@article{wu_hughes_2023, title={A Principal-Component Approach to Antenna Impedance Estimation at MISO Receivers}, volume={27}, ISSN={["1558-2558"]}, url={https://doi.org/10.1109/LCOMM.2022.3215305}, DOI={10.1109/LCOMM.2022.3215305}, abstractNote={Impedance matching between receive antenna and front-end significantly impacts channel capacity in wireless channels. To implement the optimal matching, the receiver must know the antenna impedance. This impedance varies with the antenna’s loading conditions. Techniques are derived to estimate antenna impedance, but the optimality of these techniques remains unclear. In this letter, we study antenna impedance estimation at MISO receivers over Rayleigh fading channels and derive the optimal ML estimator in closed-form. Numerical results suggest a computationally efficient, principal-components approach that estimates antenna impedance in real-time and shows sizable improvement against a reference estimator at low SNR.}, number={1}, journal={IEEE COMMUNICATIONS LETTERS}, author={Wu, Shaohan and Hughes, Brian L.}, year={2023}, month={Jan}, pages={288–292} }
 @article{wu_hughes_2018, place={Princeton, NJ}, title={Training-Based Joint Channel and Antenna Impedance Estimation}, url={http://dx.doi.org/10.1109/ciss.2018.8362280}, DOI={10.1109/ciss.2018.8362280}, abstractNote={We consider joint channel and antenna impedance estimation based on training data for single-input, single-output channels. The aim is to leverage the resources available in wireless systems for channel estimation to estimate antenna impedance as well. We assume the transmitter sends a known training sequence, during which the receiver varies its impedance in a known way. Based on this observation, we derive maximum-likelihood estimators for the channel and impedance. We show these estimators generally exhibit heavy tails, and explore estimator performance through numerical examples. Our results suggest that antenna impedance can be accurately estimated, in exchange for a small, controlled increase in channel estimation error.}, journal={2018 52ND ANNUAL CONFERENCE ON INFORMATION SCIENCES AND SYSTEMS (CISS)}, author={Wu, Shaohan and Hughes, Brian L.}, year={2018} }
 @article{kundu_hughes_2017, title={Enhancing Capacity in Compact MIMO-OFDM Systems With Frequency-Selective Matching}, volume={65}, ISSN={["1558-0857"]}, DOI={10.1109/tcomm.2017.2728063}, abstractNote={Mutual coupling among receive antennas can profoundly reduce capacity in multiple-input, multiple-output (MIMO) systems. Prior work has shown that these losses can often be significantly reduced by using sophisticated antenna matching at the receiver. However, previous studies have focused exclusively on frequency non-selective matching, and rely on assumptions that are valid only for small bandwidths. In this paper, we consider the capacity of broadband MIMO-OFDM systems with coupled receive antennas. We present upper and lower bounds on the best capacity that can be achieved by physically realizable, frequency-selective antenna matching. The upper and lower bounds coincide for most antenna configurations considered in the examples, and thus determine the performance of capacity-optimal matching. These results suggest that frequency-selective matching can significantly increase capacity in the presence of strong coupling. Moreover, these gains can be achieved by a simple class of matching networks that are easily realized in hardware.}, number={11}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Kundu, Lopamudra and Hughes, Brian L.}, year={2017}, month={Nov}, pages={4694–4703} }
 @article{taluja_hughes_2013, title={Diversity Limits of Compact Broadband Multi-Antenna Systems}, volume={31}, ISSN={["1558-0008"]}, DOI={10.1109/jsac.2013.130219}, abstractNote={In order to support multiple antennas on compact wireless devices, transceivers are often designed with matching networks that compensate for mutual coupling. Some works have suggested that when optimal matching is applied to such a system, performance at the center frequency can be improved at the expense of an apparent reduction in the system bandwidth. This paper addresses the question of how coupling impacts bandwidth in the context of circular arrays. It will be shown that mutual coupling creates eigen-modes (virtual antennas) with diverse frequency responses, using the standard matching techniques. We shall also demonstrate how common communications techniques such as Diversity-OFDM would need to be optimized in order to compensate for these effects.}, number={2}, journal={IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS}, author={Taluja, Pawandeep S. and Hughes, Brian L.}, year={2013}, month={Feb}, pages={326–337} }
 @article{stancil_adamson_alania_aliaga_andrews_del castillo_bagby_alba_bodek_boehnlein_et al._2012, title={Demonstration of communication using neutrinos}, volume={27}, number={12}, journal={Modern Physics Letters A}, author={Stancil, D. D. and Adamson, P. and Alania, M. and Aliaga, L. and Andrews, M. and Del Castillo, C. A. and Bagby, L. and Alba, J. L. B. and Bodek, A. and Boehnlein, D. and et al.}, year={2012} }
