A coded modulation scheme for deep-space optical communications is proposed, which is composed of an outer single- parity-check （SPC）-based product code, an interleaver, a bit-accumulator and a pulse-position modulation （PPM）. It is referred as SPC-APPM code, which is decoded with an iterativc demodulator-decoder using standard turbo-decoding techniques. Investigations show that the scheme has the advantages of low encoding and decoding complexities, good performance and flexible code rate for all rates above I/2. Meanwhile, simulation results demonstrate that the SPC-APPM provides the performance similar to the low-density parity-check-APPM （LDPC-APPM）, superior to the LDPC-PPM and product accumulate code-PPM （PA-PPM）, although inferior to serially concatenated PPM （SCPPM）. At the bit error rate （BER） of 105, the performance of SPC-APPM is about 0.7 dB better than LDPC-PPM and 1.2 dB better than PA-PPM.
A new decoding scheme for product accumulate (PA) code over a space optical Poisson/ pulse-position modulation (PPM) channel is investigated. In this scheme, the PPM and the accumulator of the PA code are taken as a single inner code, decoded with an iterative demodulating-decoding technique based on Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm, rather than belief propagation (BP) algorithm in the original scheme. Simulation results show that this scheme provides much better bit error rate (BER) performance. At a BER of 10-5, the new scheme has a gain of 1.8 dB more than the original one. In addition, extrinsic information transfer (EXIT) charts are employed to analyze and compare the performance. The results indicate that the new scheme has not only better BER performance, but also lower error floor.
