). Finally, the norms of the transmit precoders at the transmitters are adjusted to eliminate rate mismatch. The proposed algorithm allows for distributed implementation and has fast convergence. Numerical results show that the proposed algorithm outperforms a reasonable application of single-hop interference management strategies separately on two hops.}, number={3}, journal={IEEE Transactions on Vehicular Technology}, year={2013}, pages={1201–1215} }
 @inproceedings{ayach_heath_abu-surra_rajagopal_pi_2012, title={Low complexity precoding for large millimeter wave MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84871956153&partnerID=MN8TOARS}, DOI={10.1109/ICC.2012.6363634}, abstractNote={Millimeter wave (mmWave) systems must overcome heavy signal attenuation to support high-throughput wireless communication links. The small wavelength in mmWave systems enables beamforming using large antenna arrays to combat path loss with directional transmission. Beamforming with multiple data streams, known as precoding, can be used to achieve even higher performance. Both beamforming and precoding are done at baseband in traditional microwave systems. In mmWave systems, however, the high cost of mixed-signal and radio frequency chains (RF) makes operating in the passband and analog domains attractive. This hardware limitation places additional constraints on precoder design. In this paper, we consider single user beamforming and precoding in mmWave systems with large arrays. We exploit the structure of mmWave channels to formulate the precoder design problem as a sparsity constrained least squares problem. Using the principle of basis pursuit, we develop a precoding algorithm that approximates the optimal unconstrained precoder using a low dimensional basis representation that can be efficiently implemented in RF hardware. We present numerical results on the performance of the proposed algorithm and show that it allows mmWave systems to approach waterfilling capacity.}, booktitle={IEEE International Conference on Communications}, author={Ayach, O.E. and Heath, R.W. and Abu-Surra, S. and Rajagopal, S. and Pi, Z.}, year={2012}, pages={3724–3729} }
 @inproceedings{modeling heterogeneous network interference_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84860433126&partnerID=MN8TOARS}, DOI={10.1109/ITA.2012.6181825}, abstractNote={Cellular systems are becoming more heterogeneous with the introduction of low power nodes including femtocells, relays, and distributed antennas. Unfortunately, the resulting interference environment is also becoming more complex, making evaluation of different communication strategies for cellular systems more challenging in both analysis and simulation. This paper suggests a simplified interference model for heterogeneous network. Leveraging recent applications of stochastic geometry to analyze cellular systems, this paper propose to analyze performance in a fixed-size typical cell surrounded by an interference field consisted of superposition of marked Poisson point processes outside a guard region. The proposed model simplifies the simulation of cellular systems and may provide analytical insights for certain signaling strategies.}, booktitle={2012 Information Theory and Applications Workshop, ITA 2012 - Conference Proceedings}, year={2012}, pages={17–22} }
 @inproceedings{multi-cell coordination: a stochastic geometry approach_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84868020373&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2012.6292881}, abstractNote={Partial base station coordination is analyzed on the downlink of a cellular system with randomly deployed base stations. In contrast with existing work, a stochastic geometry framework is considered for both base station deployment and base station clustering. The cluster-cells form a Poisson tessellation of the plane generated by Poisson distributed cluster base stations. Inter-cell interference nulling is applied to coordinate interference inside each cluster-cell. Both perfect and limited channel state information (CSI) sharing among coordinating base stations are considered. The average achievable rate with perfect CSI and the mean loss in rate due to limited CSI feedback are derived for a typical user inside the cell. Numerical results show that the average achievable rate with interference coordination as well as the mean loss in rate with equal-bit partitioning increase with increasing average cluster size and increasing number of antennas at the transmitter.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2012}, pages={16–20} }
 @book{multicell handoff in mimo cellular systems_2012, volume={133 AISC}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84862067601&partnerID=MN8TOARS}, DOI={10.1007/978-3-642-27552-4_117}, abstractNote={This paper is concerned with handoff mechanisms in wireless cellular networks that employ MIMO technology. To achieve optimal performance during handoff, the mobile receiver can be linked to antennas from both the base stations of the adjacent cells. Then the handoff is termed multicell handoff. The basic idea is that in a region know as the multicell handoff region, the mobile terminal communicates simultaneously with multiple base stations. The simulation results demonstrate that the multicell handoff provides an improvement of average throughput in the high spatial correlation case.}, journal={Advances in Intelligent and Soft Computing}, year={2012}, pages={893–898} }
 @inproceedings{schwarz_heath_rupp_2012, title={Multiuser MIMO in distributed antenna systems with limited feedback}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84875641046&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2012.6477632}, abstractNote={Augmenting the central antennas of a cellular base station with remote radio units (RRUs) has the potential of providing more uniform coverage of the area served by the base station. In distributed antenna systems (DASs) these RRUs are connected to the base station via high bandwidth and low latency links, enabling coherent transmission from potentially all available antennas at the same time. Prior work has identified multiuser transmission over the entire distributed antenna array as a promising technique. In this work, we demonstrate the viability of zero forcing multiuser MIMO DASs with perfect channel state information (CSI) at the transmitter (CSIT) in an LTE simulation framework. Furthermore, the impact of imperfect CSIT, obtained via limited feedback, on the achievable transmission rates is investigated. Specifically, we modify previously derived feedback algorithms to exploit the large scale fading differences inherently existing in DASs.}, booktitle={2012 IEEE Globecom Workshops, GC Wkshps 2012}, author={Schwarz, S. and Heath, R.W. and Rupp, M.}, year={2012}, pages={546–551} }
 @inproceedings{lee_heath_2012, title={Not too delayed CSIT achieves the optimal degrees of freedom}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84875750403&partnerID=MN8TOARS}, DOI={10.1109/Allerton.2012.6483363}, abstractNote={Channel state information at the transmitter (CSIT) aids interference management in many communication systems. Due to channel state information (CSI) feedback delay and time-variation in the wireless channel, perfect CSIT is not realistic. In this paper, the CSI feedback delay-DoF gain trade-off is characterized for the multi-user vector broadcast channel. A major insight is that it is possible to achieve the optimal degrees of freedom (DoF) gain if the delay is less than a certain fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligeable. To show this, a new transmission method called space-time interference alignment is proposed, which actively exploits both the current and past CSI.}, booktitle={2012 50th Annual Allerton Conference on Communication, Control, and Computing, Allerton 2012}, author={Lee, N. and Heath, R.W.}, year={2012}, pages={1262–1269} }
 @inproceedings{lozano_andrews_heath_2012, title={On the limitations of cooperation in wireless networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84860456235&partnerID=MN8TOARS}, DOI={10.1109/ITA.2012.6181813}, abstractNote={Cooperation in a large wireless network (such as a cellular system) is shown to have certain fundamental limitations: namely, even perfect cooperation cannot in general change an interference-limited network to a noise-limited one. In contrast to existing literature that routinely assumes that the spectral efficiency scales with log P as the transmit power P grows large, we show the existence of a spectral efficiency upper bound that does not grow with P. The result uses well-accepted principles of information theory to reach the conclusion that it is not possible (or even helpful) to fully coordinate a large wireless network. Rather than simply low- and high-power regimes, there are three distinct network operating regimes: low-power; a DoF regime, where the log P scaling holds; and a saturation regime where the spectral efficiency hits a ceiling that is independent of P. Using a cellular system example, it is demonstrated that the transition to the saturation regime is operationally relevant and perhaps explains the lackluster gains from cooperation observed in practice.}, booktitle={2012 Information Theory and Applications Workshop, ITA 2012 - Conference Proceedings}, author={Lozano, A. and Andrews, J.G. and Heath, R.W.}, year={2012}, pages={123–130} }
 @article{on the overhead of interference alignment: training, feedback, and cooperation_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870494450&partnerID=MN8TOARS}, DOI={10.1109/TWC.2012.092412120588}, abstractNote={Interference alignment (IA) is a cooperative transmission strategy that, under some conditions, achieves the interference channel's maximum number of degrees of freedom. Realizing IA gains, however, is contingent upon providing transmitters with sufficiently accurate channel knowledge. In this paper, we study the performance of IA in multiple-input multiple-output systems where channel knowledge is acquired through training and analog feedback. We design the training and feedback system to maximize IA's effective sum-rate: a non-asymptotic performance metric that accounts for estimation error, training and feedback overhead, and channel selectivity. We characterize effective sum-rate with overhead in relation to various parameters such as signal-to-noise ratio, Doppler spread, and feedback channel quality. A main insight from our analysis is that, by properly designing the CSI acquisition process, IA can provide good sum-rate performance in a very wide range of fading scenarios. Another observation from our work is that such overhead-aware analysis can help solve a number of practical network design problems. To demonstrate the concept of overhead-aware network design, we consider the example problem of finding the optimal number of cooperative IA users based on signal power and mobility.}, number={11}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={4192–4203} }
 @inproceedings{khalek_caramanis_heath_2012, title={Online learning for quality-driven unequal protection of scalable video}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870688150&partnerID=MN8TOARS}, DOI={10.1109/MLSP.2012.6349781}, abstractNote={Video packet losses affect perceived video quality non-uniformly due to several factors related to video encoding such as inter-frame coding and motion compensation as well as due to psycho-visual perception of natural scenes with unequal motion. This motivates protecting video packets unequally based on their loss visibility. This paper proposes an adaptive online algorithm for unequal error protection driven by two key motivations: On one hand, for real-time video, where a video sequence is not pre-encoded, an offline approach is infeasible and determining the unequal protection levels to maintain a target video quality level must be performed online. On the other hand, an online approach enables adapting to scene changes as well as changes in video temporal and spatial characteristics. The proposed online algorithm uses local linear regression to learn the mapping between packet losses from each scalable video layer and quality degradation without assuming an underlying statistical model. The notion of locality captures the similarity in video scene characteristics as well as proximity in time. The algorithm provably guarantees an average target video quality level and converges rapidly to a stable solution. Furthermore, it provides a bias/variance tradeoff between factual estimation of loss visibility and fine adaptation to the changing video temporal characteristics.}, booktitle={IEEE International Workshop on Machine Learning for Signal Processing, MLSP}, author={Khalek, A.A. and Caramanis, C. and Heath, R.W.}, year={2012} }
 @inproceedings{pre- and post-fft interference leakage minimization for mimo ofdm networks_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84871289770&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2012.6328429}, abstractNote={Interference alignment (IA) has been shown to achieve the maximum degrees of freedom in the multiple-input multiple-output (MIMO) K-user interference channel (IFC). In the presence of frequency-selective channels, orthogonal frequency-division multiplexing (OFDM) is typically used to deal with the multipath nature of the channel. While IA techniques can be applied in a per-subcarrier basis (post-FFT), the existence of symbol timing offsets (STOs) between the desired and the interfering OFDM symbols decreases the system performance dramatically. To solve this problem, we design pre-FFT precoders and decoders for single-beam MIMO IFCs for OFDM transmissions. Since the IA decoders operate before the FFT, they mitigate the interference before synchronization takes place. We show that our proposed scheme improves the system performance when STOs occur, in comparison with traditional post-FFT IA techniques. We provide simulation results to compare post and pre-FFT beamforming techniques and to illustrate the performance of the proposed method.}, booktitle={Proceedings of the International Symposium on Wireless Communication Systems}, year={2012}, pages={556–560} }
 @inproceedings{khalek_caramanis_heath_2012, title={Prioritized multimode precoding for joint minimization of source-channel video distortions}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876271891&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2012.6489151}, abstractNote={Real-time video streaming over wireless channels suffers from channel-induced distortions due to losses and errors as well as source-induced distortions due to lossy compression. This paper proposes a cross-layer design for quality-driven packet prioritization at the physical layer combined with source and link rate adaptation to jointly minimize the source and channel distortions. With a MIMO physical layer, packet prioritization is achieved by mapping video packets to ordered spatial streams to maximize the throughput weighted by the per-packet loss visibility. The MIMO mode controls the number of priority levels and is dynamically optimized to provide the best tradeoff between high source rate and low channel distortion. Jointly with mode adaptation, source rate adaptation is driven by the client by estimating the end-to-end throughput based on the transmission rate. Results show that the proposed design enables composite video quality gains due to prioritized delivery and rate adaptation.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Khalek, A.A. and Caramanis, C. and Heath, R.W.}, year={2012}, pages={925–929} }
 @article{spatial interference cancellation for multiantenna mobile ad hoc networks_2012, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857719913&partnerID=MN8TOARS}, DOI={10.1109/TIT.2011.2178140}, abstractNote={Interference between nodes is a critical impairment in mobile ad hoc networks. This paper studies the role of multiple antennas in mitigating such interference. Specifically, a network is studied in which receivers apply zero-forcing beamforming to cancel the strongest interferers. Assuming a network with Poisson-distributed transmitters and independent Rayleigh fading channels, the transmission capacity is derived, which gives the maximum number of successful transmissions per unit area. Mathematical tools from stochastic geometry are applied to obtain the asymptotic transmission capacity scaling and characterize the impact of inaccurate channel state information (CSI). It is shown that, if each node cancels interferers, the transmission capacity decreases as as the outage probability vanishes. For fixed , as grows, the transmission capacity increases as where is the path-loss exponent. Moreover, CSI inaccuracy is shown to have no effect on the transmission capacity scaling as vanishes, provided that the CSI training sequence has an appropriate length, which we derive. Numerical results suggest that canceling merely one interferer by each node may increase the transmission capacity by an order of magnitude or more, even when the CSI is imperfect.}, number={3}, journal={IEEE Transactions on Information Theory}, year={2012}, pages={1660–1676} }
 @inproceedings{spectral efficiency limits in pilot-assisted cooperative communications_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867529979&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2012.6283030}, abstractNote={Cooperation in a large wireless network with pilotassisted coherent communication is shown to have certain fundamental limitations, namely that even perfect cooperation cannot in general change an interference-limited network to a noise-limited one. Specifically, we demonstrate the existence of a spectral efficiency upper bound that does not grow with the transmit power, when channels are estimated via pilot signals. This is because pilot-assisted channel estimation is only possible within finite cooperation clusters, resulting in out-of-cluster interference that scales with the transmit power. Making the clusters excessively large can actually worsen this effect, nor does sidestepping the pilot-assisted channel estimation via noncoherent demodulation provide an escape. Using a cellular system as an example, it is demonstrated that the spectral efficiency saturates at power levels of operational relevance, indicating that the lackluster gains from cooperation observed in practice may be based on fundamental information-theoretic limitations, rather than current technology imperfections.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2012}, pages={1132–1136} }
 @inproceedings{the capacity optimality of beam steering in large millimeter wave mimo systems_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84868031081&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2012.6292865}, abstractNote={Millimeter wave (mmWave) systems must overcome the heavy attenuation at high frequency to support high-throughput wireless communication. The small wavelength in mmWave systems enables beamforming using large antenna arrays to combat path loss with large array gain. Beamforming in traditional microwave systems is often done at baseband for maximum flexibility. Such baseband processing requires a dedicated transceiver chain per antenna element. The high cost of radio frequency (RF) chains in mmWave systems, however, makes supporting each antenna with a dedicated RF chain expensive. This mismatch between the number of antennas and transceiver chains makes baseband processing infeasible; thus mmWave systems typically rely on a traditional approach known as beam steering which can be done at RF using inexpensive phase shifters. Unlike baseband precoding, however, traditional beam steering is not explicitly designed to achieve the capacity of the mmWave channel. In this paper, we consider both beamforming and multi-stream precoding in single user systems with large mmWave antenna arrays at both transmitter and receiver. Using a realistic channel model, we show that the unconstrained capacity-achieving precoding solutions converge to simple beam steering solutions. Therefore, in large mmWave systems, no rate loss is incurred by adopting the traditional lower-complexity solution.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2012}, pages={100–104} }
 @article{transmission capacity of ad-hoc networks with multiple antennas using transmit stream adaptation and interference cancellation_2012, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84856846185&partnerID=MN8TOARS}, DOI={10.1109/TIT.2011.2173712}, abstractNote={The transmission capacity of an ad-hoc network is the maximum density of active transmitters per unit area, given an outage constraint at each receiver for a fixed rate of transmission. Assuming that the transmitter locations are distributed as a Poisson point process, this paper derives upper and lower bounds on the transmission capacity of an ad-hoc network when each node is equipped with multiple antennas. The transmitter either uses eigen multi-mode beamforming or a subset of its antennas without channel information to transmit multiple data streams, while the receiver uses partial zero forcing to cancel certain interferers using some of its spatial receive degrees of freedom (SRDOF). The receiver either cancels the nearest interferers or those interferers that maximize the post-cancellation signal-to-interference ratio. Using the obtained bounds, the optimal number of data streams to transmit, and the optimal SRDOF to use for interference cancellation are derived that provide the best scaling of the transmission capacity with the number of antennas. With beamforming, single data stream transmission together with using all but one SRDOF for interference cancellation is optimal, while without beamforming, single data stream transmission together with using a fraction of the total SRDOF for interference cancellation is optimal.}, number={2}, journal={IEEE Transactions on Information Theory}, year={2012}, pages={780–792} }
 @inproceedings{using random shape theory to model blockage in random cellular networks_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867706619&partnerID=MN8TOARS}, DOI={10.1109/SPCOM.2012.6290250}, abstractNote={Shadow fading is severe in downtown areas where buildings are densely located. This paper proposes a stochastic model to quantify blockages due to shadowing, using methods from random shape theory. Buildings inside a cell are modeled as line segments with random sizes and orientations, with locations from a spatial Poisson point process. Dense urban areas can be modeled by the parameters of the line process. Based on this construction, the distribution of the power loss caused by shadowing in a particular path is expressed in closed form. The distribution can be used to compute several performance metrics of interest in random systems. Simulations illustrate coverage and connectivity as a function of the metrics of blockages, such as the density and the average size of buildings.}, booktitle={2012 International Conference on Signal Processing and Communications, SPCOM 2012}, year={2012} }
 @inproceedings{video-aware mimo precoding with packet prioritization and unequal modulation_2012, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869779012&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, year={2012}, pages={1905–1909} }
 @inproceedings{a machine learning approach to link adaptation for sc-fde system_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857223668&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2011.6134362}, abstractNote={Single carrier frequency domain equalization (SC-FDE) uses cyclically prefixed quadrature amplitude modulation to permit simple frequency domain equalization at the receiver. Link adaptation for SC-FDE systems, where the modulation and coding rate are adapted based on the current channel state, is straightforward with perfect channel state information due to the simple analytical form of the post-processing signal-to-noise ratio (SNR). Imperfect channel state information, however, introduces adaptation errors. This paper proposes a machine learning-based approach for link adaptation in bit interleaved convolutionally encoded SC-FDE systems. To improve performance in the presence of channel uncertainty, principal component analysis is used to reduce the feature space dimensionality consisting of the channel coefficients, noise variance, and post-processing SNR. The reduced dimension feature set improves performance of the link adaptation classifier and leads to higher performance versus just the post-processing SNR estimate. Simulation results indicate that the proposed algorithm increases the goodput while maintaining the target packet error rate, achieving optimal adaptation in 95% of the tested cases.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2011} }
 @article{adaptive bit partitioning for multicell intercell interference nulling with delayed limited feedback_2011, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79960398987&partnerID=MN8TOARS}, DOI={10.1109/TSP.2011.2146777}, abstractNote={Base station cooperation can exploit knowledge of the users' channel state information (CSI) at the transmitters to manage co-channel interference. Users have to feedback CSI of the desired and interfering channels using finite-bandwidth backhaul links. Existing codebook designs for single-cell limited feedback can be used for multicell cooperation by partitioning the available feedback resources between the multiple channels. In this paper, a new feedback-bit allocation strategy is proposed, as a function of the delays in the communication links and received signal strengths in the downlink. Channel temporal correlation is modeled as a function of delay using the Gauss-Markov model. Closed-form expressions for bit partitions are derived to allocate more bits to quantize the stronger channels with smaller delays and fewer bits to weaker channels with larger delays, assuming random vector quantization. Cellular network simulations are used to show that the algorithm presented in the paper yields higher sum-rates than the equal-bit allocation technique.}, number={8}, journal={IEEE Transactions on Signal Processing}, year={2011}, pages={3824–3836} }
 @inproceedings{adaptive policies for real-time video transmission: a markov decision process framework_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863013331&partnerID=MN8TOARS}, DOI={10.1109/ICIP.2011.6116085}, abstractNote={We study the problem of adaptive video data scheduling over wireless channels. We prove that, under certain assumptions, adaptive video scheduling can be reduced to a Markov decision process over a finite state space. Therefore, the scheduling policy can be optimized via standard stochastic control techniques using a Markov decision formulation. Simulation results show that significant performance improvement can be achieved over heuristic transmission schemes.}, booktitle={Proceedings - International Conference on Image Processing, ICIP}, year={2011}, pages={2249–2252} }
 @article{decentralized precoding for multicell mimo downlink_2011, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79959570014&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.040511.100519}, abstractNote={Interference is a performance limiting factor in dense cellular networks with aggressive frequency reuse. Cooperation among base stations (BSs) is a promising approach for improving data rates by eliminating or mitigating interference. For the downlink, the highest spectral efficiency gains are achieved through precoding with full coordination, which requires complete channel state information (CSI) and data be shared among BSs at the cost of significant utilization of the backhaul. In this paper, we propose distributed precoding techniques for the multicell MIMO downlink. Unlike prior work, our proposed precoders are both decentralized with respect to the BSs as well as capable of enabling multiple users to share the same frequency carrier spatially within each cell. Specifically, each BS designs its own precoder without requiring data or downlink CSI of links from other BSs. Since CSI is unlikely to be perfect, we study the effect of imperfect CSI on our proposed precoders and propose a robust precoder in the presence of CSI uncertainty. Simulations show that our proposed methods enjoy a rate increase with SNR similar to multicell joint dirty paper coding in the high SNR regime due to effective interference mitigation. Numerical results reflect the sensitivity of each proposed precoder with respect to the imperfectness in the available CSI.}, number={6}, journal={IEEE Transactions on Wireless Communications}, year={2011}, pages={1798–1809} }
 @article{yun_caramanis_heath_2011, title={Distributed link adaptation for multicast traffic in MIMO-OFDM systems}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84866276508&partnerID=MN8TOARS}, DOI={10.1016/j.phycom.2011.10.002}, abstractNote={Multicast traffic exploits the broadcast nature of the wireless medium to deliver the same data to multiple users improving the bandwidth efficiency. Link adaptation can be used in multicast transmission to further increase data rates exploiting feedback from the users. However, it is not easy to have the quality of service (QoS) of every intended receiver met while pushing the data rate to the link capacity. Due to this difficulty, the conventional approach is to transmit isotropically with a fixed basic rate giving up the opportunity of increased throughput. For point-to-point unicast traffic, machine learning algorithms have recently found successful application in link adaptation due to their flexibility and ability to capture more environmental effects implicitly than classical adaptation algorithms. In this paper, we propose a machine learning based distributed algorithm for link adaptation for multicast traffic in multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. Our computations show that the data driven approach for link adaptation provides good prediction of the optimal modulation and coding scheme (MCS) outperforming the fixed MCS policies collectively. The distributed algorithm using dual decomposition reduces the required feedback amount significantly while maintaining the near-optimal throughput performance.}, number={4}, journal={Physical Communication}, author={Yun, S. and Caramanis, C. and Heath, R.W.}, year={2011}, pages={286–295} }
 @inproceedings{extending the reach of gps-assisted femtocell synchronization and localization through tightly-coupled opportunistic navigation_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84858410934&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2011.6162445}, abstractNote={A strategy known as Tightly-Coupled Opportunistic Navigation (TCON) is proposed for extending the penetration of GPS-assisted femtocells in weak-signal indoor environments. Standards and regulatory agencies impose strict time, frequency, and location requirements on femtocells. These can be met with GPS aiding to unparalleled time and positioning accuracies, but GPS signals are extremely weak and thus difficult to acquire indoors. The results of this paper suggest that a TCON solution fusing GPS with CDMA cellular signals offers significant sensitivity gains over state-of-the-art assisted-GPS receivers, allowing GPS-synchronized femtocells to be deployed in 90% of all residences.}, booktitle={2011 IEEE GLOBECOM Workshops, GC Wkshps 2011}, year={2011}, pages={242–247} }
 @inproceedings{grassmannian differential limited feedback for interference alignment_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863759064&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, year={2011}, pages={1075–1079} }
 @inproceedings{grassmannian predictive coding for limited feedback multiuser mimo systems_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80051658702&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2011.5946308}, abstractNote={Grassmannian beamforming is an efficient way to quantize channel state information in multiple-input multiple-output wireless systems. Unfortunately, multiuser systems require larger codebooks since the quantization error creates residual interference that limits the sum rate performance. To reduce the feedback requirements in multiuser systems, we propose Grassmannian predictive coding to exploit temporal channel correlation. The proposed algorithm exploits the differential geometric structure of the Grassmann manifold. The difference between points, prediction, and quantization are defined using the tangent space of the Grassmann manifold. We show that with practical feedback rates, a significant sum rate improvement can be obtained as a function of the channel correlation.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2011}, pages={3076–3079} }
 @inproceedings{impact of delayed limited feedback on the sum-rate of intercell interference nulling_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052149792&partnerID=MN8TOARS}, DOI={10.1109/icc.2011.5962731}, abstractNote={Base station cooperation requires knowledge of the users' channel state information (CSI) at the transmitter side to manage co-channel interference. Users in a multicell cooperative system have to feedback the CSI of desired and interfering channels using finite-bandwidth links. Hence, the available feedback resources need to be partitioned efficiently between the multiple channels. In this paper, a feedback-bit allocation strategy is proposed for intercell interference nulling by taking into account the received signal strengths and delays in the feedback and backhaul links. Closed-form expressions for bit partitions are derived, to allocate more bits to quantize the strong interferers with small delays and assign fewer bits to weak channels with large delays. Simulations show how the proposed algorithm yields higher sum-rates than equal bit allocation.}, booktitle={IEEE International Conference on Communications}, year={2011} }
 @article{improved iterative coordinated beamforming based on singular value decomposition for multiuser mimo systems with limited feedforward_2011, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84860827655&partnerID=MN8TOARS}, number={3}, journal={Journal of Applied Research and Technology}, year={2011}, pages={342–354} }
 @inproceedings{interference alignment for the multiple-antenna amplify-and-forward relay interference channel_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861325369&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2011.6190224}, abstractNote={Interference alignment is a multiplexing gain maximizing technique for the single-hop interference channel. It can be extended to multi-hop interference channels, where relays aid transmission. In addition to coverage extension and capacity enhancement, relays allow the system to achieve higher multiplexing gains. This paper proposes a relay-aided interference alignment algorithm for the multiple-antenna amplify-and-forward relay interference channel. This algorithm relies on a connection between mean square error and achievable rate to solve the end-to-end sum-rate maximization problem. Numerical results show that the proposed algorithm outperforms other strategies in terms of both end-to-end sum-rates and multiplexing gains.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2011}, pages={1288–1292} }
 @article{interference alignment with analog channel state feedback_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857366485&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.122211.101814}, abstractNote={Interference alignment (IA) is a multiplexing gain optimal transmission strategy for the interference channel. While the achieved sum rate with IA is much higher than previously thought possible, the improvement comes at the cost of requiring network channel state information at the transmitters. This can be achieved by explicit feedback, a flexible yet potentially costly approach that incurs large overhead. In this paper we propose analog feedback as an alternative to limited feedback or reciprocity based alignment. We show that the full multiplexing gain observed with perfect channel knowledge is preserved by analog feedback and that the mean loss in sum rate is bounded by a constant when signal-to-noise ratio is comparable in both forward and feedback channels. When signal-to-noise ratios are not quite symmetric, a fraction of the multiplexing gain is achieved. We consider the overhead of training and feedback and use this framework to numerically optimize the system's effective throughput. We present simulation results to demonstrate the performance of IA with analog feedback, verify our theoretical analysis, and extend our conclusions on optimal training and feedback length.}, number={2}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={626–636} }
 @inproceedings{starr_el ayach_heath_2011, title={Interference alignment with per-antenna power constraints}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054827150&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2011.6034072}, abstractNote={Interference alignment is a technique for designing transmit precoders that is known to achieve the maximum multiplexing gain in the interference channel. Prior work on interference alignment has assumed sum-power constraints in designing the transmit precoders. In practice, however, each transmit antenna has its own power amplifier which constrains the power radiated from each antenna. This paper proposes and analyzes two algorithms for performing interference alignment with per-antenna equality and inequality power constraints. It is proven that incorporating inequality constraints can be done arbitrarily and does not affect the feasibility of interference alignment. Further, we show that incorporating equality constraints is more difficult and requires more antennas for interference alignment to be feasible, especially in the case of single-stream systems. It is shown through analysis and numerical sum-rate calculations that per-antenna inequality constraints result in a systematic power loss versus the cases of per-antenna equality constraints or no constraints at all.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Starr, J. and El Ayach, O. and Heath, R.W.}, year={2011}, pages={2746–2750} }
 @article{is the phy layer dead?_2011, volume={49}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79953772591&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2011.5741160}, abstractNote={This article originates from a panel with the above title, held at IEEE VTC Spring 2009, in which the authors took part. The enthusiastic response it received prompted us to discuss for a wider audience whether research at the physical layer (PHY) is still relevant to the field of wireless communications. Using cellular systems as the axis of our exposition, we exemplify areas where PHY research has indeed hit a performance wall and where any improvements are expected to be marginal. We then discuss whether the research directions taken in the past have always been the right choice and how lessons learned could influence future policy decisions. Several of the raised issues are subsequently discussed in greater details, e.g., the growing divergence between academia and industry. With this argumentation at hand, we identify areas that are either under-developed or likely to be of impact in coming years - hence corroborating the relevance and importance of PHY research.}, number={4}, journal={IEEE Communications Magazine}, year={2011}, pages={159–165} }
 @inproceedings{joint source-channel adaptation for perceptually optimized scalable video transmission_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857213822&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2011.6134145}, abstractNote={Providing perceptual quality guarantees for video transmission over wireless channels is important for the end-user experience. The paper proposes an algorithm for perceptual quality optimization of scalable H.264 video with temporal and quality scalability. The algorithm supports adaptive unequal error protection so that different video layers are protected according to their relevance to video quality. For each video layer, a target packet error rate (PER) is selected such that a perceptual quality guarantee, measured using the multi-scale structural similarity (MS-SSIM) index, is satisfied. Given the target PER per video layer, the algorithm selects the modulation and coding scheme and the number of temporal and quality layers to transmit adaptively based on the channel state information (CSI) and source rates per video layer. Results show that the algorithm provides immunity against short term channel fluctuations, balances reliability and perceptual quality, reduces playback buffer starvation probability, and provides a convenient buffer management policy.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2011} }
 @inproceedings{chae_hwang_heath_tarokh_2011, title={Jointly optimized two-cell MIMO systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84858395349&partnerID=MN8TOARS}, DOI={10.1109/GLOCOMW.2011.6162484}, abstractNote={Jointly optimized interference aware coordinated beamforming (IA-CBF) algorithms for a two-cell system are investigated, where each base/mobile station is equipped with multiple transmit antennas. We first derive a minimum-mean-square-error IA-CBF algorithm based on a lower bound on the achievable sum rate. We next propose optimal (under an assumption of zero other-cell interference) and suboptimal transmit/receive beamforming vectors through zero-forcing IA-CBF algorithms. Numerical results confirm that the proposed system shows better sum rate performance than competitive solutions such as non-cooperative eigen-beamforming and interference nulling.}, booktitle={2011 IEEE GLOBECOM Workshops, GC Wkshps 2011}, author={Chae, C.-B. and Hwang, I. and Heath, R.W. and Tarokh, V.}, year={2011}, pages={421–425} }
 @article{link adaptation with position/motion information in vehicle-to-vehicle networks_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857370598&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.121911.110086}, abstractNote={Wireless communication networks use link adaptation to select physical layer parameters that optimize the transmission strategy as a function of the wireless channel realization. In the vehicle-to-vehicle (V2V) networks considered in this letter, the short coherence time of the wireless channel makes link adaptation based on the impulse response challenging. Consequently, link adaptation in V2V wireless networks may instead exploit the large-scale characteristics of the wireless channel (i.e. path loss) since they evolve slowly and enable less frequent feedback. Large-scale channel information may be captured through channel or position/motion measurements. We show, through the definition of new large-scale coherence expressions, that channel measurements render large-scale coherence as a function of time-change while the position/motion measurements render coherence as a function of velocity-change. This letter is concluded with highway simulations of modeled and measured channels to demonstrate the advantage of position/motion information for feedback reduction in V2V link adaptation.}, number={2}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={505–509} }
 @inproceedings{mimo interference alignment in random access networks_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861324415&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2011.6190081}, abstractNote={In this paper we analyze a multiple-input-multiple-output interference channel where nodes randomly distributed on a plane utilize interference alignment to reduce the point-to-point outage. We model the spatial distribution of the nodes as a spatial Poisson cluster point process with equal sized clusters. Each cluster uses intra-cluster interference alignment to suppress interference. We link the accuracy of channel state information to the distance between the nodes, i.e., for a fixed SNR, the quality of CSI degrades with increasing distance. Accounting for the inter-cluster unaligned interference, we compare intra-cluster interference alignment with open-loop spatial multiplexing. In our analysis we find common system setups where the benefits of using interference alignment over spatial multiplexing degrade the most due to the imperfect channel state information.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2011}, pages={641–645} }
 @article{nosrat-makouei_andrews_heath_2011, title={MIMO interference alignment over correlated channels with imperfect CSI}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79957453963&partnerID=MN8TOARS}, DOI={10.1109/TSP.2011.2124458}, abstractNote={Interference alignment (IA), given uncorrelated channel components and perfect channel state information, obtains the maximum degrees of freedom in an interference channel. Little is known, however, about how the sum rate of IA behaves at finite transmit power, with imperfect channel state information, or antenna correlation. This paper provides an approximate closed-form signal-to-interference-plus-noise-ratio (SINR) expression for IA over multiple-input-multiple-output (MIMO) channels with imperfect channel state information and transmit antenna correlation. Assuming linear processing at the transmitters and zero-forcing receivers, random matrix theory tools are utilized to derive an approximation for the postprocessing SINR distribution of each stream for each user. Perfect channel knowledge and i.i.d. channel coefficients constitute special cases. This SINR distribution not only allows easy calculation of useful performance metrics like sum rate and symbol error rate, but also permits a realistic comparison of IA with other transmission techniques. More specifically, IA is compared with spatial multiplexing and beamforming and it is shown that IA may not be optimal for some performance criteria.}, number={6}, journal={IEEE Transactions on Signal Processing}, author={Nosrat-Makouei, B. and Andrews, J.G. and Heath, R.W.}, year={2011}, pages={2783–2794} }
 @article{mimo transceiver designs for spatial sensing in cognitive radio networks_2011, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-83555172391&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.092711.101305}, abstractNote={We propose transceiver algorithms in cognitive radio networks where the cognitive users are equipped with multiple antennas. Prior work has focused on the design of precoding matrices to suppress interference to the primary receivers. This work considers designs of precoding and decoding matrices for spatial sensing to achieve two objectives: (i) to prevent interference to the primary receivers and (ii) to remove the interference, due to primary transmissions, at the secondary receiver. With single antenna primary terminals and two antenna cognitive terminals, a linear transceiver design has been introduced under a global channel state information (CSI) assumption . In this letter, multiple antenna primary and cognitive terminals and three different CSI scenarios depending upon the amount of CSI are studied: (i) local CSI, (ii) global CSI, and (iii) local CSI with side information. When local CSI is available, we leverage prior work and employ the projected-channel singular value decomposition (P-SVD). In the global CSI scenario, we propose a joint transmitter-receiver design under the assumption of full CSI of all the users at the secondary transceiver. To reduce the feedback overhead, we also propose a new iterative algorithm that exploits only local CSI with side information. In this algorithm, the secondary transmitter and receiver iteratively update precoding and decoding matrices based on the local CSI and side information (precoding/decoding matrices at the previous iteration step) to maximize the rate of the secondary link while maintaining the zero-interference constraint. Convergence is established in the special case of single stream beamforming. Numerical results confirm that the proposed joint design and the iterative algorithm show better achievable rate performance than the P-SVD technique at the expense, respectively, of CSI knowledge and side information.}, number={11}, journal={IEEE Transactions on Wireless Communications}, year={2011}, pages={3570–3576} }
