Error performance analysis for deep space laser communication system with SPAD

There has been a growing interest in employing single photon avalanche diode (SPAD) array detectors for laser communication systems in deep space communications. The most important factor affecting the performance of SPAD array in communication systems is the dead time. In this paper, we investigate the photocount distribution for a small-scale SPAD array with a fixed short dead time. We employed Gaussian, Poisson, and shifted Poisson to fit the SPAD photocount distribution and compare the goodness of fit for various array sizes. With the SPAD photocount distribution, the capacity of the PPM-SPAD system is investigated. In addition, we proposed a new log-likelihood ratio (LLR) formula using shifted Poisson fitting results. The error performance of PPM-SPAD under a deep space communication scenario is simulated. When the dead time of a SPAD to the counting interval ratio <i>θ</i> = 0.6, the new LLR provides about 2 dB coding gain over the commonly used Poisson LLR. These results give helpful insight for designing practical SPAD-based optical communication systems.