[Show abstract][Hide abstract] ABSTRACT: A novel method for estimating the power angle spectrum (PAS) is presented that decomposes the true PAS into a small set of basis functions. The basis coefficients for this sparse representation are found by enforcing equality to the covariance or Bartlett PAS subject to a minimum lscr<sub>1</sub>-norm constraint. The method, referred to as sparse PAS estimation (SPASE), can be implemented conveniently using existing linear-programming (LP) solvers. Further, because only a few clusters are required in the representation, the method enables reduced-complexity stochastic models for the channel and possibly allows reduced overhead in channel feedback schemes. Application of the method to simulated channels and multiple-input multiple-output (MIMO) propagation data demonstrates the utility of the method.
Preview · Article · Sep 2009 · IEEE Transactions on Antennas and Propagation
[Show abstract][Hide abstract] ABSTRACT: An emerging area of research in wireless communications is the generation of secret encryption keys based on the shared (or common) randomness of the wireless channel between two legitimate nodes in a communication network. However, to date, little work has appeared on methods to use the increased randomness available when the network nodes have multiple antennas. This paper provides theoretical performance bounds associated with using multi-antenna communications and proposes two practical methods for generating secret keys exploiting the increased randomness. Performance simulations reveal the efficiency of the methods.
[Show abstract][Hide abstract] ABSTRACT: Previous results for correlated block-fading MIMO channels with covariance information indicate guaranteed capacity growth with additional transmit elements and that in rapidly fading channels, vanishing element spacing maximizes capacity. However, because prior analysis neglects antenna electromagnetic coupling, the observations are not necessarily valid for small inter-element spacing. This work applies radiated power considerations to the analysis to demonstrate that additional elements do not always increase capacity and that vanishing element spacing is not optimal. An effective gain metric is introduced that quantifies the performance increase with additional transmitters in the presence of transmit correlation and mutual coupling. Performance simulations using the electromagnetic properties of uniform linear arrays characterized by closed form expressions for Hertzian dipoles and detailed finite-difference time-domain (FDTD) simulations of half-wave dipoles illustrate that capacity gains arc possible when correlation stems from directional bias in the channel but not when it arises due to compact element spacing.
Preview · Article · Mar 2008 · IEEE Transactions on Wireless Communications
[Show abstract][Hide abstract] ABSTRACT: A study on the suitability of multiple-input multiple-output (MIMO) architectures employing either single- or dual-polarization antennas is presented, with the purpose of identifying not only which architecture provides better average capacity performance, but also which is more robust for avoiding low channel rank. It is expected that systems with high robustness will be required for MIMO in demanding industrial applications. A measurement campaign employing dual-polarized 8times8 patch arrays at 2.4 and 5.2 GHz is analyzed, where measurements were performed in two very distinct indoor scenarios: an office building and an industrial environment. For both environments the performance of three 4times4 subsystems (dual-polarized, vertical- polarized and horizontal-polarized) are compared in terms of the average capacities attained by these systems and their eigenvalue distributions. Average capacities are found to be only marginally different, indicating little advantage of dual-polarized elements for average performance. However, an eigenvalue analysis indicates that the dual-polarized system is most robust for full-rank MIMO communications, by providing orthogonal channels with more equal gain.
[Show abstract][Hide abstract] ABSTRACT: The ability of correlation tensor and directional modeling strategies to accurately capture the spatial behavior of multiple-input multiple-output (MIMO) channels is investigated. Correlation tensor methods are based on the reduced order approximations of the full channel covariance, and do not require any a priori knowledge about the physical scattering mechanism or antenna array. Directional methods require knowledge about the array configuration and usually assume a double-directional wave-propagation mechanism. Five different tensor methods (Kronecker, separable maximum entropy, Weichselberger, principal hyperplane, and sparse core tensor) and one directional method (unstructured diffuse spectrum estimation) are compared in terms of the number of parameters required and the match to the true full covariance matrix. Simulations with a realistic cluster channel model indicate that tensor methods suffer from poor accuracy when too few values in the core tensor are retained, especially when no joint transmit/receive information is available. The directional method, on the other hand, has good accuracy with the same number of parameters as the simplest tensor model. The results stress the importance of including joint transmit/receive information in MIMO models and suggest that directional modeling is a logical choice for high accuracy modeling with few parameters.