 @article{multiuser mimo in distributed antenna systems with out-of-cell interference_2011, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052877913&partnerID=MN8TOARS}, DOI={10.1109/TSP.2011.2161985}, abstractNote={Distributed antenna systems (DAS) augment the base station's transmit capability by adding multiple remote radio units, connected to the base station via a high bandwidth and low latency link. With DAS, the base station operates as if it had multiple antennas, but the antennas happen to be in different geographic locations. DAS have been shown to enhance coverage and capacity in cellular systems, in a variety of different configurations. This paper proposes, analyzes, and compares several downlink multiuser multiple input multiple output (MIMO) DAS strategies in terms of per-user throughput and area spectral efficiency. Zero-forcing transmit beamforming is used for transmission, the remote radio units may have one or more antennas, and the subscriber has a single receive antenna. Techniques considered include beamforming across all remote radio units (full transmission), using the same beamforming vector for each remote radio unit (simplified transmission), and selecting a subset of remote radio units. To facilitate rapid simulation and design space exploration, approximations of the ergodic rate are proposed for each technique assuming path-loss, small-scale Rayleigh fading, and out-of-cell interference. Simulations accounting for multiple interfering cells are used to compare the different transmission techniques. Full transmission is found to have the best performance even accounting for out-of-cell interference, though gains diminish for higher numbers of active users. Simplified transmission improves over no DAS but performance degrades with more active remote radio units.}, number={10}, journal={IEEE Transactions on Signal Processing}, year={2011}, pages={4885–4899} }
 @article{noniterative coordinated beamforming for multiuser mimo systems with limited feedforward_2011, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054953445&partnerID=MN8TOARS}, DOI={10.1109/LSP.2011.2171483}, abstractNote={This letter proposes three methods to jointly calculate beamforming and combining vectors for the downlink of TDD multiuser multiple-input multiple-output systems. The algorithms use quantized information of the beamformers to get the corresponding combiners, maximizing the signal-to-interference-plus-noise-ratio. Two of the proposed methods exceed in performance those already published. The third method reduces the feedforward overhead with a light degradation in performance. Our three proposals are based on a perfect interference cancellation at the transmitter before calculating the quantized beamformers.}, number={12}, journal={IEEE Signal Processing Letters}, year={2011}, pages={701–704} }
 @inproceedings{akoum_kountouris_heath_2011, title={On imperfect CSI for the downlink of a two-tier network}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054818559&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2011.6034189}, abstractNote={In this paper, we consider a hierarchical two-tier cellular network where a macrocell is overlaid with a tier of randomly distributed femtocells. We evaluate the combined effect of uncoordinated cross-tier interference, feedback delay, and quantization errors on the achievable rate of transmit beamforming with imperfect channel state information (CSI). We model the femtocell spatial distribution as a Poisson point process (PPP) and the temporal correlation of the channel according to a Gauss-Markov model. Using stochastic geometry tools, we derive the probability of outage at the macrocell users as a function of the temporal correlation, the femtocell density, and the feedback rate. We compute the maximum average achievable rate on the downlink of the macrocell network using a properly designed rate backoff scheme. We show that transmit beamforming with imperfect CSI is a viable option for the downlink of a two-tier cellular network, and that rate backoff recovers the loss in rate due to packet outage.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Akoum, S. and Kountouris, M. and Heath, R.W.}, year={2011}, pages={553–557} }
 @article{vaze_heath_2011, title={On the capacity and diversity-multiplexing tradeoff of the two-way relay channel}, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79959534932&partnerID=MN8TOARS}, DOI={10.1109/TIT.2011.2146710}, abstractNote={In a two-way relay channel, two sources use one or more relay nodes to exchange data with each other. This paper considers a multiple input multiple output (MIMO) two-way relay channel, where each relay node has one or more antennas. Optimal relay transmission strategies for the two-way relay channel are derived to maximize the achievable rate with amplify and forward (AF) at each relay and to achieve the optimal diversity-multiplexing tradeoff (DM-tradeoff). To maximize the achievable rate with AF, an iterative algorithm is proposed which solves a power minimization problem subject to minimum signal-to-interference-and-noise ratio constraints at every step. The power minimization problem is nonconvex. The Karush Kuhn Tucker conditions, however, are shown to be sufficient for optimality. Capacity scaling law of the two-way relay channel with increasing number of relays is also established by deriving a lower and upper bound on the capacity region of the two-way relay channel. To achieve the optimal DM-tradeoff, a compress and forward strategy is proposed and its DM-tradeoff is derived. For the full-duplex two-way relay channel, the proposed strategy achieves the optimal DM-tradeoff, while for the half-duplex case the proposed strategy is shown to achieve the optimal DM-tradeoff under some conditions.}, number={7}, journal={IEEE Transactions on Information Theory}, author={Vaze, R. and Heath, R.W.}, year={2011}, pages={4219–4234} }
 @inproceedings{optimizing training and feedback for mimo interference alignment_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861321835&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2011.6190314}, abstractNote={Interference alignment (IA) is a cooperative transmission strategy that, under some conditions, achieves the interference channel's maximum multiplexing gain. Realizing the gains of IA, however, is contingent upon efficiently providing the transmitters with the accurate channel knowledge needed for alignment. In this paper we study the performance of IA in multiple-input multiple-output systems where channel knowledge is acquired by training and analog feedback. We derive throughput-maximizing training and feedback resource allocations accounting for estimation error, training and feedback overhead, and channel selectivity. We accurately characterize the effective sum rate with overhead in relation to various parameters such as signal-to-noise ratio and Doppler spread. We show that the overhead of IA can be optimized to ensure good performance in a wide range of fading scenarios.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2011}, pages={1717–1721} }
 @article{overhead reduction in coordinated beamforming for multiuser mimo-ofdm systems with limited feedforward_2011, volume={E94-B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80155138412&partnerID=MN8TOARS}, DOI={10.1587/transcom.E94.B.3168}, abstractNote={This letter proposes a method for choosing the best quantized beamforming vector that represents a subcarrier group, for coordinated beamforming in the downlink of multiuser multiple input multiple output-orthogonal frequency division multiplexing systems. The correlation between subcarriers is exploited for reducing the feedforward overhead, while maximizing the sum rate.}, number={11}, journal={IEICE Transactions on Communications}, year={2011}, pages={3168–3171} }
 @inproceedings{relay beamforming using interference pricing for the two-hop interference channel_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857216431&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2011.6134105}, abstractNote={Relays in cellular systems are sensitive to interference. A good relay design will mitigate received interference and will transmit in a way that avoids excess out-of-cell interference. This paper proposes a distributed relay beamforming algorithm for the two-hop interference channel, which models a relay-based cellular system. Interference pricing is used to provide the relays with information on how the interference impacts the end-to-end achievable rates. A new method is proposed to compute the interference prices via an approximation of the end-to-end achievable rate, which accounts for the relationship of the parameters in two hops. This alleviates the rate mismatch experienced by the naive application of single- hop interference pricing algorithms. Simulations show that the proposed algorithm outperforms single-hop interference pricing, zero forcing and maximal ratio transmission beamforming.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2011} }
 @inproceedings{robust beamforming and power control for two-tier femtocell networks_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80051971616&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2011.5956628}, abstractNote={In this paper, we consider a two-tier network consisting of a macrocell and several closed-access femtocells sharing bandwidth with the macrocell. With multiple transmit antennas at both the macrocell and femtocell base stations, we propose a downlink power control and beamforming algorithm in two steps. In the first step, the beamformer of the macrocell base station is chosen to be the maximal ratio transmission beamformer, and we determine the power allocation at the macrocell and all femtocell base stations in a centralized manner by constraining that the signal power and leakage power associated with the femtocell beamformers satisfied certain predetermined thresholds. In the second step, the femtocell beamformers are obtained in a distributed manner by taking into account these thresholds. In the presence of imperfect channel state information, we further extend our proposed algorithm in a robust manner using robust optimization methodology.}, booktitle={IEEE Vehicular Technology Conference}, year={2011} }
 @article{cui_heath_leus_2011, title={Signal processing for networking and communications}, volume={28}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052219441&partnerID=MN8TOARS}, DOI={10.1109/MSP.2011.941986}, abstractNote={The general field of signal processing for networking and communications (SPCOM) is dedicated to exploring and illuminating connections between signal processing, communications, and information theory. In its early days, SPCOM was focused on physical layer technologies. Later the emphasis shifted to wireless communication and more recently the embrace of networking. SPCOM is now exploding with advances in wireless networking as well as new research directions.}, number={5}, journal={IEEE Signal Processing Magazine}, author={Cui, S. and Heath, R.W. and Leus, G.}, year={2011}, pages={151–152} }
 @inproceedings{spatial interference mitigation for multiple input multiple output ad hoc networks: miso gains_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861325258&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2011.6190095}, abstractNote={We consider spatial interference mitigation at the transmitter for multiple input single output ad hoc networks. We apply zero forcing beamforming at the transmitter, and analyze the corresponding network throughput and transmission capacity. Assuming a network with Poisson distributed transmitting nodes and spatially independent Rayleigh fading channels, we apply mathematical tools from stochastic geometry to derive a lower bound on the probability of outage. We derive scaling laws for the transmission capacity and show that for a large number of antennas, the maximum density of concurrently transmitting nodes scales linearly with the number of antennas at the transmitter, for a given outage constraint. Numerical results show that the network throughput achieved by interference nulling at the transmitter is comparable to that achieved by interference cancellation at the receiver.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2011}, pages={708–712} }
 @article{two-way transmission capacity of wireless ad-hoc networks_2011, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79959541683&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.041311.101488}, abstractNote={The transmission capacity of an ad-hoc network is the maximum density of active transmitters per unit area, given an outage constraint at each receiver for a fixed rate of transmission. Most prior work on finding the transmission capacity of ad-hoc networks has focused only on one-way communication where a source communicates with a destination and no data is sent from the destination to the source. In practice, however, two-way or bidirectional data transmission is required to support control functions like packet acknowledgements and channel feedback. This paper extends the concept of transmission capacity to two-way wireless ad-hoc networks by incorporating the concept of a two-way outage with different rate requirements in both directions. Tight upper and lower bounds on the two-way transmission capacity are derived for frequency division duplexing. The obtained bounds are used to derive the optimal solution for bidirectional bandwidth allocation that maximizes the two-way transmission capacity, which is shown to perform better than allocating bandwidth proportional to the desired rate in both directions. Using the proposed two-way transmission capacity framework, a lower bound on the two-way transmission capacity with transmit beamforming using limited feedback is derived as a function of bandwidth and the number of bits allocated for feedback.}, number={6}, journal={IEEE Transactions on Wireless Communications}, year={2011}, pages={1966–1975} }
 @inproceedings{user admission in mimo interference alignment networks_2011, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80051633341&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2011.5947105}, abstractNote={In this paper we consider an interference channel where a set of primary active users are cooperating through interference alignment over a constant multiple-input-multiple-output channel while a set of secondary users desire access to the channel. We present the conditions under which a secondary user can be admitted to the network. For the admitted users, we derive several beamforming designs maximizing approximately the secondary users' sum-rate based on the number of the secondary users, the number of antennas at the secondary users and the total number of streams in the network of active users.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2011}, pages={3360–3363} }
 @article{user arrival in mimo interference alignment networks_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857368090&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.120511.111088}, abstractNote={In this paper we analyze a constant multiple-input multiple-output interference channel where a set of active users are cooperating through interference alignment while a set of secondary users desire access to the channel. We find the minimum number of secondary transmit antennas required so that a secondary user can use the channel without affecting the sum rate of the active users, under a zero-forcing equalization assumption. When the secondary users have enough antennas, we derive several secondary user precoders that approximately maximize the secondary users' sum rate without changing the sum rate of the active users. When the secondary users do not have enough antennas, we perform numerical optimization to find secondary user precoders that cause minimum degradation to the sum rate of the active users. Through simulations, we confirm that i) with enough antennas at the secondary users, gains equivalent to the case of all the users cooperating through interference alignment is obtainable, and ii) when the secondary users do not have enough antennas, large rate losses at the active users can be avoided.channels}, number={2}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={842–851} }
 @article{user partitioning for less overhead in mimo interference channels_2012, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84857364288&partnerID=MN8TOARS}, DOI={10.1109/TWC.2011.122211.101188}, abstractNote={This paper presents a study on multiple-antenna interference channels, accounting for general overhead as a function of the number of users and antennas in the network. The model includes both perfect and imperfect channel state information based on channel estimation in the presence of noise. Three low-complexity methods are proposed for reducing the impact of overhead in the sum network throughput by partitioning users into orthogonal groups. The first method allocates spectrum to the groups equally, creating an imbalance in the sum rate of each group. The second proposed method allocates spectrum unequally among the groups to provide rate fairness. Finally, geographic grouping is proposed for cases where some receivers do not observe significant interference from other transmitters. For each partitioning method, the optimal solution not only requires a brute force search over all possible partitions, but also requires full channel state information, thereby defeating the purpose of partitioning. We therefore propose greedy methods to solve the problems, requiring no instantaneous channel knowledge. Simulations show that the proposed greedy methods switch from time-division to interference alignment as the coherence time of the channel increases, and have a small loss relative to optimal partitioning only at moderate coherence times.}, number={2}, journal={IEEE Transactions on Wireless Communications}, year={2012}, pages={592–603} }
 @article{60 ghz wireless: up close and personal_2010, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78649845496&partnerID=MN8TOARS}, DOI={10.1109/MMM.2010.938581}, abstractNote={To meet the needs of next-generation high-data-rate applications, 60 GHz wireless networks must deliver Gb/s data rates and reliability at a low cost. In this article, we surveyed several ongoing challenges, including the design of cost-efficient and low-loss on-chip and in-package antennas and antenna arrays, the characterization of CMOS processes at millimeter-wave frequencies, the discovery of efficient modulation techniques that are suitable for the unique hardware impairments and frequency selective channel characteristics at millimeter-wave frequencies, and the creation of MAC protocols that more effectively coordinate 60 GHz networks with directional antennas. Solving these problems not only provides for wireless video streaming and interconnect replacement, but also moves printed and magnetic media such as books and hard drives to a lower cost, higher reliability semiconductor form factor with wireless connectivity between and within devices.}, number={7 SUPPL.}, journal={IEEE Microwave Magazine}, year={2010} }
 @article{bhagavatula_oestges_heath_2010, title={A new double-directional channel model including antenna patterns, array orientation, and depolarization}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953767895&partnerID=MN8TOARS}, DOI={10.1109/TVT.2010.2045775}, abstractNote={Multiple-input-multiple-output (MIMO) wireless channel models are often too simplistic to accurately model wireless propagation effects or too complex and/or site specific to be used for analytical purposes. In this paper, we develop a double-directional MIMO channel model that accounts for important propagation effects like scattering, clustering, and channel depolarization and antenna effects like antenna diversity, cross polarization, and random array orientation, while still retaining an intuitive representation. The proposed model can be parameterized using channel measurements obtained from site-specific measurement campaigns or from standard-based channel models. We show, using simulations, that the proposed model captures channel and antenna effects not included in other models, like the third-generation partnership program (3GPP) spatial channel model, the WINNER, and the IEEE 802.11n channel model. We use the model to study the impact of random orientation and channel depolarization on the data rates of a MIMO system.}, number={5}, journal={IEEE Transactions on Vehicular Technology}, author={Bhagavatula, R. and Oestges, C. and Heath, R.W.}, year={2010}, pages={2219–2231} }
 @inproceedings{a simple sinr characterization for linear interference alignment over uncertain mimo channels_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955694616&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2010.5513649}, abstractNote={This paper provides a simple closed-form SINR expression for interference alignment over MIMO channels with channel uncertainty. Assuming linear processing (specifically, zero-forcing) at the transmitters and receivers and a complex Gaussian interference channel, we show that random matrix theory can be successfully applied to find the SINR distribution of each stream for each user with channel uncertainty. Perfect channel knowledge constitutes a special case. This SINR distribution allows easy calculation of useful performance metrics like symbol error-rate and achievable sum rate.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2010}, pages={2288–2292} }
 @article{adaptive mimo transmission techniques for broadband wireless communication systems_2010, volume={48}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952154883&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2010.5458371}, abstractNote={Link adaptation is a way to increase data rates in wireless systems by adapting transmission parameters such as the modulation and coding rate. While link adaptation in single antenna systems is now mature, its application to multiple-input multiple-output communication links, presented in several emerging wireless standards, has been challenging. The main reason is that the space-time transmission strategy can also be adjusted in MIMO communication links, introducing a new dimension for adaptation. This means that practical MIMO link adaptation algorithms must also provide a dynamic adaptation between diversity and multiplexing modes of operation. This article reviews a recently proposed framework for adaptive MIMO architectures and shows how to use this framework to reduce adaptive control overhead. We also discuss practical implementation issues. Simulations in an IEEE 802.16e (mobile WiMAX) system illustrate the frame-work's potential improvements in data rates.}, number={5}, journal={IEEE Communications Magazine}, year={2010}, pages={112–118} }
 @article{adaptive limited feedback for sum-rate maximizing beamforming in cooperative multicell systems_2011, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651347966&partnerID=MN8TOARS}, DOI={10.1109/TSP.2010.2090346}, abstractNote={Base station cooperation improves the sum-rates that can be achieved in cellular systems. Conventional cooperation techniques require sharing large amounts of information over finite-capacity backhaul links and assume that base stations have full channel state information (CSI) of all the active users in the system. In this paper, a new limited feedback strategy is proposed for multicell beamforming where cooperation is restricted to sharing only the CSI of active users among base stations. The system setup considered is a linear array of cells based on the “soft hand-off model,” where each cell contains single-antenna users and multi-antenna base stations. Beamforming vectors are designed using a generalized eigenvector approach to maximize the sum-rate in a single-interferer scenario, at high signal to noise ratio. Users are assumed to feedback quantized CSI of the desired and interfering channels using a finite-bandwidth feedback link. An upper bound on the mean loss in sum rate due to random vector quantization is derived. A new feedback-bit allocation strategy, to partition the available bits between the desired and interfering channels, is developed to reduce the mean loss in sum-rate due to quantization for the soft hand-off model. The proposed feedback-bit partitioning algorithm is shown, using simulations, to yield sum-rates close to the those obtained using full CSI at base station.}, number={2}, journal={IEEE Transactions on Signal Processing}, year={2011}, pages={800–811} }
 @inproceedings{adaptive transmit antenna selection in mimo amplify-and-forward relay channels_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049376997&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5496117}, abstractNote={Obtaining the most from multiple-antenna relay systems requires adapting the number of substreams based on channel conditions. In this paper, we develop algorithms for selecting dynamically the number of data streams and the transmit antenna subsets at both the source and relay to minimize the vector symbol error rate at a fixed overall rate. An antenna selection mode of operation is defined by the number of transmit antennas and the substream-to-antenna mapping at the source and relay. We propose three suboptimal multimode algorithms that aim to select the best mode based on the knowledge of channel conditions. The algorithms are shown by Monte Carlo simulations to achieve the full diversity order and to provide considerable gains over the existing designs.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={3022–3025} }
 @inproceedings{augmenting commercial wireless transceivers with time-of-arrival positioning_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955027019&partnerID=MN8TOARS}, DOI={10.1109/PLANS.2010.5507271}, abstractNote={The augmentation of commercial wireless hardware with time-of-arrival (ToA) estimation enables pervasive wireless networks to serve additionally as positioning systems. ToA-augmented wireless links offer the ability to provide accurate localization in environments not served by global navigation satellite systems (GNSS) with greater accuracy than inertial sensing systems and wireless fingerprinting. In this paper we propose the augmentation of IEEE 802.11 commercial hardware with ToA through an add-on hardware module. Two processing architectures are introduced, loopback and loopback with probing, to leverage the ToA module. The calibration of local device clocks due to unstable frequency references is also presented. Finally, a discussion of positioning algorithms with simulations demonstrates the efficacy of the proposed ToA module to provide high-precision, low-cost positioning without the addition of significant system infrastructure.}, booktitle={Record - IEEE PLANS, Position Location and Navigation Symposium}, year={2010}, pages={982–987} }
 @article{zwingelstein-colin_akoum_heath_debbah_2011, title={Cognitive cooperation for the downlink of frequency reuse small cells}, volume={2011}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952167235&partnerID=MN8TOARS}, DOI={10.1155/2011/525271}, abstractNote={We develop a cooperative diversity protocol coded over space, time, and frequency to achieve improved quality of service for mobile users in the downlink of small-cell frequency reuse networks. The proposed protocol, called cooperative frequency reuse (CFR), leverages the cellular frequency reuse concept to create space and frequency diversity among pairs of adjacent base stations. The CFR protocol is compatible with the half-duplex mode and is distributed in the sense that each base station acts in autonomy, without the need of a centralized entity. It is implemented in two phases. During the first phase, each base station independently serves its own users on its dedicated frequency band. It simultaneously listens to the symbols transmitted by neighboring base stations. Cognitive cooperation is introduced in the second phase, where each base station transmits on two frequency bands to the scheduled users in both base stations, by means of an appropriately chosen distributed space time code based on the Golden code. We analyze and discuss the performance of the proposed protocol in terms of bit error rate, probability of outage, and ergodic sum rate under different scenarios. Simulation results show that the proposed protocol yields considerable improvement over direct transmission frequency reuse strategies.}, journal={Eurasip Journal on Advances in Signal Processing}, author={Zwingelstein-Colin, M. and Akoum, S. and Heath, R.W. and Debbah, M.}, year={2011} }
 @inproceedings{cognitive cooperation for the downlink of frequency reuse small cells_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78349284430&partnerID=MN8TOARS}, DOI={10.1109/CIP.2010.5604251}, booktitle={2010 2nd International Workshop on Cognitive Information Processing, CIP2010}, year={2010}, pages={111–115} }
 @article{cooperative algorithms for mimo interference channels_2011, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78751670063&partnerID=MN8TOARS}, DOI={10.1109/TVT.2010.2085459}, abstractNote={Interference alignment (IA) is a transmission technique for exploiting all available degrees of freedom in the frequency- or time-selective interference channel with an arbitrary number of users. Most prior work on IA, however, neglects interference from other nodes in the network that are not participating in the alignment operation. This paper proposes three generalizations of IA for the multiple-antenna interference channel with multiple users that account for colored noise, which models uncoordinated interference. First, a minimum interference-plus-noise leakage (INL) algorithm is presented and shown to be equivalent to previous subspace methods when noise is spatially white or negligible. This algorithm results in orthonormal precoders that are desirable for practical implementation with limited feedback. A joint minimum mean square error design that jointly optimizes the transmit precoders and receive spatial filters is then proposed, whereas previous designs neglect the receive spatial filter. Finally, a maximum signal-to-interference-plus-noise ratio (SINR) algorithm is developed and proven to converge, unlike previous maximum SINR algorithms. The sum throughput of these algorithms is simulated in the context of a network with uncoordinated cochannel interferers that are not participating in the alignment protocol. It is found that a network with cochannel interference can benefit from employing precoders that are designed to consider that interference, but in extreme cases, such as when only one receiver has a large amount of interference, ignoring that the cochannel interference is advantageous.}, number={1}, journal={IEEE Transactions on Vehicular Technology}, year={2011}, pages={206–218} }
 @inproceedings{coordinated single-cell vs multi-cell transmission with limited-capacity backhaul_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79957984126&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2010.5757724}, abstractNote={Base station coordination is an efficient technique to transcend the limits on spectral efficiency imposed by intercell interference. In this paper, we compare the performance of different coordination strategies with different amount of channel state information (CSI) and data sharing among the coordinating base stations. We focus on the effect of limited backhaul capacity in a two-cell network. Contrary to the common belief, we show that coordination strategies with no data and only limited CSI sharing is preferred to those with full data and CSI sharing when the backhaul capacity is relatively low and the edge SNR is high.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2010}, pages={1217–1221} }
 @inproceedings{grassmannian predictive frequency domain compression for limited feedback beamforming_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952740579&partnerID=MN8TOARS}, DOI={10.1109/ITA.2010.5454127}, abstractNote={Frequency domain channel correlation can be exploited to reduce feedback in limited feedback beamforming multiple-input multiple-output orthogonal frequency division multiplexing wireless systems. Prior methods rely on downsampling, interpolation, or clustering the channel state information in the frequency domain. The resulting compressed samples are quantized using one-shot quantization on the Grassmann manifold. The resolution, unfortunately, is limited. We propose a new frequency domain compression technique to obtain high resolution channel state information. The key idea is to use predictive coding on the Grassmann manifold, exploiting the correlation between adjacent subcarriers.}, booktitle={2010 Information Theory and Applications Workshop, ITA 2010 - Conference Proceedings}, year={2010}, pages={173–177} }
 @inproceedings{interference alignment with analog csi feedback_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79951606689&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2010.5680216}, abstractNote={Interference alignment has been used to derive and realize the maximum multiplexing gain of the MIMO interference channel. Interference alignment requires some form of channel state information at the transmitter. This paper proposes analog CSI feedback as an alternative to limited feedback or reciprocity based alignment. We show that the maximum degrees of freedom of the K-user interference channel are preserved with the suggested feedback strategy, and that the mean loss in sum rate incurred by imperfect CSI is bounded by a constant provided that the signal-to-noise ratio is comparable in both forward and reverse channels. Finally, demonstrate the performance of the proposed strategy through simulation.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2010}, pages={1644–1648} }
 @article{panah_truong_peters_heath_2011, title={Interference management schemes for the shared relay concept}, volume={2011}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78249259108&partnerID=MN8TOARS}, DOI={10.1155/2011/269817}, abstractNote={Sharing a multiantenna relay among several sectors is a simple and cost-effective way to achieving much of the gains of local interference mitigation in cellular networks. Next generation wireless systems, such as ones based on the Third Generation Partnership Projects Long-Term Evolution Advanced, will employ universal frequency reuse to simplify network deployment. This strategy is anticipated to create significant cell-edge interference in the location of the shared relays, thus rendering advanced interference management strategies a necessity. This paper proposes several interference management strategies for the shared relays ranging from simple channel inversion at the relay, to more sophisticated techniques based on channel inversion in combination with partial and full base station coordination in the downlink and uplink. Given that the relay functionality influences total interference, both amplify-and-forward and decode-and-forward type relays are considered throughout. In this context, channel cancelation techniques are investigated for one-way relaying and also the spectrally efficient two-way relaying protocol. Simulations show that strategies based on two-way shared relaying with bidirectional channel inversion at the relay often perform best in terms of total system throughput while one-way techniques are promising when the relay power is low.}, journal={Eurasip Journal on Advances in Signal Processing}, author={Panah, A.Y. and Truong, K.T. and Peters, S.W. and Heath, R.W.}, year={2011} }
 @inproceedings{limited feedback beamforming for temporally correlated mimo channels with other cell interference_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049362293&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5496108}, abstractNote={Limited feedback beamforming improves link reliability with a small amount of feedback from the receiver to the transmitter. The performance of such a closed loop MIMO system is unknown in interference limited cellular environments, when the base stations have limited or no coordination. This paper establishes the degradation in throughput due to uncoordinated other cell interference and delay on the feedback channel. Under a Markov channel assumption, the paper shows that the throughput gain of cell edge users decays doubly exponentially as the delay increases. Numerical results illustrate how the decay rate decreases when the codebook size increases.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={3054–3057} }
 @article{limited feedback for temporally correlated mimo channels with other cell interference_2010, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77956718998&partnerID=MN8TOARS}, DOI={10.1109/TSP.2010.2055860}, abstractNote={Limited feedback improves link reliability with a small amount of feedback from the receiver to the transmitter. In cellular systems, the performance of limited feedback will be degraded in the presence of other cell interference, when the base stations have limited or no coordination. This paper establishes the degradation in sum rate of users in a cellular system, due to uncoordinated other cell interference and feedback delay. A goodput metric is defined as the rate when the bits are successfully received at the mobile station, and used to derive an upper bound on the performance of limited feedback systems with delay. This paper shows that the goodput gained from having delayed limited feedback decreases doubly exponentially as the delay increases. The analysis is extended to precoded spatial multiplexing systems where it is shown that the same upper bound can be used to evaluate the decay in the achievable sum rate. To reduce the effects of interference, zero forcing interference nulling is applied at the receiver, where it is shown that the effect of the interference on the achievable sum rate can be suppressed by nulling out the interferer. Numerical results show that the decay rate of the goodput decreases when the codebook quantization size increases and when the Doppler shift in the channel decreases.}, number={10}, journal={IEEE Transactions on Signal Processing}, year={2010}, pages={5219–5232} }
 @inproceedings{limited feedback with joint csi quantization for multicell cooperative generalized eigenvector beamforming_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049376001&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5496190}, abstractNote={Existing work on limited feedback for cooperative multicell beamforming quantizes the desired and interfering channel state information (CSI) using separate codebooks. In this paper, it is shown that comparatively higher sum-rates can be obtained by jointly quantizing the desired and interfering CSI using a single codebook. A selection criterion is developed for random vector quantization (RVQ) to show that joint quantization with RVQ yields higher sum-rates than those obtained using separate codebooks. The generalized Lloyd algorithm is then used to generate codebooks using the codeword design strategy proposed in this paper. Simulations are used to show that the proposed joint quantization approaches perform almost as well as the full CSI case.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={2838–2841} }
 @inproceedings{link adaptation in mimo-ofdm with non-uniform constellation selection over spatial streams through supervised learning_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049354402&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5496020}, abstractNote={Supervised learning has been used to develop practical link adaptation algorithms for MIMO-OFDM under an equal rate per stream assumption. In this paper we develop supervised learning algorithms that select from non-uniform rates per stream. We show that the straightforward application of existing supervised learning link adaptation algorithms exhibits complexity that scales with the number of spatial streams. Therefore, we propose a decoupled stream link adaptation algorithm which reduces the complexity below the original supervised learning algorithm with uniform spatial streams. We further show that the performance loss of decoupled link adaptation is reduced in systems with non-uniform constellations per spatial stream. IEEE 802.11n and uncoded MIMO-OFDM simulations are used to validate the proposed algorithms.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={3314–3317} }
 @article{panah_heath_2010, title={MIMO two-way amplify-and-forward relaying with imperfect receiver CSI}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78149429387&partnerID=MN8TOARS}, DOI={10.1109/TVT.2010.2070087}, abstractNote={In this paper, we investigate the effects of channel estimation error on the receiver of multiple-input-multiple-output (MIMO) amplify-and-forward (AF) two-way relaying. The estimation is conducted via training using known pilot symbols transmitted from the terminals, which is a method commonly referred to as pilot symbol-aided modulation. In this context, linear minimum mean square estimation (LMMSE) of composite and individual channels is analyzed. We show that orthogonal pilots, i.e., pilot sequences that are orthogonal over the antenna arrays and the communicating terminals, minimize the composite and individual mean square errors. Using notions of worst-case noise, we obtain lower bounds on the sum rate of information transmission in both directions of the bidirectional relay link and show that the orthogonal pilots perform well against the bound.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, author={Panah, A.Y. and Heath, R.W.}, year={2010}, pages={4377–4387} }
 @inproceedings{maximum sum-rate interference alignment algorithms for mimo channels_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79551622757&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2010.5683919}, abstractNote={Alternating minimization algorithms are typically used to find interference alignment (IA) solutions for multiple-input multiple-output (MIMO) interference channels with more than K=3 users. For these scenarios many IA solutions exit, and the initial point determines which one is obtained upon convergence. In this paper, we propose a new iterative algorithm that aims at finding the IA solution that maximizes the average sum-rate. At each step of the alternating minimization algorithm, either the precoders or the decoders are moved along the direction given by the gradient of the sum-rate. Since IA solutions are defined by a set of subspaces, the gradient optimization is performed on the Grassmann manifold. The step size of the gradient ascent algorithm is annealed to zero over the iterations in such a way that during the last iterations only the interference leakage is being minimized and a perfect alignment solution is finally reached. Simulation examples are provided showing that the proposed algorithm obtains IA solutions with significant higher throughputs than the conventional IA algorithms.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2010} }
 @article{multi-mode transmission for the mimo broadcast channel with imperfect channel state information_2011, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952988005&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2011.121410.100144}, abstractNote={This paper proposes an adaptive multi-mode transmission strategy to improve the spectral efficiency achieved in the multiple-input multiple-output (MIMO) broadcast channel with delayed and quantized channel state information. The adaptive strategy adjusts the number of active users, denoted as the transmission mode, to balance transmit array gain, spatial division multiplexing gain, and residual inter-user interference. Accurate closed-form approximations are derived for the achievable rates for different modes, which help identify the active mode that maximizes the average sum throughput for given feedback delay and channel quantization error. The proposed transmission strategy can be easily combined with round-robin scheduling to serve a large number of users. As instantaneous channel information is not exploited, the proposed algorithm cannot provide multiuser diversity gain, but it is still able to provide throughput gain over single-user MIMO at moderate signal-to-noise ratio. In addition, it has a light feedback overhead and only requires feedback of instantaneous channel state information from a small number of users. In the system with a feedback load constraint, it is shown that the proposed algorithm provides performance close to that achieved by opportunistic scheduling with instantaneous feedback from a large number of users.}, number={3}, journal={IEEE Transactions on Communications}, year={2011}, pages={803–814} }
 @article{multimode antenna selection for mimo amplify-and-forward relay systems_2010, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79953813194&partnerID=MN8TOARS}, DOI={10.1109/TSP.2010.2053364}, abstractNote={Obtaining the most from multiple-antenna relay systems requires algorithms that configure the source and relay adaptively to instantaneous channel conditions. In this paper, we define an antenna selection mode of operation as the number of selected transmit antennas at the source (which is equal to the number of data substreams), the substream-to-antenna mapping at the source, the number of selected transmit antennas at the relay, and the substream-to-antenna mapping at the relay. We develop dualmode and multimode antenna selection algorithms to choose the mode that is most likely to deliver the lowest vector symbol error rate assuming the overall data rate is fixed. The effective condition numbers of both the two-hop channel and the relay channel are derived to give intuition on how the spatial characteristics of the constituent channels affect mode selection and to derive low complexity algorithms. Link-level simulations show that our proposed algorithms usually select the best mode, thus improving the diversity performance of spatial multiplexing relay systems and providing array gains over the existing single-stream relay transmission strategies. The two-hop multimode algorithms are shown by system-level simulations to improve the reliability of transmission and extend spatial multiplexing capability to cell-edge users in a multi-cell network.}, number={11}, journal={IEEE Transactions on Signal Processing}, year={2010}, pages={5845–5859} }
