@article{rusu_gonzalez-prelcic_2023, title={A Novel Approach for Unit-Modulus Least-Squares Optimization Problems}, volume={30}, ISSN={["1558-2361"]}, url={https://doi.org/10.1109/LSP.2023.3254452}, DOI={10.1109/LSP.2023.3254452}, abstractNote={We describe a novel constrained least-squares (LS) optimization problem and propose an iterative solution that deals with the constraints of placing variables on the complex unit circle. We reformulate the LS problem with unit magnitude constraints so that we obtain efficient closed-form local updates based on Procrustes orthogonal solutions that monotonically improve the objective function. We show the performance of the proposed algorithm in a synthetic experiment and an application to hybrid precoding/combining in mmWave MIMO communications.}, journal={IEEE SIGNAL PROCESSING LETTERS}, author={Rusu, Cristian and Gonzalez-Prelcic, Nuria}, year={2023}, pages={224–228} }
 @article{graff_chen_gonzalez-prelcic_shimizu_2023, title={Deep Learning-Based Link Configuration for Radar-Aided Multiuser mmWave Vehicle-to-Infrastructure Communication}, volume={72}, ISSN={["1939-9359"]}, url={https://doi.org/10.1109/TVT.2023.3239227}, DOI={10.1109/TVT.2023.3239227}, abstractNote={Configuring millimeter wave links following a conventional beam training protocol, as the one proposed in the current cellular standard, introduces a large communication overhead, especially relevant in vehicular systems, where the channels are highly dynamic. In this paper, we propose the use of a passive radar array to sense automotive radar transmissions coming from multiple vehicles on the road, and a radar processing chain that provides information about a reduced set of candidate beams for the links between the road-infrastructure and each one of the vehicles. This prior information can be later leveraged by the beam training protocol to significantly reduce overhead. The radar processing chain estimates both the timing and chirp rates of the radar signals, isolates the individual signals by filtering out interfering radar chirps, and estimates the spatial covariance of each individual radar transmission. Then, a deep network is used to translate features of these radar spatial covariances into features of the communication spatial covariances, by learning the intricate mapping between radar and communication channels, in both line-of-sight and non-line-of-sight settings. The communication rates and outage probabilities of this approach are compared against exhaustive search and pure radar-aided beam training methods (without deep learning-based mapping), and evaluated on multi-user channels simulated by ray tracing. Results show that: (i) the proposed processing chain can reliably isolate the spatial covariances for individual radars, and (ii) the radar-to-communications translation strategy based on deep learning provides a significant improvement over pure radar-aided methods in both LOS and NLOS channels.}, number={6}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, author={Graff, Andrew and Chen, Yun and Gonzalez-Prelcic, Nuria and Shimizu, Takayuki}, year={2023}, month={Jun}, pages={7454–7468} }
 @article{xie_palacios_gonzalez-prelcic_2023, title={Hybrid mmWave MIMO Systems Under Hardware Impairments and Beam Squint: Channel Model and Dictionary Learning-Aided Configuration}, volume={22}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2023.3246941}, DOI={10.1109/TWC.2023.3246941}, abstractNote={Low overhead channel estimation based on compressive sensing (CS) has been widely investigated for hybrid wideband millimeter wave (mmWave) multiple-input multiple-output (MIMO) systems. The channel sparsifying dictionaries used in prior work are built from ideal array response vectors evaluated on discrete angles of arrival/departure. In addition, these dictionaries are assumed to be the same for all subcarriers, without considering the impacts of hardware impairments and beam squint. In this manuscript, we derive a general channel and signal model that explicitly incorporates the impacts of hardware impairments, practical pulse shaping functions, and beam squint, overcoming the limitations of mmWave MIMO channel and signal models commonly used in previous work. Then, we propose a dictionary learning (DL) algorithm to obtain the sparsifying dictionaries embedding hardware impairments, by considering the effect of beam squint without introducing it into the learning process. We also design a novel CS channel estimation algorithm under beam squint and hardware impairments, where the channel structures at different subcarriers are exploited to enable channel parameter estimation with low complexity and high accuracy. Numerical results demonstrate the effectiveness of the proposed DL and channel estimation strategy when applied to realistic mmWave channels.}, number={10}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Xie, Hongxiang and Palacios, Joan and Gonzalez-Prelcic, Nuria}, year={2023}, month={Oct}, pages={6898–6913} }
 @article{bayraktar_rusu_gonzalez-prelcic_zhang_2022, title={Beam Codebook Design for Joint Initial Access and Localization in mmWave Networks}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF56349.2022.10052101}, abstractNote={Wireless networks are incorporating higher frequency bands and higher bandwidths by exploiting MIMO technology with large arrays. These large arrays and bandwidths enable high resolution estimates of the angles and delays associated to the different multipath components of the MIMO channel. Given the sparse nature of the millimeter wave (mmWave) channel, sparse recovery algorithms can extract the path parameters with reasonable accuracy. Moreover, channel sparsity also facilitates the association of these multipath components to the geometry of the environment, providing sufficient information to determine the user position. In this paper, we address the problem of designing the set of training precoders and combiners that, while providing a high accuracy channel and position estimate, also result in a reduced training overhead with respect to standardized beam training strategies. As performance metric, we consider the mutual coherence between the training hybrid precoders/combiners and the overcomplete dictionary used to represent the channel. The proposed scheme significantly reduces overhead and outperforms previous designs in terms of the accuracy of the channel estimate, which results in a higher localization accuracy and a higher spectral efficiency.}, journal={2022 56TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Bayraktar, Murat and Rusu, Cristian and Gonzalez-Prelcic, Nuria and Zhang, Charlie Jianzhong}, year={2022}, pages={919–924} }
 @article{barneto_riihonen_liyanaarachchi_heino_gonzalez-prelcic_valkama_2022, title={Beamformer Design and Optimization for Joint Communication and Full-Duplex Sensing at mm-Waves}, volume={70}, ISSN={["1558-0857"]}, url={https://doi.org/10.1109/TCOMM.2022.3218623}, DOI={10.1109/TCOMM.2022.3218623}, abstractNote={In this article, we study the joint communication and sensing (JCAS) paradigm in the context of millimeter-wave (mm-wave) mobile communication networks. We specifically address the JCAS challenges stemming from the full-duplex operation in monostatic orthogonal frequency-division multiplexing (OFDM) radars and from the co-existence of multiple simultaneous beams for communications and sensing purposes. To this end, we first formulate and solve beamforming optimization problems for hybrid beamforming based multiuser multiple-input and multiple-output JCAS systems. The cost function to be maximized is the beamformed power at the sensing direction while constraining the beamformed power at the communications directions, suppressing interuser interference and cancelling full-duplexing related self-interference (SI). We then also propose new transmitter and receiver beamforming solutions for purely analog beamforming based JCAS systems that maximize the beamforming gain at the sensing direction while controlling the beamformed power at the communications direction(s), cancelling the SI as well as eliminating the potential reflection from the communication direction and optimizing the combined radar pattern (CRP). Both closed-form and numerical optimization based formulations are provided. We analyze and evaluate the performance through extensive numerical experiments, and show that substantial gains and benefits in terms of radar transmit gain, CRP, and SI suppression can be achieved with the proposed beamforming methods.}, number={12}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Barneto, Carlos Baquero and Riihonen, Taneli and Liyanaarachchi, Sahan Damith and Heino, Mikko and Gonzalez-Prelcic, Nuria and Valkama, Mikko}, year={2022}, month={Dec}, pages={8298–8312} }