[Show abstract][Hide abstract] ABSTRACT: This paper studies two different modeling strategies for capturing the time-variant behavior of multiple-input multiple-output wireless channels. The first method generates a multi-variate random description of the channel matrix based on the measured space-time covariance. The second method describes the physical variation of the multipath propagation components with time. Information theoretic metrics are used to compare the performance of the models to measured data.
[Show abstract][Hide abstract] ABSTRACT: The goal of this work is to identify MIMO modeling strategies with high accuracy and relatively few parameters. Candidates are existing correlation tensor models (Kronecker, Weichselberger, and maximum entropy), a new generalized tensor model based on the higher-order singular value decomposition (HOSVD), and an unstructured diffuse directional model. The performance of the models is investigated by applying them to 8 x 8 MIMO reference channels generated with a realistic double- directional cluster channel model. It is shown that all of the existing models exhibit high error in the reconstructed spatial spectra (10-30%) and even higher error in the reconstructed full covariance (20-35%). Analysis of a new sparse core tensor model indicates that this error decays slowly (logarithmically) as the number of parameters is increased. For the same number of parameters as the simple Kronecker model, the directional model is able to reconstruct the spectrum with improved accuracy (about 3% error). More surprisingly, the error in the reconstructed covariance matrices (about 16%) is also substantially lower than all of the reduced-order correlation tensor methods.
[Show abstract][Hide abstract] ABSTRACT: This paper analyzes temporal variation in measured multiple- input multiple-output (MIMO) wireless channels with moving communication nodes. A wideband 8times8 sounder is employed to measure the response of indoor and outdoor channels at 2.55 and 5.2 GHz. The rate of channel temporal variation is then quantified in terms of information theoretic metrics that indicate the loss in channel quality as transmit and receive channel state information becomes increasingly outdated.
[Show abstract][Hide abstract] ABSTRACT: This paper explores the potential performance of the AdaM antenna in a set of realistic environments and over wideband operation based on experimental measurements of a prototype antenna. The results demonstrate that, when compared to a system using a traditional receive array, a 2times2 MIMO system using an AdaM receive antenna can achieve the same spectral efficiency with a significant average transmit power reduction and that the AdaM antenna enables the MIMO system to more effectively use the available frequencies in a wideband scenario.
[Show abstract][Hide abstract] ABSTRACT: A method for determining the maximum entropy (ME) estimate of the full joint transmit/receive MIMO channel covariance matrix is presented, where only knowledge of the separate transmit and receive covariances is assumed. The resulting ME covariance is similar to, but distinct from, the Kronecker modelling strategy, warranting a direct comparison of the two techniques. Application of the method to wideband indoor multiple-input multiple-output (MIMO) measurements taken at 2.4 GHz indicates a slight improvement in the modelling accuracy of the dominant covariance eigenvalues compared to the Kronecker model. On the other hand, the double-directional joint transmit/receive Bartlett spectra are nearly identical, indicating that the ME technique does not significantly improve modelling of the channel multipath structure. This fact suggests that accurate multipath modelling for MIMO systems may not be possible without detailed joint transmit/receive channel knowledge.
