Optical M-PPM signaling with binary forward error correction

Orthogonal modulation is a natural choice for free-space optical links due to the relatively simple modulation and demodulation options that are available. In particular, pulseposition modulation (PPM) takes advantage of the inherent average power limitations of some optical transmitters. In practical implementations, two types of optical PPM receivers are of interest: photon counting receivers and optical preamplified receivers. The differences between the two implementations result in different performance as well as different methods of analysis. In this paper, we consider error-correction coding options for both photon counting and preamplified versions of M-ary PPM, where M is the cardinality of the signal set. Of primary interest is the performance of commercially-available binary error-control codes, since such codes have widespread availability and near-optimum performance. We derive a capacity result to define the performance penalty from using binary decoding on M-ary orthogonal signalling.