Iterative Coded Modulation With Code Rate Flexibility for Optical Space Communications

We propose an iterative coded modulation technique with code rate flexibility for optical space communications. It is a modified version of the existing SCPPM, which consists of the serial concatenation of an outer convolutional code, an interleaver, a bit-accumulator, and pulse-position modulation (PPM), and has been demonstrated to give the best performance on an optical link modeled as PPM/Poisson channels. The primary modification is to add a single-parity-check (SPC) code. We refer to the coded modulation as SPC-SCPPM. The two codes are decoded with an iterative demodulator-decoder, using standard turbo-decoding techniques. We compare the two coded modulation techniques on a PPM/Poisson channel. The rate-1/2 SPC-SCPPM provides performance similar to the SCPPM of the same parameters. However, SPC-SCPPM has been shown to possess code rate flexibility for all rates higher than 1/2 in contrast to SCPPM.