Soft-decision multistage decoding of multilevel-coded quaternary partial-response signals

This paper deals with multilevel coding and concatenated soft-decision multistage decoding for quaternary dicode partial-response systems. Time-interleaved quaternary symbols are transmitted with precoding over the dicode channel. A single level of coding is assumed, whereby only the less significant bits of the transmitted quaternary symbols are encoded. In the receiver, an inner decoder computes approximate log-likelihood ratios for the less significant bits of the quaternary symbols input to the precoded partial-response channel by a new, reduced two-state, soft-output Viterbi decoding algorithm. With sufficient interleaving, the log-likelihood ratios represent the appropriate metrics for soft-decision decoding by an outer multistage decoder. Real coding gains over uncoded transmission with optimum maximum-likelihood sequence decoding were determined by simulation. Results are presented for various multilevel codes based on binary convolutional or Reed-Muller block codes. This scheme exhibits low decoding complexity and allows high-rate codes that achieve real coding gains of 3 to 4 dB.