 @article{domizioli_hughes_2012, title={Front-End Design for Compact MIMO Receivers: A Communication Theory Perspective}, volume={60}, ISSN={["1558-0857"]}, DOI={10.1109/tcomm.2012.072612.110541}, abstractNote={The front-end is a crucial component in modern wireless communication systems. For SISO systems minimizing the front-end noise factor is optimal, and this also applies to MIMO systems with i.i.d. noise. However, for compact MIMO receivers that may exhibit spatially correlated noise, e.g., through antenna mutual coupling, the optimal design procedure is not clear. In this paper we develop MIMO low-noise design principles from a communication theory perspective by deriving generalizations of SNR and noise factor from various MIMO communication schemes. As one result, we are able to derive optimal matching networks for a bank of uncoupled amplifiers.}, number={10}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Domizioli, Carlo P. and Hughes, Brian L.}, year={2012}, month={Oct}, pages={2938–2949} }
 @inproceedings{taluja_hughes_2012, title={Fundamental capacity limits on compact MIMO-OFDM systems}, DOI={10.1109/icc.2012.6363765}, abstractNote={We undertake an information-theoretic approach to characterize the optimal design of a broadband multi-antenna system in the presence of mutual coupling. It was shown recently that mutual coupling effectively decomposes otherwise spectrally-identical spatial modes of an antenna array into spectrally non-identical eigen-modes. We shall use Shannon's information theory and Fano's broadband matching theory to develop optimal transceiver designs for a compact broadband MIMO system. It will be shown that in the presence of channel state information, optimal transmit power allocation and matching characteristic follow a mutual space-frequency water-pouring solution.}, booktitle={2012 ieee international conference on communications (icc)}, author={Taluja, P. S. and Hughes, B. L.}, year={2012}, pages={2547–2552} }
 @article{taluja_hughes_2012, title={Optimal Antenna Selection Designs for Coupled MIMO Systems}, volume={2012}, ISSN={["1687-5877"]}, DOI={10.1155/2012/694627}, abstractNote={We consider the impact of antenna mutual coupling on receive antenna selection systems. Prior work on selection with mutual coupling has not considered the effects of the inactive (i.e., unselected) antenna terminations and spatial noise correlation. In this work, we show that the presence of inactive antennas can profoundly alter system performance when the antennas are strongly coupled. We also propose a new antenna selection technique that seeks to exploit coupling to improve performance. Simulations suggest that the new technique can significantly outperform traditional selection when coupling is present.}, journal={INTERNATIONAL JOURNAL OF ANTENNAS AND PROPAGATION}, author={Taluja, Pawandeep S. and Hughes, Brian L.}, year={2012} }
 @inproceedings{taluja_hughes_2012, title={communication theory perspective on antenna selection for compact transceivers}, DOI={10.1109/icc.2012.6364251}, abstractNote={Traditional antenna selection applied to MIMO systems ignores the presence of unselected (or inactive) antennas. However, compact arrays offer a unique advantage in that mutual coupling can be exploited to improve the system performance. Previous studies on selection with coupling have not considered the effects of the inactive-antenna terminations and lack detailed transceiver modeling. In this work, we present a communication-theoretic analysis and design of antenna selection systems for compact transceivers. We also propose a simple parasitic antenna selection technique that can significantly outperform traditional selection when coupling is present.}, booktitle={2012 ieee international conference on communications (icc)}, author={Taluja, P. S. and Hughes, B. L.}, year={2012} }
 @inproceedings{taluja_hughes_2011, title={Bandwidth limitations and broadband matching for coupled multi-antenna systems}, DOI={10.1109/glocom.2011.6134290}, abstractNote={Practical wireless devices are considered too small to support multi-antenna communications. Several studies have proposed optimal MIMO transceiver design in the presence of mutual coupling by use of matching networks. However, most studies assume a narrowband model. For broadband systems, Fano's matching theory reveals gain-bandwidth trade-offs for physically realizable networks. In this paper, we formulate matching network design for broadband coupled multi-antenna transceivers with circular arrays and investigate the role coupling plays in determining the RF bandwidth of the system. It will be shown that in the presence of coupling, the spatial modes of an antenna array have different bandwidths and that for 2-element arrays with practical bandwidths, a quarter-wavelength spacing is good enough.}, booktitle={2011 ieee global telecommunications conference (globecom 2011)}, author={Taluja, P. S. and Hughes, B. L.}, year={2011} }