 @article{lopez-valcarce_gonzalez-prelcic_2022, title={Hybrid Beamforming Designs for Frequency-Selective mmWave MIMO Systems With Per-RF Chain or Per-Antenna Power Constraints}, volume={21}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2022.3142959}, DOI={10.1109/TWC.2022.3142959}, abstractNote={Configuring precoders and combiners is a major challenge to deploy practical multiple-input multiple-output (MIMO) millimeter wave (mmWave) communication systems with large antenna arrays. Most prior work addresses the problem focusing on a total transmit power constraint. In practical transmitters, however, power amplifiers must operate within their linear range, so that a power constraint applies to each one of the input signals to these devices. Therefore, precoder and combiner designs should incorporate per-antenna or per-radio frequency (RF) chain transmit power constraints. We focus on such problem for frequency-selective channels with multicarrier modulation, and assuming hybrid analog/digital architectures based on fully connected analog blocks implemented with finite-resolution phase shifters. We first derive an all-digital solution which aims to maximize spectral efficiency. Then, we develop hybrid precoders and combiners by approximately matching the corresponding all-digital matrices while still enforcing the power constraints. Numerical results show that the proposed all-digital design performs close to the upper bound given by the standard waterfilling-based solution with a total power constraint. Additionally, the hybrid designs exhibit a moderate loss even when low-resolution phase shifters are considered.}, number={8}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Lopez-Valcarce, Roberto and Gonzalez-Prelcic, Nuria}, year={2022}, month={Aug}, pages={5770–5784} }
 @article{alves_correa_gonzalez-prelcic_klautau_2021, title={Deep Transfer Learning for Site-Specific Channel Estimation in Low-Resolution mmWave MIMO}, volume={10}, ISSN={["2162-2345"]}, DOI={10.1109/LWC.2021.3069199}, abstractNote={We consider the problem of channel estimation in low-resolution multiple-input multiple-output (MIMO) systems operating at millimeter wave (mmWave) and present a deep transfer learning (DTL) approach that exploits previously trained models to speed up site adaptation. The proposed model is composed of a feature extractor and a regressor, with only the regressor requiring training for the new environment. The DTL approach is evaluated using two 3D scenarios where ray-tracing is performed to generate the mmWave MIMO channels used in the simulations. Under the defined testing setup, the proposed DTL approach can reduce the computational cost of the training stage without decreasing the estimation accuracy.}, number={7}, journal={IEEE WIRELESS COMMUNICATIONS LETTERS}, author={Alves, Wesin and Correa, Ilan and Gonzalez-Prelcic, Nuria and Klautau, Aldebaro}, year={2021}, month={Jul}, pages={1424–1428} }
 @article{wang_myers_gonzalez-prelcic_heath_2021, title={Site-Specific Online Compressive Beam Codebook Learning in mmWave Vehicular Communication}, volume={20}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2020.3047547}, DOI={10.1109/TWC.2020.3047547}, abstractNote={Millimeter wave (mmWave) communication is one viable solution to support Gbps sensor data sharing in vehicular networks. The use of large antenna arrays at mmWave and high mobility in vehicular communication make it challenging to design fast beam alignment solutions. In this paper, we propose a novel framework that learns the channel angle-of-departure (AoD) statistics at a base station (BS) and uses this information to efficiently acquire channel measurements. Our framework integrates online learning for compressive sensing (CS) codebook learning and the optimized codebook is used for CS-based beam alignment. We formulate a CS matrix optimization problem based on the AoD statistics available at the BS. Furthermore, based on the CS channel measurements, we develop techniques to update and learn such channel AoD statistics at the BS. We use the upper confidence bound (UCB) algorithm to learn the AoD statistics and the CS matrix. Numerical results show that the CS matrix in the proposed framework provides faster beam alignment than standard CS matrix designs. Simulation results indicate that the proposed beam training technique can reduce overhead by 80% compared to exhaustive beam search, and 70% compared to standard CS solutions that do not exploit any AoD statistics.}, number={5}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Wang, Yuyang and Myers, Nitin Jonathan and Gonzalez-Prelcic, Nuria and Heath, Robert W., Jr.}, year={2021}, month={May}, pages={3122–3136} }
 @inproceedings{zheng_ali_gonzalez-prelcic_heath_klautau_pari_2020, title={5G V2X communication at millimeter wave: Rate maps and use cases}, volume={2020-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85088315072&partnerID=MN8TOARS}, DOI={10.1109/VTC2020-Spring48590.2020.9128612}, abstractNote={Millimeter wave (mmWave) has the potential to offer high data rates for vehicle-to-everything (V2X) communication. In this paper, we provide an introduction to important use cases of V2X as they pertain to fifth generation (5G) communication networks. As 5G technology is still evolving to support vehicles, and mmWave is reflected and blocked by vehicles, it remains unclear if the target rates for the use cases can be achieved. Motivated by the different data rate requirements, we introduce a methodology for evaluating rates in 5G mmWave V2X scenarios. Our approach leverages available city CAD models, realistic traffic simulators, and industry standard ray tracing tools to allow site-specific propagation evaluation. This approach may be used to devise insight into the role of traffic density, antenna array placement, and base station density in important urban propagation settings. Results are provided that highlight the application to develop a rate map for an urban intersection. They show that rate increases in dense deployments by more than ten percent per base station, but only decreases about one percent when going from light to heavy traffic.}, booktitle={IEEE Vehicular Technology Conference}, author={Zheng, W. and Ali, A. and Gonzalez-Prelcic, N. and Heath, R.W. and Klautau, A. and Pari, E.M.}, year={2020} }
 @article{yuan_liu_masouros_yuan_ng_gonzalez-prelcic_2021, title={Bayesian Predictive Beamforming for Vehicular Networks: A Low-Overhead Joint Radar-Communication Approach}, volume={20}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2020.3033776}, DOI={10.1109/TWC.2020.3033776}, abstractNote={The development of dual-functional radar-communication (DFRC) systems, where vehicle localization and tracking can be combined with vehicular communication, will lead to more efficient future vehicular networks. In this paper, we develop a predictive beamforming scheme in the context of DFRC systems. We consider a system model where the road-side unit estimates and predicts the motion parameters of vehicles based on the echoes of the DFRC signal. Compared to the conventional feedback-based beam tracking approaches, the proposed method can reduce the signaling overhead and improve the accuracy of the angle estimation. To accurately estimate the motion parameters of vehicles in real-time, we propose a novel message passing algorithm based on factor graph, which yields a near optimal performance achieved by the maximum a posteriori estimation. The beamformers are then designed based on the predicted angles for establishing the communication links. With the employment of appropriate approximations, all messages on the factor graph can be derived in a closed-form, thus reduce the complexity. Simulation results show that the proposed DFRC based beamforming scheme is superior to the feedback-based approach in terms of both estimation and communication performance. Moreover, the proposed message passing algorithm achieves a similar performance of the high-complexity particle filtering-based methods.}, number={3}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Yuan, Weijie and Liu, Fan and Masouros, Christos and Yuan, Jinhong and Ng, Derrick Wing Kwan and Gonzalez-Prelcic, Nuria}, year={2021}, month={Mar}, pages={1442–1456} }
 @inproceedings{myers_wang_gonzalez-prelcic_heath_2020, title={Deep Learning-Based Beam Alignment in Mmwave Vehicular Networks}, volume={2020-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85089217043&partnerID=MN8TOARS}, DOI={10.1109/ICASSP40776.2020.9054075}, abstractNote={Millimeter wave channels exhibit structure that allows beam alignment with fewer channel measurements than exhaustive beam search. From a compressed sensing (CS) perspective, the received channel measurements are usually obtained by multiplying a CS matrix with a sparse representation of the channel matrix. Due to the constraints imposed by analog frontends, designing CS matrices that efficiently exploit the channel structure is challenging. In this paper, we propose an end-to-end deep learning technique to design a structured CS matrix that is well suited to the underlying channel distribution, leveraging both sparsity and the particular spatial structure that appears in vehicular channels. The channel measurements acquired with the designed CS matrix are then used to predict the best beam for link configuration. Simulation results for vehicular communication channels indicate that our deep learning-based approach achieves better beam alignment than standard CS techniques that use the random phase shift-based design.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Myers, N.J. and Wang, Y. and Gonzalez-Prelcic, N. and Heath, R.W.}, year={2020}, pages={8569–8573} }
