Tunable Complexity RSSE Decoding of TCM Signals

Maximum likelihood sequence estimation (MLSE) of trellis coded modulation (TCM) signals affected by intersymbol interference (ISI) requires an optimal path search on a trellis diagram usually of a very large size. This optimal search can be performed in principle by a Viterbi algorithm, but is impractical due to the large number of trellis states. Among the various suboptimal alternatives, we concentrate on reduced state sequence estimation (RSSE), in which the trellis diagram is reduced in size by partially representing the ISI characteristics into the trellis diagram itself. A Viterbi algorithm operates on the reduced trellis diagram, taking into account the effects of residual ISI components by means of per-survivor ISI cancellation. This technique entails the cancellation of residual ISI terms, necessary for computing the branch metrics, independently on each survivor path using the code sequence uniquely associated to the survivor path itself.