Optimized iterative MAP ARQ for OFDM over rapidly-varying frequency-selective channels

In this paper we propose an optimized OFDM iterative ARQ receiver suitable for application in rapidly varying multipath channels. The receiver is an optimized MAP Turbo processor that exploits log likelihood ratio (LLR) information from previously failed transmissions. We show that this configuration achieves very high gains compared to a standard ARQ system by virtue of two co-operative mechanisms: First, by using (log likelihood ratios) LLR from previous decoding trials to improving the expectation maximization (EM) based algorithm channel tracking and LLR calculation during retransmissions and second by combining together the extrinsic detector LLR from previous reception attempts with the newly generated one from the retransmissions of a previously failed packet.