Optimum and Suboptimum Detection Mechanisms for Free-Space Binary PPM Optical Communication Systems with APD Detector Arrays

摘要

Receiver architectures are suggested for avalanche photodiode (APD) array detection of binary pulse-position modulated (PPM) in direct-detection free-space optical (FSO) communication systems. It is assumed that the received signal intensity is large enough so that the integrated currents at the outputs of APD detectors may be modelled accurately as Gaussian random variables. Receivers motivated by the maximum a posteriori (MAP) rule are suggested for APDs operating in Gaussian regime. Due to the complexity of the suggested receivers, suboptimum combining schemes are presented. In one scheme, a simple summation of the output of APDs is utilized to render decision. In alternate schemes, a weighting mechanism that combines the output of detectors in an optimum manner is suggested. The proposed weighting mechanism maximized signal-to-noise ratio (SNR). It is assumed that the signal intensity is impaired by optical scintillation and that the signal intensity is estimated at the receiver. Simulation and analytical results are presented to underscore the superiority of the weighted combining methods as compared to the simple summation algorithm, and to compare the performance of the optimum receivers with the linear receivers suggested here.