 @article{xie_gonzalez-prelcic_2020, title={Dictionary Learning for Channel Estimation in Hybrid Frequency-Selective mmWave MIMO Systems}, volume={19}, url={https://doi.org/10.1109/TWC.2020.3011126}, DOI={10.1109/TWC.2020.3011126}, abstractNote={Exploiting channel sparsity at millimeter wave (mmWave) frequencies reduces the high training overhead associated with the channel estimation stage. Compressive sensing (CS) channel estimation techniques usually adopt the (overcomplete) Fourier transform matrix as sparsifying dictionary. This may not be the best choice when considering hardware impairments in practical arrays. We propose two dictionary learning (DL) algorithms to learn the best sparsifying dictionaries for channel matrices from observations obtained with practical hybrid frequency-selective mmWave multiple-input-multiple-output (MIMO) systems. First, we optimize the combined dictionary, i.e., the Kronecker product of transmit and receive dictionaries, as it is used in practice to sparsify the channel matrix. This stage operates as a calibration phase, since all the hardware imperfections are embedded into the learnt dictionaries. Second, considering the different array structures at the transmitter and receiver, we exploit separable DL to find the best transmit and receive dictionaries. Once the channel is expressed in terms of the optimized dictionaries, various CS-based sparse recovery techniques can be applied for low overhead channel estimation. The effectiveness of the proposed DL algorithms under low SNR conditions has been corroborated via numerical simulations with different system configurations, array geometries and hardware impairments.}, number={11}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Xie, Hongxiang and Gonzalez-Prelcic, Nuria}, year={2020}, month={Nov}, pages={7407–7422} }
 @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{ali_gonzalez-prelcic_ghosh_2020, title={Passive Radar at the Roadside Unit to Configure Millimeter Wave Vehicle-to-Infrastructure Links}, volume={69}, ISSN={["1939-9359"]}, url={https://doi.org/10.1109/TVT.2020.3027636}, DOI={10.1109/TVT.2020.3027636}, abstractNote={Millimeter wave (mmWave) vehicular channels are highly dynamic, and the communication link needs to be reconfigured frequently. In this work, we propose to use a passive radar receiver at the roadside unit to reduce the training overhead of establishing an mmWave communication link. Specifically, the passive radar will tap the transmissions from the automotive radars of the vehicles on the road. The spatial covariance of the received radar signals will be estimated and used to establish the communication link. We propose a simplified radar receiver that does not require the transmitted waveform as a reference. To leverage the radar information for beamforming, the radar azimuth power spectrum (APS) and the communication APS should be similar. We outline a radar covariance correction strategy to increase the similarity between the radar and communication APS. We also propose a metric to compare the similarity of the radar and communication APS that has a connection with the achievable rate. We present simulation results based on ray-tracing data. The results show that: (i) covariance correction improves the similarity of radar and communication APS, and (ii) the radar-assisted strategy significantly reduces the training overhead, being particularly useful in non-line-of-sight scenarios.}, number={12}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Ali, Anum and Gonzalez-Prelcic, Nuria and Ghosh, Amitava}, year={2020}, month={Dec}, pages={14903–14917} }
 @article{gonzalez-prelcic_xie_palacios_shimizu_2021, title={Wideband Channel Tracking and Hybrid Precoding for mmWave MIMO Systems}, volume={20}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2020.3033514}, DOI={10.1109/TWC.2020.3033514}, abstractNote={A major source of difficulty when operating with large arrays at millimeter wave (mmWave) frequencies is to estimate the wideband channel, since the use of hybrid architectures acts as a compression stage for the received signal. Moreover, the channel has to be tracked and the antenna arrays regularly reconfigured to obtain appropriate beamforming gains when a mobile setting is considered. In this paper, we focus on the problem of channel tracking for frequency-selective mmWave channels, and propose two novel channel tracking algorithms. One of them exploits the sparsity of the mmWave channel, while the other one leverages prior statistical information about the channel parameters. We also propose a hybrid precoder and combiner design method to increase the received signal-to-noise ratio (SNR) during channel tracking, such that near-optimum data rates can be obtained with low-overhead. In our numerical results, we analyze the performance of our proposed algorithms for different system parameters. Simulation results show that our proposed channel tracking algorithms are able to achieve near-optimum data rates outperforming state-of-the-art methods.}, number={4}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Gonzalez-Prelcic, Nuria and Xie, Hongxiang and Palacios, Joan and Shimizu, Takayuki}, year={2021}, month={Apr}, pages={2161–2174} }
 @inproceedings{lopez-valcarce_gonzalez-prelcic_2019, title={Analog beamforming for full-duplex millimeter wave communication}, volume={2019-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074633972&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2019.8877288}, abstractNote={Full-duplex (FD) communication has the potential for significant improvements in spectral efficiency, as long as the problem of self-interference (SI) can be overcome. For millimeter wave (mmWave) systems with large antenna arrays, beamforming based SI mitigation is attractive because of the large number of degrees of freedom available. Previously proposed precoder- combiner designs, however, suffer from a large performance loss under the hardware-constrained architectures required for operation at mmWave. In this context, we develop a new algorithm for the design of constant-amplitude analog precoders and combiners of an FD mmWave single-stream bidirectional link, significantly reducing the aforementioned performance loss.}, booktitle={Proceedings of the International Symposium on Wireless Communication Systems}, author={Lopez-Valcarce, R. and Gonzalez-Prelcic, N.}, year={2019}, pages={687–691} }
 @inproceedings{ali_gonzalez-prelcic_ghosh_2019, title={Automotive radar radiations as signals of opportunity for millimeter wave V2I links}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083316224&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9048967}, abstractNote={Millimeter wave (mmWave) vehicular channels are highly dynamic, and the communication link needs to be reconfigured frequently. In this work, we propose to use a passive radar receiver at the roadside unit to reduce the training overhead of establishing an mmWave communication link. Specifically, the passive radar will tap the transmissions from the automotive radars of the vehicles on the road. The spatial covariance of the received radar signals will be estimated and used to establish the communication link. We propose a simplified radar receiver that does not require the transmitted waveform as a reference. We also present simulation results based on ray-tracing data. The results show that the radar-assisted strategy significantly reduces the training overhead, being particularly useful in non-line-of-sight scenarios.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Ali, A. and Gonzalez-Prelcic, N. and Ghosh, A.}, year={2019}, pages={554–558} }
