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Ground-based reception of optical signals from space suffers from degradation of the optical phase-front caused by atmospheric turbulence, leading to a reduction in the effective diameter of the receiving aperture and to random fluctuations of the point spread function in the focal plane. A proportional increase in the receiver's field of view, required to collect all of the signal, also causes a corresponding increase in the amount of interfering background radiation, resulting in degraded communications performance. These problems may be mitigated through the use of an optical detector array assembly in the focal plane that can adaptively select areas of higher signal density while ignoring areas predominated by background noise. This concept is investigated for both Poisson photon counting detector arrays and avalanche photodiode arrays. Kolmogorov phase screen simulations are used to model the sample functions of the focal-plane signal distribution due to turbulence and to generate realistic spatial distributions of the received optical field. The optimum photon counting array detector is derived and approximated by a simpler suboptimum structure that replaces the continuous weighting function of the optimal receiver by a hard decision on the selection of the signal detector elements. It is shown that for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single detector photon counting receivers, when observing turbulent optical fields in high background environments. For the avalanche photodiode detector case, it is shown that gains of up to 4 dB may be achieved by using the array receiver rather than a single APD, but that a photon-counting array still performs about 5.5 dB better than an APD array.
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