CONSTRUCTION OF TRELLIS CODES WITH A GOOD DISTANCE PROFILE

Systematic feedforward trellis codes for s-PSK and 16-QAM modulation a re constructed using a nested step by step algorithm which guarantees a good distance profile. This makes the codes suitable for use with se quential decoding, where a rapidly growing distance profile is needed to reduce the average number of computations. In addition to having a good distance profile, the new codes achieve asymptotic coding gains u p to 6.53 dB. A procedure based upon the Fano algorithm (FA) is used t o calculate the free distance of the new codes. This procedure is very effective for finding the free distances of long trellis codes becaus e of the computational and storage efficiency of the FA. From a compar ison of the new systematic feedforward codes with Ungerboeck's systema tic feedback codes, we conjecture that a systematic feedforward code o f constraint length 2nu can achieve the same free distance as a system atic feedback code of constraint length nu.