TCM on frequency-selective fading channels: a comparison of soft-output probabilistic equalizers

Suboptimal, low-complexity nonlinear equalizer structures derived from the probabilistic symbol-by-symbol maximum a posteriori probability (MAP) algorithm and the M-ary soft-output Viterbi algorithm (SOVA) are investigated. Both algorithms deliver reliability information for each symbol. The complexity of both algorithms in their simplest form is of the order of the conventional reduced-state Viterbi equalizer with hard outputs. Simulation results for a terrestrial time-varying frequency-selective fading channel are given. Realistic channel estimation at high Doppler speeds is included. The coding gain of the investigated 8-state trellis-coded 8-PSK scheme is about 6 dB at a BER (bit error rate) of 10^-3. The gain by making use of soft-decisions is about 4-5 dB at a BER of 10^-3