Cholesky Factorization-Based Adaptive BLAST DFE for Wideband MIMO Channels
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Research Article Cholesky Factorization-Based Adaptive BLAST DFE for Wideband MIMO Channels Vassilis Kekatos,1 Athanasios A. Rontogiannis,2 and Kostas Berberidis1 1 Department 2 Institute
of Computer Engineering & Informatics/C.T.I.-R&D, University of Patras, 26500 Rio-Patras, Greece for Space Applications and Remote Sensing, National Observatory of Athens, Palea Penteli, 15236 Athens, Greece
Received 11 October 2006; Accepted 23 February 2007 Recommended by Marc Moonen Adaptive equalization of wireless systems operating over time-varying and frequency-selective multiple-input multiple-output (MIMO) channels is considered. A novel equalization structure is proposed, which comprises a cascade of decision feedback equalizer (DFE) stages, each one detecting a single stream. The equalizer filters, as well as the ordering by which the streams are extracted, are updated based on the minimization of a set of least squares (LS) cost functions in a BLAST-like fashion. To ensure numerically robust performance of the proposed algorithm, Cholesky factorization of the equalizer input autocorrelation matrix is applied. Moreover, after showing that the equalization problem possesses an order recursive structure, a computationally efficient scheme is developed. A variation of the method is also described, which is appropriate for slow time-varying conditions. Theoretical analysis of the equalization problem reveals an inherent numerical deficiency, thus justifying our choice of employing a numerically robust algebraic transformation. The performance of the proposed method in terms of convergence, tracking, and bit error rate (BER) is evaluated through extensive computer simulations for time-varying and wideband channels. Copyright © 2007 Vassilis Kekatos et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1.
INTRODUCTION
To exploit the potential spectral efficiency of multiple-input multiple-output (MIMO) wireless communication systems, sophisticated receiver structures should be designed. Most of the MIMO receivers described so far deal with narrowband systems, where the channel is considered flat. Among these receivers, the BLAST (Bell Labs Layered Space-Time) architecture [1] is usually employed in high-rate spatial multiplexing systems. However, to increase transmission rate, the symbol period should be made shorter, thus giving rise to intersymbol interference (ISI). Under these circumstances a MIMO equalizer should be designed in a proper way to compensate for both intersymbol and interstream interference. Given that interference evolves in space and time, various MIMO DFE architectures have been proposed, corresponding to different detection scenarios [2, 3]. A first scenario comprises a parallel architecture, where all transmitted streams are detected simultaneously and hence only decisions on past detected symbols are available at each time instant. Due
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