 @article{seifi_viberg_heath_zhang_coldrey_2011, title={Multimode transmission in network MIMO downlink with incomplete CSI}, volume={2011}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79251648869&partnerID=MN8TOARS}, DOI={10.1155/2011/743916}, abstractNote={We consider a cooperative multicell MIMO (a.k.a network MIMO) downlink system with multiantenna base stations (BSs), which are connected to a central unit and communicate with multiantenna users. In such a network, obtaining perfect channel state information (CSI) of all users at the central unit to exploit opportunistic scheduling requires a substantial amount of feedback and backhaul signaling. We propose a scheduling algorithm based only on the knowledge of the average SNR at each user from all the cooperating BSs, denoted as incomplete CSI. Multimode transmission is applied that is able to adaptively adjust the number of data streams transmitted to each user. Utilizing the results of random matrix theory, an analytical framework is proposed to approximate the ergodic rate of each user with different number of data streams. Using these ergodic rates, a joint user and mode selection algorithm is proposed, where only the scheduled users need to feed back instantaneous CSI. Simulation results demonstrate that the developed analytical framework provides a good approximation for a practical number of antennas. While substantially reducing the feedback overhead, it is shown that the proposed scheduling algorithm performs closely to the opportunistic scheduling algorithm that requires instantaneous CSI feedback from all users.}, journal={Eurasip Journal on Advances in Signal Processing}, author={Seifi, N. and Viberg, M. and Heath, R.W. and Zhang, J. and Coldrey, M.}, year={2011} }
 @inproceedings{multiuser mimo in distributed antenna systems_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79957978588&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2010.5757721}, abstractNote={Distributed antenna systems (DAS) add remote radio units with one or more antennas to enhance coverage and capacity in cellular systems. DAS works by connecting the remote radio units to the base station via a high bandwidth and low latency link. This paper analyzes multiuser multiple input multiple output (MIMO) DAS and compares it with multiuser MIMO communication without DAS in terms of per-user throughput and area spectral efficiency. Zero-forcing transmit beamforming is used at the transmitter. To facilitate rapid simulation and design space exploration, approximations of the ergodic rate are proposed for each technique assuming path-loss, small-scale Rayleigh fading, and out-of-cell interference. Treating all the remote radio units as a super MIMO transmitter gives the best performance even accounting for out-of-cell interference, though gains diminish for higher numbers of active users.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2010}, pages={1202–1206} }
 @inproceedings{online adaptive modulation and coding with support vector machines_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77954440908&partnerID=MN8TOARS}, DOI={10.1109/EW.2010.5483527}, abstractNote={Optimizing the performance of adaptive modulation and coding (AMC) in practice has proven challenging. Prior research has struggled to find link quality metrics that are suitable for look-up-tables and simultaneously provide an injective mapping to error rate in wireless links that feature selective channels with hardware nonlinearities and non-Gaussian noise effects. This paper proposes a novel online support vector machine algorithm, compatible with accurate multidimensional link quality metrics, that is able to optimize AMC to the unique (potentially dynamic) hardware characteristics of each wireless device in selective channels. IEEE 802.11n simulations show that our proposed algorithm allows each individual wireless device to optimize the operating point in the rate/reliability tradeoff through frame-by-frame error evaluation. These simulations also show that our algorithm displays identical performance to alternative online AMC algorithms while drastically reducing complexity.}, booktitle={2010 European Wireless Conference, EW 2010}, year={2010}, pages={718–724} }
 @inproceedings{predictive limited feedback for cooperative transmission_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79957992534&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2010.5757728}, abstractNote={In multicell cooperation, knowledge of the users' channel state information (CSI) at the base stations can be used to improve sum-rates and reduce outages. Users estimate and feedback CSI of the desired and multiple interfering channels in cooperation-based systems. Existing multicell limited feedback techniques are memoryless quantization approaches, which result in large feedback overhead. In this paper, a novel technique is proposed using predictive vector quantization to reduce feedback requirements associated with multicell systems by exploiting temporal correlation in channels. An approach to further reduce feedback overhead by transmitting back only the normalized vector directions is also presented, using linear prediction. Simulations are used to show that the proposed predictive framework yields higher sum-rates with fewer bits as compared to existing multicell limited feedback strategies.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2010}, pages={1237–1241} }
 @inproceedings{sum-rate of mimo two-way relaying with imperfect csi_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049355341&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5495978}, abstractNote={In this paper we analyze pilot-aided channel estimation for the multiple-input multiple-output amplify-and-forward two-way relay channel. We use a linear minimum mean-square estimator for estimating the composite channel at each communicating terminal and by using the notion of “worst-case noise” we derive lower-bounds for the transmission sum-rate in both directions of the established bi-directional relay link.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={3418–3421} }
 @article{el ayach_peters_heath_2010, title={The feasibility of interference alignment over measured MIMO-OFDM channels}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78149417620&partnerID=MN8TOARS}, DOI={10.1109/TVT.2010.2082005}, abstractNote={Interference alignment (IA) has been shown to achieve the maximum achievable degrees of freedom in the interference channel. This results in sum rate scaling linearly with the number of users in the high signal-to-noise-ratio (SNR) regime. Linear scaling is achieved by precoding the transmitted signals to align interference subspaces at the receivers given channel knowledge of all transmit-receive pairs, effectively reducing the number of discernible interferers. The theory of IA was derived under assumptions about the richness of scattering in the propagation channel; practical channels do not guarantee such ideal characteristics. This paper presents the first experimental study of IA in measured multiple-input-multiple-output orthogonal frequency-division-multiplexing (MIMO-OFDM) interference channels. Our measurement campaign includes a variety of indoor and outdoor measurement scenarios at The University of Texas at Austin. We show that IA achieves the claimed scaling factors, or degrees of freedom, in several measured channel settings for a three-user two-antenna-per-node setup. In addition to verifying the claimed performance, we characterize the effect of Kronecker spatial correlation on sum rate and present two other correlation measures, which we show to be more tightly related to the achieved sum rate.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, author={El Ayach, O. and Peters, S.W. and Heath, R.W.}, year={2010}, pages={4309–4321} }
 @inproceedings{two-way transmission capacity of wireless ad-hoc networks_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955670406&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2010.5513314}, abstractNote={The transmission capacity of an ad-hoc network is the maximum density of active transmitters in a unit area, given an outage constraint at each receiver for a fixed rate of transmission. Most prior work on finding the transmission capacity of ad-hoc networks has focused only on one-way communication where a source communicates with destination and no data is sent from destination to the source. In practice, however, two-way or bidirectional data transmission is required to support control functions like packet acknowledgements and channel feedback. This paper develops the concept of transmission capacity for two-way wireless ad-hoc networks, by incorporating the concept of a two-way outage with different rate requirements in both directions. Upper and lower bounds on the two-way transmission capacity are derived for frequency division duplexing, under the assumption that the channel coefficients are independent on different carrier frequencies. The derived bounds are used to derive the optimal solution for bidirectional bandwidth allocation that maximizes the two-way transmission capacity, which is shown to perform better than allocating bandwidth proportional to the desired rate in both directions.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2010}, pages={1688–1692} }
 @article{a cross-layer approach to energy efficiency for adaptive mimo systems exploiting spare capacity_2009, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-73149096278&partnerID=MN8TOARS}, DOI={10.1109/TWC.2009.081123}, abstractNote={In this paper, we propose a mechanism to switch between multiple-input multiple-output (MIMO) with two transmit antennas and single-input multiple-output (SIMO) to conserve mobile terminals' energy. We focus on saving uplink RF transmission energy of mobile terminals in cellular systems supporting best effort traffic. The key idea is to judiciously slow down transmission rates when a base station is underutilized. We show that there exists a crossover point on the transmission rate below which SIMO consumes less power than MIMO when circuit power is included. The crossover point is an increasing function of the circuit power, the number of receive antennas and channel correlation, all of which increase the potential energy savings resulting from mode switching. We propose an adaptive mode switching algorithm combined with rate selection to maintain a user's target throughput while achieving energy efficiency. Extensive flow-level simulations under dynamic loads confirm that the proposed technique can reduce the transmission energy by more than 50% and enables an effective tradeoff between file transfer delay and energy conservation.}, number={8}, journal={IEEE Transactions on Wireless Communications}, year={2009}, pages={4264–4275} }
 @inproceedings{achievable throughput of multi-mode multiuser mimo with imperfect csi constraints_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449499052&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2009.5205915}, abstractNote={In the multiple-input multiple-output (MIMO) broadcast channel with imperfect channel state information (CSI), neither the capacity nor the optimal transmission technique have been fully discovered. In this paper, we derive achievable ergodic rates for a multi-antenna fading broadcast channel when CSI at the transmitter (CSIT) is delayed and quantized. It is shown that not all possible users should be supported with spatial division multiplexing due to the residual inter-user interference caused by imperfect CSIT. Based on the derived achievable rates, we propose a multi-mode transmission strategy to maximize the throughput, which adaptively adjusts the number of active users based on the channel statistics information.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2009}, pages={2659–2663} }
 @article{daniels_caramanis_heath_2010, title={Adaptation in convolutionally coded MIMO-OFDM wireless systems through supervised learning and SNR ordering}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76849102593&partnerID=MN8TOARS}, DOI={10.1109/TVT.2009.2029693}, abstractNote={Multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) wireless systems use link adaptation to exploit the dynamic nature of wireless environments. Link adaptation maximizes throughput while maintaining target reliability by adaptively selecting the modulation order and coding rate. Link adaptation is extremely challenging, however, due to the difficulty in predicting error rates in OFDM with binary convolutional codes, bit interleaving, MIMO processing, and real channel impairments. This paper proposes a new machine-learning framework that exploits past observations of the error rate and the associated channel-state information to predict the best modulation order and coding rate for new realizations of the channel state without modeling the input-output relationship of the wireless transceiver. Our approach is enabled through our new error-rate expression that is only parameterized by postprocessing signal-to-noise ratios (SNRs), ordered over subcarriers and spatial streams. Using ordered SNRs, we propose a low-dimensional feature set that enables machine learning to increase the accuracy of link adaptation. An IEEE 802.11n simulation study validates the application of this machine-learning framework in real channels and demonstrates the improved performance of SNR ordering as it compares with competing link-quality metrics.}, number={1}, journal={IEEE Transactions on Vehicular Technology}, author={Daniels, R.C. and Caramanis, C.M. and Heath, R.W.}, year={2010}, pages={114–126} }
 @article{adaptive mode switching in correlated multiple antenna cellular networks_2009, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-68149092614&partnerID=MN8TOARS}, number={3}, journal={Journal of Communications and Networks}, year={2009}, pages={279–286} }
 @article{lo_vishwanath_heath_2010, title={An energy-based comparison of long-hop and short-hop routing in MIMO networks}, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76849101090&partnerID=MN8TOARS}, DOI={10.1109/TVT.2009.2033075}, abstractNote={This paper considers the problem of selecting either routes that consist of long hops or routes that consist of short hops in a network of multiple-antenna nodes, where each transmitting node employs spatial multiplexing. This distance-dependent route-selection problem is approached from the viewpoint of energy efficiency, where a route is selected with the objective of minimizing the transmission energy consumed while satisfying a target outage criterion at the final destination. Deterministic line networks and 2-D random networks are considered. It is shown that short-hop routing requires less energy than long-hop routing when (1) the number of hops traversed between the source and destination grows large, (2) the target success probability approaches one, or (3) the number of transmit and/or receive antennas grows large. It is also shown that if both routing strategies are subject to the same delay constraint, long-hop routing requires less energy than short-hop routing as the target success probability approaches one. In addition, a numerical analysis indicates that, given loose outage constraints, only a small number of transmit antennas are needed for short-hop routing to have its maximum advantage over long-hop routing, while given stringent outage constraints, the advantage of short-hop over long-hop routing always increases with additional transmit antennas.}, number={1}, journal={IEEE Transactions on Vehicular Technology}, author={Lo, C.K. and Vishwanath, S. and Heath, R.W.}, year={2010}, pages={394–405} }
 @inproceedings{an experimental evaluation of rate adaptation for multi-antenna systems_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349668645&partnerID=MN8TOARS}, DOI={10.1109/INFCOM.2009.5062157}, abstractNote={Increasingly wireless networks use multi-antenna nodes as in IEEE 802.11n and 802.16. The Physical layer (PHY) in such systems may use the antennas to provide multiple streams of data (spatial multiplexing) or to increase the robustness of fewer streams. These physical layers also provide support for sending packets at different rates by changing the modulation and coding of transmissions. Rate adaptation is the problem of choosing the best transmission mode for the current channel and in these systems requires choosing both the level of spatial multiplexing and the modulation and coding. Hydra is an experimental wireless network node prototype in which both the MAC and PHY are highly programmable. Hydra's PHY is essentially the 802.11n PHY, and currently supports two antennas and the same modulations and codings as 802.11n. Because of limitations of our hardware platform, the actual rates are a factor of 10 smaller than 802.11n. The MAC is essentially the 802.11 MAC with extensions, including the ability to feedback channel state or rate information from the receiver. Hydra was designed to allow experimentation with real radios, PHYs, and network stacks over real-world channels and it is well suited to studying rate adaptation in multi-antenna systems. To allow controlled experimentation, we also have the ability to perform experiments over emulated channels using exactly the same MAC and PHY used for RF transmissions. We present rate control experiments based on transmission over both real and emulated channels. Our experiments include measurements for single antenna systems and two antenna systems using a single or multiple spatial streams. We study rate adaptation algorithms using both explicit and implicit feedback from the receiver. A novel aspect of our results is the first experimental study of adaptation between single and multiple spatial streams for 802.11n style systems. Increasingly wireless networking technologies, including IEEE 802.11n and IEEE 802.16, support radios with multiple an- tennas. These antennas can be used to support multiple data streams (spatial multiplexing) or to increase robustness by tak- ing advantage of channel diversity (1), (2). Choosing between}, booktitle={Proceedings - IEEE INFOCOM}, year={2009}, pages={2313–2321} }
 @inproceedings{an online learning framework for link adaptation in wireless networks_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349289675&partnerID=MN8TOARS}, DOI={10.1109/ITA.2009.5044935}, abstractNote={Current and future wireless networks require the selection of a plurality of parameters at different layers of the communication system to optimize network throughput while satisfying certain reliability constraints. In prior work, mathematical input/output models along with system performance expressions have been used to perform the parameter selection. In practice, however, impairments such as interference and analog circuit nonlinearities are difficult to model in a simple and tractable framework. Moreover, these impairments are in flux due to environmental factors. This paper summarizes an online machine learning approach to parameter selection through the real-time capturing of performance related data. Online learning is advantageous, not only because changes in the system model can be captured in the data observations, but also for its ability to learn system operation details not provided by current system models. A modified version of k-nearest neighbor is developed to enable both the real-time capture of training data and the performance criterion for physical layer adaptation.}, booktitle={Information Theory and Applications Workshop, ITA 2009}, year={2009}, pages={138–140} }
 @inproceedings{block diagonalization in the mimo broadcast channel with delayed csit_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951583643&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2009.5426132}, abstractNote={This paper investigates the impact of delayed channel state information at the transmitter (CSIT) on the MIMO broadcast channel with block diagonalization (BD) precoding. First, an upper bound for the achievable throughput is provided, which shows that BD is more robust to imperfect CSIT than zeroforcing precoding as it has fewer inter-user interfering streams. Due to residual inter-user interference, the throughput of BD still saturates at high SNR, which motivates switching between single-user and multi-user precoding. An accurate closed-form approximation is derived for the achievable throughput of the BD system, which provides guidance on the preferred transmission technique for a given scenario.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2009} }
 @article{vaze_heath_2009, title={End-to-End joint antenna selection strategy and distributed compress and forward strategy for relay channels}, volume={2009}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-68949105570&partnerID=MN8TOARS}, DOI={10.1155/2009/295418}, abstractNote={Multihop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Previously, distributed space-time codes were proposed to maximize the achievable diversity-multiplexing tradeoff; however, they fail to achieve all the points of the optimal diversity-multiplexing tradeoff. In the presence of a low-rate feedback link from the destination to each relay stage and the source, this paper proposes an end-to-end antenna selection (EEAS) strategy as an alternative to distributed space-time codes. The EEAS strategy uses a subset of antennas of each relay stage for transmission of the source signal to the destination with amplifying and forwarding at each relay stage. The subsets are chosen such that they maximize the end-to-end mutual information at the destination. The EEAS strategy achieves the corner points of the optimal diversity-multiplexing tradeoff (corresponding to maximum diversity gain and maximum multiplexing gain) and achieves better diversity gain at intermediate values of multiplexing gain, versus the best-known distributed space-time coding strategies. A distributed compress and forward (CF) strategy is also proposed to achieve all points of the optimal diversity-multiplexing tradeoff for a two-hop relay channel with multiple relay nodes.}, journal={Eurasip Journal on Wireless Communications and Networking}, author={Vaze, R. and Heath, R.W.}, year={2009} }
 @article{frequency-domain channel estimation and equalization for continuous-phase modulations with superimposed pilot sequences_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70450190510&partnerID=MN8TOARS}, DOI={10.1109/TVT.2009.2024017}, abstractNote={This paper proposes a new frequency-domain channel-estimation and equalization method for continuous-phase modulation (CPM) block transmissions with superimposed pilot signals. Our method provides spectral and power-efficient broadband CPM wireless communications with less complexity than previous methods. The proposed frequency-domain channel estimation uses the superimposed pilot sequence as a reference signal to reduce the throughput loss caused by traditionally multiplexed pilots. The proposed CPM frequency-domain decision feedback equalizer (DFE) eliminates the complexity overhead of conventional decomposition-based CPM receivers.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={4903–4908} }
 @inproceedings{geodesic prediction for limited feedback multiuser mimo systems in temporally correlated channels_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67650524116&partnerID=MN8TOARS}, DOI={10.1109/RWS.2009.4957311}, abstractNote={Limited feedback in multiuser multi-antenna systems is a practical technique for providing partial channel state information (CSI) to the transmitter. Unfortunately, it is known that the number of feedback bits per mobile terminal must be scaled with the number of transmit antennas and system signal to noise ratio. Therefore very high resolution codebooks are needed to approach the theoretically achievable sum rate. In this paper, we propose a geodesic prediction algorithm to 1) reduce the feedback overhead under temporally correlated channels, and 2) improve on the sum rate ceiling observed with a fixed codebook size. The proposed algorithm provides local refinement of the CSI thereby achieving higher resolution CSI at the transmitter despite of using a fixed codebook size. Furthermore, feedback can be performed infrequently to lower the average feedback rate. Numerical results are provided to illustrate the performance of the proposed algorithm.}, booktitle={RWS 2009 IEEE Radio and Wireless Symposium, Proceedings}, year={2009}, pages={167–170} }
 @inproceedings{grassmannian predictive coding for delayed limited feedback mimo systems_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77949758202&partnerID=MN8TOARS}, DOI={10.1109/ALLERTON.2009.5394938}, abstractNote={Limited feedback in multiple antenna wireless systems is a practical technique to obtain channel state information at the transmitter. When the channel is time-varying with memory, however, selected codeword may become outdated before its use at the transmitter. To overcome this problem, we propose a Grassmannian prediction and predictive coding framework for delayed feedback systems by exploiting the memory in the channel. A prediction step size optimization criterion for correlated time-series evolving on the Grassmann manifold is derived. The proposed predictive coding framework uses optimized prediction to account for the feedback delay. Application to delayed limited feedback multiuser multiple antenna system shows sum rate improvement and robustness to delay.}, booktitle={2009 47th Annual Allerton Conference on Communication, Control, and Computing, Allerton 2009}, year={2009}, pages={783–788} }
 @inproceedings{interference alignment via alternating minimization_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349218041&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2009.4960116}, abstractNote={Using interference alignment, it has been shown that the number of degrees of freedom in the interference channel scales linearly with the number of users. Unfortunately, closed-form solutions for interference alignment over constant-coefficient channels with more than 3 users are difficult to derive. This paper proposes an algorithm for interference alignment in the MIMO interference channel with an arbitrary number of users, antennas, or spatial streams. The algorithm is an alternating minimization over the precoding matrices at the transmitters and the interference subspaces at the receivers, and is proven to converge. Numerical results show how the algorithm is useful for simulation and can give insight into the limitations of interference alignment.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2009}, pages={2445–2448} }
 @article{kerdock codes for limited feedback precoded mimo systems_2009, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-69349085232&partnerID=MN8TOARS}, DOI={10.1109/TSP.2009.2020761}, abstractNote={Codebook based limited feedback is a practical way to obtain partial channel state information at the transmitter in a precoded multiple-input multiple-output wireless system. In this correspondence, we propose a systematic codebook design. The proposed Kerdock codebook consists of multiple mutually unbiased unitary basis matrices with quaternary alphabet and the identity matrix. We propose to derive the beamforming and precoding codebooks from this base codebook, eliminating the requirement to store multiple codebooks. Furthermore, we show that quaternary alphabet facilitates efficient codebook storage and codeword search. We derive the chordal distance for two and four antenna codebooks, showing that the proposed codebooks compare favorably with prior designs. Monte Carlo simulations compare achievable rates and error rates for different codebooks sizes.}, number={9}, journal={IEEE Transactions on Signal Processing}, year={2009}, pages={3711–3716} }
 @article{limited feedback beamforming over temporally-correlated channels_2009, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65649116192&partnerID=MN8TOARS}, DOI={10.1109/TSP.2009.2014272}, abstractNote={Feedback of quantized channel state information (CSI), called limited feedback, enables transmit beamforming in multiple-input-multiple-output (MIMO) wireless systems with a small amount of overhead. Due to its efficiency, beamforming with limited feedback has been adopted in several wireless communication standards. Prior work on limited feedback commonly adopts the block fading channel model where temporal correlation in wireless channels is neglected. In this paper, we consider temporally correlated channels and design single-user transmit beamforming with limited feedback. Analytical results concerning CSI feedback are derived by modeling quantized CSI as a first-order finite-state Markov chain. These results include the information rate of the CSI quantizer output, the bit rate a CSI feedback channel is required to support, and the effect of feedback delay on throughput. In particular, based on the theory of Markov chain convergence rate, feedback delay is proved to reduce the throughput gain due to CSI feedback at least exponentially. Furthermore, an algorithm is proposed for CSI feedback compression in time. Combining the results in this work leads to a new method for designing limited feedback beamforming as demonstrated by a design example.}, number={5}, journal={IEEE Transactions on Signal Processing}, year={2009}, pages={1959–1975} }
 @inproceedings{linear network coordinated beamforming for cell-boundary users_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449597916&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2009.5161842}, abstractNote={In this paper, low complexity linear network coordinated beamforming (N-CBF) algorithms under a zero inter-user interference constraint are proposed for the multi-cell downlink channel. We consider a downlink scenario with three base stations (BSs) equipped with one transmit antenna each and three mobile users, each of which has more than one receive antenna and is located on the cell-boundary. A single data stream is transmitted to all three cell-boundary users where each user receives his/her own independent data stream (called full broadcast channel), or where two users receive the same data stream while the third user receives a different data stream (called clustered broadcast channel). For the full and clustered broadcast channel scenarios, we propose low complexity linear N-CBF algorithms approaching the sum capacity realized by multi-cell dirty paper coding. Monte Carlo simulations are carried out to verify the proposed algorithms.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2009}, pages={534–538} }
 @inproceedings{mimo spatial mode adaptation at the cell edge using interferer spatial correlation_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349694235&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2009.5073673}, abstractNote={A system and method is proposed for adapting the spatial transmission strategy in a cellular MIMO (multiple input multiple output) communication system for the downlink. Spatial mode adaptation, the choice of multiplexing, transmit diversity, number of streams, space-time code family, and the like are performed slowly based on side information from other base stations. Base stations exchange their transmission plans with neighboring base stations and broadcast this information to active users. Each user measures its susceptibility to spatial interference and returns this information to the base station. The base station then schedules active users according to the decisions made in interfering base stations and the preferred transmission strategies of its own users.}, booktitle={IEEE Vehicular Technology Conference}, year={2009} }
 @inproceedings{leus_heath_2009, title={Message from the technical co-chairs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449558998&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2009.5161732}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Leus, G. and Heath, R.W.}, year={2009} }
 @article{zhang_heath_kountouris_andrews_2009, title={Mode switching for the multi-antenna broadcast channel based on delay and channel quantization}, volume={2009}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-68949136720&partnerID=MN8TOARS}, DOI={10.1155/2009/802548}, abstractNote={Imperfect channel state information degrades the performance of multiple-input multiple-output (MIMO) communications; its effects on single-user (SU) and multiuser (MU) MIMO transmissions are quite different. In particular, MU-MIMO suffers from residual interuser interference due to imperfect channel state information while SU-MIMO only suffers from a power loss. This paper compares the throughput loss of both SU and MU-MIMO in the broadcast channel due to delay and channel quantization. Accurate closed-form approximations are derived for achievable rates for both SU and MU-MIMO. It is shown that SU-MIMO is relatively robust to delayed and quantized channel information, while MU-MIMO with zero-forcing precoding loses its spatial multiplexing gain with a fixed delay or fixed codebook size. Based on derived achievable rates, a mode switching algorithm is proposed, which switches between SU and MU-MIMO modes to improve the spectral efficiency based on average signal-to-noise ratio (SNR), normalized Doppler frequency, and the channel quantization codebook size. The operating regions for SU and MU modes with different delays and codebook sizes are determined, and they can be used to select the preferred mode. It is shown that the MU mode is active only when the normalized Doppler frequency is very small, and the codebook size is large.}, journal={Eurasip Journal on Advances in Signal Processing}, author={Zhang, J. and Heath, R.W. and Kountouris, M. and Andrews, J.G.}, year={2009} }
 @article{multiuser mimo transmission with limited feedback, cooperation, and coordination_2009, volume={2009}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-73449093805&partnerID=MN8TOARS}, DOI={10.1155/2009/480179}, journal={Eurasip Journal on Advances in Signal Processing}, year={2009} }
 @article{network coordinated beamforming for cell-boundary users: linear and nonlinear approaches_2009, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76149123233&partnerID=MN8TOARS}, DOI={10.1109/JSTSP.2009.2035857}, abstractNote={In this paper, low-complexity linear and nonlinear network coordinated beamforming (N-CBF) algorithms are proposed for the multi-cell downlink channel. We consider a downlink scenario with three base stations (BSs) equipped with one transmit antenna each and three mobile users, each of which has more than one receive antenna and is located on the cell-boundary. A single data stream is transmitted to all three cell-boundary users where each user receives his/her own independent data stream (called full broadcast channel), or where two users receive the same data stream while the third user receives a different data stream (called clustered broadcast channel). For full and clustered broadcast channel scenarios, we propose low-complexity linear N-CBF algorithms approaching the sum capacity realized by multi-cell dirty paper coding. We also propose low complexity nonlinear N-CBF algorithms that avoid power enhancement and maximize the effective channel gain for enhancing the bit error rate (BER). The impact of the number of receive antennas on the behavior of the proposed system is also discussed. Monte Carlo simulations are carried out to verify the sum rate and BER performances of the proposed algorithms.}, number={6}, journal={IEEE Journal on Selected Topics in Signal Processing}, year={2009}, pages={1094–1105} }
 @article{zhang_chen_andrews_ghosh_heath_2009, title={Networked MIMO with clustered linear precoding}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65949119326&partnerID=MN8TOARS}, DOI={10.1109/TWC.2009.080180}, abstractNote={A clustered base transceiver station (BTS) coordination strategy is proposed for a large cellular MIMO network, which includes full intra-cluster coordination-to enhance the sum rate-and limited inter-cluster coordination-to reduce interference for the cluster edge users. Multi-cell block diagonalization is used to coordinate the transmissions across multiple BTSs in the same cluster. To satisfy per-BTS power constraints, three combined precoder and power allocation algorithms are proposed with different performance and complexity tradeoffs. For inter-cluster coordination, the coordination area is chosen to balance fairness for edge users and the achievable sum rate. It is shown that a small cluster size (about 7 cells) is sufficient to obtain most of the sum rate benefits from clustered coordination while greatly relieving channel feedback requirement. Simulations show that the proposed coordination strategy efficiently reduces interference and provides a considerable sum rate gain for cellular MIMO networks.}, number={4}, journal={IEEE Transactions on Wireless Communications}, author={Zhang, J. and Chen, R. and Andrews, J.G. and Ghosh, A. and Heath, R.W.}, year={2009}, pages={1910–1921} }
 @article{on the optimality of linear multiuser mimo beamforming for a two-user two-input multiple-output broadcast system_2009, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-59149090427&partnerID=MN8TOARS}, DOI={10.1109/LSP.2008.2008937}, abstractNote={The asymptotic optimality of a jointly optimized linear multiuser beamforming system with two transmit and many receive antennas for the fading multiple-input multiple-output (MIMO) Gaussian broadcast channel is investigated. We derive the asymptotic sum capacity of the independent and identically-distributed (i.i.d.) MIMO broadcast channel and the asymptotic sum rate of a linear multiuser beamforming system with respect to the number of receive antennas per user, where two users are served with a zero inter-user interference constraint. Based on this result, we show that linear multiuser beamforming with many receive antennas can asymptotically achieve the sum capacity of the two-user i.i.d. MIMO broadcast channel even without opportunistic scheduling gain. Numerical results confirm that as the number of receive antennas increases the sum rate of linear multiuser beamforming system converges to the sum capacity.}, number={2}, journal={IEEE Signal Processing Letters}, year={2009}, pages={117–120} }
 @inproceedings{optimal amplify and forward strategy for two-way relay channel with multiple relays_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77950679295&partnerID=MN8TOARS}, DOI={10.1109/ITWNIT.2009.5158567}, abstractNote={An iterative algorithm is proposed to achieve the optimal rate region in a two-way relay channel, where two nodes want to exchange data with each other using multiple relays, and each relay employs an amplify and forward strategy. The iterative algorithm solves a power minimization problem at every step, subject to minimum signal-to-interference-and-noise ratio constraints, which is non-convex, however, for which the Karush Kuhn Tucker conditions are sufficient for optimality. Using simulations, the achievable rate region of the iterative algorithm is compared with the cut-set upper bound; the gap is shown to be quite small for most cases.}, booktitle={Proceedings - 2009 IEEE Information Theory Workshop on Networking and Information Theory, ITW 2009}, year={2009}, pages={181–185} }
 @article{performance analysis of maximum ratio combining with imperfect channel estimation in the presence of cochannel interferences_2009, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-62949172273&partnerID=MN8TOARS}, DOI={10.1109/TWC.2009.080114}, abstractNote={In this paper, we analyze the performance of maximum ratio combining (MRC) systems with imperfect channel estimation in the presence of cochannel interference (CCI) with an arbitrary power interference-to-noise ratio (INR). The maximum combining weights are the imperfect estimates of the desired user's fading channel coefficients and are assumed to be complex Gaussian distributed. The quantified measure for estimation error is the correlation coefficient between the true fading channel coefficients and their estimates. Exact closedform expressions are derived for the probability density function(pdf) of the signal-to-interference-plus-noise ratio (SINR), as well as performance metrics including outage probability and the average symbol error probability (ASEP) for some modulation formats. Simulation results demonstrate the accuracy of our theoretic analysis.}, number={3}, journal={IEEE Transactions on Wireless Communications}, year={2009}, pages={1080–1085} }
 @article{performance of orthogonal beamforming for sdma with limited feedback_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-59649125050&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.925003}, abstractNote={On the multiantenna broadcast channel, the spatial degrees of freedom support simultaneous transmission to multiple users. The optimal multiuser transmission, which is known as dirty paper coding, is not directly realizable. Moreover, close-to-optimal solutions such as Tomlinson-Harashima precoding are sensitive to channel state information (CSI) inaccuracy. This paper considers a more practical design called per user unitary and rate control (PU2RC), which has been proposed for emerging cellular standards. PU2RC supports multiuser simultaneous transmission, enables limited feedback, and is capable of exploiting multiuser diversity. Its key feature is an orthogonal beamforming (or precoding) constraint, where each user selects a beamformer (or precoder) from a codebook of multiple orthonormal bases. In this paper, the asymptotic throughput scaling laws for PU2RC with a large user pool are derived for different regimes of the signal-to-noise ratio (SNR). In the multiuser interference-limited regime, the throughput of PU2RC is shown to logarithmically scale with the number of users. In the normal SNR and noise-limited regimes, the throughput is found to scale double logarithmically with the number of users and linearly with the number of antennas at the base station. In addition, numerical results show that PU2RC achieves higher throughput and is more robust against CSI quantization errors than the popular alternative of zero-forcing beamforming if the number of users is sufficiently large.}, number={1}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={152–164} }
 @article{performance of vector perturbation multiuser mimo systems with limited feedback_2009, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70350405704&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2009.09.080041}, abstractNote={This paper considers the multiuser multiple-input multiple-output (MIMO) Rayleigh fading broadcast channel. We consider the case where the multiple transmit antennas are used to deliver independent data streams to multiple users via a multiuser technique known as vector perturbation. We propose lattice-theoretic and rate-distortion based approaches to analyze the performance of these systems, taking into account the practical restrictions imposed by limited feedback and training. We show that performance is primarily determined by the ratio between the number of users and the number of transmit antennas. We then propose a new practical low-complexity low-rate feedback scheme, and show that the performance approaches the ideal rate-distortion based scheme.}, number={9}, journal={IEEE Transactions on Communications}, year={2009}, pages={2633–2644} }
 @article{progressive refinement of beamforming vectors for high-resolution limited feedback_2009, volume={2009}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76649104048&partnerID=MN8TOARS}, DOI={10.1155/2009/463823}, abstractNote={Limited feedback enables the practical use of channel state information in multiuser multiple-input multiple-output (MIMO) wireless communication systems. Using the limited feedback concept, channel state information at the receiver is quantized by choosing a representative element from a codebook known to both the receiver and transmitter. Unfortunately, achieving the high resolution required with multiuser MIMO communication is challenging due to the large number of codebook entries required. This paper proposes to use a progressively scaled local codebook to enable high resolution quantization and reconstruction for multiuser MIMO with zero-forcing precoding. Several local codebook designs are proposed including one based on a ring and one based on mutually unbiased bases; both facilitate efficient implementation. Structure in the local codebooks is used to reduce search complexity in the progressive refinement algorithm. Simulation results illustrate sum rate performance as a function of the number of refinements.}, journal={Eurasip Journal on Advances in Signal Processing}, year={2009} }
 @article{quantized antenna combining for multiuser mimo-ofdm with limited feedback_2009, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349588468&partnerID=MN8TOARS}, DOI={10.1109/LSP.2009.2028942}, abstractNote={A method for reducing feedback in a downlink multiuser MIMO-OFDM cellular system, with zero forcing beamforming at the base (BS) and antenna combining at each mobile (MS) is proposed. The method consists of partitioning the subcarriers into groups, choosing the quantized channel vector that better represents each group in terms of average quantization error. Numerical results show that savings up to 93% in overhead bits are obtained for SNR equal to 15 dB, with reductions in channel capacity that goes from 30% to 50% depending on the codebook size used for quantization.}, number={12}, journal={IEEE Signal Processing Letters}, year={2009}, pages={1027–1030} }
 @inproceedings{real world feasibility of interference alignment using mimo-ofdm channel measurements_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951452323&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2009.5379982}, abstractNote={Interference alignment (IA) has been shown to achieve linear sum capacity growth, at high SNR, with the number of users in the interference channel by cooperatively precoding transmitted signals to align interference subspaces at the receivers. The theory of IA was derived under assumptions about the richness of the propagation channel; practical channels do not guarantee such ideal decorrelation. This paper presents the first experimental study of IA in measured interference channel and shows that IA achieves the claimed scaling factors in a variety of measured channel settings for a 3 user, 2 antennas per node setup.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2009} }
