Optical Receiver for Coherently Detected Pulse-Position Modulated Signals in the Presence of Atmospheric Turbulence

Performance analysis and experimental verification of a coherent free-space opti-cal communications receiver in the presence of simulated atmospheric turbulence is presented. Bit-error rate (BER) performance of ideal coherent detection is analyzed in Section II, and the laboratory equipment and experimental setup used to carry out these experiments are described. The key components include two lasers oper-ating at a 1064-nm wavelength for use with coherent detection, a 16-element (4×4) focal-plane detector array, and a data acquisition and signal processing assembly needed to sample and collect the data and analyze the results. The detected signals are combined using the least-mean-square (LMS) algorithm. In Section III, conver-gence of the algorithm for experimentally obtained signal tones in the presence of atmospheric turbulence is demonstrated. In Section IV, adaptive combining of experimentally obtained heterodyned pulse-position modulated (PPM) signals with pulse-to-pulse coherence, in the presence of simulated spatial distortions resembling atmospheric turbulence, is demonstrated. The adaptively combined PPM signals are phased up via an LMS algorithm suit-ably optimized to operate with PPM in the presence of additive shot noise. A convergence analysis of the algorithm is presented, and results with both computer-simulated and experimentally obtained PPM signals are analyzed. I