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 partial-response class-IV (PRIV) systems. Time-interleaved multilevel-coded quaternary symbols are transmitted with precoding over the PRIV channel. In the receiver, an inner decoder computes approximate log-likelihood ratios for the less significant bits of the quaternary symbols input to the precoded PRIV channel by a new reduced two-state soft-output Viterbi decoding algorithm. An outer multistage decoder employs the obtained log-likelihood ratios which, with sufficient interleaving, represent appropriate metrics for soft-decision decoding. Real coding gains over uncoded transmission with maximum-likelihood sequence decoding are determined by simulation. Results are presented for various multilevel codes that employ a single level of convolutional or Reed-Muller block coding. The scheme considered here exhibits low decoding complexity and allows for high-rate codes that achieve real coding gains of 3 to 4 dB