 @inproceedings{lopez-valcarce_gonzalez-prelcic_2019, title={Beamformer design for full-duplex amplify-and-forward millimeter wave relays}, volume={2019-August}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074653567&partnerID=MN8TOARS}, DOI={10.1109/ISWCS.2019.8877269}, abstractNote={We consider the design of Amplify & Forward Full-Duplex (FD) relay-assisted communication systems. The FD mode has potential for significant improvements in spectral efficiency, but it suffers from large self-interference (SI) levels. Beamforming-based SI mitigation is attractive for millimeter wave (mmWave) systems due to the large number of degrees of freedom available with large antenna arrays. We first develop an all-digital beamformer design by imposing a zero-forcing constraint on SI, performing quasi-optimally in terms of spectral efficiency. The design is then modified to fit the hardware-related constraints usually found when operating at mmWave, so that the beamformers can be implemented in the analog domain by using phase shifters, with an acceptable performance loss.}, booktitle={Proceedings of the International Symposium on Wireless Communication Systems}, author={Lopez-Valcarce, R. and Gonzalez-Prelcic, N.}, year={2019}, pages={86–90} }
 @article{rodriguez-fernandez_gonzalez-prelcic_2019, title={Channel Estimation for Hybrid mmWave MIMO Systems With CFO Uncertainties}, volume={18}, url={https://doi.org/10.1109/TWC.2019.2924004}, DOI={10.1109/TWC.2019.2924004}, abstractNote={Channel estimation at millimeter wave (mmWave) allows designing hybrid precoders and combiners to optimize performance metrics, such as the spectral efficiency. One major source of difficulty when estimating the high-dimensional and large-bandwidth channel, however, is the carrier frequency offset (CFO) that impairs the received signal. Most of the prior work assumes perfect time–frequency synchronization when estimating the channel. However, in practical systems, this assumption does not hold, and joint time–frequency synchronization is an important issue to deal with. In this paper, we propose a multi-stage algorithm for joint CFO and channel estimation. Using a training protocol between a transmitter and a receiver, we estimate first the equivalent low-SNR beamformed channel and the CFO, using the maximum-likelihood (ML) criterion. Thereafter, using these estimates, the high-dimensional multiple-input multiple-output (MIMO) channel is estimated leveraging its angular sparsity. The simulation results show that, using our proposed strategy, small estimation errors and near-optimal values of spectral efficiency can be achieved, without incurring in significant overhead and/or computational complexity.}, number={10}, journal={IEEE Transactions on Wireless Communications}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Rodriguez-Fernandez, Javier Javier and Gonzalez-Prelcic, Nuria}, year={2019}, month={Oct}, pages={4636–4652} }
 @inproceedings{frequency-selective hybrid precoding and combining for mmwave mimo systems with per-antenna power constraints_2019, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85069475587&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2019.8683831}, abstractNote={Configuring hybrid precoders and combiners is the main challenge to be solved to operate at millimeter wave (mmWave) frequencies. The use of hybrid architectures imposse hardware constraints on the analog precoder that need to be carefully dealt with. In this paper, we develop hybrid precoders and combiners aiming at minimizing the Euclidean distance with respect to the approximate all-digital precoders and combiners maximizing the spectral efficiency under per-antenna power constraints. Numerical results demonstrate the effectiveness of the proposed design method, whose performance is close to that of the all-digital solution.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2019}, pages={4794–4798} }
 @inproceedings{palacios_rodriguez-fernandez_gonzalez-prelcic_2019, title={Hybrid precoding and combining for full-duplex millimeter wave communication}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85081948437&partnerID=MN8TOARS}, DOI={10.1109/GLOBECOM38437.2019.9013484}, abstractNote={Full-duplex (FD) communication is an enabling technology to increase spectral efficiency. The self- interference (SI) resulting from sharing time and frequency resources between different transceivers in FD mode must, however, be managed. At millimeter wave (mmWave) frequencies, FD communication is different than at sub-6 GHz frequencies, because non conventional MIMO architectures and large antenna arrays are often used. In this paper, we address a major challenge for FD mmWave systems: the design of hybrid precoders and combiners that simultaneously maximize the sum spectral efficiency and cancel the SI in the analog domain, to keep control of the signal level at the input of the analog- to-digital converters (ADCs). The optimal joint design is a very difficult problem, since it involves the optimization of eight precoding/combining matrices with several constraints, some of them non convex. We derive two suboptimal solutions which exhibit near- optimum spectral efficiency and significantly outperform prior work in terms of SI cancelation.}, booktitle={2019 IEEE Global Communications Conference, GLOBECOM 2019 - Proceedings}, author={Palacios, J. and Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N.}, year={2019} }
 @inproceedings{zheng_gonzalez-prelcic_2019, title={Joint Position, Orientation and Channel Estimation in Hybrid mmWAVE MIMO Systems}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083310683&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9048769}, abstractNote={Millimeter wave MIMO communication signals can provide accurate user localization in addition to high data rates. Previous work derives methods for position and orientation estimation assuming, however, unrealistic mmWave MIMO architectures or channel models. In this paper, we consider a band-limited frequency selective channel model and a hybrid MIMO architecture at the base station (BS) and the user equipment (UE). We develop a time domain algorithm that provides an estimation of the channel and the user position and orientation. The proposed approach is based on a compressive channel estimation stage that operates in parallel in the angular and delay domains. Experimental results show that the proposed approach outperforms previous work when operating with hybrid architectures and realistic channel realizations.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Zheng, W. and Gonzalez-Prelcic, N.}, year={2019}, pages={1453–1458} }
 @article{klautau_gonzalez-prelcic_heath_2019, title={LIDAR Data for Deep Learning-Based mmWave Beam-Selection}, volume={8}, url={https://doi.org/10.1109/LWC.2019.2899571}, 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}, month={Jun}, pages={909–912} }
 @inproceedings{palacios_gonzalez-prelcic_widmer_2019, title={Managing Hardware Impairments in Hybrid Millimeter Wave Mimo Systems: A Dictionary Learning-Based Approach}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083336521&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9049078}, abstractNote={Compressed sensing-based strategies have been derived in prior work to reduce training overhead when estimating the high dimensional millimeter wave MIMO channel. These techniques rely on a channel model based on a sparsifying dictionary which does not account for hardware impairments such as calibration errors, mutual coupling effects, or manufacturing errors in the inter-spacing between the array elements. In this paper, we propose a learning strategy for the sparsifying dictionary that considers a channel model with hardware impairments, embedding these effects into the dictionary itself. This way, a sparser representation of the channel can be obtained even when considering realistic implementations of the antenna array and the radio frequency chains. Numerical simulations with different system configurations and parameters of the hardware impairments, show the effectiveness of the proposed dictionary learning algorithm for channel estimation at millimeter wave frequencies with hybrid MIMO architectures.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Palacios, J. and Gonzalez-Prelcic, N. and Widmer, J.}, year={2019}, pages={168–172} }
 @inproceedings{graff_ali_gonzalez-prelcic_2019, title={Measuring radar and communication congruence at millimeter wave frequencies}, volume={2019-November}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083334929&partnerID=MN8TOARS}, DOI={10.1109/IEEECONF44664.2019.9048969}, abstractNote={Millimeter wave (mmWave) communication enables high-rate vehicle-to-infrastructure (V2I) communications by using larger bandwidths, but configuring mmWave links requires significant training overhead. This overhead can be greatly reduced by using either prior knowledge of the channel or information about the target’s location. In this paper, we conduct a measurement based study of vehicle positioning accuracy using roadside unit (RSU) mounted mmWave radar and GPS. Additionally, we analyze the spatial congruence between the communication and radar power spectra from the results of a measurement campaign. Finally, we compare the performance of radar-assisted and GPS-assisted beam-selection.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Graff, A. and Ali, A. and Gonzalez-Prelcic, N.}, year={2019}, pages={925–929} }