 @inproceedings{simpson_cox_krier_cochenour_hughes_muth_2010, title={5 mbps optical wireless communication with error correction coding for underwater sensor nodes}, DOI={10.1109/oceans.2010.5664429}, abstractNote={One issue with underwater sensors is how to efficiently transfer large amounts of data collected by the node to an interrogating platform such as an underwater vehicle. It is often impractical to make a physical connection between the node and the vehicle which suggests an acoustic or optical wireless solution. For large amounts of data, the high bandwidth of underwater optical wireless is an advantage. A small, low-cost platform to demonstrate the potential of an optical wireless communications interface for underwater sensor nodes is demonstrated. To enhance the reliability and robustness of the optical wireless communication digital signal processing and error correction techniques are used. The system was tested in 3 and 7.7 meter tanks at 5 Mbps with the turbidity of the water controlled by the addition of Maalox.}, booktitle={Oceans 2010}, author={Simpson, J. A. and Cox, W. C. and Krier, J. R. and Cochenour, B. and Hughes, B. L. and Muth, J. F.}, year={2010} }
 @inproceedings{cox_gray_simpson_cochenour_hughes_muth_2010, title={A MEMS Blue/Green retroreflecting modulator for underwater optical communications}, DOI={10.1109/oceans.2010.5664432}, abstractNote={Short range, high bandwidth underwater optical communications links are potentially useful for retrieving stored information from underwater systems. However many underwater systems have limited power capacity or limited weight budget. The use of a modulating retroreflector eliminates the need for a transmitting laser on the data bearing platform and reduces the pointing requirements by retroreflecting the modulated light back to the interrogating source. Blue/green operation of a MEMS Fabry-Perot modulator was demonstrated at data rates of 500 kbps and 1 Mbps in a 7.7 meter tank with water conditions varied by the addition of Maalox as a scattering agent.}, booktitle={Oceans 2010}, author={Cox, W. C. and Gray, K. F. and Simpson, J. A. and Cochenour, B. and Hughes, B. L. and Muth, J. F.}, year={2010} }
 @inproceedings{taluja_hughes_2010, title={Information theoretic optimal broadband matching for communication systems}, DOI={10.1109/glocom.2010.5683273}, abstractNote={We present an information theoretic perspective on optimal broadband matching. Most of the broadband matching and communication theory literature assumes a frequency-flat reflection coefficient. We derive the optimal capacity and matching network characteristic for a broadband system from an information theoretic view point while incorporating the broadband matching limitations. We also propose an iterative algorithm to arrive at the optimal solution. We illustrate by an example that a white characteristic for a matching network is often sub- optimal for a system with a non-white transfer function.}, booktitle={2010 ieee global telecommunications conference globecom 2010}, author={Taluja, P. S. and Hughes, B. L.}, year={2010} }
 @article{domizioli_hughes_gard_lazzi_2010, title={Noise Correlation in Compact Diversity Receivers}, volume={58}, ISSN={["1558-0857"]}, DOI={10.1109/tcomm.2010.05.080601}, abstractNote={The impact of antenna mutual coupling on signal correlation in multi-antenna receivers has been studied in great detail. By contrast, there has been little work on how mutual coupling affects noise. In this paper we present a noise model for a diversity receiver that includes noise generated by the antennas, front-end amplifiers, and other receiver components. This model shows that noise in a compact diversity receiver may be spatially correlated. Expressions relating noise correlation to properties of the antennas and amplifiers are derived and its impact on the outage probability of an optimal diversity combiner is studied. Examples illustrating the relationship between mutual coupling and noise correlation demonstrate how different noise sources may impact performance in profoundly different ways.}, number={5}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Domizioli, Carlo P. and Hughes, Brian L. and Gard, Kevin G. and Lazzi, Gianluca}, year={2010}, month={May}, pages={1426–1436} }
 @article{zhang_dai_hughes_2009, title={Analysis on the Diversity-Multiplexing Tradeoff for Ordered MIMO SIC Receivers}, volume={57}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2009.0901.060321}, abstractNote={The diversity-multiplexing tradeoff for multiple-input multiple-output (MIMO) point-to-point channels and multiple access channels were first proposed and studied by Zheng and Tse recently. While the optimal tradeoff curves for MIMO channels have been explicitly explored, those corresponding to some suboptimal and practical MIMO schemes are still open. One such important problem is the diversity-multiplexing tradeoff for a V-BLAST type system employing ordered successive interference cancellation (SIC) receivers with zero forcing (ZF) or minimum mean square error (MMSE) processing at each stage. In this paper, we take a novel geometrical approach and rigorously verify that under general settings, the optimal ordering rule for a V-BLAST SIC receiver will not improve its performance regarding diversity-multiplexing tradeoff in point-to- point channels. The same geometrical tool is then applied to MIMO spatial-division multiple access channels, leading to some first results in this area. Particularly, we reveal that when the rates of data streams are fixed (i.e., zero spatial multiplexing gain), the diversity order is not improved by user ordering.}, number={1}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Zhang, Hongyuan and Dai, Huaiyu and Hughes, Brian L.}, year={2009}, month={Jan}, pages={125–133} }
