Soft output MAP equalization and trellis coded modulation for severe frequency-selective fading channels

The authors propose an algorithm that makes maximum a posteriori (MAP) equalization feasible for severe frequency-selective fading channels. In the algorithm the fully blown up intersymbol interference trellis is transformed into a set of parallel trellises with few states by means of tentative decisions. MAP equalization is performed on each subtrellis. The authors study the full MAP algorithm as well as reduced-complexity variants operating on parallel subtrellises. The proposed equalizer gives soft decisions which can be exploited by a successive decoder. The performance of the proposed algorithm was compared to M-algorithm equalization by extensive computer simulations. Results are presented for uncoded and trellis-coded signals for typical HF fading channels. The proposed soft output equalizer offers an additional soft decision coding gain of about 2.5 dB for short channel impulse responses and an improvement in bit error rate of two orders of magnitude for long channel impulse responses with a simple trellis code