Deep-Space Optical Communications Downlink Budget: Modulation and Coding

A link budget for a deep-space optical channel depends in part on the choice of modulation format and error-control coding scheme. This article describes several properties of the channel capacity that lead to an appropriate selection of modulation format, pulse-position modulation (PPM) order, and error-control code rate. It also describes performance limits when additional constraints—such as bounds on average power, peak power, and uncoded symbol-error rate—are imposed. We compare these limits to the performance of Reed–Solomon codes and convolutional codes concatenated with PPM, and show that, when iteratively decoded, the concatenated convolutional codes operate approximately 0.5 dB from capacity over a wide range of signal levels, about 2.5 dB better than Reed–Solomon codes.

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