Capacity of Coherent Laser Downlinks

Accurate knowledge of information data capacity between a spacecraft and optical ground stations is of utmost importance for assessing the feasibility of laser downlinks in future high bandwidth communication systems from space. For the additional cost and complexity of the downlink systems, there is a growing interest for the sensitivity and spectral efficiency potential of coherent optical receivers. But in practice, as the link includes part of the atmosphere, the potential of coherent receivers is seriously affected by phase distortions and fading to the link induced by clear-air turbulence. Conveniently, coherent channel-matched array receivers consisting of multiple subapertures can reduce signal fading. Also, wavefront distortions can be mitigated in principle with adaptive optics. Here we provide a comprehensive, unified analysis of these two fundamental techniques used in atmospheric downlink coherent system. Our results demonstrate how array technologies, as an alternative to adaptive optics, are expected to expand capacity of laser downlinks by tracking and correcting atmospherically distorted signals.