 @article{dong_domizioli_hughes_2009, title={Effects of Mutual Coupling and Noise Correlation on Downlink Coordinated Beamforming with Limited Feedback}, ISSN={["1687-6180"]}, DOI={10.1155/2009/807830}, abstractNote={We consider the impact of receiver correlation, antenna coupling, matching, and noise on the performance of coordinated beamforming systems. We present a new coordinated beamforming technique for two receivers that is suitable for MIMO broadcast channels with signal and noise correlation at the receiver. We then apply this technique to the specific type of signal and noise correlation that occurs in the presence of receiver mutual coupling. Numerical results suggest that, even in the presence of strong coupling, most of the benefits of coordinated beamforming can be preserved by using appropriate matching networks and linear beamforming. Moreover, these benefits can be achieved even when feedback is limited.}, journal={EURASIP JOURNAL ON ADVANCES IN SIGNAL PROCESSING}, author={Dong, Yuhan and Domizioli, Carlo P. and Hughes, Brian L.}, year={2009} }
 @article{dong_hughes_lazzi_2008, title={Mutual Coupling Effects in MIMO MRC Systems with Limited Feedback}, ISSN={["1930-529X"]}, DOI={10.1109/glocom.2008.ecp.692}, abstractNote={We consider the impact of transmitter correlation, mutual coupling and matching networks on the design and performance of MIMO MRC systems with limited feedback. We present codebook design techniques for three measures of transmitted power and we investigate the impact of antenna matching on the performance of these codebooks. Numerical results suggest that, regardless of the power measure and matching networks, the benefits of MIMO MRC with limited feedback can be achieved with antennas spaced as close as 0.2 - 0.3 wavelengths apart.}, journal={GLOBECOM 2008 - 2008 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE}, author={Dong, Yuhan and Hughes, Brian L. and Lazzi, Gianluca}, year={2008} }
 @article{gupta_hughes_lazzi_2008, title={On the degrees of freedom in linear array systems with tri-polarized antennas}, volume={7}, ISSN={["1558-2248"]}, DOI={10.1109/TWC.2008.070160}, abstractNote={In this paper, we provide a theoretical, simulation and experimental characterization of the performance of linear arrays of tri-polarized dipole antennas in multiple input multiple output (MIMO) systems in terms of degrees of freedom and channel capacity. Specifically, we formulate three distinct approaches, theoretical and experimental, that lead to the conclusion that linear arrays of tri-polarized dipole antennas employed at the transmit and receive end of a MIMO system can achieve a three-fold increase in channel capacity with respect to analogous arrays of linearly polarized antennas.}, number={7}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Gupta, Gaurav and Hughes, Brian L. and Lazzi, Gianluca}, year={2008}, month={Jul}, pages={2458–2462} }
 @article{domizioli_hughes_gard_lazzi_2008, title={Optimal Front-End Design for MIMO Receivers}, ISSN={["1930-529X"]}, DOI={10.1109/glocom.2008.ecp.717}, abstractNote={The effect of antenna mutual coupling on fading correlation in compact MIMO arrays has received considerable attention. By contrast, relatively little attention has been paid to the noise. In this paper we present a circuit model for a noisy MIMO receiver front-end that includes arbitrary coupling and noise correlation. The noise figure for two-port networks is extended to multiport networks, from which theorems for optimal front-end design are derived. Numerical results are presented which provide insight into receiver noise behavior in the presence of coupled antennas.}, journal={GLOBECOM 2008 - 2008 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE}, author={Domizioli, Carlo P. and Hughes, Brian L. and Gard, Kevin G. and Lazzi, Gianluca}, year={2008} }
 @article{rajagopalan_gupta_konanur_hughes_lazzi_2007, title={Increasing channel capacity of an ultrawideband MIMO system using vector antennas}, volume={55}, ISSN={["0018-926X"]}, DOI={10.1109/TAP.2007.905938}, abstractNote={Recently it was shown that co-located vector antennas can provide the same capacity increase as spatial array antennas in multiple-input multiple-output (MIMO) systems. This paper describes the design of a vector antenna consisting of a loop antenna and two orthogonal bowtie antennas for a ultrawideband (UWB) MIMO system that operates in the frequency range 3.6-8.5 GHz. Channel capacity calculations performed with the antenna operating in a rich scattering environment show that spectral efficiency of the UWB vector antenna system approaches that of the traditional spatial array. This demonstrates that vector antennas can be used in place of spatial arrays where compact UWB multiantenna systems are needed.}, number={10}, journal={IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION}, author={Rajagopalan, Ajit and Gupta, Gaurav and Konanur, Anand S. and Hughes, Brian and Lazzi, Gianluca}, year={2007}, month={Oct}, pages={2880–2887} }
