A Receiver for Differential Space-Time -Shifted BPSK Modulation Based on Scalar-MSDD and the EM Algorithm

In this paper, we consider the issue of blind detection of Alamouti-type differential space-time (ST) modulation in static Rayleighfading channels. We focus our attention on a π/2-shifted BPSK constellation, introducing a novel transformation to the receivedsignal such that this binary ST modulation, which has a second-order transmit diversity, is equivalent to QPSK modulation withsecond-order receive diversity. This equivalent representation allows us to apply a low-complexity detection technique specificallydesigned for receive diversity, namely, scalar multiple-symbol differential detection (MSDD). To further increase receiver performance,we apply an iterative expectation-maximization (EM) algorithm which performs joint channel estimation and sequencedetection. This algorithm uses minimum mean square estimation to obtain channel estimates and the maximum-likelihood principleto detect the transmitted sequence, followed by differential decoding.With receiver complexity proportional to the observationwindow length, our receiver can achieve the performance of a coherent maximal ratio combining receiver (with differentialdecoding) in as few as a single EM receiver iteration, provided that the window size of the initialMSDD is sufficiently long. To furtherdemonstrate that the MSDD is a vital part of this receiver setup, we show that an initial ST conventional differential detectorwould lead to a strange convergence behavior in the EM algorithm.