 @inproceedings{ali_gonzalez-prelcic_ghosh_2019, title={Millimeter wave V2I beam-training using base-station mounted radar}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073121668&partnerID=MN8TOARS}, DOI={10.1109/RADAR.2019.8835615}, abstractNote={Reducing the beam-training overhead is a major challenge in realizing millimeter wave (mmWave) V2I communication systems. In this work, we use location of the vehicle to reduce the beam-training overhead of line-of-sight V2I links. Specifically, we consider a base-station (BS) supporting an analog mmWave communication system and an off-the-shelf automotive radar. Using Crámer-Rao lower bound on the DoA estimation error of a radar, we establish that an off-the-shelf radar has better localization than global navigation satellite system (GNSS). The spectral efficiency results show that beam-training based on radar based localization is robust to vehicle speeds.}, booktitle={2019 IEEE Radar Conference, RadarConf 2019}, author={Ali, A. and Gonzalez-Prelcic, N. and Ghosh, A.}, year={2019} }
 @article{venugopal_gonzalez-prelcic_heath_2019, title={Optimal Frequency-Flat Precoding for Frequency-Selective Millimeter Wave Channels}, volume={18}, url={https://doi.org/10.1109/TWC.2019.2932932}, 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}, month={Nov}, pages={5098–5112} }
 @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{rodriguez-fernandez_gonzalez-prelcic_pamplona-trindade_klautau_2019, title={Position-Aided Compressive Channel Estimation and Tracking for Millimeter Wave Multi-User MIMO Air-to-Ground Communications}, volume={2019-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072346816&partnerID=MN8TOARS}, DOI={10.1109/SPAWC.2019.8815594}, abstractNote={Configuring the antenna arrays from the channel estimates in millimeter wave (mmWave) communications is challenging in the presence of highly mobile users, as in the case of air-to-ground (A2G) networks. In this paper, we address the problem of channel tracking in the context of a multi-user A2G mmWave network. We propose to obtain channel information using the Maximum Likelihood (ML) criterion, while leveraging the sparse nature of the mmWave channel and using prior position information on the UAVs. Simulation results show that, even under small Rician factors of the A2G channel, obtained from ray-tracing, the proposed algorithm is able to keep track of the multi-user channel and maintain near-optimal data rates, thereby making it a suitable candidate for upcoming cellular-supported mmWave A2G deployments.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N. and Pamplona-Trindade, I. and Klautau, A.}, year={2019} }
 @inproceedings{rodriguez-fernandez_gonzalez-prelcic_shimizu_2019, title={Position-Aided Compressive Channel Tracking for Wideband Millimeter Wave Multi-User Communication}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85070222520&partnerID=MN8TOARS}, DOI={10.1109/ICC.2019.8761967}, abstractNote={A major challenge in millimeter wave (mmWave) communications is to configure the antenna arrays in the transceivers. The mobility of the users in mmWave cellular networks makes it necessary to periodically reconfigure the precoders and combiners, according to the variations of the channel. In this paper, we propose a new strategy to track the channel variations in a multi-user (MU) hybrid mmWave MIMO communication cellular network. Besides exploiting the sparse nature of the mmWave channel, this approach also leverages statistical knowledge of the channel parameters, which enables further overhead reductions. Simulation results show that using the proposed algorithm, it is possible to maintain an effective high data rate even in high mobility scenarios.}, booktitle={IEEE International Conference on Communications}, author={Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N. and Shimizu, T.}, 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} }
 @article{park_ali_gonzalez-prelcic_heath_2020, title={Spatial Channel Covariance Estimation for Hybrid Architectures Based on Tensor Decompositions}, volume={19}, url={https://doi.org/10.1109/TWC.2019.2951135}, 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}, month={Feb}, 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={https://doi.org/10.1109/TWC.2019.2932404}, 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}, month={Dec}, pages={5471–5485} }
 @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{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{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={https://doi.org/10.1109/JSTSP.2018.2819130}, 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 of 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}, month={May}, pages={353–367} }
 @inproceedings{channel estimation for frequency-selective mmwave mimo systems with beam-squint_2018, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063423123&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2018.8647269}, abstractNote={Acquiring channel informaiton is necessary to design the precoders and combiners in hybrid MIMO (Multiple-Input Multiple-Output) systems operating at millimeter wave (mmWave). Most of prior work on channel estimation with hybrid MIMO architectures neglects the beam-squint effect that appears when the system bandwidth is very large. To consider beam-squint, it is necessary to assume frequency- dependent array steering vectors, which further complicates the estimation of the mmWave wideband channel. In this paper, we propose al algorithm that approximates the optimal solution to the dictionary-constrained Maximum-Likelihood (ML) estimator for frequency-selective mmWave channels considering frequency-dependent array responses. Further, we develop a suboptimal channel estimation strategy to consider the beam squint effect as well. Simulation results based on channel realizations extracted from Quadriga show the effectiveness of the proposed methods at the low Signal-to-Noise Ratio (SNR) regime without assuming the channel sparsity is known.}, booktitle={2018 IEEE Global Communications Conference, GLOBECOM 2018 - Proceedings}, year={2018} }
 @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} }
 @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} }
 @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={https://doi.org/10.1109/TWC.2018.2804943}, 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}, month={May}, pages={2946–2960} }
 @inproceedings{rodriguez-fernandez_gonzalez-prelcic_2019, title={Joint Synchronization and Compressive Estimation for Frequency-Selective mmWave MIMO Systems}, volume={2018-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062980180&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2018.8645093}, abstractNote={A major challenge to establish and maintain millimeter wave (mmWave) links amounts as to how to obtain channel information to configure hybrid precoders and combiners in a hybrid architecture. Owing to lack of prior information on the wireless channel, achieving time-frequency synchronization between transmitter and receiver is one of the greatest challenges that needs to be solved in order to acquire Channel State Information (CSI). In this paper, we study and find the Maximum Likelihood (ML) solution to the joint problem of time-frequency synchronization and compressive channel estimation for broadband mmWave MIMO systems with hybrid architectures. Simulation results show that, using as few training symbols as in the 5G New Radio (NR) communications standard, near-optimum data rates can be achieved even at the very low SNR regime, thereby emphasizing the suitability of compressive channel estimation even when synchronization has not been performed at the receiver side.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N.}, year={2019}, pages={1280–1286} }
 @inproceedings{boas_fonseca_klautau_gonzalez-prelcic_2019, title={Low Complexity Transform Coding for Millimeter Wave MIMO CSI Compression}, volume={2018-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062978496&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2018.8645142}, abstractNote={Channel state information (CSI) obtained at the user equipment (UE) can be made available at the base stations (BS) using limited feedback strategies. The design of feedback techniques is critical in millimeter wave (mmWave) multiple-input multiple-output (MIMO) systems, due to the high dimension of the channel matrix. In this paper, we assume that the mmWave MIMO channel has been estimated, and develop a compression method that allows trading off rate and distortion. The proposed method, which exhibits low complexity, is based on transform coding and uses a customized significance map to encode the relevant coefficients without using a fixed ordering scheme, such as the zig-zag scanning used in image coding. We show the performance gain of our method compared to previous algorithms using several datasets of MIMO channels obtained with ray tracing simulations, for mobile and fixed access scenarios.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, author={Boas, B.V. and Fonseca, N. and Klautau, A. and Gonzalez-Prelcic, N.}, year={2019}, pages={1582–1586} }