 @article{zhang_dai_zhou_hughes_2006, title={On the diversity order of spatial multiplexing systems with transmit antenna selection: A geometrical approach}, volume={52}, ISSN={["1557-9654"]}, DOI={10.1109/TIT.2006.885531}, abstractNote={In recent years, the remarkable ability of multiple-input-multiple-output (MIMO) wireless communication systems to provide spatial diversity or multiplexing gains has been clearly demonstrated. For MIMO diversity schemes, it is well known that antenna selection methods that optimize the postprocessing signal-to-noise ratio (SNR) can preserve the diversity order of the original full-size MIMO system. On the other hand, the diversity order achieved by antenna selection in spatial multiplexing systems, especially those exploiting practical coding and decoding schemes, has not thus far been rigorously analyzed. In this paper, a geometrical framework is proposed to theoretically analyze the diversity order achieved by transmit antenna selection for separately encoded spatial multiplexing systems with linear and decision-feedback receivers. When two antennas are selected from the transmitter, the exact achievable diversity order is rigorously derived, which previously only appears as conjectures based on numerical results in the literature. If more than two antennas are selected, we give lower and upper bounds on the achievable diversity order. Furthermore, the same geometrical approach is used to evaluate the diversity-multiplexing tradeoff in spatial multiplexing systems with transmit antenna selection}, number={12}, journal={IEEE TRANSACTIONS ON INFORMATION THEORY}, author={Zhang, Hongyuan and Dai, Huaiyu and Zhou, Quan and Hughes, Brian L.}, year={2006}, month={Dec}, pages={5297–5311} }
 @inproceedings{zhang_dai_hughes_2006, title={On the diversity-multiplexing Tradeoff for ordered SIC receivers over MIMO channels}, DOI={10.1109/icc.2006.254939}, abstractNote={The diversity-multiplexing tradeoff for MIMO point-to-point channels and multiple access channels are first proposed and studied in [4][5]. While the optimal tradeoff curves for MIMO channels have been explicitly explored, those corresponding to some practical MIMO schemes are still open. One such example, as mentioned in [4][5], is the diversity-multiplexing tradeoff problem for ordered successive interference cancellation (SIC) receivers, which is the focus of this paper. In literature, the impact of the optimal ordering on the diversity order for V-BLAST SIC receivers is analyzed for 2-layer scenarios [2][3][6], but only conjectured for larger number of layers through numerical results [2][7]. In this paper, based on a novel geometrical analysis, we prove that under general settings, any ordering rule for a V-BLAST SIC receiver will not improve its performance regarding diversity-multiplexing tradeoff. Furthermore, extending the study to multiple access channels, we show that the two extreme points of the tradeoff curve remain unchanged regardless of ordering, which motivates us to predict that the whole tradeoff curve is the same as that of fixed-order detectors.}, booktitle={2006 ieee international conference on communications, vols 1-12}, author={Zhang, H. Y. and Dai, H. Y. and Hughes, B. L.}, year={2006}, pages={1368–1373} }
 @article{konanur_gosalia_krishnamurthy_hughes_lazzi_2005, title={Increasing wireless channel capacity through MIMO systems employing co-located antennas}, volume={53}, ISSN={["1557-9670"]}, DOI={10.1109/TMTT.2005.848105}, abstractNote={Wireless networks consisting of compact antennas find applications in diverse areas such as communication systems, direction of arrival estimation, sensor networks, and imaging. The effectiveness of many of these systems depend on maximizing the reception of RF power and extracting maximum information from the incident electromagnetic (EM) wave. Traditionally, this has been achieved through multiple-input multiple-output (MIMO) systems employing a spatial array of antennas that enhance the channel capacity. In this paper, we report similar increases in channel capacity obtained through the use of vector antennas consisting of co-located loops and dipoles, which can respond to more than one component of the EM field. It is shown that systems with three- and four-element vector antennas at both the transmitter and receiver operating around the frequency of 2.25 GHz support three and four times more information, respectively, as compared to conventional systems consisting of sensors with single antennas. Comparison with a simplified theoretical model of a MIMO system with co-located antennas in a rich multipath environment shows good agreement.}, number={6}, journal={IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES}, author={Konanur, AS and Gosalia, K and Krishnamurthy, SH and Hughes, B and Lazzi, G}, year={2005}, month={Jun}, pages={1837–1844} }
