Estimation and Direct Equalization of Doubly Selective Channels
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Estimation and Direct Equalization of Doubly Selective Channels Imad Barhumi,1 Geert Leus,2 and Marc Moonen3 1 Electrical
Engineering Department, United Arab Emirates University, Al-Ain 17555, United Arab Emirates of Electrical Engineering, Mathematics, and Computer Science, Delft University of Technology, Mekelweg 4, 2628CD Delft, The Netherlands 3 ESAT/SCD-SISTA, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, 3001 Leuven, Belgium 2 Faculty
Received 15 June 2005; Revised 9 June 2006; Accepted 13 August 2006 We propose channel estimation and direct equalization techniques for transmission over doubly selective channels. The doubly selective channel is approximated using the basis expansion model (BEM). Linear and decision feedback equalizers implemented by time-varying finite impulse response (FIR) filters may then be used to equalize the doubly selective channel, where the time-varying FIR filters are designed according to the BEM. In this sense, the equalizer BEM coefficients are obtained either based on channel estimation or directly. The proposed channel estimation and direct equalization techniques range from pilot-symbol-assistedmodulation- (PSAM-) based techniques to blind and semiblind techniques. In PSAM techniques, pilot symbols are utilized to estimate the channel or directly obtain the equalizer coefficients. The training overhead can be completely eliminated by using blind techniques or reduced by combining training-based techniques with blind techniques resulting in semiblind techniques. Numerical results are conducted to verify the different proposed channel estimation and direct equalization techniques. Copyright © 2006 Imad Barhumi 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.
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INTRODUCTION
Over the last decade, the mobile wireless telecommunication industry has undergone tremendous changes and experienced rapid growth. The reason behind this growth is the increasing demand for bandwidth hungry multimedia applications. This demand for even higher data rates at the user’s terminal is expected to continue for the coming years as more and more applications are emerging. Therefore, current cellular systems have been designed to provide date rates that range from a few megabits per second for stationary or low mobility users to a few hundred kilobits per second for high mobility users. In addition to the frequency-selectivity characteristics caused by multipath propagation, the channel often exhibits time-variant characteristics caused by the user’s mobility. This results in the so-called doubly selective (timeand frequency-selective) channels. In [1, 2], linear and decision feedback equalizers have been developed for single carrier transmission over doubly selective channels. There, the time-varying channel was approximated using the basis expansion model (BEM). The BEM coefficients are then used to design the equalizer (
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