 @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} }
 @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} }
 @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{gonzalez-coma_rodriguez-fernandez_gonzalez-prelcic_castedo_2017, title={Channel Estimation and Hybrid Precoding/Combining for Frequency Selective Multiuser mmWave Systems}, volume={2018-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85046467360&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2017.8254863}, abstractNote={In this work we consider the problem of designing the precoders and combiners in a multiuser hybrid frequency selective millimeter wave (mmWave) system. We propose two methods to design the hybrid analog-digital filters when only an estimate of the channel is available at both the base station (BS) and the users. The first one is a projected gradient method that factorizes the all-digital solution, while the second approach is based on an alternating minimization (AM) iterative strategy. The channel is estimated using an algorithm that takes into account the hardware limitations of the hybrid architecture and leverages the sparse structure of mmWave channels. The proposed methods are evaluated in terms of achievable sum rate, outperforming the state of the art solutions. The negligible impact of the hybrid structure and the imperfect channel knowledge on the system performance are shown via numerical experiments.}, booktitle={2017 IEEE Global Communications Conference, GLOBECOM 2017 - Proceedings}, author={Gonzalez-Coma, J.P. and Rodriguez-Fernandez, J. and Gonzalez-Prelcic, N. and Castedo, L.}, year={2017}, pages={1–6} }
 @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} }
 @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{kumari_choi_gonzalez-prelcic_heath_2018, title={IEEE 802.11ad-Based Radar: An Approach to Joint Vehicular Communication-Radar System}, volume={67}, url={https://doi.org/10.1109/TVT.2017.2774762}, 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}, month={Apr}, pages={3012–3027} }
 @article{ali_gonzalez-prelcic_heath_2018, title={Millimeter Wave Beam-Selection Using Out-of-Band Spatial Information}, volume={17}, url={https://doi.org/10.1109/TWC.2017.2773532}, 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}, month={Feb}, 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{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{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} }
 @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} }
 @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{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} }
 @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} }
 @article{designing incoherent frames through convex techniques for optimized compressed sensing_2016, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84963988165&partnerID=MN8TOARS}, DOI={10.1109/TSP.2016.2521607}, abstractNote={The construction of highly incoherent frames, sequences of vectors placed on the unit hyper sphere of a finite dimensional Hilbert space with low correlation between them, has proven very difficult. Algorithms proposed in the past have focused on minimizing the absolute value off-diagonal entries of the Gram matrix of these structures. Recently, a method based on convex optimization that operates directly on the vectors of the frame has been shown to produce promising results. This paper gives a detailed analysis of the optimization problem at the heart of this approach and, based on these insights, proposes a new method that substantially outperforms the initial approach and all current methods in the literature for all types of frames, with low and high redundancy. We give extensive experimental results that show the effectiveness of the proposed method and its application to optimized compressed sensing.}, number={9}, journal={IEEE Transactions on Signal Processing}, year={2016}, pages={2334–2344} }
 @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} }
 @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{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{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{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{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} }
 @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{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} }
 @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{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{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{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{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} }
 @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} }
 @inproceedings{dommel_boccolini_raschkowski_jaeckel_thiele_haustein_prelcic_2014, title={5G in space: PHY-Layer design for satellite communications using non-orthogonal multi-carrier transmission}, volume={2014-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84984984829&partnerID=MN8TOARS}, DOI={10.1109/ASMS-SPSC.2014.6934543}, abstractNote={For the 5th generation of terrestrial mobile communications, Multi-Carrier (MC) transmission based on non-orthogonal waveforms is a promising technology component compared to orthogonal frequency division multiplex (OFDM) in order to achieve higher throughput and enable flexible spectrum management. Coverage extension and service continuity can be provided considering satellites as additional components in future networks by allowing vertical handover to terrestrial radio interfaces. In this paper, the properties of Filter Bank Multicarrier (FBMC) as potential MC transmission scheme is discussed taking into account the requirements for the satellite-specific PHY-Layer like non-linear distortions due to High Power Amplifiers (HPAs). The performance for specific FBMC configurations is analyzed in terms of peak-to-average power ratio (PAPR), computational complexity, non-linear distortions as well as carrier frequency offsets sensitivity (CFOs). Even though FBMC and OFDM have similar PAPR and suffer comparable spectral regrowth at the output of the non linear amplifier, simulations on link level show that FBMC still outperforms OFDM in terms of CFO sensitivity and symbol error rate in the presence of non-linear distortions.}, booktitle={2014 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space Communications Workshop, ASMS/SPSC 2014}, author={Dommel, J. and Boccolini, G. and Raschkowski, L. and Jaeckel, S. and Thiele, L. and Haustein, T. and Prelcic, N.G.}, year={2014}, pages={190–196} }
 @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{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} }
 @inproceedings{gonzález-prelcic_domínguez-jiménez_2014, title={Circular sparse rulers based on co-prime sampling for compressive power spectrum estimation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988227706&partnerID=MN8TOARS}, DOI={10.1109/GLOCOM.2014.7037272}, abstractNote={One of the main challenges in cognitive radio systems is the development of fast algorithms for spectrum sensing of wideband signals. To alleviate sampling requirements, several strategies have been proposed, relying on the fact that only covariance information is of interest. Under this premise, this paper solves the design of low rate covariance samplers by developing a multicoset sampling strategy based on co-prime samplers, achieving higher compression ratios than previous solutions. The designed sampling patterns constitute a family of circular sparse rulers, which can be implemented with only two analog-to-digital converters working in parallel, and operating at sampling frequencies significantly lower than the Nyquist rate.}, booktitle={2014 IEEE Global Communications Conference, GLOBECOM 2014}, author={González-Prelcic, N. and Domínguez-Jiménez, M.E.}, year={2014}, pages={3044–3050} }
 @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{tausiesakul_gonzalez-prelcic_2014, title={Power spectrum blind sampling using optimal multicoset sampling patterns in the MSE sense}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905230295&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2014.6853758}, abstractNote={We consider the design of a multicoset sampling pattern to be used in power spectrum blind sampling (PSBS). The criterion for the PSBS pattern design that we propose is based on the minimization of the mean square error of the power spectrum estimate. The design framework appears as a constrained optimization problem, whose complexity increases with the pattern length. We solve such a constrained optimization problem in terms of nonlinear integer programming by using exhaustive search.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Tausiesakul, B. and Gonzalez-Prelcic, N.}, year={2014}, pages={1055–1059} }