 @article{peters_panah_truong_heath_2009, title={Relay architectures for 3GPP LTE-advanced}, volume={2009}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-69249230901&partnerID=MN8TOARS}, DOI={10.1155/2009/618787}, abstractNote={The Third Generation Partnership Project's Long Term Evolution-Advanced is considering relaying for cost-effective throughput enhancement and coverage extension. While analog repeaters have been used to enhance coverage in commercial cellular networks, the use of more sophisticated fixed relays is relatively new. The main challenge faced by relay deployments in cellular systems is overcoming the extra interference added by the presence of relays. Most prior work on relaying does not consider interference, however. This paper analyzes the performance of several emerging half-duplex relay strategies in interference-limited cellular systems: one-way, two-way, and shared relays. The performance of each strategy as a function of location, sectoring, and frequency reuse are compared with localized base station coordination. One-way relaying is shown to provide modest gains over single-hop cellular networks in some regimes. Shared relaying is shown to approach the gains of local base station coordination at reduced complexity, while two-way relaying further reduces complexity but only works well when the relay is close to the handset. Frequency reuse of one, where each sector uses the same spectrum, is shown to have the highest network throughput. Simulations with realistic channel models provide performance comparisons that reveal the importance of interference mitigation in multihop cellular networks.}, journal={Eurasip Journal on Wireless Communications and Networking}, author={Peters, S.W. and Panah, A.Y. and Truong, K.T. and Heath, R.W.}, year={2009} }
 @article{relay-assisted user scheduling in wireless networks with hybrid arq_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70450188405&partnerID=MN8TOARS}, DOI={10.1109/TVT.2009.2027711}, abstractNote={This paper studies the problem of relay-assisted user scheduling for downlink wireless transmission. The base station or access point employs hybrid automatic repeat request (HARQ) with the assistance of a set of fixed relays to serve a set of mobile users. By minimizing a cost function of the queue lengths at the base station and the number of retransmissions of the head-of-line packet for each user, the base station can schedule an appropriate user in each time slot and an appropriate transmitter to serve it. It is shown that a priority-index policy is optimal for a linear cost function with packets arriving according to a Poisson process and for an increasing convex cost function where packets must be drained from the queues at the base station.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={5284–5288} }
 @article{panah_heath_2009, title={Single-user and multicast OFDM power loading with nonregenerative relaying}, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70450167535&partnerID=MN8TOARS}, DOI={10.1109/TVT.2009.2025954}, abstractNote={In this paper, we derive power-loading strategies for single-user and multicast orthogonal frequency-division multiplexing (OFDM) wireless links in the presence of a nonregenerative relay node. Our approach is based on the minimization of the vector error rate at the destination subject to individual power constraints at the source and the relay. For the single-user case, we propose a joint max-min power-loading strategy that is optimal at large signal-to-noise ratios (SNRs), and in doing so, we show how the original problem may be recast into an effective (no-relay) OFDM power-loading problem with a previously known solution. Using bounds on effective SNRs, we propose disjoint power-loading strategies that require channel state information either at the source or at the relay but not at both. For the multicast scenario, we propose a max-min equalizing solution that yields a minimum effective SNR at each user terminal. We also formulate several linear solutions based on prioritizing the users in the network. Our single-user simulations show considerable error-rate performance gains for the proposed joint max-min solution and gives hints as to where disjoint power loading is the most effective. Our multicast simulations show that the max-min equalizing solution is best suited for networks with random user locations, whereas the linear prioritizing solutions work best for networks with fixed user locations.}, number={9}, journal={IEEE Transactions on Vehicular Technology}, author={Panah, A.Y. and Heath, R.W.}, year={2009}, pages={4890–4902} }
 @inproceedings{sum-rate maximizing beamforming in multicell systems with limited feedback_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953866809&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2009.5470216}, abstractNote={Multicell cooperative transmit strategies typically require full channel state information (CSI) at the base stations. In this paper, a more practical limited feedback scenario is considered for a recently proposed multicell beamforming approach that approximately maximizes the sum-rates in a two-cell two-user network. In the limited feedback system presented, the desired and interfering CSI is quantized by the users and fed back to the base stations over a finite-bandwidth link. Each receiver is assumed to use two separate codebooks to quantize the desired and interfering channels. It is proposed to vary the codebook sizes adaptively as a function of the relative strength of the channels to minimize the loss in sum-rate. To this end, the mean loss in sum-rate due to random vector quantization is quantified. Closed-form expressions are derived to partition the available feedback bits adaptively between the desired and interfering channels to minimize the mean loss in sum-rate. Finally, simulations are used to verify the results presented in the paper.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2009}, pages={1838–1842} }
 @article{peters_heath_2009, title={The Future of WiMAX: Multihop Relaying with IEEE 802.16j}, volume={47}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85008031242&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2009.4752686}, abstractNote={Relaying and cooperation have re-emerged as important research topics in wireless communication over the past half-decade. Although multihop relaying for coverage extension in wireless networks is an old concept, it became practical only recently. Nowhere is this better illustrated than in the IEEE 802.16 working group, which has devoted a task group to incorporating relay capabilities in the foundation of mobile WiMAX-IEEE 802.16e-2005. Currently, this task group is in the process of finishing IEEE 802.16j, the multihop relay specification for 802.16. This amendment will be fully compatible with 802.16e-2005 mobile and subscriber stations, but a BS specific to 802.16j will be required for relays to operate. This article presents an introduction to the upcoming IEEE 802.16j amendment and provides insight about the obstacles that practical system designers face when incorporating relaying into a wireless broadband network.}, number={1}, journal={IEEE Communications Magazine}, author={Peters, S.W. and Heath, R.W.}, year={2009}, pages={104–111} }
 @article{the impact of channel feedback on opportunistic relay selection for hybrid-arq in wireless networks_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-64049112628&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.928896}, abstractNote={This paper presents a decentralized relay selection protocol for a dense wireless network and describes channel feedback strategies that improve its performance. The proposed selection protocol supports hybrid automatic repeat request transmission where relays forward parity information to the destination in the event of a decoding error. Channel feedback is employed for refining the relay selection process and to select an appropriate transmission mode in a proposed adaptive modulation transmission framework. An approximation of the throughput of the proposed adaptive modulation strategy is presented, and the dependence of the throughput on system parameters such as the relay contention probability and the adaptive modulation switching point is illustrated via maximization of this approximation. Simulations show that the throughput of the proposed selection strategy is comparable with that yielded by a centralized selection approach that relies on geographic information.}, number={3}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={1255–1268} }
 @article{the performance of space-time block codes from coordinate interleaved orthogonal designs over nakagami-m fading channels_2009, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-63649111565&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2009.03.060716}, abstractNote={Space-time block codes (STBCs) from coordinate interleaved orthogonal designs (CIODs) offer several advantages including full-diversity and single-symbol decodability. In an effort to assess their performance in quasi-static frequency nonselective i.i.d. Nakagami-m fading channels, we analyze the error rate, outage capacity, and information outage probability. First, based on an accurate closed-form formula for the average symbol pairwise error rate (SPER), we derive tight union upper and lower bounds on the symbol-error rate (SER). Second, we apply Gaussian and Gamma approximations to provide closed form expressions for the outage capacity. Third, using high signal-to-noise ratio (SNR) and moment-matching approximation techniques, we also derive accurate closed-form approximations for the information outage probability (IOP). Finally, we show that STBCs from CIODs provide full-diversity by deriving SER based and IOP-based asymptotic and instantaneous diversity orders. Monte-Carlo simulations show that the analytical results agree with simulation experiments.}, number={3}, journal={IEEE Transactions on Communications}, year={2009}, pages={653–664} }
 @article{to code in space and time or not in multihop relay channels_2009, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67650178762&partnerID=MN8TOARS}, DOI={10.1109/TSP.2009.2017002}, abstractNote={Multihop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Previously, distributed space-time coding was used to maximize diversity gain. Assuming a low-rate feedback link from the destination to each relay stage and the source, this paper proposes end-to-end antenna selection strategies as an alternative to distributed space-time coding. One-way (where only the source has data for destination) and two-way (where the destination also has data for the source) multihop relay channels are considered with both the full-duplex and half-duplex relay nodes. For the full-duplex case, end-to-end antenna selection strategies are designed and proven to achieve maximum diversity gain by using a single-antenna path (using single antenna of the source, each relay stage and the destination) with the maximum signal-to-noise ratio at the destination. For the half-duplex case, two paths with the two best signal-to-noise ratios in alternate time slots are used to overcome the rate loss with half-duplex nodes, with a small diversity gain penalty. A multiple stream end-to-end antenna selection strategy for full-duplex multihop relay channel is also proposed to obtain a lower bound on the diversity multiplexing tradeoff of multihop relay channels. Finally, to answer the question of whether to code in space and time or not in a multihop relay channel, end-to-end antenna selection strategy and distributed space-time coding is compared with respect to several important performance metrics such as encoding/decoding complexity, rate of transmission, latency, bit-error-rate performance, and resource requirements.}, number={7}, journal={IEEE Transactions on Signal Processing}, year={2009}, pages={2736–2747} }
 @inproceedings{transmission capacity of ad-hoc networks with multiple antennas using transmit stream adaptation and interference cancelation_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953848580&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2009.5469710}, abstractNote={The transmission capacity of an ad-hoc network is the maximum density of active transmitters in an unit area, given an outage constraint at each receiver for a fixed rate of transmission. Assuming channel state information is available at the receiver, this paper presents bounds on the transmission capacity as a function of the number of antennas used for transmission, and the spatial receive degrees of freedom used for interference cancelation at the receiver. Canceling the strongest interferers, using a single antenna for transmission together with using all but one spatial receive degrees of freedom for interference cancelation is shown to maximize the transmission capacity. Canceling the closest interferers, using a single antenna for transmission together with using a fraction of the total spatial receive degrees of freedom for interference cancelation depending on the path loss exponent, is shown to maximize the transmission capacity.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2009}, pages={1709–1713} }
 @inproceedings{transmission capacity of two-way communication in wireless ad hoc networks_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449465298&partnerID=MN8TOARS}, DOI={10.1109/ICC.2009.5199540}, abstractNote={Wireless ad hoc networks require bidirectional data transmission to support two-way traffic and control functions like packet acknowledgement. Most prior work on the capacity of wireless ad hoc networks, however, has focused only on one-way communication. In this paper we develop the concept of transmission capacity of two-way communication in wireless ad hoc networks. The transmission capacity has been used extensively to analyze one-way ad hoc networks - in this paper we provide a generalization that incorporates the concept of a two-way outage. We derive an upper bound and an approximation for the two-way transmission capacity, which are shown to be relatively tight for small outage probability constraints. We also quantify how the two-way success requirement reduces network capacity. Our numerical and simulation results show that for certain two-way networks the capacity loss is considerable.}, booktitle={IEEE International Conference on Communications}, year={2009} }
 @inproceedings{transmission capacity of wireless ad-hoc networks with multiple antennas using multi-mode precoding and interference cancelation_2009, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449562784&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2009.5161816}, abstractNote={The transmission capacity of an ad-hoc network is the maximum density of active transmitters in an unit area, given an outage constraint at each receiver for a fixed rate of transmission. This paper addresses the problem of maximizing the transmission capacity of an ad-hoc network when each node is equipped with multiple antennas. Bounds on the transmission capacity are derived as a function of the number of antennas used for transmission, and the number of antennas used for interference cancellation. Both the transmitter and receiver are assumed to have channel state information (CSI). CSI at the transmitter is used to transmit multiple streams using multi-mode precoding, while CSI at the receiver is used for interference cancellation. Canceling the strongest or the closest interferers, single stream transmission on the best eigenmode together with using all but one receiver antennas for interference cancellation is shown to maximize the transmission capacity.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2009}, pages={404–408} }
 @article{unequal power allocation for jpeg transmission over mimo systems_2010, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76849110426&partnerID=MN8TOARS}, DOI={10.1109/TIP.2009.2032346}, abstractNote={With the introduction of multiple transmit and receive antennas in next generation wireless systems, real-time image and video communication are expected to become quite common, since very high data rates will become available along with improved data reliability. New joint transmission and coding schemes that explore advantages of multiple antenna systems matched with source statistics are expected to be developed. Based on this idea, we present an unequal power allocation scheme for transmission of JPEG compressed images over multiple-input multiple-output systems employing spatial multiplexing. The JPEG-compressed image is divided into different quality layers, and different layers are transmitted simultaneously from different transmit antennas using unequal transmit power, with a constraint on the total transmit power during any symbol period. Results show that our unequal power allocation scheme provides significant image quality improvement as compared to different equal power allocations schemes, with the peak-signal-to-noise-ratio gain as high as 14 dB at low signal-to-noise-ratios.}, number={2}, journal={IEEE Transactions on Image Processing}, year={2010}, pages={410–421} }
 @inproceedings{a lattice-theoretic analysis of vector perturbation for multi-user mimo systems_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51249089192&partnerID=MN8TOARS}, DOI={10.1109/ICC.2008.628}, abstractNote={This paper considers the use of multiple transmit antennas to deliver independent data streams to multiple users. In particular, we examine a multi-user technique known as vector perturbation. We provide a new lattice-theoretic approach to analyze its performance in the presence of Rayleigh fading. Vector perturbation is based on performing a channel inversion, with the additional step of perturbing the data signal prior to linear preceding to significantly reduce the required transmit power. To analyze such systems it is necessary to calculate the resulting average energy of the sphere-encoded signal vector, as this determines the signal-to-noise ratio (SNR) at the output of the demodulator. Previous results presented in the literature were partially analytic, requiring further numerical evaluation. Here, we derive a concise approximation to the output SNR. We also provide tight upper and lower bounds on the bit error rate for the reception of QAM symbols using the required modulo demodulator, as a function of the average energy of the sphere- encoded signal vector.}, booktitle={IEEE International Conference on Communications}, year={2008}, pages={3340–3344} }
 @inproceedings{a low complexity linear multiuser mimo beamforming system with limited feedback_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51849084424&partnerID=MN8TOARS}, DOI={10.1109/CISS.2008.4558563}, abstractNote={In this paper, we propose a low complexity linear multiuser beamforming system for the multiple-input multiple-output (MIMO) broadcast channel. We consider the specific case of transmission of a single information stream to two users with two or more receive antennas. Unlike past work in which an iterative algorithm is required to design the beamformers, we first provide a low complexity non-iterative solution via the generalized eigenvector decomposition to jointly optimize transmit beamforming and receive combining vectors. The proposed beamforming technique works for two or more transmit and receive antennas where perfect channel state information is available at the transmitter. To enable practical implementation, a new non-uniform limited feedback algorithm is also proposed that exploits the structure of the algorithm to avoid full channel quantization especially for two transmit antenna systems. The feedback overhead is independent of the number of receive antennas. Simulation results show that the proposed method performs close to the sum capacity of the MIMO broadcast channel even with limited feedback.}, booktitle={CISS 2008, The 42nd Annual Conference on Information Sciences and Systems}, year={2008}, pages={418–422} }
 @inproceedings{a new mimo channel representation including spatial diversity, array orientation and depolarization effects_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-55749101729&partnerID=MN8TOARS}, DOI={10.1109/APS.2008.4619984}, abstractNote={This paper proposes an analytical channel model that accounts for spatial, pattern, polarization diversities, channel depolarization, random mobile terminal orientations and mutual coupling effects. by utilizing simulations, this study verifies that the model accounts for more propagation effects than the 3GPP SCM and IEEE 802.11n model.}, booktitle={2008 IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting, APSURSI}, year={2008} }
 @article{tang_heath_2008, title={A space-time receiver with joint synchronization and interference cancellation in asynchronous MIMO-OFDM systems}, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-53149154875&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.915522}, abstractNote={We consider the scenario in which multiple transmitters send signals to a receiver in the asynchronous multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) mode. A receiver structure that can perform joint synchronization and space-time equalization for the asynchronous MIMO-OFDM systems is proposed. We first introduce a novel space-time equalization algorithm based on the channel state information. This algorithm aims to null the cochannel interference and combat asynchronism, as well as minimize a lower bound to the symbol error rate in the asynchronous MIMO-OFDM system. In the second part of this paper, we propose a training-based minimizing mean-square-error (MMSE) algorithm to jointly obtain frequency offsets and equalization delays. We compensate for the effects of the frequency offsets and the propagation delays with these estimated parameters and then use the proposed direct training-based equalizer design algorithm to obtain the equalizer coefficients. The simulation section illustrates the desirable bit-error-rate (BER) performance of our proposed algorithms.}, number={5}, journal={IEEE Transactions on Vehicular Technology}, author={Tang, T. and Heath, R.W.}, year={2008}, pages={2991–3005} }
 @inproceedings{a supervised learning approach to adaptation in practical mimo-ofdm wireless systems_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67249165041&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.878}, abstractNote={MIMO-OFDM wireless systems require adaptive modulation and coding based on channel state information (CSI) to maximize throughput in changing wireless channels. Traditional adaptive modulation and coding attempts to predict the best rate available by estimating the packet error rate for each modulation and coding scheme (MCS) by using CSI, which has shown to be challenging. This paper considers supervised learning with the k-nearest neighbor (k-NN) algorithm as a new framework for adaptive modulation and coding. Practical k-NN operation is enabled through feature space dimensionality reduction using subcarrier ordering techniques based on postprocessing SNR. Simulation results of an IEEE 802.11n draft-compatible physical layer in flat and frequency selective wireless channels shows the k-NN with an ordered subcarrier feature space performs near ideal adaptation under packet error rate constraints.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2008}, pages={4583–4587} }
 @inproceedings{adaptive mode switching in the mimo broadcast channel_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949084658&partnerID=MN8TOARS}, DOI={10.1109/RWS.2008.4463591}, abstractNote={This paper proposes an adaptive mode switching between single user and multi-user multiple input multiple output (MIMO) transmissions for Gaussian MIMO broadcast channels with partial channel state information at the transmitter. With full channel state information, it is known that multi-user transmission outperforms all other policies. With alternate channel state information available at the transmitter, it is not true that one transmission strategy always dominates the other. In particular, under some practical side information assumptions, there is instead a transition point - when the number of users is below this point, single user transmission outperforms multi-user transmission, and the reverse is true when the number of users exceeds this critical value. In this paper, we determine this mode switching point both analytically and numerically for a MIMO channel with transmit side correlation.}, booktitle={2008 IEEE Radio and Wireless Symposium, RWS}, year={2008}, pages={711–714} }
 @article{an overview of limited feedback in wireless communication systems_2008, volume={26}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54249161889&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2008.081002}, abstractNote={It is now well known that employing channel adaptive signaling in wireless communication systems can yield large improvements in almost any performance metric. Unfortunately, many kinds of channel adaptive techniques have been deemed impractical in the past because of the problem of obtaining channel knowledge at the transmitter. The transmitter in many systems (such as those using frequency division duplexing) can not leverage techniques such as training to obtain channel state information. Over the last few years, research has repeatedly shown that allowing the receiver to send a small number of information bits about the channel conditions to the transmitter can allow near optimal channel adaptation. These practical systems, which are commonly referred to as limited or finite-rate feedback systems, supply benefits nearly identical to unrealizable perfect transmitter channel knowledge systems when they are judiciously designed. In this tutorial, we provide a broad look at the field of limited feedback wireless communications. We review work in systems using various combinations of single antenna, multiple antenna, narrowband, broadband, single-user, and multiuser technology. We also provide a synopsis of the role of limited feedback in the standardization of next generation wireless systems.}, number={8}, journal={IEEE Journal on Selected Areas in Communications}, year={2008}, pages={1341–1365} }
 @article{block diagonalization for multi-user mimo with other-cell interference_2008, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48149110501&partnerID=MN8TOARS}, DOI={10.1109/TWC.2008.070093}, abstractNote={Block diagonalization is one approach for linear preceding in the multiple-input multiple-output broadcast channel that sends multiple interference free data streams to different users in the same cell. Unfortunately, block diagonalization neglects other-cell interference (OCI), which limits the performance of users at the edge of the cell. This paper presents an OCI-aware enhancement to block diagonalization that uses a whitening filter for interference suppression at the receiver and a novel precoder using the interference-plus-noise covariance matrix for each user at the transmitter. For complex Gaussian matrix channels, the asymptotic sum rate of the proposed system is analyzed under a large antenna assumption for isotropic inputs and compared to conventional block diagonalization. The capacity loss due to OCI is quantified in terms of results from single-user MIMO capacity. Several numerical examples compare achievable sum rates, the proposed asymptotic rates, and the capacity loss, in low and high interference regimes.}, number={7}, journal={IEEE Transactions on Wireless Communications}, year={2008}, pages={2671–2681} }
 @article{block diagonalized vector perturbation for multiuser mimo systems_2008, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57149123543&partnerID=MN8TOARS}, DOI={10.1109/T-WC.2008.070262}, abstractNote={Precoding with block diagonalization (BD) is an attractive technique for approaching the sum capacity in the multiuser multiple-input multiple-output (MIMO) broadcast channel. Unfortunately, BD requires either global channel state information at every receiver or an additional training phase, which demands additional control overhead and additional system planning. In this paper we propose a new multiuser MIMO algorithm that combines BD with vector perturbation (VP). The proposed algorithm avoids the second training phase, reduces each user is receiver complexity thanks to pre-equalization with VP at the transmitter, and has comparable diversity performance to BD with maximum likelihood decoding algorithm. A bound on the achievable sum rate for the proposed technique is derived and used to show that BD with VP approaches the achievable sum rate of BD with water-filling. Numerical simulations confirm that the proposed technique provides better bit error rate and diversity performance than BD with a zero-forcing receiver as well as BD with zero-forcing precoding.}, number={11}, journal={IEEE Transactions on Wireless Communications}, year={2008}, pages={4051–4057} }
 @inproceedings{computing the receive spatial correlation for a multi-cluster mimo channel using different array configurations_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67449093948&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.759}, abstractNote={Spatial correlation among received signals has a significant impact on the performance of a multiple- input multiple-output (MIMO) system. In prior research, expressions were derived for the spatial correlation between the signals received at a uniform linear array (ULA) by assuming that the propagation environment has a single cluster, low angular spread or that the received rays arrive only along the broadside of the array. In this paper, analytical expressions are derived for the spatial correlation between the received signals of a ULA and uniform circular array without making any of the above mentioned assumptions. The derivation can be extended to suit most planar antenna array configurations. This makes the derived expressions valid for most practical propagation channels. Finally, we show using simulations that the derived expression evaluates spatial correlation more accurately than the existing models.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2008}, pages={3959–3963} }
 @article{dhillon_heath_strohmer_tropp_2008, title={Constructing packings in grassmannian manifolds via alternating projection}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44249089528&partnerID=MN8TOARS}, DOI={10.1080/10586458.2008.10129018}, abstractNote={This paper describes a numerical method for finding good packings in Grassmannian manifolds equipped with various metrics. This investigation also encompasses packing in projective spaces. In each case, producing a good packing is equivalent to constructing a matrix that has certain structural and spectral properties. By alternately enforcing the structural condition and then the spectral condition, it is often possible to reach a matrix that satisfies both. One may then extract a packing from this matrix. This approach is both powerful and versatile. In cases in which experiments have been performed, the alternating projection method yields packings that compete with the best packings recorded. It also extends to problems that have not been studied numerically. For example, it can be used to produce packings of subspaces in real and complex Grassmannian spaces equipped with the Fubini–Study distance; these packings are valuable in wireless communications. One can prove that some of the novel configurations constructed by the algorithm have packing diameters that are nearly optimal.}, number={1}, journal={Experimental Mathematics}, author={Dhillon, I.S. and Heath, R.W. and Strohmer, T. and Tropp, J.A.}, year={2008}, pages={9–35} }
 @article{coordinated beamforming for the multiuser mimo broadcast channel with limited feedforward_2008, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57149124760&partnerID=MN8TOARS}, DOI={10.1109/TSP.2008.929869}, abstractNote={Linear transmit beamforming and receive combining is a low complexity approach for communications in the multiuser multiple-input multiple-output (MIMO) channel. This paper proposes a full search and two low complexity coordinated algorithms where each receiver uses a quantized combining vector. Since the optimization is performed at the transmitter with channel state information from all the receivers, the receive combining vectors are (either jointly or independently) quantized at the transmitter and sent to the receivers via a low-rate feedforward control channel. Rate bounds are provided to estimate the impact of quantization loss on the achievable rate in Rayleigh channels. Simulations show that the sum rate of the proposed methods using Grassmannian codebooks is close to the sum capacity of the MIMO broadcast channel, especially in the high signal-to-noise ratio (SNR) regime.}, number={12}, journal={IEEE Transactions on Signal Processing}, year={2008}, pages={6044–6056} }
 @article{coordinated beamforming with limited feedback in the mimo broadcast channel_2008, volume={26}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54249102174&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2008.081016}, abstractNote={In this paper, we propose a new joint optimization of linear transmit beamforming and receive combining vectors for the multiple-input multiple-output (MIMO) broadcast channel. We consider the transmission of a single information stream to two users with two or more receive antennas. Unlike past work in which iterative computation is required to design the beamformers, we derive specific formulations for the transmit beamformers for two active users via a power iteration and a generalized eigen analysis. To enable practical implementation, a new limited feedback algorithm is proposed that exploits the structure of the algorithm to avoid full channel quantization. The feedback overhead of the proposed algorithm is independent of the number of receive antennas. Monte Carlo simulations are used to evaluate the bit error rate and the sum rate performances of the proposed algorithm. Simulation results show that the proposed method performs close to the sum capacity of the MIMO broadcast channel even with limited feedback.}, number={8}, journal={IEEE Journal on Selected Areas in Communications}, year={2008}, pages={1505–1515} }
 @article{piazza_kirsch_forenza_heath_dandekar_2008, title={Design and evaluation of a reconfigurable antenna array for MIMO systems}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41649111335&partnerID=MN8TOARS}, DOI={10.1109/TAP.2008.916908}, abstractNote={New reconfigurable antenna array is demonstrated for multiple input multiple output (MIMO) communication systems that improves link capacity in closely spaced antenna arrays. The antenna system consists of an array of two printed dipoles separated by a distance of a quarter wavelength. Each of the dipoles can be reconfigured in length using PIN diode switches. The switch configuration can be modified in a manner adaptive to changes in the environment. The configuration of switches effects the mutual coupling between the array elements, and subsequently, the radiation pattern of each antenna, leading to different degrees of pattern diversity which can be used to improve link capacity. The PIN diode-based reconfigurable antenna solution is first motivated through a capacity analysis of the antenna in a clustered MIMO channel model. A new definition of spatial correlation coefficient is introduced to include the effects of antenna mismatch and radiation efficiency when quantifying the benefit of pattern diversity. Next, the widespread applicability of the proposed technique is demonstrated, relative to conventional half wavelength printed dipoles, using computational electromagnetic simulation in an outdoor and indoor environment and field measurements in an indoor laboratory environment. It is shown for the 2 times 2 system considered in this paper, that an average improvement of 10% and 8% is achieved in link capacity for a signal to noise ratio (SNR) respectively of 10 dB and 20 dB in an indoor environment compared to a system employing non reconfigurable antenna arrays.}, number={3}, journal={IEEE Transactions on Antennas and Propagation}, author={Piazza, D. and Kirsch, N.J. and Forenza, A. and Heath, R.W. and Dandekar, K.R.}, year={2008}, pages={869–881} }
 @article{design of linear equalizers optimized for the structural similarity index_2008, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44649189765&partnerID=MN8TOARS}, DOI={10.1109/TIP.2008.921328}, abstractNote={We propose an algorithm for designing linear equalizers that maximize the structural similarity (SSIM) index between the reference and restored signals. The SSIM index has enjoyed considerable application in the evaluation of image processing algorithms. Algorithms, however, have not been designed yet to explicitly optimize for this measure. The design of such an algorithm is nontrivial due to the nonconvex nature of the distortion measure. In this paper, we reformulate the nonconvex problem as a quasi-convex optimization problem, which admits a tractable solution. We compute the optimal solution in near closed form, with complexity of the resulting algorithm comparable to complexity of the linear minimum mean squared error (MMSE) solution, independent of the number of filter taps. To demonstrate the usefulness of the proposed algorithm, it is applied to restore images that have been blurred and corrupted with additive white gaussian noise. As a special case, we consider blur-free image denoising. In each case, its performance is compared to a locally adaptive linear MSE-optimal filter. We show that the images denoised and restored using the SSIM-optimal filter have higher SSIM index, and superior perceptual quality than those restored using the MSE-optimal adaptive linear filter. Through these results, we demonstrate that a) designing image processing algorithms, and, in particular, denoising and restoration-type algorithms, can yield significant gains over existing (in particular, linear MMSE-based) algorithms by optimizing them for perceptual distortion measures, and b) these gains may be obtained without significant increase in the computational complexity of the algorithm.}, number={6}, journal={IEEE Transactions on Image Processing}, year={2008}, pages={857–872} }
 @inproceedings{end-to-end antenna selection strategies for multi-hop relay channels_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349655555&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2008.5074672}, abstractNote={Multi-hop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Assuming a low-rate feedback link from the destination to each relay stage and the source, this paper proposes end-to-end antenna selection strategies to achieve maximum diversity gain in a multi-hop relay channel. For the full-duplex case, end-to-end antenna selection strategies are designed and proven to achieve maximum diversity gain by using a single antenna path (using single antenna of the source, each relay stage and the destination) with the maximum signal-to-noise ratio at the destination. For the half-duplex case, two paths with the two best signal-to-noise ratios in alternate time slots are used to overcome the rate loss with half-duplex nodes, with a small diversity gain penalty. To increase the multiplexing gain, a multiple stream end-to-end antenna selection strategy for full-duplex multi-hop relay channel is also proposed, where multiple data streams are transmitted simultaneously using multiple paths from the source to the destination.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2008}, pages={1506–1510} }
 @inproceedings{energy-efficient adaptive mimo systems leveraging dynamic spare capacity_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51849090250&partnerID=MN8TOARS}, DOI={10.1109/CISS.2008.4558497}, abstractNote={In this paper, we propose an adaptive technique exploiting transmission mode switching between multiple input multiple output (MIMO) and single input multiple output (SIMO) with antenna selection to conserve mobile terminals' energy. We focus on saving uplink RF transmission energy in cellular systems supporting dynamic best effort (file transfer) traffic loads. The key idea is to judiciously slow down file transfer rates when a base station is underutilized. Due to the DC power components associated with the multiplicity of transmission chains, MIMO may have higher power consumption than SIMO. Thus, considering a desired user perceived target throughput as well as energy-efficiency, we propose an algorithm for mode switching (MIMO/SIMO) and rate selection. Extensive flow-level simulations under dynamic loads and Rayleigh fading channels confirm that the proposed technique can save more than 50 % of the mobile terminals' transmission energy and enable an effective tradeoff between performance and energy conservation.}, booktitle={CISS 2008, The 42nd Annual Conference on Information Sciences and Systems}, year={2008}, pages={68–73} }
 @article{erratum: "sdma with a sum feedback rate constraint" (ieee transactions on signal processing)_2008, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48849108990&partnerID=MN8TOARS}, DOI={10.1109/TSP.2008.920140}, abstractNote={This correspondence corrects an error in the proof for theorem, the concavity of the logarithmic function was incorrectly applied to obtain the sum-rate lower bound even though it leads to an upper bound. The revised proof is provided in section II-A, where the aforementioned error has been removed. The changes include replacing the average of a set of signal-to-noise- interference (SINR) terms with an arbitrary term in this set, which allows the use of the concavity of the logarithm function for obtaining a sum-rate lower bound [(5)]. In addition, the proof for the sum-rate upper bound in [1, App. E] has been also modified to replace with a more appropriate one. The modified proof is presented in section II-B.}, number={8 I}, journal={IEEE Transactions on Signal Processing}, year={2008}, pages={3800–3801} }
 @article{guest editorial exploiting limited feedback in tomorrow's wireless communication networks_2008, volume={26}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54249138610&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2008.081001}, abstractNote={Recent research has demonstrated that by utilizing channel state information at the transmitter, the physical layer can be optimized to provide higher link capacity and throughput, more efficiently share the channel with multiple users, increase range by exploiting diversity due to spatial and frequency selectivity, and simplify multi-user receivers through known interference cancellation. Unfortunately, acquiring channel state information at the transmitter is difficult. In most systems, the only opportunity for the transmitter to learn about the channel is through a feedback control channel. Because feedback information is control overhead, the rate of the feedback channel is limited. This motivates the study of limited feedback techniques where only partial or quantized information from the receiver is conveyed back to the transmitter.}, number={8}, journal={IEEE Journal on Selected Areas in Communications}, year={2008}, pages={1337–1340} }
 @article{impact of mutual coupling on adaptive switching between mimo transmission strategies and antenna configurations_2010, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-73349116581&partnerID=MN8TOARS}, DOI={10.1007/s11277-008-9513-2}, number={1}, journal={Wireless Personal Communications}, year={2010}, pages={69–87} }
 @article{information outage probability and diversity order of alamouti transmit diversity in time-selective fading channels_2008, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57049090464&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.919985}, abstractNote={We analyze the effect of time-selective fading on the information outage probability and diversity order of Alamouti transmit diversity with joint maximum likelihood, symbolwise linear maximum likelihood, zero forcing, and decision feedback detectors. In contrast to the existing bit error rate (BER) and bit error outage (BEO) analyses, we evaluate the information outage probabilities and corresponding asymptotic diversity orders for each detector. From the theoretical and simulation results, we demonstrate that, when the channel is time selective, the asymptotic diversity orders for the four detectors approach two, zero, one, and one, respectively.}, number={6}, journal={IEEE Transactions on Vehicular Technology}, year={2008}, pages={3890–3895} }
 @book{tulino_mckay_andrews_collings_heath_2008, title={Joint Detection for Multi-Antenna Channels}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84889472209&partnerID=MN8TOARS}, DOI={10.1002/9780470473818.ch6}, abstractNote={Jeffrey G. Andrews, Iain Collings, Robert W. Heath, Jr., Matthew McKay, Antonia Tulino Dept. of Electrical and Comp. Engineering, University of Texas at Austin Email: {jandrews,rheath}@ece.utexas.edu 2 Commonwealth Scientific and Research Organization (CSIRO) Email: iain.collings@csiro.au 3 Dept. of Electronic and Computer Engineering Hong Kong University of Science and Technology Dip. di Ing. Elettronica e delle Telecomunicazioni, Universita’ degli Studi di Napoli Email: atulino@princeton.edu}, journal={Advances in Multiuser Detection}, author={Tulino, A. and Mckay, M.R. and Andrews, J.G. and Collings, I.B. and Heath, R.W.}, year={2008}, pages={311–364} }
 @article{joint source-channel distortion modeling for mpeg-4 video_2009, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-58049195560&partnerID=MN8TOARS}, DOI={10.1109/TIP.2008.2005819}, abstractNote={Multimedia communication has become one of the main applications in commercial wireless systems. Multimedia sources, mainly consisting of digital images and videos, have high bandwidth requirements. Since bandwidth is a valuable resource, it is important that its use should be optimized for image and video communication. Therefore, interest in developing new joint source-channel coding (JSCC) methods for image and video communication is increasing. Design of any JSCC scheme requires an estimate of the distortion at different source coding rates and under different channel conditions. The common approach to obtain this estimate is via simulations or operational rate-distortion curves. These approaches, however, are computationally intensive and, hence, not feasible for real-time coding and transmission applications. A more feasible approach to estimate distortion is to develop models that predict distortion at different source coding rates and under different channel conditions. Based on this idea, we present a distortion model for estimating the distortion due to quantization and channel errors in MPEG-4 compressed video streams at different source coding rates and channel bit error rates. This model takes into account important aspects of video compression such as transform coding, motion compensation, and variable length coding. Results show that our model estimates distortion within 1.5 dB of actual simulation values in terms of peak-signal-to-noise ratio.}, number={1}, journal={IEEE Transactions on Image Processing}, year={2009}, pages={90–105} }