 @article{kamath_hughes_2005, title={The asymptotic capacity of multiple-antenna Rayleigh-fading channels}, volume={51}, ISSN={["1557-9654"]}, DOI={10.1109/tit.2005.858966}, abstractNote={We consider the asymptotic behavior of the capacity of multiple-antenna Rayleigh-fading channels in the limit as the transmit and receive arrays become large. We show that the capacity converges in distribution to a Gaussian random variable, and give closed-form formulas for its mean and variance. These results enable us to derive the first asymptotic formula for outage rates, as well as a sharper estimate of the error in previously reported asymptotic formulas for ergodic capacity. Although these formulas are asymptotic, we show by simulation that they are often quite accurate, even for relatively small arrays.}, number={12}, journal={IEEE TRANSACTIONS ON INFORMATION THEORY}, author={Kamath, MA and Hughes, BL}, year={2005}, month={Dec}, pages={4325–4333} }
 @article{hong_hughes_2004, title={Bit-interleaved space-time coded modulation with iterative decoding}, volume={3}, ISSN={["1558-2248"]}, DOI={10.1109/twc.2004.837450}, abstractNote={Bit-interleaved space-time coded modulation (BI-STC), which combines serial concatenation of bit-interleaved coded modulation (BICM) with space-time block codes, can effectively exploit the available diversity in space and time under various fading conditions. In this letter, we propose to use iterative decoding to further improve the performance of BI-STC by exploiting the concatenating structure of the codes. The decoding metric is therefore modified to fit for the iterative process, and the derived error bounds suggest that set-partition labeling instead of gray labeling should be used when considering iterative decoding.}, number={6}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Hong, ZH and Hughes, BL}, year={2004}, month={Nov}, pages={1912–1917} }
 @article{cozzo_hughes_2003, title={Joint channel estimation and data detection in space-time communications}, volume={51}, ISSN={["0090-6778"]}, DOI={10.1109/TCOMM.2003.815062}, abstractNote={The paper considers joint channel estimation and data sequence detection for multipath radio channels with multiple antennas at the transmitter and/or receiver. An iterative space-time receiver based on the expectation-maximization algorithm is proposed. We examine the performance of this receiver for transmit diversity and space-time coding methods over Rayleigh fading channels. Simulation results show that the receiver can often achieve near-coherent performance with modest complexity and using very few pilot symbols.}, number={8}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Cozzo, C and Hughes, BL}, year={2003}, month={Aug}, pages={1266–1270} }
 @article{hughes_2003, title={Optimal space-time constellations from groups}, volume={49}, ISSN={["0018-9448"]}, DOI={10.1109/TIT.2002.807283}, abstractNote={We consider the design of space-time constellations based on group codes for fading channels with multiple transmit and receive antennas. These codes can be viewed as multiantenna extensions of phase-shift keying (PSK), in the sense that all codewords have equal energy, all are rotations of a fixed codeword, and there is a simple differential transmission rule that allows data to be sent without channel estimates at the transmitter or receiver. For coherent detection, we show that all optimal full-rank space-time group codes are unitary (each code matrix has equal-energy, orthogonal rows). This leads to a simpler code design criterion and suggests that unitary codes may play an important role in coherent as well as noncoherent communication. For any number of transmit antennas t, we then use the design criterion to characterize all full-rank unitary space-time group codes of minimum block length (also t) which have 2/sup p/ codewords. These results allow us to characterize all optimal 2/sup p/-ary unitary group codes with square code matrices. This restricted class of block codes matches the class proposed for differential modulation by Hughes (see IEEE Trans. Inform. Theory, vol.46, p.2567-78, Nov. 2000), and by Hochwald and Sweldens (see IEEE Trans. Commun., vol.48, p.2041-2052, Dec. 2000).}, number={2}, journal={IEEE TRANSACTIONS ON INFORMATION THEORY}, author={Hughes, BL}, year={2003}, month={Feb}, pages={401–410} }
