A suboptimal Iterative Method for Maximum-Likelihood Sequence Estimation in a Multipath Context
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A Suboptimal Iterative Method for Maximum-Likelihood Sequence Estimation in a Multipath Context Olivier Rousseaux Katholieke Universiteit Leuven, ESAT-SISTA, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgium Email: [email protected]
Geert Leus Katholieke Universiteit Leuven, ESAT-SISTA, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgium Email: [email protected]
Marc Moonen Katholieke Universiteit Leuven, ESAT-SISTA, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgium Email: [email protected] Received 12 February 2002 and in revised form 20 June 2002 We present an iterative semiblind suboptimal maximum-likelihood sequence estimation (MLSE) method for single-carrier block transmission over stationary multipath channels. This suboptimal ML detector is based on an iterative least squares with projection (ILSP) algorithm exploiting both the finite alphabet properties of the transmitted signal and its cyclic prefixed structure in order to approach ML detection in a cheap way. Since the initial channel estimate is crucial for the convergence speed of the ILSP algorithm, we propose a new low-complexity stochastic method for providing an initial channel estimate. We therefore rely on some known symbols that are provided by a variant of cyclic prefix only (CP-Only) transmission, known as the known symbol padding only (KSP-Only) technique. The resulting channel model is sufficiently accurate to be used as a starting point for the iterations. The final result is a direct symbol estimation method that is characterised by its low computational complexity and its promising results in terms of bit error rate (BER). Keywords and phrases: block transmission, OFDM, maximum-likelihood symbol detection, known symbol padding, iterative least squares with projections.
1.
INTRODUCTION
The constantly increasing need for high data rate transmission systems has driven the research in broadband communications in the last years. Multipath effects, resulting in frequency-selective fading, are a major impediment of these broadband communication systems since they introduce intersymbol interference (ISI), which needs to be tackled by appropriate techniques. However, these are most often computationally demanding. In this context, block transmission techniques (multicarrier or single-carrier) based on the use of a cyclic prefix (CP) have attracted a lot of attention in the last years for they allow an efficient and computationally cheap ISI cancellation procedure [1, 2]. ISI can be suppressed by a single-tap “frequency-domain” equalization on blocks of data symbols using FFT and IFFT operations relying on the fact that the blocks of transmitted data are made cyclic by the use of a CP, whose length is at least equal to
the channel order. More details on these techniques are presented in Section 1.1. In this paper, we consider the case where the transmitter sends data over an unknown convolutive channel and where the receiver has no a priori information on that channel. The problem in this case consists of
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