Optical space communications employing pulse position modulation

The authors review optical technologies for space communications. Initially the advantages of this technology are described and the tradeoffs between direct detection and coherent detection explained. As a model for deep space communications, the ideal photon counting channel is used and it is illustrated that the most efficient modulation format is pulse position modulation (PPM). They go on to consider optical sources and photodetectors and explain the reasoning that has led to the selection of GaAlAs semiconductor lasers and avalanche Si photodiode detectors. The channel configurations that are currently being pursued by NASA and ESA are described