 @article{chandra_hughes_2003, title={Optimizing information efficiency in a direct-sequence mobile packet radio network}, volume={51}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2002.807607}, abstractNote={We consider a direct-sequence multihop packet radio network using slotted ALOHA in a Rayleigh fading environment. We are interested in optimally choosing the transmission range, code rate, and slotted ALOHA transmission probability to be used by each node. We use a new performance measure, information efficiency, to analyze the network and show that the information efficiency of the low-cost mobile packet radio network can be improved approximately 20 fold by using the optimum parameters. We suggest a practical trellis-coded modulation scheme which comes close to realizing the theoretic limits found.}, number={1}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Chandra, MW and Hughes, BL}, year={2003}, month={Jan}, pages={22–24} }
 @article{blum_bolcskei_fitz_hughes_paulraj_2003, title={Special Issue on MIMO Wireless Communications}, volume={51}, ISSN={["1053-587X"]}, DOI={10.1109/TSP.2003.818198}, abstractNote={HE major challenges in future wireless communications system design are increased spectral efficiency and improved link reliability. The radio channel constitutes a hostile propagation medium, which suffers from fading (signal level fluctuations due to destructive addition of multipaths) and interference from other users. Diversity provides the receiver with several ideally independent replicas of the transmitted signal and is therefore a powerful technique to mitigate fading and interference from other users, thereby improving link reliability. The use of spatial or antenna diversity has recently become very popular since it can be provided without loss in spectral efficiency. The corresponding signal processing and coding techniques are known as space-time coding and require that multiple antennas at the transmitter and one or more antennas at the receiver are employed. The use of multiple antennas at both the transmitter and the receiver of a wireless link [multiple-input multiple-output (MIMO) technology] has recently been demonstrated, both theoretically and practically, to achieve an order of magnitude improvement in terms of spectral efficiency when compared with existing systems. MIMO wireless is evolving into one of the most active research segments in wireless communications, and the first standards based on MIMO technology are being defined. Despite the significant research efforts in the MIMO area, numerous problems related to ultimate performance limits, code design, and receiver design remain open. The present special issue is an attempt to cover a wide range of topics in MIMO wireless by publishing 26 high-quality papers reporting on recent results in the above mentioned areas. As an indication of the level of activity in this research area, there were a significant number of high-quality papers we could not include in this issue due to space constraints. The papers in this special issue correspond to advances in the understanding of six different areas of MIMO communication. The first two are analytical and include understanding the ultimate capacity that MIMO wireless links can provide as well as understanding the performance of MIMO systems. Performance and capacity are being assessed with ever increasingly sophisticated models to reflect the complex environment that MIMO communication represents. The second two relate}, number={11}, journal={IEEE TRANSACTIONS ON SIGNAL PROCESSING}, author={Blum, RS and Bolcskei, H and Fitz, MP and Hughes, B and Paulraj, AJ}, year={2003}, month={Nov}, pages={2709–2709} }
 @article{cozzo_hughes_2002, title={An adaptive receiver for space-time trellis codes based on per-survivor processing}, volume={50}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2002.801457}, abstractNote={An adaptive receiver based on per-survivor processing is proposed for detecting space-time trellis codes transmitted over time-selective flat-fading channels, and its performance is examined through simulations. Results suggest that the new receiver can outperform iterative approaches based on Kalman filtering and the expectation-maximization algorithm, at the cost of increased receiver complexity.}, number={8}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Cozzo, C and Hughes, BL}, year={2002}, month={Aug}, pages={1213–1216} }
 @article{al-dhahir_giannakis_hochwald_hughes_marzetta_2002, title={Untitled}, volume={50}, ISSN={["1053-587X"]}, DOI={10.1109/tsp.2002.804127}, number={10}, journal={IEEE TRANSACTIONS ON SIGNAL PROCESSING}, author={Al-Dhahir, N and Giannakis, GB and Hochwald, B and Hughes, BL and Marzetta, TL}, year={2002}, month={Oct}, pages={2381–2384} }
 @article{liu_giannakis_hughes_2001, title={Double differential space-time block coding for time-selective fading channels}, volume={49}, ISSN={["1558-0857"]}, DOI={10.1109/26.950340}, abstractNote={Most existing space-time coding schemes assume time-invariant fading channels and offer antenna diversity gains relying on accurate channel estimates at the receiver. Other single differential space-time block coding schemes forego channel estimation but are less effective in rapidly fading environments. Based on a diagonal unitary matrix group, a novel double differential space-time block coding approach is derived in this paper for time-selective fading channels. Without estimating the channels at the receiver, information symbols are recovered with antenna diversity gains regardless of frequency offsets. The resulting transceiver has very low complexity and is applicable to an arbitrary number of transmit and receive antennas. Approximately optimal space-time codes are also designed to minimize bit error rate. System performance is evaluated both analytically and with simulations.}, number={9}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Liu, ZQ and Giannakis, GB and Hughes, BL}, year={2001}, month={Sep}, pages={1529–1539} }
