Maximum-likelihood spatial-diversity reception on correlated turbulent free-space optical channels

In free-space optical links using intensity modulation and direct detection, atmospheric turbulence can cause signal fading. Spatial-diversity with multiple receivers can mitigate this fading, improving the system performance. When the instantaneous fading state is not known, the conventional space-diversity detector must employ equal-gain combining (EGC) and simple threshold detection, but the correlation of fading at different receivers can reduce the diversity gain achieved by EGC. We derive expressions for the fading correlation at different receivers by considering a physical model for optical wave propagation. Using this correlation model, we derive the maximum-likelihood (ML) decision rule under the assumption that the fading correlation properties are known, but the instantaneous fading state is not known. We have performed numerical simulations for the dual-receiver case. Our results show that the ML scheme provides better performance than EGC, particularly when the fading at different receivers is highly correlated and/or when the average SNR is high