 @inproceedings{méndez-rial_caurel-carrera_gonzález-prelcic_2014, title={Sensitivity of compressive sensing architectures based on time interleaved analog-to-digital converters to channel mismatches}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84914688262&partnerID=MN8TOARS}, DOI={10.1109/NEWCAS.2014.6933981}, abstractNote={Recent advances in the context of compressed sensing (CS) have lead to the development of a wide variety of applications that process sparse signals from sub-Nyquist sampled sequences. However, only a few architectures have been proposed to acquire sparse analog signals, and their sensitivity to impairments in the hardware components of the systems has not been studied in detail. In this paper we study the performance of analog to information converters based on time interleaved analog to digital converters (TI-ADCs) when real components are used as the core of the sampling devices. Mathematical models for the different TI-ADC error sources are presented, and numerical simulations of A2I converters based on commercially available TI-ADCs show the influence of these parameters on the quality of the reconstructed signal.}, booktitle={2014 IEEE 12th International New Circuits and Systems Conference, NEWCAS 2014}, author={Méndez-Rial, R. and Caurel-Carrera, J.M. and González-Prelcic, N.}, year={2014}, pages={45–48} }
 @inproceedings{dominguez-jimenez_gonzalez-prelcic_2013, title={A class of circular sparse rulers for compressive power spectrum estimation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901365760&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, author={Dominguez-Jimenez, M.E. and Gonzalez-Prelcic, N.}, year={2013} }
 @inproceedings{power spectrum blind sampling using minimum mean square error and weighted least squares_2013, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901252003&partnerID=MN8TOARS}, DOI={10.1109/ACSSC.2013.6810249}, abstractNote={We present a new power spectrum recovery method in the context of power spectrum blind sampling. As sampling device we propose a multicoset sampler, which provides sub-Nyquist rate samples. A weighted least squares (WLS) criterion is adopted with the aim to define a power spectrum recovery algorithm that minimizes the mean square error (MSE) of the correlation estimate of the input signal. It is analytically shown that the optimal weighting matrix is equal to the inverse of the covariance matrix of the correlation estimate of the sub-Nyquist rate samples. The derived weight can also be shown to be optimal in MSE sense for power spectrum estimation. We also provide an optimization framework for the design of multicoset sampling patterns that minimize the MSE of the compressive WLS power spectrum estimator. The resulting integer nonlinear programming problem is solved by using exhaustive search.}, booktitle={Conference Record - Asilomar Conference on Signals, Systems and Computers}, year={2013}, pages={153–157} }
 @inproceedings{domínguez-jitnénez_gonzález-prelcic_2012, title={Analysis and design of multirate synchronous sampling schemes for sparse multiband signals}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869776929&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, author={Domínguez-Jitnénez, M.E. and González-Prelcic, N.}, year={2012}, pages={1184–1188} }
 @inproceedings{domínguez-jiménez_gonzález-prelcic_vazquez-vilar_lópez-valcarce_2012, title={Design of universal multicoset sampling patterns for compressed sensing of multiband sparse signals}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867606886&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2012.6288630}, abstractNote={Many problems in digital communications involve wideband radio signals. As the most recent example, the impressive advances in Cognitive Radio systems make even more necessary the development of sampling schemes for wideband radio signals with spectral holes. This is equivalent to considering a sparse multiband signal in the framework of Compressive Sampling theory. Starting from previous results on multicoset sampling and recent advances in compressive sampling, we analyze the matrix involved in the corresponding reconstruction equation and define a new method for the design of universal multicoset codes, that is, codes guaranteeing perfect reconstruction of the sparse multiband signal.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Domínguez-Jiménez, M.E. and González-Prelcic, N. and Vazquez-Vilar, G. and López-Valcarce, R.}, year={2012}, pages={3337–3340} }
 @inproceedings{wideband spectral estimation from compressed measurements exploiting spectral a priori information in cognitive radio systems_2010, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78049400013&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2010.5496152}, abstractNote={In Cognitive Radio scenarios channelization information from primary network may be available to the spectral monitor. Under this assumption we propose a spectral estimation algorithm from compressed measurements of a multichannel wideband signal. The analysis of the Cramer-Rao Lower Bound (CRLB) for this estimation problem shows the importance of detecting the underlaying sparsity pattern of the signal. To this end we describe a Bayesian based iterative algorithm that discovers the set of active signals conforming the band and simultaneously reconstructs the spectrum. This iterative spectral estimator is shown to perform close to a Genie-Aided CRLB that includes full knowledge about the sparsity pattern of the channels.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, year={2010}, pages={2958–2961} }
 @article{jiménez_prelcic_2004, title={Linear boundary extensions for finite length signals and paraunitary two-channel filterbanks}, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-7544241230&partnerID=MN8TOARS}, DOI={10.1109/TSP.2004.836526}, abstractNote={In this paper, we introduce a novel and general matrix formulation of artificial linear boundary extension methods for removing border effects inherent to any paraunitary two-channel size-limited filterbank. This new characterization of the transformation operator allows us to prove that perfect reconstruction (PR) of finite signals can be ensured under some conditions without using extra subband coefficients; in other words, we characterize the signal extension methods that lead to nonexpansive transforms. The necessary and sufficient condition we find allows us to show that some traditional extension techniques that are being used in an expansive way, such as the polynomial extension, lead in fact to nonexpansive invertible transforms; moreover, we can also prove that in contradiction to previous literature, not every transformation matrix associated with a linear extension is invertible even if using prototype filters of the same length. Apart from these invertibility criteria, we propose the first algorithm for the design of all linear extensions and their associated biorthogonal boundary filters that lead to nonexpansive and invertible transforms. Analogously, we provide the first method for the design of all linear extensions that yield orthogonal transforms: We construct an infinite number of orthogonal extensions, apart from the commonly used periodic extension, and their associated orthogonal boundary filters. The final contribution of the paper is a new algorithm for the design of smooth orthogonal extensions, which keep the orthogonality property and overcome the main drawback of periodization, that is, the introduction of subband coefficients of great amplitude near the boundaries in the transform domain.}, number={11}, journal={IEEE Transactions on Signal Processing}, author={Jiménez, M.E.D. and Prelcic, N.G.}, year={2004}, pages={3213–3226} }
 @inproceedings{dominguez jimenez_gonzalez prelcic_2000, title={Design of non-expansionist and orthogonal extension methods for tree-structured filter banks}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033707911&partnerID=MN8TOARS}, DOI={10.1109/ICASSP.2000.862035}, abstractNote={Signal extension methods have been extensively used when applying filter banks to finite length sequences. However, not every extension guarantees perfect reconstruction without considering extra subband samples, that is, in some cases the associated transform is expansionist. In this paper we introduce first the most general method for designing signal extensions which yield non-expansionist subband transforms. Secondly, all orthogonal extension methods are constructed. We conclude by analyzing some examples of the new boundary filters associated to the proposed extensions.}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Dominguez Jimenez, M.E. and Gonzalez Prelcic, N.}, year={2000}, pages={532–535} }