[Show abstract][Hide abstract] ABSTRACT: A recent study on correlated block-fading MIMO channels with transmit covariance information indicates guaranteed capacity growth with additional transmit elements, in contrast to previous results for uncorrelated channels. Also, for very rapidly fading channels, the results indicate the optimality of placing antenna elements as close together as possible. In this work, application of radiated power considerations indicates that transmit beamforming only increases capacity when this gain results from correlation inherent in the channel, not correlation that is simply offset by increased antenna coupling. The new analysis reveals that when multipath is directionally biased, antenna spacings of 0.3 to 0.6 wavelengths are optimal, and that when no such bias is present, antennas should be placed as far apart as possible. As expected, the electromagnetic analysis shows that at zero element separation, the array yields no capacity gain over a single antenna.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we characterize the extent of channel time variation in an outdoor campus environment by taking narrowband measurements at 2.45 GHz in a number of representative locations and present MIMO metrics that quantify the rate of channel time variation, thus allowing channels to be classified based on their time variability
[Show abstract][Hide abstract] ABSTRACT: Multiple-input multiple-output (MIMO) double-directional spatial channel responses for co-located indoor measurements at 2.4 and 5.2 GHz using eight element uniform circular arrays are compared. Correlation coefficients of the power spectra demonstrate a linear dependence, indicating similarity in propagation mechanisms at the two frequencies.
Preview · Article · Dec 2005 · Electronics Letters
[Show abstract][Hide abstract] ABSTRACT: The extent of time variation of measured multipleinput multiple-output (MIMO) wireless channels is explored. A prototype MIMO channel sounder developed at Brigham Young University, capable of probing 8×8 MIMO channels from 2-8 GHz with up to 100 MHz of instantaneous bandwidth, is used to measure representative indoor and outdoor scenarios at 2.4 and 5.2 GHz. New metrics are proposed to quantify the time variation observed in the channel measurements: eigenvalue level crossing rate, eigenvalue fade duration, rate of eigenvector angular deviation, and temporal capacity degradation. Two different models are used to fit the measurements: (1) a simple random matrix model and (2) a physical time-varying cluster model. The performance of these models is assessed in terms of their ability to reproduce the measured time-variation metrics.
[Show abstract][Hide abstract] ABSTRACT: Multiantenna systems such as devices for multiple-input-multiple-output (MIMO) communication can theoretically use array superdirectivity to optimally exploit the propagation channel. In traditional analyses of MIMO systems, such superdirectivity is not observed due to the commonly applied constraint that limits the excitation current magnitudes. However, when an electromagnetically appropriate constraint on the power radiated by the array is applied, the computed capacity can include effects of transmit superdirectivity. A similar result occurs at the receiver for spatially colored noise. This paper formulates the MIMO system capacity under these circumstances and provides a framework for computing this capacity when the level of tolerable superdirectivity (as measured by the superdirectivity Q factor) is constrained. Example computations using the framework illustrate the impact that superdirectivity can have on achievable MIMO system performance.
Preview · Article · Oct 2005 · IEEE Transactions on Antennas and Propagation
[Show abstract][Hide abstract] ABSTRACT: Studies of wireless multiple-input multiple-output (MIMO) channel capacity generally treat the channel as a propagation environment coupled with an antenna array. However, it is well known that altering antenna properties can have a dramatic impact on the MIMO performance. This motivates definition of a capacity bound that depends only on the propagation environment and is independent of the antenna. geometry. This paper provides a framework for formulating this capacity, and demonstrates its behavior for several representative scenarios.
[Show abstract][Hide abstract] ABSTRACT: A cost-effective, experimental MIMO channel sounder is presented, capable of performing measurements with up to eight transmit and eight receive elements over 80 MHz of system bandwidth. The system provides the wideband channel transfer matrix for both indoor and outdoor wireless communication scenarios, allowing direct measurement of key MIMO system parameters. The system is described, fundamental hardware building blocks are outlined, and post-processing data algorithms are presented. Initial investigations on the effect of frequency scaling and array configuration at 5.2 GHz and 2.4 GHz are also presented.
[Show abstract][Hide abstract] ABSTRACT: Multiple-input multiple-output (MIMO) spatial correlation functions for co-located measurements at 2.4 and 5.2 GHz using 8-element linear arrays are compared. A single-parameter exponential model for the magnitude correlation coefficient provides a good fit, with an average mean squared error of 0.019. Results demonstrate that a linear model can describe the dependence of decorrelation at the two frequencies.