Soft decision output decoder for decoding convolutionally encoded codewords

摘要

A soft decision output decoder (20) and decoding method. The decoder (20) decodes a sequence of signals output by an encoder (12) and transmitted over a channel (17). The soft decision output decoder (20) includes a first "generalized" Viterbi decoder (24) for decoding the sequence of signals received over the channel during a forward iteration through a trellis representing the encoder output having a block length T. The first "generalized" Viterbi encoder (24) begins at an initial state to and provides a plurality of forward iteration state metrics a for each state at each time interval over a window of length 2L, where L is on the order of a few constraint lengths and 2L is less than a block length T. A second "generalized" Viterbi decoder decodes the sequence of signals received over the channel (17) during a backward iteration through the trellis. The second decoder (26) starts at a second time t 2L and provides a plurality of backward iteration state metrics ² for each state at each time interval. A processor (28) then performs a dual maxima computation at each state using the forward state metric, the backward state metric and the branch metric for same to provide a measure of the likelihood that a particular sequence of data was transmitted by the encoder (12). By performing forward and backward Viterbi decoding with dual maxima computations at each node within a window moved over the trellis, the inventive decoder (20) provides the performance benefits associated with a LOG-MAP decoder while avoiding the excessive memory requirements of same.