 @inproceedings{jimenez_prelcic_2002, title={Polynomial extension method for size-limited paraunitary filter banks}, volume={2002-March}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84960896268&partnerID=MN8TOARS}, booktitle={European Signal Processing Conference}, author={Jimenez, M.E.D. and Prelcic, N.G.}, year={2002} }
 @inproceedings{jiménez_prelcic_2002, title={Smooth orthogonal signal extensions for paraunitary tree-structured filter banks}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036297198&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Jiménez, M.E.D. and Prelcic, N.G.}, year={2002} }
 @article{prelcic_pena_2001, title={Adaptive tiling of the time-frequency plane with application to multiresolution-based perceptive audio coding}, volume={81}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035251036&partnerID=MN8TOARS}, DOI={10.1016/S0165-1684(00)00209-7}, abstractNote={The research efforts recently devoted to the theoretical treatment of time-varying filter bank structures have created an adequate environment for the development of adaptive analysis tools, useful in many signal processing problems. In this paper we present a complete adaptive analysis system suitable for the compression of audio signals; we would like to remark that our goal is not to develop a complete audio coder, just the appropriate analysis/synthesis scheme. For basis selection, we propose a search algorithm and a new cost function based on perceptual considerations. In addition, a procedure that determines the optimum length of the analysis segment is described. Examples are given of the performance of these algorithms on some representative audio signals. When the search method is compared with previous basis selection algorithms, we find and show that, for the application of audio compression, the perceptually driven basis selection algorithm leads to a much higher compression efficiency and better quality of the reconstructed signal.}, number={2}, journal={Signal Processing}, author={Prelcic, N.G. and Pena, A.S.}, year={2001}, pages={301–319} }
 @inproceedings{domínguez jiménez_gonzález prelcic_2001, title={Orthogonal extensions of AR processes without artificial discontinuities for size-limited filter banks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035556105&partnerID=MN8TOARS}, booktitle={IEEE Workshop on Statistical Signal Processing Proceedings}, author={Domínguez Jiménez, M.E. and González Prelcic, N.}, year={2001}, pages={579–582} }
 @article{prelcic_gonzález_jiménez_2001, title={Wavelet packet-based subband adaptive equalization}, volume={81}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035426571&partnerID=MN8TOARS}, DOI={10.1016/S0165-1684(01)00077-9}, abstractNote={This paper proposes adaptive algorithms for channel equalization in the wavelet packet transform domain. Adaptation to highly correlated time-varying channels is achieved by the use of the orthogonal tree-structured time-varying filter bank associated to this transform. The filter bank structure, that is, the type of decomposition, is obtained, for a fixed number of bands, as the one that leads to a quasi-optimum convergence rate of the adaptive filters in the different subbands. Then, we propose an NLMS-type adaptive strategy for two possible implementations, a block algorithm, BWPKNLMS, and a non-block approach, WPKNLMS. A theoretical analysis of both schemes is also provided: we obtain the mean squared error after the adaptive algorithm has converged and we find the expression for the optimal solution, to finally show that faster convergence is achieved. Experimental results for the learning curves show the efficiency of the proposed schemes and illustrate the previously presented theoretical results.}, number={8}, journal={Signal Processing}, author={Prelcic, N.G. and González, F.P. and Jiménez, M.ElenaD.}, year={2001}, pages={1641–1662} }
 @inproceedings{jiménez_prelcic_2000, title={New orthogonal extension methods for tree-structured filter banks}, volume={2015-March}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84937047372&partnerID=MN8TOARS}, number={March}, booktitle={European Signal Processing Conference}, author={Jiménez, M.E.D. and Prelcic, N.G.}, year={2000} }
 @inproceedings{dominguez jimenez_gonzalez prelcic_1999, title={Processing finite length signals via filter banks without border distortions: A non-expansionist solution}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032639753&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Dominguez Jimenez, M.E. and Gonzalez Prelcic, N.}, year={1999}, pages={1481–1484} }
 @inproceedings{prelcic_gonzalez_gonzalez_1998, title={Flexible structure for wavelet packet-based subband adaptive equalization}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031632111&partnerID=MN8TOARS}, booktitle={Proceedings of the IEEE-SP International Symposium on Time-Frequency and Time-Scale Analysis}, author={Prelcic, N.Gonzalez and Gonzalez, F.Perez and Gonzalez, A.B.Mosquera}, year={1998}, pages={377–380} }
 @inproceedings{serantes_pena_prelcic_1997, title={Fast noise-scaling algorithm for uniform quantization in audio coding schemes}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030644364&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Serantes, Carlos A. and Pena, Antonio S. and Prelcic, Nuria G.}, year={1997}, pages={339–342} }
 @inproceedings{pena_prelcic_serantes_1997, title={Flexible tiling of the time axis for adaptive wavelet packet decompositions}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030649048&partnerID=MN8TOARS}, booktitle={ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings}, author={Pena, Antonio S. and Prelcic, Nuria Gonzalez and Serantes, Carlos}, year={1997}, pages={2137–2140} }
 @article{farrow_xydeas_oakley_khorabi_prelcic_1996, title={A comparison of system architectures for intelligent document understanding}, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030289020&partnerID=MN8TOARS}, DOI={10.1016/S0923-5965(96)00002-1}, abstractNote={Intelligent document understanding (IDU) is the process of converting scanned document images into a high level representation which describes the document's layout and logical structure, in addition to providing its information content. In this paper we discuss IDU in general and address a specific problem within this domain concerning the extraction of the layout structure of pages from a technical journal. Three different architectural approaches to accomplishing this task are proposed. Firstly we describe a novel document understanding system (System A) which exploits a hybrid bottom-up/top-down control architecture. The system uses a variety of image processing algorithms in a bottom-up manner. Conversely, a system based on a pure top-down architecture (System B) is then proposed which produces a segmentation of the page via projection profile analysis and achieves classification of image regions via procedural deduction. Finally, an alternative top-down architecture (System C) is described in which an optimised segmentation scheme is applied to produce partitioned blocks. These are then classified in a goal driven manner using a decision tree. A comparison of the three systems is made by measuring system performance on images obtained from a specific class of input document. The performance of document understanding systems has been quantified in terms of an object identification rate and the percentage of column area successfully interpreted. Using these measures, System A has given superior results to the two top-down systems presented. System A also performs significantly better than a previously reported top-down system operating on a comparable problem (Viswanathan, 1990).}, number={1}, journal={Signal Processing: Image Communication}, author={Farrow, G.S.D. and Xydeas, C.S. and Oakley, J.P. and Khorabi, A. and Prelcic, N.G.}, year={1996}, pages={1–19} }
 @article{gonzález-prelcic_docampo_1996, title={A multipulse-like wavelet-based speech coder}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17744406405&partnerID=MN8TOARS}, number={2}, journal={Eurasip Journal on Applied Signal Processing}, author={González-Prelcic, N. and Docampo, D.}, year={1996}, pages={78–87} }
 @inproceedings{gonzalez prelcic_pena_1996, title={Adaptive tree search algorithm with application to multiresolution based perceptive audio coding}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029712709&partnerID=MN8TOARS}, booktitle={Proceedings of the IEEE-SP International Symposium on Time-Frequency and Time-Scale Analysis}, author={Gonzalez Prelcic, Nuria and Pena, Antonio S.}, year={1996}, pages={117–120} }
 @inproceedings{marquez_prelcic_1996, title={Interactive software for a hypertext course in wavelets}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029728710&partnerID=MN8TOARS}, booktitle={IEEE Digital Signal Processing Workshop}, author={Marquez, Oscar W. and Prelcic, Nuria Gonzalez}, year={1996}, pages={105–108} }