 @article{hughes_2000, title={Differential space-time modulation}, volume={46}, DOI={10.1109/18.887864}, abstractNote={Space-time coding and modulation exploit the presence of multiple transmit antennas to improve the performance on multipath radio channels. Thus far, most work on space-time coding has assumed that perfect channel estimates are available at the receiver. In certain situations, however, it may be difficult or costly to estimate the channel accurately, in which case it is natural to consider the design of modulation techniques that do not require channel estimates at the transmitter or receiver. We propose a general approach to differential modulation for multiple transmit antennas based on group codes. This approach ran be applied to any number of transmit and receive antennas, and any signal constellation. We also derive low-complexity differential receivers, error bounds, and modulator design criteria, which we use to construct optimal differential modulation schemes for two transmit antennas. These schemes can be demodulated with or without channel estimates. This permits the receiver to exploit channel estimates when they are available. The performance degrades by approximately 3 dB when estimates are not available.}, number={7}, journal={IEEE Transactions on Information Theory}, author={Hughes, B. L.}, year={2000}, pages={2567–2578} }
 @article{murali_hughes_1998, title={Coding and stability in frequency-hop packet radio networks}, volume={46}, ISSN={["0090-6778"]}, DOI={10.1109/26.659478}, abstractNote={A fully connected radio network is considered in which packets are sent using slow frequency-hop (FH) modulation, slotted ALOHA random access, and Reed-Solomon (RS) error-control coding. For this network, the dependence of throughput, delay, and drift on the code rate and block length is examined. It is shown that the drift approaches a simple limiting form as the block length becomes large. This form suggests that, in a bistable FH network, the undesirable stable point can usually be eliminated without increasing the delay or reducing the throughput at the desirable stable point. In particular, bistability can be eliminated by increasing the code block length and retransmission delay, and does not require the use of decentralized control or channel traffic estimates.}, number={2}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Murali, R and Hughes, BL}, year={1998}, month={Feb}, pages={191–199} }
 @article{hizlan_hughes_1998, title={Worst-case error probability of a spread-spectrum system in energy-limited interference}, volume={46}, ISSN={["0090-6778"]}, DOI={10.1109/26.659487}, abstractNote={We consider a communication channel corrupted by thermal noise and by an unknown and arbitrary interference of bounded energy. For this channel, we derive a simple upper bound to the worst-case error probability suffered by a direct sequence (DS) communication system with error-correction coding, pseudorandom interleaving, and a correlation receiver. This bound is exponentially tight as the block length of the error correcting code becomes large. Numerical examples are given that illustrate the dependence of the bound on the choice of error correcting code, the type of interleaving used, and the relative energy of the Gaussian noise and arbitrary interference.}, number={2}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Hizlan, M and Hughes, BL}, year={1998}, month={Feb}, pages={286–296} }
 @article{murali_hughes_1997, title={Random access with large propagation delay}, volume={5}, ISSN={["1063-6692"]}, DOI={10.1109/90.650150}, abstractNote={Random access to a packet broadcast channel with large propagation delay is investigated. A protocol is presented that combines slotted ALOHA random access with the use of forward-error-correction (FEC) across transmitted packets. Expressions for the throughput, delay, and drift of this protocol are derived. Numerical studies and asymptotic analyses of the drift indicate that the protocol has a maximum throughput of e/sup -1/ and exhibits bistability and saturation behavior similar to that of slotted ALOHA with immediate feedback. However, unlike ALOHA, bistability and saturation in the code protocol can be eliminated with the proper choice of protocol parameters without increasing the packet delay. It is further shown that, when compared to slotted ALOHA, the code protocol typically achieves a higher throughput and lower delay at system equilibrium with no loss in maximum throughput.}, number={6}, journal={IEEE-ACM TRANSACTIONS ON NETWORKING}, author={Murali, R and Hughes, BL}, year={1997}, month={Dec}, pages={924–935} }