Understanding the performance of atmospheric free-space laser communications systems using coherent detection

We introduce a realistic model for the impact of atmospheric phase and amplitude fluctuations on free-space links using either synchronous or nonsynchronous detection. We compare options for atmospheric compensation, including active modal methods and diversity combining techniques. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various parameters, including the ratio of receiver aperture diameter to wavefront coherence diameter, the scintillation index, the number of modes compensated, and the number of independent diversity branches combined at the receiver. We analyze outage Shannon capacity, placing upper bounds on the achievable spectral efficiency and enabling the performance of specific system designs to be predicted.