 @inproceedings{kerdock codes for limited feedback mimo systems_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51449098050&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2008.4518309}, abstractNote={A codebook based limited feedback strategy is a practical way to obtain partial channel state information at the transmitter in precoded multiple-input multiple-output (MIMO) wireless systems. Construction of conventional codebooks, such as Grassmannian and Fourier codebooks, relies on nonlinear search and iterative algorithms which often does not exhibit structure to ease storage or online search computation. Furthermore, multiple codebooks are needed to support beamforming and spatial multiplexing. In this paper, we propose a new codebook design based on Kerdock codes and mutually unbiased bases which enjoys performance similar to previously known codebooks. The proposed Kerdock codebook with quaternary alphabet has systematic construction, reduced storage, and reduced online search computation. Special structure in the codebook is used to derive a spatial multiplexing codebook from multiple columns of the beamforming codebook resulting in further storage reduction.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2008}, pages={3113–3116} }
 @inproceedings{gulati_chopra_heath_evans_tinsley_lin_2008, title={MIMO receiver design in the presence of radio frequency interference}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67249086026&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.641}, abstractNote={Multi-input multi-output (MIMO) receivers have been designed and their communication performance analyzed under the assumption of additive Gaussian noise. Wireless transceivers, however, may be affected by radio frequency interference (RFI) that is well modeled using non-Gaussian impulsive statistics. In this paper, we consider the problem of receiver design for a two transmit, two receive antenna MIMO system in the presence of RFI. First, we show that RFI is well modeled using a bivariate Middleton Class A model and validate the model with measured data. Using this RFI model, we demonstrate that conventional MIMO receivers experience significant degradation in communication performance. Then we derive the maximum likelihood (ML) receiver assuming bivariate Middleton Class A noise. Furthermore, we develop a parameter estimation method for this noise model and propose two sub-optimal ML receivers with reduced computational complexity. Simulations show significant improvement in symbol error rate performance of the proposed techniques over receivers designed assuming additive Gaussian noise.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, author={Gulati, K. and Chopra, A. and Heath, R.W. and Evans, B.L. and Tinsley, K.R. and Lin, X.E.}, year={2008}, pages={3340–3344} }
 @article{mimo relaying with linear processing for multiuser transmission in fixed relay networks_2008, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39649100853&partnerID=MN8TOARS}, DOI={10.1109/TSP.2007.907821}, abstractNote={In this paper, a novel relaying strategy that uses multiple-input multiple-output (MIMO) fixed relays with linear processing to support multiuser transmission in cellular networks is proposed. The fixed relay processes the received signal with linear operations and forwards the processed signal to multiple users creating a multiuser MIMO relay. This paper proposes upper and lower bounds on the achievable sum rate for this architecture assuming zero-forcing dirty paper coding at the base station, neglecting the direct links from the base station to the users, and with certain structure in the relay. These bounds are used to motivate an implementable multiuser precoding strategy that combines Tomlinson-Harashima precoding at the base station and linear signal processing at the relay, adaptive stream selection, and QAM modulation. Reduced complexity algorithms based on the sum rate lower bounds are used to select a subset of users. We compare the sum rates achieved by the proposed system architecture and algorithms with the sum rate upper bound and the sum rate achieved by the decode-and-forward relaying.}, number={2}, journal={IEEE Transactions on Signal Processing}, year={2008}, pages={727–738} }
 @inproceedings{machine learning for physical layer link adaptation in multiple-antenna wireless networks_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-60349088484&partnerID=MN8TOARS}, DOI={10.1145/1410077.1410106}, abstractNote={Prototyping and experimentation are key to understanding the operation of wireless systems in practice. In this extended abstract we present an implementation of physical layer link adaptation, or data rate selection, through machine learning on Hydra: an IEEE 802.11n draft standard multihop wireless networking prototype. This implementation highlights both the utility of learning-based link adaptation in practical networks as well as the flexibility of Hydra.}, booktitle={Proceedings of the Annual International Conference on Mobile Computing and Networking, MOBICOM}, year={2008}, pages={114–115} }
 @inproceedings{vaze_heath_2008, title={Maximizing reliability in multi-hop wireless networks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-52349115107&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2008.4594938}, abstractNote={Distributed space-time block coding is a diversity technique to mitigate the effects of fading in multi-hop wireless networks, where multiple relay stages are used by a source to communicate with its destination. This paper proposes a new distributed space-time block code called the cascaded orthogonal space-time block code (COSTBC) for the case where the source and destination are equipped with multiple antennas and each relay stage has one or more single antenna relays. Each relay stage is assumed to have receive channel state information (CSI) for all the channels from the source to itself, while the destination is assumed to have receive CSI for all the channels. To construct COSTBC, multiple orthogonal space-time block codes are used in cascade by the source and each relay stage. COSTBC is shown to achieve the maximum diversity gain in a multi-hop wireless network with flat Rayleigh fading channels. An explicit construction of COSTBCs is also provided. It is also shown that COSTBC requires minimum decoding complexity thanks to the connection to orthogonal space-time block codes.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Vaze, R. and Heath, R.W.}, year={2008}, pages={11–15} }
 @article{multimode transmission for multiuser mimo systems with block diagonalization_2008, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-46649116275&partnerID=MN8TOARS}, DOI={10.1109/TSP.2008.920453}, abstractNote={A low-complexity multimode transmission technique for downlink multiuser multiple-input-multiple-output (MIMO) systems with block diagonalization (BD) is proposed. The proposed technique adaptively configures the number of data streams for each user by adjusting its number of active receive antenna and switching between single-stream beamforming and multistream spatial multiplexing, as a means to exploit the multimode switching diversity. We consider a highly loaded system where there are a large number of users, hence a subset of users need to be selected. Joint user and antenna selection has been proposed as a multiuser multimode switching technique, where the optimal subset of receive antennas and users are chosen to maximize the sum throughput. The brute-force search, however, is prohibitively complicated. In this paper, two low-complexity near-optimal user/antenna selection algorithms are developed. The first algorithm aims at maximizing a capacity lower bound, derived in terms of the sum Frobenius norm of the channel, while the second algorithm greedily maximizes the sum capacity. We analytically evaluate the complexity of the proposed algorithms and show that it is orders of magnitude lower than that of the exhaustive search. Simulation results demonstrate that the proposed algorithms achieve up to 98% of the sum throughput of the exhaustive search, for most system configurations, while the complexity is substantially reduced.}, number={7 II}, journal={IEEE Transactions on Signal Processing}, year={2008}, pages={3294–3302} }
 @inproceedings{multiuser mimo downlink with limited feedback using transmit-beam matching_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51249084931&partnerID=MN8TOARS}, DOI={10.1109/ICC.2008.659}, abstractNote={The multiuser multiple-input multiple-output (MIMO) broadcast channel uses multiple antennas at the transmitter to simultaneously deliver information to multiple users. Despite the theoretical capacity gains, the actual throughput of the system with limited feedback of channel state information suffers due to interspatial stream interference caused by quantization errors. This paper introduces a new method to mitigate the interference: transmit-beam matching. Using multiple receive antennas, transmit-beam matching minimizes the inter-spatial stream interference by each user's effective channel to the transmit-beam. It is evaluated based on a recent MIMO broadcast channel technique with limited feedback, per-user unitary rate control (PU2RC) and shown to effectively mitigate the interference.}, booktitle={IEEE International Conference on Communications}, year={2008}, pages={3506–3510} }
 @inproceedings{non-iterative multiuser mimo coordinated beamforming with limited feedforward_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51449085298&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2008.4518129}, abstractNote={This paper proposes non-iterative coordinated beamforming algorithms for a multiuser MIMO (multiple input multiple output) system with multiple antennas at the transmitter and multiple users, each with multiple receive antennas. The transmitter uses linear beamforming to convey information to each user, while each receiver uses a quantized combining vector, sent from the transmitter via a low-rate feedforward control channel. Two different algorithms for optimizing transmit beamformers and receive combining vectors are proposed: a joint optimization and a greedy search. Simulations show that the proposed methods using quantized codebooks approach the sum capacity of the MIMO broadcast channel.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2008}, pages={2393–2396} }
 @article{non-redundant precoding-based blind and semi-blind channel estimation for mimo block transmission with a cyclic prefix_2008, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44849093431&partnerID=MN8TOARS}, DOI={10.1109/TSP.2007.914954}, abstractNote={By combining multiple-input-multiple-output (MIMO) communication with block transmission using a cyclic prefix (CP), MIMO block transmission systems can not only achieve high data rates but also simplify channel estimation and equalization. In this paper, to provide a bandwidth-efficient solution for channel estimation in MIMO block transmission systems with a CP, we present a framework for blind channel estimation based on a general non-redundant precoding. Using this framework, we propose a blind estimator exploiting a simplified non-redundant precoding that is robust against overestimates of a true MIMO channel order. Furthermore, in the case with the number of transmit antennas greater than that of receive antennas, we show under specific system conditions that the proposed blind algorithms can be used for estimation of the MIMO channel without oversampling the received signals. With the simplified precoding conditions established in this paper, the proposed method does not impose the strict identifiability conditions on the MIMO channel, which are required for the existing methods. Furthermore the proposed estimator achieves accurate channel estimation with a small number of symbol blocks. In addition, we derive a simplified precoder that is optimized in the sense of minimizing the impact of unknown additive noise. We discuss the tradeoff between channel estimation accuracy and bit error rate performance associated with the simplified precoder. Finally, we develop a technique for resolving the channel ambiguity in the proposed blind estimator to create a semi-blind channel estimator relying on only a few known symbols.}, number={6}, journal={IEEE Transactions on Signal Processing}, year={2008}, pages={2509–2523} }
 @article{peters_heath_2008, title={Nonregenerative MIMO relaying with optimal transmit antenna selection}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67650126118&partnerID=MN8TOARS}, DOI={10.1109/LSP.2008.921466}, abstractNote={We derive optimal SNR-based transmit antenna selection rules at the source and relay for the nonregenerative half-duplex MIMO relay channel. While antenna selection is a suboptimal form of beamforming, it has the advantage that the optimization is tractable and can be implemented with only a few bits of feedback from the destination to the source and relay. We compare the bit error rate of optimal antenna selection at both the source and relay to other proposed beamforming techniques and propose methods for performing the necessary limited feedback.}, journal={IEEE Signal Processing Letters}, author={Peters, S.W. and Heath, R.W.}, year={2008}, pages={421–424} }
 @article{opportunistic scheduling in multiuser ofdm systems with clustered feedback_2010, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-73349120952&partnerID=MN8TOARS}, DOI={10.1007/s11277-008-9502-5}, number={1}, journal={Wireless Personal Communications}, year={2010}, pages={209–225} }
 @article{forenza_heath_2008, title={Optimization methodology for designing 2-CPAs exploiting pattern diversity in clustered MIMO channels}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54949087491&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2008.060582}, abstractNote={Arrays designed for multiple-input multiple-output (MIMO) communication systems must provide good performance in terms of information theoretic and microwave theoretic performance measures for a large number of channel scenarios. Consequently, the general MIMO array design problem is challenging. This paper proposes a novel optimization methodology for designing circular patch arrays (CPAs), under a clustered MIMO channel model assumption. Based on the ergodic capacity, it is found that the single-cluster scenario can approximate CPA performance in clustered channels, reducing the number of channel scenarios that need to be considered. Using a new definition of channel spatial correlation that allows for pattern diversity, metrics are derived for evaluating average and outage capacity performance of CPAs, which can be expressed as a function of only the antenna parameters. With this expression, an optimization is formulated and solved numerically for optimizing the parameters of a 2-CPA on a capacity based objective function and microwave theory performance constraints. An example design is given corresponding to a 2-CPA in the 2.4 GHz unlicensed frequency band.}, number={10}, journal={IEEE Transactions on Communications}, author={Forenza, A. and Heath, R.W.}, year={2008}, pages={1748–1759} }
 @article{optimizing pilot locations using feedback in ofdm systems_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67549115605&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.2008655}, abstractNote={Pilot-aided orthogonal frequency-division multiplexing (OFDM) channel estimation has, so far, been optimized only for open-loop systems for which the uniform spacing of pilots is optimal. In this paper, we examine closed-loop OFDM systems and show how uniform pilots may no longer be optimal. In doing so, we propose a feedback technique in which the pilot allocation mechanism is adapted to the mobile channel. After deriving a general expression for the symbol error rate as a function of pilot allocation, we optimize the pilot locations and propose two practical solutions using bounds on the objective function. We proceed to formulate pilot allocation based on the maximal average OFDM channel capacity. We examine the feedback overhead that is associated with our method and make comparisons with the conventional uniform framework. Finally, we suggest the use of vector quantization in the context of the generalized Lloyd algorithm to reduce feedback and extend our method to a multiple-input-multiple-output (MIMO)-OFDM system based on the Alamouti space-time block code. Monte-Carlo simulations illustrate the error-rate/capacity performance gains via nonuniform pilots and illustrate the effects of Doppler frequency and carrier frequency offset.}, number={6}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={2803–2814} }
 @inproceedings{perceptual soft thresholding using the structural similarity index_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-69949151027&partnerID=MN8TOARS}, DOI={10.1109/ICIP.2008.4711818}, abstractNote={In this paper, we present a novel algorithm for wavelet domain image denoising using the soft thresholding function. The thresholds are designed to be locally optimal with respect to the structural similarity (SSIM) index. The SSIM Index is first expressed in terms of wavelet transform coefficients of orthogonal wavelet transforms. The wavelet domain representation of the SSIM Index, along with the assumption of a Gaussian prior for the wavelet coefficients is used to formulate the soft thresholding optimization problem. A locally optimal solution is found using a quasi-Newton approach. This solution is applied to denoise images in the wavelet domain. The visual quality of the images denoised using the proposed algorithm is shown to be higher compared to the MSE-optimal soft thresholding denoising solution, as measured by the SSIM Index.}, booktitle={Proceedings - International Conference on Image Processing, ICIP}, year={2008}, pages={569–572} }
 @inproceedings{progressive refinement for high resolution limited feedback multiuser mimo beamforming_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349678838&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2008.5074507}, abstractNote={Limited feedback enables the practical use of channel state information in single user and multiple user MIMO (multiple input multiple output) wireless communication systems. Using the limited feedback concept, channel state information at the receiver is quantized by choosing a representative element from a codebook known to both the receiver and transmitter. Unfortunately, achieving the high resolution required by multiuser MIMO communication is challenging due to the large size codebooks required. This paper proposes a progressive refinement concept that uses a base codebook quantization followed by a progressively scaled local codebook to enable high resolution quantization and reconstruction.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2008}, pages={743–747} }
 @article{rate bounds for mimo relay channels_2008, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47249142481&partnerID=MN8TOARS}, number={2}, journal={Journal of Communications and Networks}, year={2008}, pages={194–203} }
 @inproceedings{rate bounds on ssim index of quantized image dct coefficients_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50249146927&partnerID=MN8TOARS}, DOI={10.1109/DCC.2008.91}, abstractNote={In this paper, we derive bounds on the structural similarity (SSIM) index as a function of quantization rate for fixed-rate uniform quantization of image discrete cosine transform (DCT) coefficients under the high rate assumption. The space domain SSIM index is first expressed in terms of the DCT coefficients of the space domain vectors. The transform domain SSIM Index is then used to derive bounds on the average SSIM index as a function of quantization rate for Gaussian and Laplacian sources. As an illustrative example, uniform quantization of the DCT coefficients of natural images is considered. We show that the SSIM index between the reference and quantized images fall within the bounds for a large set of natural images. Further, we show using a simple example that the proposed bounds could be very useful for rate allocation problems in practical image and video coding applications.}, booktitle={Data Compression Conference Proceedings}, year={2008}, pages={352–361} }
 @article{rate bounds on ssim index of quantized images_2008, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50549091559&partnerID=MN8TOARS}, DOI={10.1109/TIP.2008.2001400}, abstractNote={In this paper, we derive bounds on the structural similarity (SSIM) index as a function of quantization rate for fixed-rate uniform quantization of image discrete cosine transform (DCT) coefficients under the high-rate assumption. The space domain SSIM index is first expressed in terms of the DCT coefficients of the space domain vectors. The transform domain SSIM index is then used to derive bounds on the average SSIM index as a function of quantization rate for uniform, Gaussian, and Laplacian sources. As an illustrative example, uniform quantization of the DCT coefficients of natural images is considered. We show that the SSIM index between the reference and quantized images fall within the bounds for a large set of natural images. Further, we show using a simple example that the proposed bounds could be very useful for rate allocation problems in practical image and video coding applications.}, number={9}, journal={IEEE Transactions on Image Processing}, year={2008}, pages={1624–1639} }
 @article{relay subset selection in wireless networks using partial decode-and-forward transmission_2009, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-61449257403&partnerID=MN8TOARS}, DOI={10.1109/TVT.2008.929466}, abstractNote={This paper considers the problem of selecting a subset of nodes in a two-hop wireless network to act as relays in aiding the communication between the source-destination pair. Optimal relay subset selection with the objective of maximizing the overall throughput is a difficult problem that depends on multiple factors, including node locations, queue lengths, and power consumption. A partial decode-and-forward strategy is applied in this paper to improve the tractability of the relay selection problem and the performance of the overall network. Note that the number of relays that are selected ultimately determines the performance of the network. This paper benchmarks this performance by determining the net diversity that is achieved using the relays that are selected and the partial decode-and-forward strategy. This framework is subsequently used to further transform relay selection into a simpler relay placement problem, and two proximity-based approximation algorithms are developed to determine the appropriate set of relays to be selected in the network. Other selection strategies, such as random relay selection and a greedy algorithm that relies on channel state information, are also presented. This paper concludes by showing that the proposed proximity-based relay selection strategies yield near-optimal expected rates for a small number of selected relays.}, number={2}, journal={IEEE Transactions on Vehicular Technology}, year={2009}, pages={692–704} }
 @inproceedings{relay subset selection in wireless networks using partial decode-and-forward transmission_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47749084311&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2008.530}, abstractNote={This paper considers the problem of selecting a set of relay nodes to assist a transmitting node in a two-hop wireless network. Throughput-maximizing relay subset selection is a difficult problem that depends on variables such as node locations and power constraints. It is proposed that all relays employ partial decode-and-forward operations to improve the tractability of the relay selection problem. This allows relay selection to be transformed into a simpler relay placement problem which motivates two proximity-based relay selection algorithms. These algorithms are compared with a greedy algorithm based on relay channel gains to the sink and an algorithm that randomly selects relays. The diversity gain achieved by employing multiple relay nodes is derived. The proposed proximity-based algorithms offer good performance in terms of the expected achieved rate.}, booktitle={IEEE Vehicular Technology Conference}, year={2008}, pages={2395–2399} }
 @article{andrews_shakkottai_heath_jindal_haenggi_berry_guo_neely_weber_jafar_et al._2008, title={Rethinking information theory for mobile ad hoc networks}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57749093128&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2008.4689214}, abstractNote={The subject of this article is the long standing open problem of developing a general capacity theory for wireless networks, particularly a theory capable of describing the fundamental performance limits of mobile ad hoc networks. A MANET is a peer-to-peer network with no preexisting infrastructure. MANETs are the most general wireless networks, with single-hop, relay, interference, mesh, and star networks comprising special cases. The lack of a MANET capacity theory has stunted the development and commercialization of many types of wireless networks, including emergency, military, sensor, and community mesh networks. Information theory, which has been vital for links and centralized networks, has not been successfully applied to decentralized wireless networks. Even if this was accomplished, for such a theory to truly characterize the limits of deployed MANETs it must overcome three key roadblocks. First, most current capacity results rely on the allowance of unbounded delay and reliability. Second, spatial and timescale decompositions have not yet been developed for optimally modeling the spatial and temporal dynamics of wireless networks. Third, a useful network capacity theory must integrate rather than ignore the important role of overhead messaging and feedback. This article describes some of the shifts in thinking that may be needed to overcome these roadblocks and develop a more general theory.}, number={12}, journal={IEEE Communications Magazine}, author={Andrews, J. and Shakkottai, S. and Heath, R. and Jindal, N. and Haenggi, M. and Berry, R. and Guo, D. and Neely, M. and Weber, S. and Jafar, S. and et al.}, year={2008}, pages={94–101} }
 @inproceedings{ssim-optimal linear image restoration_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51449113836&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2008.4517722}, abstractNote={In this paper, we present an algorithm for designing a linear equalizer that is optimal with respect to the structural similarity (SSIM) index. The optimization problem is shown to be non-convex, thereby making it non-trivial. The non-convex problem is first converted to a quasi-convex problem and then solved using a combination of first order necessary conditions and bisection search. To demonstrate the usefulness of this solution, it is applied to image denoising and image restoration examples. We show using these examples that optimizing equalizers for the SSIM index does indeed result in higher perceptual image quality compared to equalizers optimized for the ubiquitous mean squared error (MSE).}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2008}, pages={765–768} }
 @article{shannon capacity and symbol error rate of space-time block codes in mimo rayleigh channels with channel estimation error_2008, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-38349110208&partnerID=MN8TOARS}, DOI={10.1109/TWC.2008.060539}, abstractNote={Space-time block coding (STBC) is an attractive solution for improving quality in wireless links. In this paper, we analyze the impact of channel estimation error on the ergodic capacity and symbol error rate (SER) for space-time block coded multiple-input multiple-output (MIMO) systems. We derive a closed-form capacity expression over MIMO Rayleigh channels with channel estimation error. Moreover, we derive an exact closed-form SER for general PAM/PSK/QAM with channel estimation error. We show that, as expected, channel estimation error introduces the capacity loss and diversity gain loss. Furthermore, as the number of transmit and receive antennas increases, the sensitivity of the STBC system to channel estimation error increases. Simulation results demonstrate the accuracy of our analysis.}, number={1}, journal={IEEE Transactions on Wireless Communications}, year={2008}, pages={324–333} }
 @inproceedings{single-user mimo vs. multiuser mimo in the broadcast channel with csit constraints_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-64549116322&partnerID=MN8TOARS}, DOI={10.1109/ALLERTON.2008.4797572}, abstractNote={This paper considers fundamental limits on adaptive multiple-input multiple-output (MIMO) transmission with imperfect channel state information at the transmitter (CSIT). First, the achievable ergodic rates are derived for both single-user and multi-user MIMO systems with different CSIT assumptions, which shows that single-user MIMO is robust to imperfect CSIT while multiuser MIMO loses spatial multiplexing gain in proportion to delay and inverse with channel quantization codebook size. A single-user/multiuser mode switching rule is then proposed based on the average SNR, the amount of delay, and the quantization codebook size. The operating regions for single-user and multiuser modes are determined, which can be used to select the preferred mode. It is shown that the multiuser mode should not be activated when the delay is large or the codebook size is small.}, booktitle={46th Annual Allerton Conference on Communication, Control, and Computing}, year={2008}, pages={309–314} }
 @article{bhagavatula_heath_vishwanath_forenza_2008, title={Sizing up MIMO arrays}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-62949097971&partnerID=MN8TOARS}, DOI={10.1109/MVT.2008.930299}, abstractNote={In this article, we illustrate the proposed methodology by applying it to wireless deployment in aircraft, to reduce expenditure associated with the installation and maintenance of wires for seatback entertainment. The large data rates required can be easily supported by MIMO systems.}, number={4}, journal={IEEE Vehicular Technology Magazine}, author={Bhagavatula, R. and Heath, R. and Vishwanath, S. and Forenza, A.}, year={2008}, pages={31–38} }
 @article{smart antennas for next generation wireless systems_2007, volume={2007}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-43949115154&partnerID=MN8TOARS}, DOI={10.1155/2007/20427}, journal={Eurasip Journal on Wireless Communications and Networking}, year={2007} }
 @article{space-time block codes with limited feedback using antenna grouping_2008, volume={E91-B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67651027862&partnerID=MN8TOARS}, DOI={10.1093/ietcom/e91-b.10.3387}, abstractNote={We propose an antenna grouping method that improves the error rate performance of space-time codes in a wide range of mobility environments. The idea is to group symbols to antennas based on limited feedback from the mobile station to utilize all antennas. Our approach requires only two bits of feedback information to achieve better link performance and full rate for a certain four transmit antenna system. Numerical results confirm the bit/frame error gains over the Alamouti-based space-time block code and antenna subset selection strategies.}, number={10}, journal={IEICE Transactions on Communications}, year={2008}, pages={3387–3390} }
 @inproceedings{spatial interference cancelation for mobile ad hoc networks: imperfect csi_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349671481&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2008.5074377}, abstractNote={Spatial interference cancelation based on zero-forcing beamforming significantly improves the throughput of a mobile ad hoc network. Nevertheless, the performance of spatial interference cancelation can be degraded by imperfect channel estimation at receivers. We analyze the impact of inaccurate channel state information (CSI) on network transmission capacity by modeling transmitters as a Poisson point process and applying mathematical tools from stochastic geometry. CSI inaccuracy leads to smaller throughput and larger outage probability of each data link. Such performance loss can be controlled by choosing the length of training sequences as derived. Furthermore, increasing this length logarithmically with the inverse of the decreasing target outage probability achieves the same network-capacity scaling laws as for perfect CSI. Simulation results show that spatial interference cancelation increases transmission capacity by an order of magnitude or more despite CSI inaccuracy.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2008}, pages={131–135} }
 @inproceedings{spatial interference cancellation for mobile ad hoc networks: perfect csi_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67249094173&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.134}, abstractNote={Interference between nodes directly limits the capacity of mobile ad hoc networks. This paper focuses on spatial interference cancellation with perfect channel state information (CSI), and analyzes the corresponding network capacity. Specifically, by using multiple antennas, zero-forcing beamforming is applied at each receiver for canceling the strongest interferers. Given spatial interference cancellation, the network transmission capacity is analyzed in this paper, which is defined as the maximum transmitting node density under constraints on outage and the signal-to-interference-plus-noise ratio. Assuming that the locations of network nodes are Poisson distributed and spatially i.i.d. Rayleigh fading channels, mathematical tools from stochastic geometry are applied for deriving scaling laws for transmission capacity. Specifically, for a large number of antennas per node, the transmission capacity scales with the number of antennas raised to a fractional power, which depends only on the path-loss exponent. Moreover, for small target outage probability, transmission capacity is proved to increase following a power law, where the exponent is the inverse of the size of antenna array or larger depending on the pass-loss exponent. As shown by simulations, spatial interference cancellation increases transmission capacity by an order of magnitude or more even if only one extra antenna is added to each node.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2008}, pages={666–670} }
 @inproceedings{switching between antenna selection and spatial multiplexing in the nonregenerative mimo relay channel_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349661729&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2008.5074665}, abstractNote={The use of a relay to aid communication between a data source and its desired destination is an attractive solution for low-cost coverage extension, capacity enhancement, and reliability improvement. The relays need not even decode the signal in order to aid the transmission. Multi-antenna relays, in particular, may apply a linear transfer matrix to improve transmission while maintaining a simple design. The usage of multiple antennas, however, raises questions about which of the multitude of transmission strategies should be used for a given channel realization. In this paper, the use of pre-existing adaptation techniques is shown to be inapplicable to the linear MIMO relay channel, motivating the derivation of new techniques. To simplify the task, the source is limited to transmitting using either spatial multiplexing with no channel state information, or antenna selection based on a limited amount of feedback from the destination. Transmission is assumed to be fixed-rate and adaptation is based on minimizing the symbol error rate. A sufficient condition for multiplexing to outperform antenna selection is derived and, out of this, a value that behaves as an effective condition number of the relay channel is found.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2008}, pages={1469–1473} }
 @inproceedings{throughput/delay measurements of limited feedback beamforming in indoor wireless networks_2008, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67249111478&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.880}, abstractNote={This paper investigates the tradeoff between throughput and feedback delay of limited feedback beamforming in indoor wireless channels with a practical MIMO-OFDM prototype. Past descriptions of this tradeoff are largely based on simplified models of the wireless channel. Unfortunately, wireless channel models may not accurately represent the complexities of a real wireless channel. Furthermore, system impairments, including channel estimation error, only exacerbate the problem of modeling a real wireless system. One such analytical result predicts the performance of limited feedback beamforming as an exponential function of feedback delay. This analytical result has been confirmed through Monte Carlo simulation under Rayleigh fading channel models. Through rigorous measurements and experimentation this paper both evaluates the performance of limited feedback beamforming under feedback delay and confirms the accuracy of the analytical results.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2008}, pages={4593–4598} }
 @inproceedings{deyoung_heath_evans_2008, title={Using higher order cyclostationarity to identify space-time block codes}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67249083389&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2008.ECP.647}, abstractNote={Research in cognitive radios has renewed interest in tools, such as spectrum estimation and modulation identification, to characterize the radio frequency (RF) environment. The use of multiple antennas for multiple-input multiple-output (MIMO) communications presents a new challenge in detecting and classifying signals. In this paper, we propose a cyclostationarity-based statistical test to detect space-time block codes, focusing on the two transmitter Alamouti space-time block code (STBC). Our test exploits a new characterization of the Alamouti code using fourth order cyclic frequencies. The test requires only a single receive antenna, and does not require any symbol synchronization.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, author={Deyoung, M.R. and Heath, R.W. and Evans, B.}, year={2008}, pages={3370–3374} }
 @article{60 ghz wireless communications: emerging requirements and design recommendations_2007, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-40349094869&partnerID=MN8TOARS}, DOI={10.1109/MVT.2008.915320}, abstractNote={Multiple GHz of internationally available, unlicensed spectrum surrounding the 60 GHz carrier frequency has the ability to accommodate high-throughput wireless communications. While the size and availability of this free spectrum make it very attractive for wireless applications, 60 GHz implementations must overcome many challenges. For example, the high attenuation and directional nature of the 60 GHz wireless channel as well as limited gain amplifiers and excessive phase noise in 60 GHz transceivers are explicit implementation difficulties. The challenges associated with this channel motivate commercial deployment of short-range wireless local area networks, wireless personal area networks, and vehicular networks. In this paper we detail design tradeoffs for algorithms in the 60 GHz physical layer including modulation, equalization, and space-time processing. The discussion is enhanced by considering the limitations in circuit design, characteristics of the effective wireless channel (including antennas), and performance requirements to support current and next generation 60 GHz wireless communication applications.}, number={3}, journal={IEEE Vehicular Technology Magazine}, year={2007}, pages={41–50} }
 @inproceedings{a mimo demonstration of hydra_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37849023156&partnerID=MN8TOARS}, DOI={10.1145/1287767.1287790}, abstractNote={The evaluation of cross-layer protocols for wireless networking requires analysis, simulation, and experimentation. The final step in this process is often overlooked, but is equally important. Prototyping and experimentation with testbeds not only provides a mechanism for evaluating the performance of wireless networks, but also provide keen insight into the design of new wireless protocols. Hydra, a flexible wireless network testbed developed at UT Austin, was designed to investigate cross-layer issues in wireless protocol design. In this paper, we propose a demonstration of the multiple-input multiple-output (MIMO) functionality of Hydra. We will investigate rate and spatial mode adaptation using this MIMO system.}, booktitle={International Conference on Mobile Computing and Networking, MobiCom'07 Co-Located Workshops -Proceedings of the the Second ACM International Workshop on Wireless Network Testbeds, Experimental Evalua}, year={2007}, pages={101–102} }
 @inproceedings{a mimo demonstration of hydra_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70450228536&partnerID=MN8TOARS}, booktitle={Proceedings of the 2nd ACM International Workshop on Wireless Network Testbeds, Experimental Evaluation and Characterization, WiNTECH '07, Co-located with the ACM MobiCom 2007 Conference}, year={2007}, pages={101–102} }
 @article{a diversity guarantee and snr performance for unitary limited feedback mimo systems_2008, volume={2008}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41249098886&partnerID=MN8TOARS}, DOI={10.1155/2008/594928}, abstractNote={A multiple-input multiple-output (MIMO) wireless channel formed by antenna arrays at the transmitter and at the receiver offers high capacity and significant diversity. Linear precoding may be used, along with spatial multiplexing (SM) or space-time block coding (STBC), to realize these gains with low-complexity receivers. In the absence of perfect channel knowledge at the transmitter, the precoding matrices may be quantized at the receiver and informed to the transmitter using a feedback channel, constituting a limited feedback system. This can possibly lead to a performance degradation, both in terms of diversity and array gain, due to the mismatch between the quantized precoder and the downlink channel. In this paper, it is proven that if the feedback per channel realization is greater than a threshold, then there is no loss of diversity due to quantization. The threshold is completely determined by the number of transmit antennas and the number of transmitted symbol streams. This result applies to both SM and STBC with unitary precoding and confirms some conjectures made about antenna subset selection with linear receivers. A closed form characterization of the loss in SNR (transmit array gain) due to precoder quantization is presented that applies to a precoded orthogonal STBC system and generalizes earlier results for single-stream beamforming.}, journal={Eurasip Journal on Advances in Signal Processing}, year={2008} }
 @article{adaptive mimo transmission for exploiting the capacity of spatially correlated channels_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34147175447&partnerID=MN8TOARS}, DOI={10.1109/TVT.2007.891427}, abstractNote={We consider a novel low-complexity adaptive multiple-input multiple-output (MIMO) transmission technique. The approach is based on switching between low-complexity transmission schemes, including statistical beamforming, double space-time transmit diversity, and spatial multiplexing, depending on the changing channel statistics, as a practical means of approaching the spatially correlated MIMO channel capacity. We first derive new ergodic capacity expressions for each MIMO transmission scheme in spatially correlated channels. Based on these results, we demonstrate that adaptive switching between MIMO schemes yields significant capacity gains over fixed transmission schemes. We also derive accurate analytical approximations for the optimal signal-to-noise-ratio switching thresholds, which correspond to the crossing-points of the capacity curves. These thresholds are shown to vary, depending on the spatial correlation, and are used to identify key switching parameters. Finally, we propose a practical switching algorithm that is shown to yield significant spectral efficiency improvements over nonadaptive schemes for typical channel scenarios}, number={2}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={619–630} }
 @article{blind channel estimation for mimo-ofdm systems_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34147190010&partnerID=MN8TOARS}, DOI={10.1109/TVT.2007.891429}, abstractNote={By combining multiple-input multiple-output (MIMO) communication with the orthogonal frequency division multiplexing (OFDM) modulation scheme, MIMO-OFDM systems can achieve high data rates over broadband wireless channels. In this paper, to provide a bandwidth-efficient solution for MIMO-OFDM channel estimation, we establish conditions for channel identifiability and present a blind channel estimation technique based on a subspace approach. The proposed method unifies and generalizes the existing subspace-based methods for blind channel estimation in single-input single-output OFDM systems to blind channel estimation for two different MIMO-OFDM systems distinguished according to the number of transmit and receive antennas. In particular, the proposed method obtains accurate channel estimation and fast convergence with insensitivity to overestimates of the true channel order. If virtual carriers (VCs) are available, the proposed method can work with no or insufficient cyclic prefix (CP), thereby potentially increasing channel utilization. Furthermore, it is shown under specific system conditions that the proposed method can be applied to MIMO-OFDM systems without CPs, regardless of the presence of VCs, and obtains an accurate channel estimate with a small number of OFDM symbols. Thus, this method improves the transmission bandwidth efficiency. Simulation results illustrate the mean-square error performance of the proposed method via numerical experiments}, number={2}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={670–685} }
 @inproceedings{capacity scaling for mimo two-way relaying_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51649103066&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2007.4557134}, abstractNote={This paper considers capacity scaling in the recently proposed two-way MIMO (multiple input multiple output) relay channel. In the two-way relay channel, two nodes use a relay for exchanging data with each other. Under the assumption that each node has perfect receive channel state information and all nodes work only in half duplex mode, this paper shows that the sum capacity scales linearly with the number of transmit antennas and logarithmically with the number of relays, as the number of relays grows large. This result shows that with two- way relay channels it is possible to asymptotically (in the number of relays) obtain full-duplex performance while using only half-duplex nodes.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2007}, pages={1451–1455} }
 @inproceedings{conference record - asilomar conference on signals, systems and computers: foreword_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50249186426&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2007.4487150}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2007} }
 @article{congruent voronoi tessellations from equiangular lines_2007, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547579650&partnerID=MN8TOARS}, DOI={10.1016/j.acha.2007.03.005}, abstractNote={It is proven that the Voronoi tessellations of the real projective space generated by equiangular lines are congruent. Two implications of this result are mentioned—an equiangular set of lines forms the best N -point representation of an isotropically distributed one-dimensional subspace in terms of mutual information and a subspace quantizer defined by equiangular lines provides equal partial distortion.}, number={2}, journal={Applied and Computational Harmonic Analysis}, year={2007}, pages={254–258} }
