Compared merits of hard/soft decoding for coherent optical communications impaired by phase noise

Phase noise of semiconductor lasers severely degrades the performance of coherent optical fiber communication systems, imposing restrictions on the admissible laser linewidth. The application of a forward error controlling (FEC) scheme is a suitable means to combat the bit-error-rate (BER) degradation due to phase noise and to relax the laser linewidth requirement. In this paper, a heterodyne coherent binary phase-shift keying (BPSK) system and a heterodyne differential phase-shift keying (DPSK) system, both with a Viterbi decoded convolutional coding scheme, are investigated to illustrate the improvements of the system performance. In particular, for the first time, the use of soft decision decoding in such systems is analyzed and its advantage over hard decision decoding is demonstrated