Signal Intensity Estimators for Free-Space Optical Communication With Array Detectors

Estimators motivated by the minimum mean square error estimation criterion are presented for the estimation of the optical signal intensity in free space optical communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected by using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected by using an avalanche photodiode (APD) array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected by using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a lognormal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood estimators for small scintillation indices while providing a performance similar to the maximum a posteriori estimator.