 @article{coordinate interleaved orthogonal design with two transmit antennas in spatially correlated rayleigh fading channels: symbol-error rate and diversity order_2007, volume={E90-B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67651052062&partnerID=MN8TOARS}, DOI={10.1093/ietcom/e90-b.11.3294}, abstractNote={Full-diversity transmission for space-time block codes (STBCs) with multiple transmit antennas can be achieved by using coordinate interleaved orthogonal designs (CIODs). To effectively evaluate the performance of CIODs, we derive union upper and lower bounds on the symbol-error rate (SER) and a corresponding asymptotic diversity order of symmetric structured CIOD, in particular, with two transmit antennas over quasi-static spatially uncorrelated/correlated frequency-nonselective Rayleigh fading channels. Some numerical results are provided to verify our analysis.}, number={11}, journal={IEICE Transactions on Communications}, year={2007}, pages={3294–3297} }
 @inproceedings{coordinated multi-cell mimo systems with cellular block diagonalization_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50249093779&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2007.4487516}, abstractNote={A clustered base transceiver station (BTS) coordination strategy is proposed to realize the gains of multiuser MIMO communication in interference-limited cellular systems. In the proposed coordination strategy, users are divided into two groups: a full intra-cluster coordination group to enhance the sum rate gain and a limited inter-cluster coordination group to reduce interference for the cluster edge users. Multi-cell block diagonalization is used to coordinate the transmissions across multiple BTSs in the same cluster. Because of the per-BTS power constraints, three combined precoder and power allocation algorithms are considered in this paper with different performance complexity tradeoffs. Simulations show that the proposed coordination strategy improves the sum rate over conventional systems and reduces the impact of interference for the cluster-edge users.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2007}, pages={1669–1673} }
 @book{heath_salvekar_2011, title={Digital Communication Basics}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84886185650&partnerID=MN8TOARS}, DOI={10.1002/9781118256053.ch41}, abstractNote={This chapter contains sections titled: Introduction Fundamentals of Digital Communication Important Concepts in Digital Communication A Performance Example Practical Discussion of Wimax: A Digital Communication Standard Conclusion Glossary Cross References References Further Reading}, journal={Handbook of Computer Networks}, author={Heath, R.W. and Salvekar, A.A.}, year={2011}, pages={627–641} }
 @inproceedings{downlink mimo block diagonalization in the presence of other-cell interference_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39349091848&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2007.828}, abstractNote={Block diagonalization is one approach for linear preceding in the multiple-input multiple-output broadcast channel that sends multiple interference free data streams to different users in the same cell. Unfortunately, block diagonalization neglects other cell interference, which limits the performance of users at the edge of the cell. This paper presents an OCI-aware enhancement to block diagonalization that uses a whitening filter for interference suppression at the receiver and a novel precoder using the interference-plus-noise covariance matrix for each user at the transmitter. For complex Gaussian matrix channels, the asymptotic sum rate of the proposed system is analyzed under a large antenna assumption for isotropic inputs and compared to conventional block diagonalization.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2007}, pages={4354–4358} }
 @inproceedings{early results on hydra: a flexible mac/phy multihop testbed_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547325148&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2007.393}, abstractNote={Hydra is a flexible wireless network testbed being developed at UT Austin. Our focus is networks that support multiple wireless hops and where the network, especially the MAC, takes advantage of sophisticated PHY techniques, such as OFDM and MIMO. We argue that for this domain simulation alone is not adequate and that working prototypes are needed to validate algorithms and protocols. Hydra nodes consist of a flexible RF front-end and a general purpose machine with a software based MAC and PHY. Using the frameworks of the Click modular router and GNU radio and coding in C++ makes it relatively easy to implement working prototypes of cross-layer designs that require custom MACs and PHYs. We present the architecture and implementation of Hydra, as well as a preliminary cross-layer design experiment for a rate-adaptive MAC. These early results show Hydra is a capable prototyping tool for wireless network research.}, booktitle={IEEE Vehicular Technology Conference}, year={2007}, pages={1896–1900} }
 @inproceedings{efficient transmit antenna selection for multiuser mimo systems with block diagonalization_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39349115145&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2007.664}, abstractNote={Block diagonalization is a preceding technique for multiuser MIMO systems that pre-cancels inter-user interference at the transmitter side. When there are a large number of base station antennas but a limited number of RF amplifiers, the system performance can be significantly improved by switching a subset of antennas to the RF chains and exploiting antenna selection diversity. The optimal antenna subset can be obtained by exhaustively searching over all possible antenna combinations. This brute-force search, however, is prohibitively complicated and impractical. To reduce the complexity, in this paper we propose several low-complexity suboptimal transmit selection algorithms that minimize a symbol error rate (SER) upper bound or maximize a capacity lower bound. Simulation results show that our proposed algorithms perform very close to the optimal exhaustive search, while the complexity is much lower.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2007}, pages={3499–3503} }
 @inproceedings{ergodic capacity of spatial multiplexing mimo systems with zf receivers for log-normal shadowing and rayleigh fading channels_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44349087569&partnerID=MN8TOARS}, DOI={10.1109/PIMRC.2007.4394313}, abstractNote={This paper presents a derivation of an expression for the ergodic capacity of spatial multiplexing multiple-input-multiple-output (MIMO) systems with zero-forcing (ZF) receivers and independent substream detection. Assuming that the channel is unknown at the transmitter but known at the receiver, the ergodic capacity is formulated as a function of log-normal shadowing and Rayleigh fading. Gauss-Hermite quadrature integration is used to approximate the ergodic capacity expression in a concise form. The proposed analytical approach allows investigation of the effects of the shadowing standard deviation and the transmit correlation. Numerical and simulation results confirm that under various composite channel scenarios, the analytical results match well with the simulation results.}, booktitle={IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC}, year={2007} }
 @inproceedings{hybrid-arq in multihop networks with opportunistic relay selection_2007, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547513223&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2007.366755}, abstractNote={This paper develops a contention-based opportunistic feedback technique towards relay selection in a dense wireless network. This technique enables the forwarding of additional parity information from the selected relay to the destination. For a given network, the effects of varying key parameters such as the feedback probability are presented and discussed. A primary advantage of the proposed technique is that relay selection can be performed in a distributed way. Simulation results find its performance to closely match that of centralized schemes that utilize GPS information, unlike the proposed method. The proposed relay selection method is also found to achieve throughput gains over a point-to-point transmission strategy.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2007} }
 @inproceedings{impact of mutual coupling and antenna efficiencies on adaptive switching between mimo transmission strategies_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47649132725&partnerID=MN8TOARS}, DOI={10.1109/VETECF.2007.166}, abstractNote={Previous research has shown that adaptive switching between multiple-input multiple-output (MIMO) transmission strategies like spatial multiplexing and beamforming increases link reliability and capacity gains, as compared to fixed transmission strategies. To get the full benefit of adaptive switching it is necessary to obtain accurate estimates of the SNR values when we switch between the transmission strategies. In this paper, it is shown that (relatively more) accurate switching point estimates can be obtained by taking into account real-life effects like mutual coupling and antenna efficiencies, for switching between statistical beamforming and spatial multiplexing. Using simulations, it is shown that accounting for these effects can make the switching point estimate more accurate by as much as 12 dB, compared to the case when the practical effects are not considered.}, booktitle={IEEE Vehicular Technology Conference}, year={2007}, pages={749–753} }
 @article{khaled_mondal_leus_heath_petré_2007, title={Interpolation-Based Multi-Mode Precoding for MIMO-OFDM Systems with Limited Feedback}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85008055165&partnerID=MN8TOARS}, DOI={10.1109/TWC.2007.05334}, abstractNote={Spatial multiplexing with multi-mode precoding provides a means to achieve both high capacity and high reliability in multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. Multi-mode precoding uses linear transmit precoding that adapts the number of spatial multiplexing data streams or modes, according to the transmit channel state information (CSI). As such, it typically requires complete knowledge of the multi-mode precoding matrices for each subcarrier at the transmitter. In practical scenarios where the CSI is acquired at the receiver and fed back to the transmitter through a low-rate feedback link, this requirement may entail a prohibitive feedback overhead. In this paper, we propose to reduce the feedback requirement by combining codebook-based precoder quantization, to efficiently quantize and represent the optimal precoder on each subcarrier, and multi-mode precoder frequency down-sampling and interpolation, to efficiently reconstruct the precoding matrices on all subcarriers based on the feedback of the indexes of the quantized precoders only on a fraction of the subcarriers. To enable this efficient interpolation-based quantized multimode precoding solution, we introduce (1) a novel precoder codebook design that lends itself to precoder interpolation, across subcarriers, followed by mode selection, (2) a new precoder interpolator and, finally, (3) a clustered mode selection approach that significantly reduces the feedback overhead related to the mode information on each subcarrier. Monte-Carlo bit-error rate (BER) performance simulations demonstrate the effectiveness of the proposed quantized multi-mode precoding solution, at reasonable feedback overhead}, number={3}, journal={IEEE Transactions on Wireless Communications}, author={Khaled, N. and Mondal, B. and Leus, G. and Heath, R.W. and Petré, F.}, year={2007}, pages={1003–1013} }
 @article{joint interference cancellation and channel shortening in multiuser-mimo systems_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34147111699&partnerID=MN8TOARS}, DOI={10.1109/TVT.2006.889572}, abstractNote={In this paper, a two-stage receiver structure for interference cancellation in multiuser spatially multiplexed multiple-antenna systems is presented. A space-time equalizer is used in the first stage for joint coantenna/cochannel-interference suppression and shortening of the effective channel for each transmit stream of the desired user. The channel shortening, combined with independent detection, helps reduce the complexity of the second stage Viterbi equalizer, which is used for separate intersymbol-interference equalization for each of the streams. Three objective functions are proposed for determining the coefficients of the space-time equalizer using a direct training data-based approach, which does not require estimation of the interferer's channel. Simulation results show good symbol error performance as compared to existing algorithms with asynchronous multiple-input multiple-output interferers}, number={2}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={652–660} }
 @inproceedings{jointly optimized multiuser beamforming for the mimo broadcast channel with limited feedback_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44349103076&partnerID=MN8TOARS}, DOI={10.1109/PIMRC.2007.4394188}, abstractNote={This paper considers the joint optimization of the transmitter beamforming filters and receiver combining filters for the multiple-input multiple-output (MIMO) broadcast channel. A low-complexity iterative algorithm to compute the filters is proposed and then a closed-form expression of the filters for the downlink with two transmit antennas is derived, avoiding the need for iterative computation in this case. To enable practical implementation, a new limited feedback algorithm is proposed that exploits the channel structure in the closed form solution, and is independent of the number of receive antennas. Performance is evaluated by Monte Carlo simulations as a function of the amount of feedback. With two receive antennas, the proposed method performs close to the sum capacity of the MIMO broadcast channel, without the need for multiuser diversity and with only limited feedback.}, booktitle={IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC}, year={2007} }
 @inproceedings{low-complexity user and antenna selection for multiuser mimo systems with block diagonalization_2007, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547534112&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2007.366754}, abstractNote={Block diagonalization is a downlink preceding technique that pre-cancels inter-user interference in multiuser MIMO systems. When there are a large number of users, the system throughput can be significantly increased by selecting a subset of users and a subset of receive antennas for each user. The optimal user and antenna subset can be obtained by exhaustively searching over all possible user and antenna combinations to find the one with the highest sum throughput. This brute-force solution, however, is prohibitively complicated. To reduce the complexity, in this paper we propose a low-complexity suboptimal user and antenna selection algorithm. For most system configurations, we show that our proposed algorithm achieves up to 98% of the optimal sum throughput of the exhaustive search, where the complexity is orders of magnitude lower than the exhaustive search method.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2007} }
 @article{multiplexing/beamforming switching for coded mimo in spatially correlated channels based on closed-form ber approximations_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54949131046&partnerID=MN8TOARS}, DOI={10.1109/TVT.2007.900353}, abstractNote={This paper considers low-complexity coded multiple-input-multiple-output transmission in Rayleigh channels with correlation between antennas at both the transmitter and receiver. We consider statistical beamforming (SB) and spatial multiplexing (SM) with a zero-forcing receiver. We calculate the link-level capacity of both schemes with bit-interleaved coded modulation and derive accurate closed-form approximations to the bit error rate. We then show how the resulting expressions can be used in an adaptive algorithm to select the best combination of code rate, modulation format, and transmission scheme (SB or SM) in order to maximize throughput. Unlike other mode-switching schemes that require empirical lookup tables, this approach applies to any correlation scenario. Numerical studies are used to demonstrate the performance as a function of signal-to-noise ratio and correlation parameters.}, number={5 I}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={2555–2567} }
 @article{multiuser antenna partitioning for cellular mimo-cdma systems_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37249089377&partnerID=MN8TOARS}, DOI={10.1109/TVT.2007.898396}, abstractNote={Improving the capacity of code-division-multiple-access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple-output (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO-CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. In this paper, we consider the downlink of a heavily loaded multicell CDMA system with multiple transmit and receive antennas. Straightforward application of the known MIMO techniques to such a system does not substantially increase the number of supportable users. To overcome this, a novel MIMO-CDMA system design based on user partitioning is developed, in which each user is assigned to a single transmit antenna either without regard to channel knowledge (static) or based on antenna-selection feedback bits (dynamic). These proposed multiuser-antenna-partitioning techniques have a minimal increase in complexity and would require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems are derived, and it is shown that, particularly, the dynamic partitioning scheme has a large gain over both the conventional CDMA and the static MIMO-CDMA scheme. This gain can be credited to multiuser antenna selection diversity. Unlike prior research, the multiuser antenna selection diversity gain is achieved without the typical expense of a loss in spatial multiplexing gain.Improving the capacity of code-division-multiple-access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple-output (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO-CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. In this paper, we consider the downlink of a heavily loaded multicell CDMA system with multiple transmit and receive antennas. Straightforward application of the known MIMO techniques to such a system does not substantially increase the number of supportable users. To overcome this, a novel MIMO-CDMA system design based on user partitioning is developed, in which each user is assigned to a single transmit antenna either without regard to channel knowledge (static) or based on antenna-selection feedback bits (dynamic). These proposed multiuser-antenna-partitioning techniques have a minimal increase in complexity and would require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems are derived, and it is shown that, particularly, the dynamic partitioning scheme has a large gain over both the conventional CDMA and the static MIMO-CDMA scheme. This gain can be credited to multiuser antenna selection diversity. Unlike prior research, the multiuser antenna selection diversity gain is achieved without the typical expense of a loss in spatial multiplexing gain.}, number={5 I}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={2448–2456} }
 @inproceedings{multiuser limited feedback for wireless multi-antenna communication_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51649091653&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2007.4557520}, abstractNote={For a wireless multi-antenna network with a large number of users, the sum capacity scales at most linearly with the number of antennas and double logarithmically the number of users. Achieving this optimal capacity scaling potentially requires feedback of channel state information (CSI) from all users, leading to overflow of the feedback channel. This paper proposes a limited feedback strategy that provides feedback control such that a sum CSI feedback rate constraint is satisfied. It is proved that a wireless multi-antenna network using the proposed limited feedback strategy achieves the optimal capacity scaling for the broadcast channel.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2007}, pages={2036–2040} }
 @article{on the existence of equiangular tight frames_2007, volume={426}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547906516&partnerID=MN8TOARS}, DOI={10.1016/j.laa.2007.05.043}, abstractNote={An equiangular tight frame (ETF) is a d×N matrix that has unit-norm columns and orthogonal rows of norm N/d. Its key property is that the absolute inner products between pairs of columns are (i) identical and (ii) as small as possible. ETFs have applications in communications, coding theory, and sparse approximation. Numerical experiments indicate that ETFs arise for very few pairs (d, N), and it is an important challenge to develop restrictions on the pairs for which they can exist. This article uses field theory to provide detailed conditions on real and complex ETFs. In particular, it describes restrictions on harmonic ETFs, a specific type of complex ETF that appears in applications. Finally, the article offers empirical evidence that these conditions are sharp or nearly sharp, especially in the real case.}, number={2-3}, journal={Linear Algebra and Its Applications}, year={2007}, pages={619–635} }
 @article{opportunistic feedback for multiuser mimo systems with linear receivers_2007, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34249075993&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2007.896013}, abstractNote={A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station}, number={5}, journal={IEEE Transactions on Communications}, year={2007}, pages={1020–1032} }
 @inproceedings{opportunistic relay selection with limited feedback_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547348290&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2007.40}, abstractNote={It has been shown that a decentralized relay selection protocol based on opportunistic feedback from the relays yields good throughput performance in dense wireless networks. This selection strategy supports a hybrid-ARQ transmission approach where relays forward parity information to the destination in the event of a decoding error. Such an approach, however, suffers a loss compared to centralized strategies that select relays with the best channel gain to the destination. This paper closes the performance gap by adding another level of channel feedback to the decentralized relay selection problem. It is demonstrated that only one additional bit of feedback is necessary for good throughput performance. The performance impact of varying key parameters such as the number of relays and the channel feedback threshold is discussed. An accompanying bit error rate analysis demonstrates the importance of relay selection}, booktitle={IEEE Vehicular Technology Conference}, year={2007}, pages={135–139} }
 @article{opportunistic space-division multiple access with beam selection_2007, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41249083578&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2007.910702}, abstractNote={In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcast channel is proposed that allows simultaneous transmission to multiple users under a limited feedback requirement. During a training phase, the base station modulates a training sequence on multiple sets of randomly generated orthogonal beamforming vectors. Then, based on the users' feedback, the base station opportunistically selects the users and corresponding orthogonal vectors that maximize the sum capacity. From theoretical analysis, the optimal amount of training to maximize the sum capacity is derived as a function of the system parameters. The main advantage of the proposed system is that it provides throughput gains for the MIMO broadcast channel with a small feedback overhead, and provides these gains even with a small number of active users. Numerical simulations show that a 20% gain in sum capacity is achieved (for a small number of users) over conventional opportunistic space division multiple access, and a 100% gain (for a large number of users) over conventional opportunistic beamforming.}, number={12}, journal={IEEE Transactions on Communications}, year={2007}, pages={2371–2380} }
 @inproceedings{optimizing mimo antenna placement and array configurations for multimedia delivery in aircraft_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547325984&partnerID=MN8TOARS}, DOI={10.1109/VETECS.2007.99}, abstractNote={In this paper, the feasibility of multiple-input multiple-output (MIMO) systems in aircraft is examined for the specific application of seatback entertainment, using an approach built around a site-specific capacity analysis methodology. The average capacity is evaluated as a function of the seat location, using the proposed methodology that relies on inputs from measurement results or ray-tracing simulations. The extension of the methodology to optimize the access point and client antenna placement locations and array configurations is also described in the paper. While the specific results are for the deployment of MIMO communication in aircraft for seatback entertainment, the same methodology can be applied to other deployment scenarios as well.}, booktitle={IEEE Vehicular Technology Conference}, year={2007}, pages={425–429} }
 @inproceedings{orthogonal beamforming for sdma downlink with limited feedback_2007, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547540872&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2007.366481}, abstractNote={On a multi-antenna downlink channel, separation of multiple users by transmit beamforming enables simultaneous transmission from the base station to the users, resulting in high sum throughput. This paper proposes and analyzes a practical algorithm for joint scheduling and orthogonal beamforming, which is enabled by feedback of quantized channel state information (CSI). In this approach, each user quantizes CSI using a codebook comprised of multiple orthonormal vector sets and sends back quantized CSI. Using feedback CSI, the base station jointly selects a set of orthogonal beamforming vectors and schedules a subset of feedback users for downlink transmission such that the throughput is maximized. For moderate to large numbers of users, the proposed algorithm achieves higher sum capacities than the conventional ones.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2007} }
 @article{overcoming interference in spatial multiplexing mimo cellular networks_2007, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37249050361&partnerID=MN8TOARS}, DOI={10.1109/MWC.2007.4407232}, abstractNote={Multi-antenna transmission and reception (known as MIMO) is widely touted as the key technology for enabling wireless broadband services, whose widespread success will require 10 times higher spectral efficiency than current cellular systems, at 10 times lower cost per bit. Spectrally efficient, inexpensive cellular systems are by definition densely populated and interference-limited. But spatial multiplexing MIMO systems- whose principal merit is a supposed dramatic increase in spectral efficiency- lose much of their effectiveness in high levels of interference. This article overviews several approaches to handling interference in multicell MIMO systems. The discussion is applicable to any multi-antenna cellular network, including 802.16e/WiMAX, 3GPP (HSDPA and 3GPP LTE), and 3GPP2 (lxEVDO). We argue that many of the traditional interference management techniques have limited usefulness (or are even counterproductive) when viewed in concert with MIMO. The problem of interference in MIMO systems is too large in scope to be handled with a single technique: in practice a combination of complementary countermeasures will be needed. We overview emerging system-level interference-reducing strategies based on cooperation, which will be important for overcoming interference in future spatial multiplexing cellular systems.}, number={6}, journal={IEEE Wireless Communications}, year={2007}, pages={95–104} }
 @inproceedings{physical concerns for cross-layer prototyping and wireless network experimentation_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37849027235&partnerID=MN8TOARS}, DOI={10.1145/1287767.1287771}, abstractNote={The performance of a wireless network protocol is inseparably linked to the physical layer algorithms on which it is built, the hardware used to implement the radio, and the wireless environment in which it operates. This paper identifies three features of wireless networking protocols impacted by these lower-level characteristics that are often overlooked or misunderstood by many researchers developing wireless protocols or using testbed-based evaluation methods. These features are temporal scaling, measurement reciprocity, and cross-layer adaptation. Temporal scaling refers to the time resolution with which events, such as broadcast or feedback, occur in the wireless network. This feature is tightly coupled with processing time at the physical layer and time selectivity in the wireless channel. Measurement reciprocity is an assumption used to estimate parameters of the forward link of a bidirectional communication channel, based on observations from the reverse link. This assumption directly depends on the interference properties and hardware symmetry of nodes in a wireless network. System adaptation, based on reciprocity or feedback, inevitably requires careful scrutiny of power and rate control applied to physical wireless devices. This paper also provides recommendations to guide researchers in setting up interesting and useful wireless experiments. Three concerns for wireless experimentation are addressed, namely: ambient interference, RF hardware profiling, and fading properties of the wireless channel. The motivation for this paper stems from experience prototyping and experimenting with Hydra, a wireless cross-layer testbed developed at the University of Texas at Austin.}, booktitle={International Conference on Mobile Computing and Networking, MobiCom'07 Co-Located Workshops -Proceedings of the the Second ACM International Workshop on Wireless Network Testbeds, Experimental Evalua}, year={2007}, pages={11–18} }
 @inproceedings{physical concerns for cross-layer prototyping and wireless network experimentation_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70450225415&partnerID=MN8TOARS}, booktitle={Proceedings of the 2nd ACM International Workshop on Wireless Network Testbeds, Experimental Evaluation and Characterization, WiNTECH '07, Co-located with the ACM MobiCom 2007 Conference}, year={2007}, pages={11–18} }
 @article{quantization on the grassmann manifold_2007, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547894696&partnerID=MN8TOARS}, DOI={10.1109/TSP.2007.896112}, abstractNote={The problem of quantization in an Euclidean space with unitary constraints can be formulated as an unconstrained problem on a Grassmann manifold. Such constraints arise in areas such as wireless communication with multiple antennas at the transmitter and at the receiver. Due to the constraints, the distortion rate analysis developed for Euclidean spaces cannot be applied directly. This paper extends Gersho's asymptotic (large rate, small distortion) distortion bounds to the case when the source is distributed on the complex Grassmann manifold. The special structure of the Grassmann manifold and the distortion measures defined on it differentiate this problem from the traditional vector quantization in Euclidean spaces.}, number={8}, journal={IEEE Transactions on Signal Processing}, year={2007}, pages={4208–4216} }
 @article{reduced complexity signal detection for ofdm systems with transmit diversity_2007, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247103991&partnerID=MN8TOARS}, number={1}, journal={Journal of Communications and Networks}, year={2007}, pages={75–82} }
 @inproceedings{reduced rank signaling in spatially correlated mimo channels_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51649105163&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2007.4557367}, abstractNote={The optimal input covariance matrix Q that achieves the ergodic capacity under the coherent assumption in a point-to-point, multi-antenna setting is a function of the spatial correlation and the transmit SNR. While the eigenvectors of Q can be characterized in closed-form for many realistic correlation models, the eigenvalues have to be determined numerically. However, it is well-known that the rank of Q is a non-decreasing function of SNR. Motivated by this fact, in this work, we study communication with a low-complexity family of input covariance matrices that are characterized by their rank, assuming uniform power allocation over the smaller-dimensional eigen-space. We quantify the impact of spatial correlation on the M-th transition-SNR which is defined as the smallest SNR at which rank-M transmission becomes optimal.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2007}, pages={1081–1085} }
 @inproceedings{sdma with a sum feedback rate constraint_2007, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547525655&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2007.366482}, abstractNote={Space division multiple access (SDMA) is capable of achieving sum capacity that grows double logarithmically with the number of users. The sum rate for channel state information (CSI) feedback, however, increases linearly with the number of users, reducing the effective uplink capacity. To address this problem, a novel SDMA design is proposed, where the sum feedback rate is upper-bounded by a constant. This design consists of algorithms for CSI quantization, threshold based CSI feedback, and joint beamforming and scheduling. The key feature of the proposed approach is the use of feedback thresholds to select feedback users with large channel gains and small CSI quantization errors such that the sum feedback rate constraint is satisfied. Despite this constraint, the proposed SDMA design is shown to achieve a sum capacity growth rate close to the optimal one. Numerical results show that the proposed SDMA design is capable of attaining higher sum capacities than existing ones, even though the sum feedback rate is bounded.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2007} }
 @article{shifting the mimo paradigm_2007, volume={24}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36249021932&partnerID=MN8TOARS}, DOI={10.1109/MSP.2007.904815}, abstractNote={Multi-user MIMO (MU-MIMO) networks reveal the unique opportunities arising from a joint optimization of antenna combining techniques with resource allocation protocols. Furthermore, it brings robustness with respect to multipath richness, allowing for compact antenna spacing at the BS and, crucially, yielding the diversity and multiplexing gains without the need for multiple antenna user terminals. To realize these gains, however, the BS should be informed with the user's channel coefficients, which may limit practical application to TDD or low-mobility settings. To circumvent this problem and reduce feedback load, combining MU-MIMO with opportunistic scheduling seems a promising direction. The success for this type of scheduler is strongly traffic and QoS-dependent, however.}, number={5}, journal={IEEE Signal Processing Magazine}, year={2007}, pages={36–46} }
 @article{simplified spatial correlation models for clustered mimo channels with different array configurations_2007, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547879584&partnerID=MN8TOARS}, DOI={10.1109/TVT.2007.897212}, abstractNote={An approximate spatial correlation model for clustered multiple-input multiple-output (MIMO) channels is proposed in this paper. The two ingredients for the model are an approximation for uniform linear and circular arrays to avoid numerical integrals and a closed-form expression for the correlation coefficients that is derived for the Laplacian azimuth angle distribution. A new performance metric to compare parametric and nonparametric channel models is proposed and used to show that the proposed model is a good fit to the existing parametric models for low angle spreads (i.e., smaller than 10deg). A computational-complexity analysis shows that the proposed method is a numerically efficient way of generating the spatially correlated MIMO channels.}, number={4 II}, journal={IEEE Transactions on Vehicular Technology}, year={2007}, pages={1924–1934} }
 @article{space division multiple access with a sum feedback rate constraint_2007, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34347372828&partnerID=MN8TOARS}, DOI={10.1109/TSP.2007.894245}, abstractNote={On a multiantenna broadcast channel, simultaneous transmission to multiple users by joint beamforming and scheduling is capable of achieving high throughput, which grows double logarithmically with the number of users. The sum rate for channel state information (CSI) feedback, however, increases linearly with the number of users, reducing the effective uplink capacity. To address this problem, a novel space division multiple access (SDMA) design is proposed, where the sum feedback rate is upper bounded by a constant. This design consists of algorithms for CSI quantization, threshold-based CSI feedback, and joint beamforming and scheduling. The key feature of the proposed approach is the use of feedback thresholds to select feedback users with large channel gains and small CSI quantization errors such that the sum feedback rate constraint is satisfied. Despite this constraint, the proposed SDMA design is shown to achieve a sum capacity growth rate close to the optimal one. Moreover, the feedback overflow probability for this design is found to decrease exponentially with the difference between the allowable and the average sum feedback rates. Numerical results show that the proposed SDMA design is capable of attaining higher sum capacities than existing ones, even though the sum feedback rate is bounded.}, number={7 II}, journal={IEEE Transactions on Signal Processing}, year={2007}, pages={3879–3891} }
 @article{sum capacity of multiuser mimo broadcast channels with block diagonalization_2007, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547772139&partnerID=MN8TOARS}, DOI={10.1109/TWC.2007.05831}, abstractNote={The sum capacity of a Gaussian broadcast MIMO channel can be achieved with dirty paper coding (DPC). However, algorithms that approach the DPC sum capacity do not appear viable in the forseeable future, which motivates lower complexity interference suppression techniques. Block diagonalization (BD) is a linear preceding technique for downlink multiuser MIMO systems. With perfect channel knowledge at the transmitter, BD can eliminate other users' interference at each receiver. In this paper, we study the sum capacity of BD with and without receive antenna selection. We analytically compare BD without receive antenna selection to DPC for a set of given channels. It is shown that (1) if the user channels are orthogonal to each other, then BD achieves the same sum capacity as DPC; (2) if the user channels lie in the same subspace, then the gain of DPC over BD can be upper bounded by the minimum of the number of transmit and receive antennas. These observations also hold for BD with receive antenna selection. Further, we study the ergodic sum capacity of BD with and without receive antenna selection in a Rayleigh fading channel. Simulations show that BD can achieve a significant part of the total throughput of DPC. An upper bound on the ergodic sum capacity gain of DPC over BD is proposed for easy estimation of the gap between the sum capacity of DPC and BD without receive antenna selection.}, number={6}, journal={IEEE Transactions on Wireless Communications}, year={2007}, pages={2040–2045} }
 @article{systematic codebook designs for quantized beamforming in correlated mimo channels_2007, volume={25}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548557559&partnerID=MN8TOARS}, DOI={10.1109/JSAC.2007.070804}, abstractNote={The full diversity gain provided by a multi-antenna channel can be achieved by transmit beamforming and receive combining. This requires the knowledge of channel state information (CSI) at the transmitter which is difficult to obtain in practice. Quantized beamforming where fixed codebooks known at both the transmitter and the receiver are used to quantize the CSI has been proposed to solve this problem. Most recent works focus attention on limited feedback codebook design for the uncorrelated Rayleigh fading channel. Such designs are sub-optimal when used in correlated channels. In this paper, we propose systematic codebook design for correlated channels when channel statistical information is known at the transmitter. This design is motivated by studying the performance of pure statistical beamforming in correlated channels and is implemented by maps that can rotate and scale spherical caps on the Grassmannian manifold. Based on this study, we show that even statistical beamforming is near-optimal if the transmitter covariance matrix is ill-conditioned and receiver covariance matrix is well-conditioned. This leads to a partitioning of the transmit and receive covariance spaces based on their conditioning with variable feedback requirements to achieve an operational performance level in the different partitions. When channel statistics are difficult to obtain at the transmitter, we propose a universal codebook design (also implemented by the rotation-scaling maps) that is robust to channel statistics. Numerical studies show that even few bits of feedback, when applied with our designs, lead to near perfect CSI performance in a variety of correlated channel conditions.}, number={7}, journal={IEEE Journal on Selected Areas in Communications}, year={2007}, pages={1298–1310} }
 @inproceedings{throughput scaling of uplink sdma with limited feedback_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50249109554&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2007.4487216}, abstractNote={In a multi-antenna system with a large number of users, combining space division multiple access (SDMA) and scheduling achieves high throughput. The implementation of joint SDMA and scheduling requires limited feedback of channel sate information (CSI) from users to a base station. In this paper, an uplink SDMA system with limited feedback and a large number of users is considered. The scaling of uplink throughput with different system parameters is analyzed. These parameters include the numbers of users, feedback bits and antennas. Our results provide guidelines for designing uplink SDMA to achieve optimum throughput scaling. For instance, in the high SNR regime, scheduling should select users with small CSI quantization error.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2007}, pages={292–296} }
 @article{transmit selection diversity for unitary precoded multiuser spatial multiplexing systems with linear receivers_2007, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947380186&partnerID=MN8TOARS}, DOI={10.1109/TSP.2006.887147}, abstractNote={Multiuser spatial multiplexing is a downlink transmission technique that uses linear transmit precoding to multiplex multiple users and precancel interuser interference. In such a system, the spatial degrees of freedom are used for interference mitigation and generally come at the expense of diversity gain. This paper proposes two precoding methods that use extra transmit antennas, beyond the minimum required, to provide additional degrees of diversity. The approach taken is to solve for a unitary transmit precoder, under a zero interuser interference constraint, that minimizes an upper bound on the symbol error rate (SER) for each user. Solutions where all transmit antennas are employed, as well as subsets of antennas (to reduce analog components), are described. Numerical results confirm a dramatic improvement in terms of SER and mutual information over single-user multiple-input multiple-output (MIMO) methods and static allocation methods. For example, the proposed techniques achieve a signal-to-noise ratio (SNR) improvement of 6-10 dB at an uncoded SER of 10-3, with only one extra transmit antenna}, number={3}, journal={IEEE Transactions on Signal Processing}, year={2007}, pages={1159–1171} }
 @article{uplink sdma with limited feedback: throughput scaling_2008, volume={2008}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41249092963&partnerID=MN8TOARS}, DOI={10.1155/2008/479357}, abstractNote={Combined space division multiple access (SDMA) and scheduling exploit both spatial multiplexing and multiuser diversity, increasing throughput significantly. Both SDMA and scheduling require feedback of multiuser channel sate information (CSI). This paper focuses on uplink SDMA with limited feedback, which refers to efficient techniques for CSI quantization and feedback. To quantify the throughput of uplink SDMA and derive design guidelines, the throughput scaling with system parameters is analyzed. The specific parameters considered include the numbers of users, antennas, and feedback bits. Furthermore, different SNR regimes and beamforming methods are considered. The derived throughput scaling laws are observed to change for different SNR regimes. For instance, the throughput scales logarithmically with the number of users in the high SNR regime but double logarithmically in the low SNR regime. The analysis of throughput scaling suggests guidelines for scheduling in uplink SDMA. For example, to maximize throughput scaling, scheduling should use the criterion of minimum quantization errors for the high SNR regime and maximum channel power for the low SNR regime.}, journal={Eurasip Journal on Advances in Signal Processing}, year={2008} }
 @article{uplink power control in multi-cell spatial multiplexing wireless systems_2007, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547396826&partnerID=MN8TOARS}, DOI={10.1109/TWC.2007.051007}, abstractNote={This paper proposes a power control strategy for the uplink of cellular MIMO spatial multiplexing systems, with a linear MMSE receiver applied at the base station and a single active user per time instant. A fixed per-stream SINR target is employed that allows guaranteed QoS for delay-sensitive applications. A straightforward application of single antenna power control is not possible in the MIMO context due to coordination between receive antennas and nonlinear dependence between interference and eigenspaces of the channel matrices. Two schemes are proposed to solve the problem. The first equally allocates power to all transmit antennas. Deriving an SINR lower bound based on an eigenvalue approximation of the composite interference, allows application of the conventional single antenna power control framework to solve this problem. To improve the feasibility performance, a second scheme is proposed that adaptively allocates power on the transmit antennas, where an iterative algorithm based on game theory is used to sequentially update each user's power distribution. The optimal solution with full channel knowledge, and a practical near-optimal solution requiring only partial channel knowledge, are both derived. Numerical results show that power control, compared to supposedly optimal waterfilling strategies, actually achieves higher throughput at the low SINRs typical in cellular systems, with significantly lower overhead and complexity. Due to its better exploitation of spatial diversity and reduced transmit power (and hence reduced interference), adaptive power allocation increases the achievable SINR by an order of magnitude over equal power allocation, resulting in far better coverage.}, number={7}, journal={IEEE Transactions on Wireless Communications}, year={2007}, pages={2700–2711} }
