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Numerical Evaluation of PPM for Deep-Space Links

We numerically evaluate the deep-space communication performance in a broadband lossy channel of coherent pulse position modulation (PPM) with an on/off receiver, single-symbol square root detection, and Holevo information. We also consider quadrature amplitude modulation (QAM) signals and phase-shift keying signals with dyne-type detections. We show the quantitative gap between these detection strategies in terms of the capacity, particularly in the quantum-limited region where the quantum noise seriously limits the transmission rate. In particular, we find that for an extremely weak signal input power, use of a multilevel PPM system is a good strategy, whereas for an extremely strong signal, use of a multilevel QAM system is recommended.