Coherent Communications for Free Space Optical Low-Earth Orbit Downlinks

Abstract

This work addresses physical layer design aspects of coherent free-space optical downlinks from low-earth orbit satellites to ground. Achievable information rates are derived and assessed that include the availability of diversity, shaping, bitmetric decoding, repetition coding and automatic repeat request with maximum-ratio combining. A channel coding scheme is presented that approaches the theoretic limits within 1 dB. Extrinsic information transfer analysis for the free-space optical fading channel shows that a code design tailored to the additive white Gaussian noise channel is robust for fading channels with various parameters.