 @inproceedings{heath_bottomley_2007, title={Welcome message from the technical program co-chairs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47649108327&partnerID=MN8TOARS}, DOI={10.1109/VETECF.2007.5}, booktitle={IEEE Vehicular Technology Conference}, author={Heath, R. and Bottomley, G.}, year={2007} }
 @article{a joint source-channel distortion model for jpeg compressed images_2006, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646877013&partnerID=MN8TOARS}, DOI={10.1109/TIP.2006.871118}, abstractNote={The need for efficient joint source-channel coding (JSCC) is growing as new multimedia services are introduced in commercial wireless communication systems. An important component of practical JSCC schemes is a distortion model that can predict the quality of compressed digital multimedia such as images and videos. The usual approach in the JSCC literature for quantifying the distortion due to quantization and channel errors is to estimate it for each image using the statistics of the image for a given signal-to-noise ratio (SNR). This is not an efficient approach in the design of real-time systems because of the computational complexity. A more useful and practical approach would be to design JSCC techniques that minimize average distortion for a large set of images based on some distortion model rather than carrying out per-image optimizations. However, models for estimating average distortion due to quantization and channel bit errors in a combined fashion for a large set of images are not available for practical image or video coding standards employing entropy coding and differential coding. This paper presents a statistical model for estimating the distortion introduced in progressive JPEG compressed images due to quantization and channel bit errors in a joint manner. Statistical modeling of important compression techniques such as Huffman coding, differential pulse-coding modulation, and run-length coding are included in the model. Examples show that the distortion in terms of peak signal-to-noise ratio (PSNR) can be predicted within a 2-dB maximum error over a variety of compression ratios and bit-error rates. To illustrate the utility of the proposed model, we present an unequal power allocation scheme as a simple application of our model. Results show that it gives a PSNR gain of around 6.5 dB at low SNRs, as compared to equal power allocation.}, number={6}, journal={IEEE Transactions on Image Processing}, year={2006}, pages={1349–1364} }
 @inproceedings{a lattice-based mimo broadcast precoder with block diagonalization for multi-stream transmission_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949096066&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2006.648}, abstractNote={Preceding with block diagonalization is an attractive approach for approaching sum capacity in multiuser MIMO (multiple input multiple output) broadcast channels. This method though requires either global channel state information at every receiver or an additional training phase, which requires additional system planning. This paper proposes a lattice based multi-user precoder that uses block diagonalization combined with pre-equalization and perturbation for the multiuser MIMO broadcast channel. Achievable rates are computed and used to show that the proposed technique approaches the capacity with block diagonalization and water-filling but does not require the additional channel state information at the receiver. Monte Carlo simulations under the case of equal power allocation show that the proposed method provides better diversity and BER (bit error rate) performance than block diagonalization with a zero-forcing receiver.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2006} }
 @inproceedings{a linear estimator optimized for the structural similarity index and its application to image denoising_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449616133&partnerID=MN8TOARS}, DOI={10.1109/ICIP.2006.313051}, abstractNote={We use a perceptual distortion metric-the structural similarity (SSIM) index, to derive a new linear estimator for estimating zero-mean Gaussian sources distorted by additive white Gaussian noise (AWGN). We use this estimator in an image denoising application and compare its performance with the traditional linear least squared error (LLSE) estimator. Although images denoised using the SSIM-optimized estimator have a lower peak signal-to-noise ratio (PSNR) compared to their LLSE counterparts, the SSIM-optimized estimator clearly outperforms the LLSE estimator in terms of the visual quality of the denoised images.}, booktitle={Proceedings - International Conference on Image Processing, ICIP}, year={2006}, pages={2637–2640} }
 @inproceedings{a space-time receiver for mimo-ofdm ad hoc networks_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847363093&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2005.1605874}, abstractNote={In this paper, we propose a receiver architecture that can demodulate multiple data streams with different frequency offsets and propagation delays in multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) ad hoc networks. The receiver performs joint synchronization and interference cancellation to separate these interfering data streams and achieve synchronization for each stream. We propose a novel training-based minimizing mean square error (MMSE) algorithm to jointly obtain the frequency offsets and the equalization delays. The effects of frequency offsets and propagation delays are compensated with these estimated parameters. We also propose a direct training-based equalizer design algorithm to obtain the equalizer coefficients. This algorithm involves solving an optimization problem that considers symbol error rate. Simulation shows good uncoded bit error rate performance for our proposed algorithms}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2005} }
 @inproceedings{a throughput-based adaptive mimo-bicm approach for spatially-correlated channels_2006, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-42549116739&partnerID=MN8TOARS}, DOI={10.1109/ICC.2006.254940}, abstractNote={This paper considers low complexity transmission for MIMO bit-interleaved coded modulation (BICM) in spatially-correlated Rayleigh channels. We consider both statistical beam-forming (SB) and spatial-multiplexing with a zero-forcing (ZF) receiver. We derive tight closed-form bit error rate (BER) expressions based on a saddlepoint approximation. We then propose a practical adaptive algorithm which selects, based on the analytical results, the combination of code-rate, modulation format, and MIMO transmission scheme (SB or ZF) that maximizes throughput whilst maintaining a pre-defined BER.}, booktitle={IEEE International Conference on Communications}, year={2006}, pages={1374–1379} }
 @inproceedings{chae_heath_mazzarese_2006, title={Achievable sum rate bounds of zero-forcing based linear multi-user MIMO systems}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84940659014&partnerID=MN8TOARS}, booktitle={44th Annual Allerton Conference on Communication, Control, and Computing 2006}, author={Chae, C.-B. and Heath, R.W. and Mazzarese, D.}, year={2006}, pages={1134–1140} }
 @article{benefit of pattern diversity via two-element array of circular patch antennas in indoor clustered mimo channels_2006, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646898446&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2006.873978}, abstractNote={In this paper, we analyze a multiple-input multiple-output (MIMO) array consisting of two circular microstrip antennas, designed to exploit pattern diversity. We analytically derive the spatial correlation coefficients of this array as a function of the mode excited, for realistic clustered MIMO channel models. We compare the performance of the circular patch array (CPA) against an array of two spaced dipoles. In particular, we compute a theoretical tradeoff to predict when the pattern diversity provided by the CPA is more effective than space diversity from the uniform linear array (ULA), based on the eigenvalues of the spatial correlation matrix. Through simulations, we show that CPAs yield better performance or satisfy more restrictive size constraints than ULAs in clustered MIMO channels, depending on the element spacing of the ULA. These results make the CPA an attractive solution for miniaturized MIMO arrays for portable devices or access points.}, number={5}, journal={IEEE Transactions on Communications}, year={2006}, pages={943–954} }
 @inproceedings{capacity of opportunistic space division multiple access with beam selection_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949109255&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2006.909}, abstractNote={In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcast channel is proposed that allows simultaneous transmission to multiple users under a limited feedback requirement. During a training phase, the base station modulates a training sequence on multiple sets of randomly generated orthogonal beamforming vectors. Then, based on the users' feedback, the base station opportunistically selects the users and corresponding orthogonal vectors that maximize the sum capacity. From theoretical analysis, the optimal amount of training to maximize the sum capacity is derived as a function of the system parameters. The main advantage of the proposed system is that it provides throughput gains for the MIMO broadcast channel with a small feedback overhead, and provides these gains even with a small number of active users. Numerical simulations show that a 20% gain in sum capacity is achieved (for a small number of users) over conventional opportunistic space division multiple access, and a 100% gain (for a large number of users) over conventional opportunistic beamforming. I. INTRODUCTION Multiple-input multiple-output (MIMO) technology can in- crease throughput in cellular systems by exploiting the spatial dimensions of the MIMO broadcast channel (MIMO-BC) to support transmissions to multiple users. The theoretical sum capacity achievable over the MIMO-BC has been recently}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2006} }
 @article{channel adaptive quantization for limited feedback mimo beamforming systems_2006, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947108808&partnerID=MN8TOARS}, DOI={10.1109/TSP.2006.880041}, abstractNote={Multiple-input multiple-output (MIMO) wireless systems can achieve significant diversity and array gain by using transmit beamforming and receive combining techniques. In the absence of full channel knowledge at the transmitter, the transmit beamforming vector can be quantized at the receiver and sent to the transmitter using a low-rate feedback channel. In the literature, quantization algorithms for the beamforming vector are designed and optimized for a particular channel distribution, commonly the uncorrelated Rayleigh distribution. When the channel is not uncorrelated Rayleigh, however, these quantization strategies result in a degradation of the receive signal-to-noise ratio (SNR). In this paper, switched codebook quantization is proposed where the codebook is dynamically chosen based on the channel distribution. The codebook adaptation enables the quantization to exploit the spatial and temporal correlation inherent in the channel. The convergence properties of the codebook selection algorithm are studied assuming a block-stationary model for the channel. In the case of a nonstationary channel, it is shown using simulations that the selected codebook tracks the distribution of the channel resulting in improvements in SNR. Simulation results show that in the case of correlated channels, the SNR performance of the link can be significantly improved by adaptation, compared with nonadaptive quantization strategies designed for uncorrelated Rayleigh-fading channels}, number={12}, journal={IEEE Transactions on Signal Processing}, year={2006}, pages={4717–4729} }
 @inproceedings{coordinated beamforming for multiuser mimo systems with limited feedforward_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44349131273&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2006.355010}, abstractNote={Jointly optimized linear transmit beamforming and receive combining is a low complexity approach for communication in the multiuser MIMO (multiple input multiple output) broadcast channel. This paper proposes an iterative algorithm for jointly designing the beamforming and combining vectors, which enforces a zero interference requirement after combining. Since the optimization is performed at the base station with channel state information for all the users, the receive beamformers are quantized at the base station and sent to the users via a low- rate feedforward control channel. Rate bounds are provided to estimate the impact of quantization loss on the achievable rate in Rayleigh channels is performed. Simulations show that the proposed approach using Grassmannian codebooks approaches the sum capacity of the MIMO broadcast channel.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2006}, pages={1511–1515} }
 @inproceedings{effect of feedback delay on multi-antenna limited feedback for temporally-correlated channels_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949134173&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2006.70}, abstractNote={A novel method based on Markov chain theory is proposed for analyzing the effect of feedback delay on a transmit beamforming system with limited feedback. Using this method, the ergodic capacity with delayed feedback of channel state information is derived. The capacity gain with respect to the case of no feedback is shown to decrease at least exponentially with the feedback delay. From these results, useful design guidelines can be derived for choosing system parameters including the vehicular speed and the tolerable feedback delay.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2006} }
 @inbook{love_heath_2006, title={Feedback techniques for MIMO channels}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78650263108&partnerID=MN8TOARS}, booktitle={MIMO System Technology for Wireless Communications}, author={Love, D.J. and Heath, R.W.}, year={2006}, pages={113–146} }
 @article{interpolation based unitary preceding for spatial multiplexing mimo-ofdm with limited feedback_2006, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947101967&partnerID=MN8TOARS}, DOI={10.1109/TSP.2006.881251}, abstractNote={<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Spatial multiplexing with linear precoding is a simple technique for achieving high spectral efficiency in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Linear precoding requires channel state information for each OFDM subcarrier, which can be achieved using feedback. To reduce the amount of feedback, this paper proposes a limited feedback architecture that combines precoder quantization with a special matrix interpolator. In the proposed system, the receiver sends information about a fraction of the precoding matrices to the transmitter and the transmitter reconstructs the precoding matrices for all the subcarriers. A new interpolator is proposed inspired by spherical interpolation that respects the orthogonal columns of the precoding matrices and the performance invariance to right multiplication by a unitary matrix. The interpolator is parameterized by a set of unitary matrices; a construction of a suitable set is briefly described. Simulations illustrate the performance of limited feedback precoding with coding, estimation or prediction error, and time variation for bit error rate (BER), mutual information, and mean squared error (MSE). </para>}, number={12}, journal={IEEE Transactions on Signal Processing}, year={2006}, pages={4730–4740} }
 @inproceedings{joint source-channel distortion modelling for mpeg-4 video_2006, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947671283&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2006} }
 @article{limited feedback diversity techniques for correlated channels_2006, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33645240298&partnerID=MN8TOARS}, DOI={10.1109/TVT.2005.863346}, abstractNote={Employing multiple antennas at the transmitter is a well-established technique for providing diversity advantage in wireless systems. Transmit beamforming relies on the assumption of current channel knowledge at the transmitter, which is unrealistic when the forward and reverse links are separated in frequency. One solution to this problem is for the receiver to send a small number of feedback bits that convey channel information to the transmitter. Feedback design techniques have been proposed over the past few years, but they were derived using the assumption of spatially uncorrelated Rayleigh fading. This correspondence addresses the design of limited feedback beamformers when the channel is correlated.}, number={2}, journal={IEEE Transactions on Vehicular Technology}, year={2006}, pages={718–722} }
 @article{low complexity user selection algorithms for multiuser mimo systems with block diagonalization_2006, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947098857&partnerID=MN8TOARS}, DOI={10.1109/TSP.2006.879269}, abstractNote={Block diagonalization (BD) is a precoding technique that eliminates interuser interference in downlink multiuser multiple-input multiple-output (MIMO) systems. With the assumptions that all users have the same number of receive antennas and utilize all receive antennas when scheduled for transmission, the number of simultaneously supportable users with BD is limited by the ratio of the number of base station transmit antennas to the number of user receive antennas. In a downlink MIMO system with a large number of users, the base station may select a subset of users to serve in order to maximize the total throughput. The brute-force search for the optimal user set, however, is computationally prohibitive. We propose two low-complexity suboptimal user selection algorithms for multiuser MIMO systems with BD. Both algorithms aim to select a subset of users such that the total throughput is nearly maximized. The first user selection algorithm greedily maximizes the total throughput, whereas the criterion of the second algorithm is based on the channel energy. We show that both algorithms have linear complexity in the total number of users and achieve around 95% of the total throughput of the complete search method in simulations}, number={9}, journal={IEEE Transactions on Signal Processing}, year={2006}, pages={3658–3663} }
 @inproceedings{markov models for limited feedback mimo systems_2006, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947693199&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2006} }
 @inproceedings{multi-antenna limited feedback for temporally-correlated channels: feedback compression_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949100413&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2006.839}, abstractNote={A novel method is proposed for reducing the feedback rate of a transmit beamforming system with feedback of quantized channel state information. Specifically, the channel is modeled as a Markov chain and the feedback bits are compressed by truncating the Markov chain transition probabilities. Using the proposed method, the feedback rate can be compressed by more than 100% without degrading the system performance.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2006} }
 @inproceedings{multichannel feedback in ofdm ad hoc networks_2006, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44049084648&partnerID=MN8TOARS}, DOI={10.1109/SAHCN.2006.288534}, abstractNote={We propose a multichannel feedback protocol to enable local scheduling with channel state information for wireless networks with orthogonal frequency division multiplexing (OFDM). In our proposed protocol, the frequency subcarrier domain is shared by multiple control channels, on which request-to-send (RTS) and clear-to-send (CTS) are exchanged. These control channels are created using random spreading signatures. Channel state information of the simultaneous transmissions, which defines the gains of these channels, is exchanged on these control channels and channel state information tables that contain the channel information of both the desired link and the interfering links are created at the transmit nodes, then scheduling decisions are made based on the channel information tables. We show that the proposed protocol improves the network throughput compared to IEEE 802.11 style protocols for a wireless local area network topology}, booktitle={2006 3rd Annual IEEE Communications Society on Sensor and Adhoc Communications and Networks, Secon 2006}, year={2006}, pages={701–706} }
 @inproceedings{multiple description image coding using natural scene statistics_2005, volume={II}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646782604&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2005.1415398}, abstractNote={The statistics of natural scenes in the wavelet domain are accurately characterized by the Gaussian scale mixture (GSM) model. The model lends itself easily to analysis and many applications that use this model are emerging (e.g., denoising, watermark detection). We present an error-resilient image communications application that uses the GSM model and multiple description coding (MDC) to provide error-resilience. We derive a rate-distortion bound for GSM random variables, derive the redundancy rate-distortion function, and finally implement an MD image communication system.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2005} }
 @inproceedings{on achievable sum rates of a multiuser mimo relay channel_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39049153896&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2006.261883}, abstractNote={In this paper, we investigate a multiple input multiple output (MIMO) multiuser relay channel, where a source with multiple antennas sends data to multiple users via a relay with multiple antennas. The relay applies linear processing to the received signal and forwards the processed signal to multiple users. In our system model, the direct links from the source to the users are neglected. We propose algorithms to compute achievable sum rates of this system based on dirty paper coding. An achievable sum rate defines a sum rate that can be achieved in the MIMO multiuser relay channel with zero error probability for any user, hence it is also a lower bound of the capacity of this channel. These algorithms also produce coefficients of the precoder at the source node and the coefficients of the linear processing unit at the relay. Simulations show that the proposed system architecture and algorithms achieve sum rate performance that is close to the derived performance upper bound}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2006}, pages={1026–1030} }
 @article{on quasi-orthogonal signatures for cdma systems_2006, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947367440&partnerID=MN8TOARS}, DOI={10.1109/TIT.2005.864469}, abstractNote={Sum capacity optimal signatures in synchronous code-division multiple-access (CDMA) systems are functions of the codebook length as well as the number of active users. A new signature set must be assigned every time the number of active users changes. This correspondence considers signature sets that are less sensitive to changes in the number of active users. Equiangular signature sequences are proven to solve a certain max-min signal-to-interference-plus-noise problem, which results from their interference invariance. Unions of orthonormal bases have subsets that come close to satisfying the Welch bound. Bounds on the maximum number of bases with minimum maximum correlation are derived and a new construction algorithm is provided. Connections are made between these signature design problems, Grassmannian line packing, frame theory, and algebraic geometry}, number={3}, journal={IEEE Transactions on Information Theory}, year={2006}, pages={1217–1226} }
 @inproceedings{on the snr and diversity of quantized precoded mimo systems_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48749124842&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2006.346416}, abstractNote={A multiple-input multiple-output (MIMO) wireless channel formed by antenna arrays at the transmitter and at the receiver offers high capacity and diversity benefits. Spatial multiplexing (SM) and space-time block coding (STBC) are attractive because they can provide high data-rate and full diversity, respectively, with low receiver complexity. Unitary linear precoding, which generalizes antenna subset selection, may be used to improve the diversity performance of a SM system and provide transmit array gain for a STBC system assuming perfect downlink channel state information (CSI) at the transmitter. This assumption can be relaxed by devising a MIMO system where the precoding matrices are quantized at the receiver and informed to the transmitter using a feedback channel. This leads to a performance degradation due to the mismatch between the quantized precoder and the downlink channel. In this paper, the loss in channel gain (transmit array gain) due to precoder quantization is characterized as a function of the feedback data-rate for precoded SM system with linear receivers and STBC systems. A sufficient condition is derived that will guarantee no loss in diversity due to quantization for such systems. The diversity order of a precoded SM system with perfect CSI is also established in the case when the number of receive antennas is equal to the number of transmitted streams}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2006} }
 @inproceedings{opportunistic feedback and online optimization for multiuser mimo systems with linear receivers_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47049124627&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2006.354858}, abstractNote={This paper proposes a scheduling and feedback strategy for the multiuser multiple input multiple output (MIMO) downlink. The base station uses a per-antenna scheduling that maps each transmit antenna to a different user in the downlink. The scheduling is performed based on the channel information obtained via a contention based feedback mechanism. Two design parameters, i.e., a channel quality threshold and a feedback probability, are proposed to adjust the scheduling and feedback behavior of this system. They are updated through a novel online updating procedure using empirically observed distribution information. The main advantages of the proposed protocol are that it substantially reduces the feedback requirements, still achieves multiuser diversity gain, and provides some fairness without a priori assumptions about the channel distribution.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2006}, pages={794–798} }
 @inproceedings{forenza_wan_heath_2018, title={Optimization of 2-element arrays of circular patch antennas in spatially correlated MIMO channels}, volume={2018-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050908274&partnerID=MN8TOARS}, DOI={10.1109/WDD.2006.8321481}, abstractNote={We present a novel optimization methodology to design compact circular patch arrays (CPAs). We first introduce a new definition of the spatial correlation coefficient suitable for analysis of MIMO arrays exploiting pattern diversity, in clustered channel models. From these correlation coefficients we derive the average and outage capacity performance of CPAs in realistic propagation scenarios, as a function of the antenna parameters. We also measure the performance of CPAs in terms of bandwidth, accounting for mutual coupling effects. Finally, we propose an optimization algorithm to design CPAs by jointly maximizing MIMO communication and microwave theory performance metrics. The proposed design methodology can be generalized to any MIMO array exploiting pattern diversity.}, booktitle={2006 International Waveform Diversity and Design Conference, WDD 2006 - Proceedings}, author={Forenza, A. and Wan, G. and Heath, R.W.}, year={2018}, pages={1–9} }
 @article{performance analysis of quantized beamforming mimo systems_2006, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947124451&partnerID=MN8TOARS}, DOI={10.1109/TSP.2006.881243}, abstractNote={Multiple-input multiple-output (MIMO) wireless systems can achieve significant diversity and array gain by using single-stream transmit beamforming and receive combining. A MIMO beamforming system with feedback using a codebook based quantization of the beamforming vector allows practical implementation of such a strategy in a single-user scenario. The performance of this system in uncorrelated Rayleigh flat fading channels is studied from the point-of-view of signal-to-noise ratio (SNR) and outage probability. In this paper, lower bounds are derived on the expected SNR loss and the outage probability of systems that have a single receive antenna or two transmit antennas. For arbitrary transmit and receive antennas, approximations for the SNR loss and outage are derived. In particular, the SNR loss in a quantized MIMO beamforming system is characterized as a function of the number of quantization bits and the number of transmit and receive antennas. The analytical expressions are proved to be tight with asymptotically large feedback rate. Simulations show that the bounds and approximations are tight even at low feedback rates, thereby providing a benchmark for feedback system design}, number={12}, journal={IEEE Transactions on Signal Processing}, year={2006}, pages={4753–4766} }
 @inproceedings{performance evaluation of 2-element arrays of circular patch antennas in indoor clustered mimo channels_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847403480&partnerID=MN8TOARS}, DOI={10.1109/MILCOM.2005.1605655}, abstractNote={In this paper we present a computationally efficient method to evaluate the performance of circular patch arrays (CPAs) in clustered MIMO channel models. The proposed method is based on bounds to the eigenvalues of the spatial correlation matrix that are shown to depend only on the channel angle spread. From these bounds, we derive a closed-form expression of the MIMO ergodic capacity as a function of the angle spread, which is used to estimate the performance of CPAs in different propagation environments. This method yields dramatic reduction in computational complexity due to the reduced number of channel parameters required for performance evaluation of CPAs. Simulation results show that through this method it is possible to predict the performance of CPAs, with negligible capacity loss (below 2%), for the most practical channel scenarios}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2005} }
 @inproceedings{power control for cellular mimo systems_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50949108461&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2006.677}, abstractNote={Power control is an important technique for interference management in interference-limited multiuser communication systems. In this paper, we propose a novel multidimensional power control technique for the uplink of cellular MIMO systems. Similar to prior single antenna power control work, we employ a fixed SINR target that allows guaranteed quality-of-service for delay-sensitive data applications. The proposed power control scheme is a more general technique because it incorporates single antenna, orthogonal multi-carrier, and utility-based power control schemes as special cases. Two approaches are proposed, the first where each user's power is equally allocated to its antenna array, in the second the allocation is adaptive. The optimal solution with full channel knowledge, and a practical near-optimal solution requiring only partial channel knowledge, are both derived. Numerical results show that power control actually achieves higher throughput a.t the low SINRs typical in cellular systems, compared to supposedly optimal water-filling strategies, with lower overhead and complexity. In the higher SINR ranges, iterative water-filling indeed slightly outperforms adaptive power allocation. Due to its better exploitation of spatial diversity and reduced transmit power (and hence reduced other-cell interference), adaptive power allocation increases the achievable SINR by an order of magnitude over equal power allocation, resulting in far better coverage.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2006} }
 @inproceedings{quantization on the complex projective space_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41249087124&partnerID=MN8TOARS}, DOI={10.1109/DCC.2006.68}, abstractNote={This paper derives bounds on the asymptotic expected distortion (high rate) for quantization on the complex projective space denoted as /spl Copf/P/sup n-1/. In essence the problem of quantization in an Euclidean space with constraints can be posed as an unconstrained problem on an appropriate manifold. /spl Copf/P/sup n-1/ is a non-linear manifold that represents the constraints that arise in areas such as communication with multiple antennas at the transmitter and receiver. Due to the constraints, the distortion analysis developed for Euclidean spaces cannot be applied directly. The special structure of /spl Copf/P/sup n-1/ and the distortion measures that are defined on it differentiate this problem from traditional vector quantization in Euclidean spaces.}, booktitle={Data Compression Conference Proceedings}, year={2006}, pages={242–251} }
 @inproceedings{quantization on the grassmann manifold: applications to precoded mimo wireless systems_2005, volume={V}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646786793&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2005.1416481}, abstractNote={This paper studies the problem of quantization of a source that lives on the complex Grassmann manifold. The special structure of the Grassmann manifold and the distortion measures that are defined on it differentiates this problem from the traditional problem of vector quantization in Euclidean spaces. Assuming a uniform source distribution along with a distortion based on chordal distance, codebook design algorithms are mentioned and rate distortion tradeoffs are studied. The expected distortion for such a quantizer is approximately characterized. These results are then applied to the performance analysis of a multiple antenna wireless communication system.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2005} }
 @inproceedings{khaled_mondal_heath_leus_petre_2005, title={Quantized multi-mode precoding for spatial multiplexing MIMO-OFDM system}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84931464076&partnerID=MN8TOARS}, DOI={10.1109/VETECF.2005.1558048}, abstractNote={Spatial multiplexing with multi-mode precoding can achieve both high capacity and high reliability in multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Multimode precoding uses linear transmit precoding but adapts the number of transmit streams or modes according to the channel conditions. Multi-mode precoding typically requires complete knowledge of the transmit precoding matrices for each subcarrier at the transmitter. In this paper we propose to reduce the feedback requirements by sending back the quantized precoding matrices of a fraction of the subcarriers and obtaining the other precoders using interpolation. Two algorithms are proposed for the interpolation of unitary matrices. Bit error rates simulations demonstrate the performance improvements of the proposed algorithms as a function of the feedback rate.}, booktitle={IEEE Vehicular Technology Conference}, author={Khaled, N. and Mondal, B. and Heath, R.W. and Leus, G. and Petre, F.}, year={2005}, pages={867–871} }
 @article{space-time water-filling for composite mimo fading channels_2006, volume={2006}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646340411&partnerID=MN8TOARS}, DOI={10.1155/WCN/2006/16281}, abstractNote={We analyze the ergodic capacity and channel outage probability for a composite MIMO channel model, which includes both fast fading and shadowing effects. The ergodic capacity and exact channel outage probability with space-time water-filling can be evaluated through numerical integrations, which can be further simplified by using approximated empirical eigenvalue and maximal eigenvalue distribution of MIMO fading channels. We also compare the performance of space-time water-filling with spatial water-filling. For MIMO channels with small shadowing effects, spatial water-filling performs very close to space-time water-filling in terms of ergodic capacity. For MIMO channels with large shadowing effects, however, space-time water-filling achieves significantly higher capacity per antenna than spatial water-filling at low to moderate SNR regimes, but with a much higher channel outage probability. We show that the analytical capacity and outage probability results agree very well with those obtained from Monte Carlo simulations.}, journal={Eurasip Journal on Wireless Communications and Networking}, year={2006} }
 @inproceedings{sum capacity of multiuser mimo broadcast channels with block diagonalization_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39049095775&partnerID=MN8TOARS}, DOI={10.1109/ISIT.2006.261786}, abstractNote={The sum capacity of a Gaussian broadcast MIMO channel can be achieved with dirty paper coding (DPC). Deploying DPC in real-time systems is, however, impractical. Block diagonalization (BD) is an alternative precoding technique for downlink multiuser MIMO systems, which can eliminate interuser interference at each receiver, at the expense of suboptimal sum capacity vs. DPC. In this paper, we study the sum capacity loss of BD for a fixed channel. We show that 1) if the user channels are orthogonal to each other, then BD achieves the complete sum capacity; and 2) if the user channels lie in a common row vector space, then the gain of DPC over BD can be bounded by the minimum of the number of transmit and receive antennas and the number of users. We also compare the ergodic sum capacity of DPC with that of BD in a Rayleigh fading channel. Simulations show that BD can achieve a significant part of the total throughput of DPC. An upper bound on the ergodic sum capacity gain of DPC over BD is derived, which can be evaluated with a few numerical integrations. With this bound, we can easily estimate how far away BD is from being optimal in terms of ergodic sum capacity, which is useful in directing practical system designs}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, year={2006}, pages={886–890} }
 @inproceedings{switching between ostbc and spatial multiplexing with linear receivers in spatially correlated mimo channels_2006, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34047147890&partnerID=MN8TOARS}, booktitle={IEEE Vehicular Technology Conference}, year={2006}, pages={1387–1391} }
 @inproceedings{tomlinson-harashima precoding with adaptive modulation for fixed relay networks_2006, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48749105800&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2006.346437}, abstractNote={In this paper, a relaying strategy that uses multiple input multiple output (MIMO) fixed relays with linear processing for multiuser transmission in cellular networks is proposed. This approach applies to the two hop relaying scenario for coverage enhancement, where the base station transmits data to multiple users through one fixed relay (multiuser transmission). The fixed relay processes the received signal by applying a linear transformation and forwards the processed signal to multiple users. We propose an implementable multiuser precoding strategy that combines Tomlinson-Harashima precoding at the base station and linear signal processing at the relay with adaptive stream selection and QAM modulation. We propose a reduced complexity algorithm to select a subset of users to avoid an exhaustive search over all user permutations. Simulations illustrate the sum rate using Tomlinson-Harashima precoding without coding}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2006} }
 @inproceedings{transmit precoding for the multiple antenna broadcast channel_2006, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34047133270&partnerID=MN8TOARS}, booktitle={IEEE Vehicular Technology Conference}, year={2006}, pages={1396–1400} }
 @inproceedings{a joint mimo-ofdm transceiver and mac design for mobile ad hoc networking_2005, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-28244475399&partnerID=MN8TOARS}, booktitle={2004 International Workshop on Wireless Ad-Hoc Networks}, year={2005}, pages={315–319} }
 @inproceedings{adaptive mimo transmission scheme: exploiting the spatial selectivity of wireless channels_2005, volume={61}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-26444553593&partnerID=MN8TOARS}, number={5}, booktitle={IEEE Vehicular Technology Conference}, year={2005}, pages={3188–3192} }
 @inproceedings{algorithms for quantized precoded mimo-ofdm systems_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847639400&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2005}, pages={381–385} }
 @inproceedings{algorithms for quantized precoding in mimo ofdm beamforming systems_2005, volume={5847}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-28444496353&partnerID=MN8TOARS}, DOI={10.1117/12.612595}, abstractNote={Multiple input multiple output (MIMO) wireless systems can offer significant diversity and transmit beamforming with receive combining provides a method to achieve this diversity with simple receive processing. The maximum gains in terms of array gain and diversity, however, requires perfect channel knowledge at the transmitter. In the absence of perfect channel knowledge, the channel information can be quantized at the receiver and sent back to the transmitter using a low-rate feedback link. In the case of narrowband channels, considerable work has been done in reducing the feedback information while maintaining bit-error-rate performance close to the case of perfect channel knowledge. This work, however, does not naturally extend to the case of frequency selective channels and leads to an explosion in the feedback overhead. In this paper, orthogonal frequency division multiplexing (OFDM)is considered as a low complexity implementation of MIMO beamforming combining over frequency selective channels. Two broad classes of algorithms are discussed for quantizing channel information - clustering and transform. The clustering algorithms group the subcarriers and choose a common frequency-domain representation of the channel information for each group. Thus the feedback rate depends on the number of groups and not on the number of subcarriers. The transform algorithms quantize the channel information in time-domain where the transform essentially decorrelates the channel information. Both the algorithms provide significant compression of channel information maintaining bit-error-rate performance close to the case of perfect channel knowledge.}, booktitle={Proceedings of SPIE - The International Society for Optical Engineering}, year={2005}, pages={80–87} }
 @inproceedings{antenna partitioning for multiuser mimo-cdma_2005, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33846576637&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2005.1577908}, abstractNote={Improving downlink CDMA capacity has been an area of intensive research for the past decade, especially as the downlink has become the capacity limiting link. Multi-antenna technologies are an obvious candidate for increasing the downlink capacity, but successfully decoding spatially multiplexed signals is very challenging in an interference-limited environment, such as that of CDMA cellular systems. In this paper, a simple and novel MIMO-CDMA system design is developed, in which users are assigned to a transmit antenna either without regard to channel knowledge (static) or based on antenna selection feedback bits (dynamic). These proposed antenna partitioning techniques have a minimal increase in complexity and require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems are derived and it is shown that they outperform conventional CDMA systems regardless of the number of antennas or antenna partitioning technique.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2005}, pages={1535–1539} }
 @inproceedings{capacity enhancement via multi-mode adaptation in spatially correlated mimo channels_2005, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34047199879&partnerID=MN8TOARS}, booktitle={IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC}, year={2005}, pages={754–758} }
 @inproceedings{codebook adaptation for quantized mimo beamforming systems_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847689465&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2005}, pages={376–380} }
 @article{designing structured tight frames via an alternating projection method_2005, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-12444320397&partnerID=MN8TOARS}, DOI={10.1109/TIT.2004.839492}, abstractNote={Tight frames, also known as general Welch-bound- equality sequences, generalize orthonormal systems. Numerous applications - including communications, coding, and sparse approximation- require finite-dimensional tight frames that possess additional structural properties. This paper proposes an alternating projection method that is versatile enough to solve a huge class of inverse eigenvalue problems (IEPs), which includes the frame design problem. To apply this method, one needs only to solve a matrix nearness problem that arises naturally from the design specifications. Therefore, it is the fast and easy to develop versions of the algorithm that target new design problems. Alternating projection will often succeed even if algebraic constructions are unavailable. To demonstrate that alternating projection is an effective tool for frame design, the paper studies some important structural properties in detail. First, it addresses the most basic design problem: constructing tight frames with prescribed vector norms. Then, it discusses equiangular tight frames, which are natural dictionaries for sparse approximation. Finally, it examines tight frames whose individual vectors have low peak-to-average-power ratio (PAR), which is a valuable property for code-division multiple-access (CDMA) applications. Numerical experiments show that the proposed algorithm succeeds in each of these three cases. The appendices investigate the convergence properties of the algorithm.}, number={1}, journal={IEEE Transactions on Information Theory}, year={2005}, pages={188–209} }
 @inproceedings{frame based multiple description image coding in the wavelet domain_2005, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33749236975&partnerID=MN8TOARS}, DOI={10.1109/ICIP.2005.1530543}, abstractNote={Multiple description codes generated by quantized frame expansions have been shown to perform well on erasure channels when compared to traditional channel codes. In this paper we propose a multiple description image coding scheme in the wavelet domain using quantized frame expansions. We form zerotrees from wavelet coefficients and apply a tight frame operator to the zerotrees. We then group appropriate expansions to form packets and evaluate the performance of the scheme over an erasure channel. We compare the performance of the proposed scheme with a conventional channel coding scheme.}, booktitle={Proceedings - International Conference on Image Processing, ICIP}, year={2005}, pages={920–923} }
 @inproceedings{frame theoretic quantization for limited feedback mimo beamforming systems_2005, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547236839&partnerID=MN8TOARS}, DOI={10.1109/WIRLES.2005.1549560}, abstractNote={In an uncorrelated Rayleigh fading channel, multiple-input multiple-output (MIMO) beamforming-combining systems can attain full diversity and significant array gain using channel knowledge at the transmitter. In the absence of channel knowledge at the transmitter, a beamforming vector can be quantized at the receiver using a fixed pre-determined codebook of beamforming vectors and the codeword index can be sent to the transmitter. Designing optimal codebooks (that maximize expected SNR) is an open problem and vector quantization (VQ) and Grassmannian line packings (GLP) have been proposed as possible solutions. In this paper, a mutual information based codebook design criterion is introduced and equiangular frames (EF) are proposed as beamforming codebooks satisfying this criterion. Besides providing a systematic way to design codebooks, EFs result in higher receive signal-to-noise ratio compared to codebooks designed using VQ or GLP.}, booktitle={2005 International Conference on Wireless Networks, Communications and Mobile Computing}, year={2005}, pages={1065–1070} }
 @article{generalized finite algorithms for constructing hermitian matrices with prescribed diagonal and spectrum_2006, volume={27}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33144482555&partnerID=MN8TOARS}, DOI={10.1137/S0895479803438183}, abstractNote={In this paper, we present new algorithms that can replace the diagonal entries of a Hermitian matrix by any set of diagonal entries that majorize the original set without altering the eigenvalues of the matrix. They perform this feat by applying a sequence of (N-1) or fewer plane rotations, where N is the dimension of the matrix. Both the Bendel--Mickey and the Chan--Li algorithms are special cases of the proposed procedures. Using the fact that a positive semidefinite matrix can always be factored as $\mtx{X^\adj X}$, we also provide more efficient versions of the algorithms that can directly construct factors with specified singular values and column norms. We conclude with some open problems related to the construction of Hermitian matrices with joint diagonal and spectral properties.}, number={1}, journal={SIAM Journal on Matrix Analysis and Applications}, year={2006}, pages={61–71} }
 @article{interpolation based transmit beamforming for mimo-ofdm with limited feedback_2005, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744559310&partnerID=MN8TOARS}, DOI={10.1109/TSP.2005.857019}, abstractNote={Transmit beamforming and receive combining are simple methods for exploiting spatial diversity in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. Optimal beamforming requires channel state information in the form of the beamforming vectors for each OFDM subcarrier. This paper proposes a limited feedback architecture that combines beamforming vector quantization and smart vector interpolation. In the proposed system, the receiver sends a fraction of information about the optimal beamforming vectors to the transmitter and the transmitter computes the beamforming vectors for all subcarriers through interpolation. A new spherical interpolator is developed that exploits parameters for phase rotation to satisfy the phase invariance and unit norm properties of the transmitted beamforming vectors. The beamforming vectors and phase parameters are quantized at the receiver and the quantized information is provided to the transmitter. The proposed quantization system provides only a moderate increase in complexity versus over comparable approaches. Numerical simulations show that the proposed scheme performs better than existing diversity techniques with the same feedback data rate.}, number={11}, journal={IEEE Transactions on Signal Processing}, year={2005}, pages={4125–4135} }
 @inproceedings{interpolation-based multi-mode precoding for mimo-ofdm systems_2005, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863701197&partnerID=MN8TOARS}, booktitle={13th European Signal Processing Conference, EUSIPCO 2005}, year={2005}, pages={1552–1555} }
 @article{limited feedback unitary precoding for spatial multiplexing systems_2005, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-23844475192&partnerID=MN8TOARS}, DOI={10.1109/TIT.2005.850152}, abstractNote={Multiple-input multiple-output (MIMO) wireless systems use antenna arrays at both the transmitter and receiver to provide communication links with substantial diversity and capacity. Spatial multiplexing is a common space-time modulation technique for MIMO communication systems where independent information streams are sent over different transmit antennas. Unfortunately, spatial multiplexing is sensitive to ill-conditioning of the channel matrix. Precoding can improve the resilience of spatial multiplexing at the expense of full channel knowledge at the transmitter-which is often not realistic. This correspondence proposes a quantized precoding system where the optimal precoder is chosen from a finite codebook known to both receiver and transmitter. The index of the optimal precoder is conveyed from the receiver to the transmitter over a low-delay feedback link. Criteria are presented for selecting the optimal precoding matrix based on the error rate and mutual information for different receiver designs. Codebook design criteria are proposed for each selection criterion by minimizing a bound on the average distortion assuming a Rayleigh-fading matrix channel. The design criteria are shown to be equivalent to packing subspaces in the Grassmann manifold using the projection two-norm and Fubini-Study distances. Simulation results show that the proposed system outperforms antenna subset selection and performs close to optimal unitary precoding with a minimal amount of feedback.}, number={8}, journal={IEEE Transactions on Information Theory}, year={2005}, pages={2967–2976} }
 @inproceedings{low complexity user selection algorithms for multiuser mimo systems with block diagonalization_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847644173&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2005}, pages={628–632} }
 @inproceedings{min-ser space-time equalization in asynchronous mimo-ofdm systems_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33745502195&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2005.1506063}, abstractNote={In this paper, we propose an algorithm for the design of space-time equalizer in asynchronous MIMO-OFDM systems. The equalizer is designed to null the co-channel interference and combat asynchronism as well as minimize a lower bound to the symbol error rate. Numerical simulations show good uncoded bit error rate performance for our proposed algorithm.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2005}, pages={440–444} }
 @article{multimode antenna selection for spatial multiplexing systems with linear receivers_2005, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-25444488050&partnerID=MN8TOARS}, DOI={10.1109/TSP.2005.851109}, abstractNote={Spatial multiplexing is a simple transmission technique for multiple-input multiple-output (MIMO) wireless communication links in which data is multiplexed across the transmit antennas. In Rayleigh fading matrix channels, however, spatial multiplexing with low-complexity linear receivers suffers due to a lack of diversity advantage. This paper proposes multimode antenna selection, which uses a low-rate feedback channel to improve the error rate performance of spatial multiplexing systems with linear receivers. In the proposed technique, both the number of substreams and the mapping of substreams to antennas are dynamically adjusted, for a fixed total data rate, to the channel based on limited feedback from the receiver. Dual-mode selection, where spatial multiplexing or selection diversity is adaptively chosen, dramatically improves the diversity gain achieved. Multimode selection (i.e., allowing any number of substreams to be dynamically selected) provides additional array gain. Various criteria for selecting the number of substreams and the optimal mapping of substreams to transmit antennas are derived. Relationships are made between the selection criteria and the eigenmodes of the channel. A probabilistic analysis of the selection criteria are provided for Rayleigh fading channels. Applications to nonlinear receivers are mentioned. Monte Carlo simulations demonstrate significant performance improvements in independent and identically distributed (i.i.d.) flat-fading Rayleigh matrix channels with minimal feedback.}, number={8}, journal={IEEE Transactions on Signal Processing}, year={2005}, pages={3042–3056} }
 @article{multimode precoding for mimo wireless systems_2005, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27844521745&partnerID=MN8TOARS}, DOI={10.1109/TSP.2005.855107}, abstractNote={Multiple-input multiple-output (MIMO) wireless systems obtain large diversity and capacity gains by employing multielement antenna arrays at both the transmitter and receiver. The theoretical performance benefits of MIMO systems, however, are irrelevant unless low error rate, spectrally efficient signaling techniques are found. This paper proposes a new method for designing high data-rate spatial signals with low error rates. The basic idea is to use transmitter channel information to adaptively vary the transmission scheme for a fixed data rate. This adaptation is done by varying the number of substreams and the rate of each substream in a precoded spatial multiplexing system. We show that these substreams can be designed to obtain full diversity and full rate gain using feedback from the receiver to transmitter. We model the feedback using a limited feedback scenario where only finite sets, or codebooks, of possible precoding configurations are known to both the transmitter and receiver. Monte Carlo simulations show substantial performance gains over beamforming and spatial multiplexing.}, number={10 I}, journal={IEEE Transactions on Signal Processing}, year={2005}, pages={3674–3687} }
 @article{necessary and sufficient conditions for full diversity order in correlated rayleigh fading beamforming and combining systems_2005, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-19944392275&partnerID=MN8TOARS}, DOI={10.1109/TWC.2004.840197}, abstractNote={Transmit beamforming and receive combining are low complexity, linear techniques that make use of the spatial diversity advantage provided by transmitters and/or receivers employing multiple antennas. There has been a growing interest in designing beamforming schemes for frequency division duplexing systems that use a limited amount of feedback from the receiver to the transmitter. This limited feedback conveys a beamforming vector chosen from a finite set known to both the transmitter and receiver. These techniques often use a set of beamforming vectors where the probability of error expression can not be easily formulated or bounded. It is of utmost importance to guarantee that the sets of beamforming and combining vectors are chosen such that full diversity order is achieved. For this reason, necessary and sufficient conditions on the sets of possible beamformers and combiners are derived that guarantee full diversity order in correlated Rayleigh fading.}, number={1}, journal={IEEE Transactions on Wireless Communications}, year={2005}, pages={20–23} }
 @article{ofdm power loading using limited feedback_2005, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-29044437088&partnerID=MN8TOARS}, DOI={10.1109/TVT.2005.853473}, abstractNote={Orthogonal frequency division multiplexing (OFDM) is a practical broadband signaling technique for use in multipath fading channels. Over the past ten years, research has shown that power loading, where the power allocations on the OFDM frequency tones are jointly optimized, can improve error rate or capacity performance. The implementation of power loading, however, is dependent on the presence of complete forward link channel knowledge at the transmitter. In systems using frequency division duplexing (FDD), this assumption is unrealistic. In this paper, we propose power loading for OFDM symbols using a limited number of feedback bits sent from the receiver to the transmitter. The power loading vector is designed at the receiver, which is assumed to have perfect knowledge of the forward link channel, and conveyed back to the transmitter over a limited rate feedback channel. To allow for the vector to be represented by a small number of bits, the power loading vector is restricted to lie in a finite set, or codebook, of power loading vectors. This codebook is designed offline and known a priori to both the transmitter and receiver. We present two power allocation selection algorithms that optimize the probability of symbol error and capacity, respectively. Simulation results show that the proposed limited feedback techniques provide performance close to full channel knowledge power loading.}, number={5}, journal={IEEE Transactions on Vehicular Technology}, year={2005}, pages={1773–1780} }
 @article{opportunistic feedback for downlink multiuser diversity_2005, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27644435489&partnerID=MN8TOARS}, DOI={10.1109/LCOMM.2005.10002}, abstractNote={This paper proposes a random access based feedback protocol for achieving multiuser diversity with a fixed number of feedback slots. Active users send a feedback message with some probability if their channel power is above a threshold. The threshold and random access probability are jointly designed to maximize the average sum rate of all users.}, number={10}, journal={IEEE Communications Letters}, year={2005}, pages={948–950} }
 @inproceedings{pattern diversity with multi-mode circular patch antennas in clustered mimo channels_2005, volume={3 B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33846908339&partnerID=MN8TOARS}, DOI={10.1109/APS.2005.1552537}, abstractNote={The throughput that a MIMO channel can support is a function of different array parameters. Another technique, for miniaturized antenna designs, is polarization/pattern diversity where the antennas are designed to radiate with orthogonal radiation patterns and polarizations as a means to create uncorrelated channels across different array elements. The benefits of pattern diversity have been shown by L. Dong et al. (2003). That analysis, however, did not use realistic channel models. We aim to extend the analysis of Dong et al. to clustered channel models, adopted by the IEEE 802.11n standard body for wireless local area networks (WLANs). We analyze MIMO arrays consisting of circular microstrip antennas to enable pattern diversity and compare their performance against conventional uniform linear arrays. We also model the effect of mutual coupling and measure the performance degradation produced by the near-field effects. We briefly review the channel model and the properties of circular microstrip antennas. Then, we analytically compute the spatial correlation coefficients and evaluate the MIMO channel capacity. Pattern diversity yields better performance than space diversity.}, booktitle={IEEE Antennas and Propagation Society, AP-S International Symposium (Digest)}, year={2005}, pages={438–441} }
 @inproceedings{performance of the mimo downlink channel with multi-mode adaptation and scheduling_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33745485346&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2005.1506229}, abstractNote={This paper presents an algorithm for switching transmission methods (whether for diversity or data rate maximization) over a multiple antenna broadcast channel. The proposed approach exploits long-term spatial selectivity of the user channels to decide between different diversity and multiplexing modes. We investigate the performance of this adaptive transmission method with different scheduling polices (both sensitive to user queue sizes and independent of user queue sizes). While the methods outlined in this paper are general, we present our results in the context of a broadcast channel with four transmit antennas at the central transmitter, and four antennas at each receiver. Our results indicate a gain of approximately 20% in average system throughput with the proposed algorithm over realistic channel models.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2005}, pages={695–699} }
 @inproceedings{rate bounds for mimo relay channels using precoding_2005, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33846594834&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2005.1577838}, abstractNote={Relay channels plays a central role in next-generation multihop wireless systems. This paper considers the MIMO relay channel where multiple antennas are employed by each terminal. New lower bounds on the capacity of a Gaussian MIMO relay channel are derived under the assumption that the transmitter employs either superposition coding or dirty-paper coding. The proposed lower bounds improve on a previously proposed lower bound that arises from a simple transmit strategy.}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2005}, pages={1172–1176} }
 @article{receiver designs for alamouti coded ofdm systems in fast fading channels_2005, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17144375611&partnerID=MN8TOARS}, DOI={10.1109/TWC.2004.843042}, abstractNote={In this paper, receiver designs for orthogonal frequency-division multiplexing (OFDM) systems that exploit the Alamouti transmit diversity technique are addressed. In Alamouti space-time coded OFDM systems, the simple Alamouti decoding at the receiver relies on the assumption that the channels do not change over an Alamouti codeword period (two consecutive OFDM symbol periods). Unfortunately, when the channel is fast fading, the assumption is not met, resulting in severe performance degradation. In this paper, a sequential decision feedback sequence estimation (SDFSE) scheme with an adaptive threshold (AT), a traditionally single-carrier equalization technique, is used to mitigate the performance degradation. A new method to set the threshold value is proposed. For small signal constellations like BPSK and QPSK, the SDFSE with an AT requires much lower complexity than a previous minimum mean square error approach at the cost of a small performance degradation. Furthermore, we show that the performance difference becomes smaller when channel estimation error is included. An adaptive effort sequence estimation (AESE) scheme is also proposed to furthur reduce the average complexity of the SDFSE scheme with an AT. The AESE scheme is based on the observation that a high Doppler frequency does not necessarily mean significant instantaneous channel variation. Simulations show the efficacy of the proposed SDFSE with an AT and the AESE.}, number={2}, journal={IEEE Transactions on Wireless Communications}, year={2005}, pages={550–559} }
 @article{simulation of mimo channel capacity with antenna polarization diversity_2005, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744541073&partnerID=MN8TOARS}, DOI={10.1109/TWC.2005.850318}, abstractNote={A simulation study of the channel capacity of a multiple-input multiple-output (MIMO) antenna system exploiting multiple polarizations is carried out. We focus on a simple yet realistic trimonopole antenna structure, taking into account all the mutual coupling and casing effects using the computational electromagnetics solver-numerical electromagnetics code. Simulation results show that, with a special transmit geometry, using the collocated trimonopole antennas at a size-constrained receiver can offer channel capacity that approaches the capacity of an uncorrelated MIMO Rayleigh channel. In addition, it is shown that the capacity increase is mainly attributed to polarization diversity instead of pattern diversity. Furthermore, we find that the mutual coupling and casing effects in the trimonopole system can actually provide a large capacity increase with less constraint on the antenna configurations than the idealized tridipole system.}, number={4}, journal={IEEE Transactions on Wireless Communications}, year={2005}, pages={1869–1872} }
 @article{space-time chase decoding_2005, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-28444490067&partnerID=MN8TOARS}, DOI={10.1109/TWC.2005.853830}, abstractNote={Multiple-antenna wireless systems are of interest because they provide increased capacity over single-antenna systems. Several space-time signaling schemes have been proposed to make use of this increased capacity. Space-time techniques, such as space-time block coding and spatial multiplexing, can all be viewed as signaling with a multidimensional constellation. Because of the large capacity of multiple-input multiple-output (MIMO) channels, these multidimensional constellations often have large cardinalities. For this reason, it is impractical to perform optimal maximum-likelihood (ML) decoding for space-time systems, even for a moderate number of transmit antennas. In this paper, we propose a modified version of the classic Chase decoder for multiple-antenna systems. The decoder applies successive detection to yield an initial estimate of the transmitted bit sequence, constructs a list of candidate symbol vectors using this initial estimate, and then computes bit likelihood information over this list. Three algorithms are presented for constructing the candidate vector list. This decoder can be adjusted to have a fixed or variable complexity, while maintaining performance close to that of an ML decoder.}, number={5}, journal={IEEE Transactions on Wireless Communications}, year={2005}, pages={2035–2039} }
 @article{space-time interference cancellation in mimo-ofdm systems_2005, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-29044433176&partnerID=MN8TOARS}, DOI={10.1109/TVT.2005.851299}, abstractNote={In this paper, a two stage hybrid interference cancellation and equalization framework is proposed for interference cancellation in the uplink of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The first stage uses time domain equalization to suppress cochannel interference, mitigate asynchronism, and shorten the postequalization channel response to be no longer than the length of the cyclic prefix. The second stage performs low-complexity single tap equalization and detection in the frequency domain. The framework is developed specifically for spatial multiplexing and is applied to multiuser MIMO-OFDM systems with asynchronism between users as well as to single-user MIMO-OFDM systems. Various equalizer design methods are proposed that determine the coefficients directly from the training data and are compared with methods based on channel estimates. The equalizer coefficients and postequalization channel response are found by solving a joint optimization that maximizes the signal to interference-plus-noise ratio (SINR) in the frequency domain. Simulations compare various training-based methods and show the proposed methods provide good bit error rate (BER) and SINR performance in a variety of interference scenarios.}, number={5}, journal={IEEE Transactions on Vehicular Technology}, year={2005}, pages={1802–1816} }
 @article{switching between diversity and multiplexing in mimo systems_2005, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-22144485992&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2005.849774}, abstractNote={Multiple-input multiple-output (MIMO) wireless communication systems can offer high data rates through spatial multiplexing or substantial diversity using transmit diversity. In this letter, switching between spatial multiplexing and transmit diversity is proposed as a simple way to improve the diversity performance of spatial multiplexing. In the proposed approach, for a fixed rate, either multiplexing or diversity is chosen based on the instantaneous channel state and the decision is conveyed to the transmitter via a low-rate feedback channel. The minimum Euclidean distance at the receiver is computed for spatial multiplexing and transmit diversity and is used to derive the selection criterion. Additionally, the Demmel condition number of the matrix channel is shown to provide a sufficient condition for multiplexing to outperform diversity. Monte Carlo simulations demonstrate improvement over either multiplexing or diversity individually in terms of bit error rate.}, number={6}, journal={IEEE Transactions on Communications}, year={2005}, pages={962–972} }
 @inproceedings{unequal power allocation for jpeg transmission over mimo systems_2005, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847652903&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2005}, pages={1608–1612} }
 @inproceedings{sabir_sheikh_heath_bovik_2004, title={A joint source-channel distortion model for JPEG compressed images}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-20444476685&partnerID=MN8TOARS}, booktitle={Proceedings - International Conference on Image Processing, ICIP}, author={Sabir, M.F. and Sheikh, H.R. and Heath, R.W. and Bovik, A.C.}, year={2004}, pages={3249–3252} }
 @inproceedings{a low complexity algorithm to simulate the spatial covariance matrix for clustered mimo channel models_2004, volume={59}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-15544385927&partnerID=MN8TOARS}, number={2}, booktitle={IEEE Vehicular Technology Conference}, year={2004}, pages={889–893} }
 @inproceedings{a lower bound on outage probability of limited feedback mimo beamforming systems_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21644441092&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2004}, pages={876–880} }
 @inproceedings{a space-time fir equalizer training algorithm for mimo-ofdm systems_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-28244490587&partnerID=MN8TOARS}, booktitle={2004 International Conference on Signal Processing and Communications, SPCOM}, year={2004}, pages={550–554} }
 @inproceedings{adaptive feedback for mimo beamforming systems_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21644470893&partnerID=MN8TOARS}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, year={2004}, pages={213–217} }
 @inproceedings{an upper bound on snr for limited feedback mimo beamforming systems_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-19544380000&partnerID=MN8TOARS}, booktitle={2004 IEEE Information Theory Workshop - Proceedings, ITW}, year={2004}, pages={408–412} }
 @inproceedings{chase decoding for space-time codes_2004, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17144430439&partnerID=MN8TOARS}, number={3}, booktitle={IEEE Vehicular Technology Conference}, year={2004}, pages={1663–1667} }
 @inproceedings{comparison of space-time water-filling and spatial water-filling for mimo fading channels_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144376745&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={431–435} }
 @inproceedings{construction of equiangular signatures for synchronous cdma systems_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-26944493737&partnerID=MN8TOARS}, booktitle={IEEE International Symposium on Spread Spectrum Techniques and Applications}, year={2004}, pages={708–712} }
 @article{diversity performance of precoded orthogonal space-time block codes using limited feedback_2004, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-2942536692&partnerID=MN8TOARS}, DOI={10.1109/LCOMM.2004.827381}, abstractNote={Orthogonal space-time block codes (OSTBCs) are simple space-time codes that can be used for open-loop transmit diversity systems. OSTBCs, however, can only be designed for certain numbers of transmit antennas. Channel-dependent linear precoders have been proposed to overcome this deficiency, but it is not clear what conditions the precoder design must satisfy to guarantee full diversity order. In this letter, we show necessary and sufficient conditions for linear precoded OSTBCs to provide full diversity order. We show that limited feedback precoding can achieve full diversity order using fewer bits than limited feedback beamforming. We also present a simplified version of antenna subset selection for OSTBCs that can provide full diversity order with low complexity and only a small amount of feedback.}, number={5}, journal={IEEE Communications Letters}, year={2004}, pages={305–307} }
 @article{finite-step algorithms for constructing optimal cdma signature sequences_2004, volume={50}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-8144223740&partnerID=MN8TOARS}, DOI={10.1109/TIT.2004.836698}, abstractNote={A description of optimal sequences for direct-spread code-division multiple access (DS-CDMA) is a byproduct of recent characterizations of the sum capacity. This paper restates the sequence design problem as an inverse singular value problem and shows that the problem can be solved with finite-step algorithms from matrix theory. It proposes a new one-sided algorithm that is numerically stable and faster than previous methods.}, number={11}, journal={IEEE Transactions on Information Theory}, year={2004}, pages={2916–2921} }
 @inproceedings{grassmannian beamforming on correlated mimo channels_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144375936&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={106–110} }
 @inproceedings{impact of antenna geometry on mimo communication in indoor clustered channels_2004, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4544277232&partnerID=MN8TOARS}, booktitle={IEEE Antennas and Propagation Society, AP-S International Symposium (Digest)}, year={2004}, pages={1700–1703} }
 @inproceedings{improving throughput and fairness for mimo ad hoc networks using antenna selection diversity_2004, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144426381&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={3363–3367} }
 @inproceedings{interpolation based transmit beamforming for mimo-ofdm with limited feedback_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143145417&partnerID=MN8TOARS}, booktitle={IEEE International Conference on Communications}, year={2004}, pages={249–253} }
 @inproceedings{interpolation based unitary precoding for spatial multiplexing mimo-ofdm with limited feedback_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144408720&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={214–218} }
 @inproceedings{joint frequency offset estimation and interference cancellation for mimo-ofdm systems_2004, volume={60}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17144392892&partnerID=MN8TOARS}, number={3}, booktitle={IEEE Vehicular Technology Conference}, year={2004}, pages={1553–1557} }
 @inproceedings{limited feedback power loading for ofdm_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744510283&partnerID=MN8TOARS}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2004}, pages={71–77} }
 @inproceedings{limited feedback precoding for orthogonal space-time block codes_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144429497&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={561–565} }
 @article{limited feedback unitary precoding for orthogonal space-time block codes_2005, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-11844289606&partnerID=MN8TOARS}, DOI={10.1109/TSP.2004.838928}, abstractNote={Orthogonal space-time block codes (OSTBCs) are a class of easily decoded space-time codes that achieve full diversity order in Rayleigh fading channels. OSTBCs exist only for certain numbers of transmit antennas and do not provide array gain like diversity techniques that exploit transmit channel information. When channel state information is available at the transmitter, though, precoding the space-time codeword can be used to support different numbers of transmit antennas and to improve array gain. Unfortunately, transmitters in many wireless systems have no knowledge about current channel conditions. This motivates limited feedback precoding methods such as channel quantization or antenna subset selection. This paper investigates a limited feedback approach that uses a codebook of precoding matrices known a priori to both the transmitter and receiver. The receiver chooses a matrix from the codebook based on current channel conditions and conveys the optimal codebook matrix to the transmitter over an error-free, zero-delay feedback channel. A criterion for choosing the optimal precoding matrix in the codebook is proposed that relates directly to minimizing the probability of symbol error of the precoded system. Low average distortion codebooks are derived based on the optimal codeword selection criterion. The resulting design is found to relate to the famous applied mathematics problem of subspace packing in the Grassmann manifold. Codebooks designed by this method are proven to provide full diversity order in Rayleigh fading channels. Monte Carlo simulations show that limited feedback precoding performs better than antenna subset selection.}, number={1}, journal={IEEE Transactions on Signal Processing}, year={2005}, pages={64–73} }
 @inproceedings{wong_forenza_heath_evans_2004, title={Long range channel prediction for adaptive OFDM systems}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21644486341&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Wong, I.C. and Forenza, A. and Heath, R.W. and Evans, B.L.}, year={2004}, pages={732–736} }
 @inproceedings{multi-mode precoding using linear receivers for limited feedback mimo systems_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143110330&partnerID=MN8TOARS}, booktitle={IEEE International Conference on Communications}, year={2004}, pages={448–452} }
 @inproceedings{multi-user diversity for the multiple antenna broadcast channel with linear receivers: asymptotic analysis_2004, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21644458808&partnerID=MN8TOARS}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2004}, pages={886–890} }
 @inproceedings{multiuser space-time block coded mimo system with downlink precoding_2004, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143114693&partnerID=MN8TOARS}, booktitle={IEEE International Conference on Communications}, year={2004}, pages={2689–2693} }
 @inproceedings{optimal cdma signatures: a finite-step approach_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-26944485134&partnerID=MN8TOARS}, booktitle={IEEE International Symposium on Spread Spectrum Techniques and Applications}, year={2004}, pages={335–339} }
 @inproceedings{practical costa precoding for the multiple antenna broadcast channel_2004, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144372344&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={3942–3946} }
 @inproceedings{rapid mimo-ofdm software defined radio system prototyping_2004, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17044378192&partnerID=MN8TOARS}, booktitle={IEEE Workshop on Signal Processing Systems, SiPS: Design and Implementation}, year={2004}, pages={182–187} }
 @article{heath_larsson_murch_nehorai_uysal_2004, title={Special issue: Multiple-input multiple-output (MIMO) communications}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-13244267097&partnerID=MN8TOARS}, DOI={10.1002/wcm.247}, abstractNote={Wireless Communications and Mobile ComputingVolume 4, Issue 7 p. 693-696 EditorialFree Access Special Issue: Multiple-Input Multiple-Output (MIMO) Communications Robert W. Heath, Robert W. Heath Guest Editor University of Texas, USASearch for more papers by this authorErik G. Larsson, Erik G. Larsson Guest Editor University of Florida, USASearch for more papers by this authorRoss Murch, Ross Murch Guest Editor The Hong Kong University of Science and Technology, Hong KongSearch for more papers by this authorArye Nehorai, Arye Nehorai Guest Editor The University of Illinois at Chicago, USASearch for more papers by this authorMurat Uysal, Murat Uysal Guest Editor University of Waterloo, CanadaSearch for more papers by this author Robert W. Heath, Robert W. Heath Guest Editor University of Texas, USASearch for more papers by this authorErik G. Larsson, Erik G. Larsson Guest Editor University of Florida, USASearch for more papers by this authorRoss Murch, Ross Murch Guest Editor The Hong Kong University of Science and Technology, Hong KongSearch for more papers by this authorArye Nehorai, Arye Nehorai Guest Editor The University of Illinois at Chicago, USASearch for more papers by this authorMurat Uysal, Murat Uysal Guest Editor University of Waterloo, CanadaSearch for more papers by this author First published: 09 November 2004 https://doi.org/10.1002/wcm.247Citations: 3AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume4, Issue7Special Issue: Special Issue: Multiple-Input Multiple-Output (MIMO) CommunicationsNovember 2004Pages 693-696 ReferencesRelatedInformation}, number={7}, journal={Wireless Communications and Mobile Computing}, author={Heath, R.W. and Larsson, E.G. and Murch, R. and Nehorai, A. and Uysal, M.}, year={2004}, pages={593–696} }
 @inproceedings{heath_schmidt_2003, title={Student generated course demos}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946072134&partnerID=MN8TOARS}, DOI={10.1109/FIE.2003.1264663}, abstractNote={Actively engaging engineering students in classroom instruction often results in project-centered activities. Generally these projects are selected because of student interest or topic availability. In an advanced graduate level course, EE 381K-9 Advanced Digital Signal Processing, in the Department of Electrical Engineering at The University of Texas at Austin, students are assembling technology-based projects that will be used by other students in the class and in subsequent years. Thus, these projects serve a dual educational purpose: to teach those students who are creating them and to serve as resources to teach future students. This paper describes some of the challenges of developing student generated educational projects and course demos. Outcomes and observations are presented based on a trial run in the fall 2002 class. Specific topics such as the parameters of the assignment, necessary incentives, and mechanisms for collecting feedback are discussed. Numerous suggestions for incorporating these ideas into other classes are provided.}, booktitle={Proceedings - Frontiers in Education Conference, FIE}, author={Heath, R.W. and Schmidt, K.J.}, year={2003}, pages={F1B13-F1B18} }
 @inproceedings{transmit selection diversity for multiuser spatial multiplexing systems_2004, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18144397203&partnerID=MN8TOARS}, booktitle={GLOBECOM - IEEE Global Telecommunications Conference}, year={2004}, pages={2625–2629} }
 @article{what is the value of limited feedback for mimo channels?_2004, volume={42}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-7244221856&partnerID=MN8TOARS}, DOI={10.1109/MCOM.2004.1341261}, abstractNote={Feedback in a communications system can enable the transmitter to exploit channel conditions and avoid interference. In the case of a multiple-input multiple-output channel, feedback can be used to specify a precoding matrix at the transmitter, which activates the strongest channel modes. In situations where the feedback is severely limited, important issues are how to quantize the information needed at the transmitter and how much improvement in associated performance can be obtained as a function of the amount of feedback available. We give an overview of some recent work in this area. Methods are presented for constructing a set of possible precoding matrices, from which a particular choice can be relayed to the transmitter. Performance results show that even a few bits of feedback can provide performance close to that with full channel knowledge at the transmitter.}, number={10}, journal={IEEE Communications Magazine}, year={2004}, pages={54–59} }
 @article{a decision-directed receiver for alamouti coded ofdm systems_2003, volume={E86-B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0242412915&partnerID=MN8TOARS}, number={10}, journal={IEICE Transactions on Communications}, year={2003}, pages={3141–3143} }
 @inproceedings{a decision directed receiver for alamouti coded ofdm systems_2003, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143073630&partnerID=MN8TOARS}, number={1}, booktitle={IEEE Vehicular Technology Conference}, year={2003}, pages={662–665} }
 @inproceedings{a symbol estimation scheme for alamouti coded ofdm systems_2003, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-3142749994&partnerID=MN8TOARS}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, year={2003}, pages={1225–1230} }
 @inproceedings{cdma signature sequences with low peak-to-average-power ratio via alternating projection_2003, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143116538&partnerID=MN8TOARS}, booktitle={Conference Record of the Asilomar Conference on Signals, Systems and Computers}, year={2003}, pages={475–479} }
 @inproceedings{dual-mode antenna selection for spatial multiplexing systems with linear receivers_2003, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4143086337&partnerID=MN8TOARS}, booktitle={Conference Record of the Asilomar Conference on Signals, Systems and Computers}, year={2003}, pages={1085–1089} }
 @article{equal gain transmission in multiple-input multiple-output wireless systems_2003, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0042524487&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2003.814195}, abstractNote={Multiple-input multiple-output (MIMO) wireless systems are of interest due to their ability to provide substantial gains in capacity and quality. The paper proposes equal gain transmission (EGT) to provide diversity advantage in MIMO systems experiencing Rayleigh fading. The applications of EGT with selection diversity combining, equal gain combining, and maximum ratio combining are addressed. It is proven that systems using EGT with any of these combining schemes achieve full diversity order when transmitting over a memoryless, flat-fading Rayleigh matrix channel with independent entries. Since, in practice, full channel knowledge at the transmitter is difficult to realize, a quantized version of EGT is proposed. An algorithm to construct a beamforming vector codebook that guarantees full diversity order is presented. Monte-Carlo simulation comparisons with various beamforming and combining systems illustrate the performance as a function of quantization.}, number={7}, journal={IEEE Transactions on Communications}, year={2003}, pages={1102–1110} }
 @article{erratum: equal gain transmission in multiple-input multiple-output wireless systems (ieee trans. commun. (2003) 51 (694-703))_2003, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0141989510&partnerID=MN8TOARS}, DOI={10.1109/TCOMM.2003.818386}, number={9}, journal={IEEE Transactions on Communications}, year={2003} }
 @inproceedings{grassmannian signatures for cdma systems_2003, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0842310825&partnerID=MN8TOARS}, booktitle={Conference Record / IEEE Global Telecommunications Conference}, year={2003}, pages={1553–1557} }
 @article{grassmannian beamforming for multiple-input multiple-output wireless systems_2003, volume={49}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0242334025&partnerID=MN8TOARS}, DOI={10.1109/TIT.2003.817466}, abstractNote={Transmit beamforming and receive combining are simple methods for exploiting the significant diversity that is available in multiple-input multiple-output (MIMO) wireless systems. Unfortunately, optimal performance requires either complete channel knowledge or knowledge of the optimal beamforming vector; both are hard to realize. In this article, a quantized maximum signal-to-noise ratio (SNR) beamforming technique is proposed where the receiver only sends the label of the best beamforming vector in a predetermined codebook to the transmitter. By using the distribution of the optimal beamforming vector in independent and identically distributed Rayleigh fading matrix channels, the codebook design problem is solved and related to the problem of Grassmannian line packing. The proposed design criterion is flexible enough to allow for side constraints on the codebook vectors. Bounds on the codebook size are derived to guarantee full diversity order. Results on the density of Grassmannian line packings are derived and used to develop bounds on the codebook size given a capacity or SNR loss. Monte Carlo simulations are presented that compare the probability of error for different quantization strategies.}, number={10}, journal={IEEE Transactions on Information Theory}, year={2003}, pages={2735–2747} }
 @inproceedings{grassmannian beamforming for multiple-input multiple-output wireless systems_2003, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0038648277&partnerID=MN8TOARS}, booktitle={IEEE International Conference on Communications}, year={2003}, pages={2618–2622} }
 @article{grassmannian frames with applications to coding and communication_2003, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0242323185&partnerID=MN8TOARS}, DOI={10.1016/S1063-5203(03)00023-X}, abstractNote={For a given class F of unit norm frames of fixed redundancy we define a Grassmannian frame as one that minimizes the maximal correlation |〈fk,fl〉| among all frames {fk}k∈I∈F. We first analyze finite-dimensional Grassmannian frames. Using links to packings in Grassmannian spaces and antipodal spherical codes we derive bounds on the minimal achievable correlation for Grassmannian frames. These bounds yield a simple condition under which Grassmannian frames coincide with unit norm tight frames. We exploit connections to graph theory, equiangular line sets, and coding theory in order to derive explicit constructions of Grassmannian frames. Our findings extend recent results on unit norm tight frames. We then introduce infinite-dimensional Grassmannian frames and analyze their connection to unit norm tight frames for frames which are generated by group-like unitary systems. We derive an example of a Grassmannian Gabor frame by using connec