Receiver Architecture for Channel-Aided, OOK, APD-Based FSO Communications Through Turbulent Atmosphere

The problem of optimum signal detection for the channel-aided free-space optics (FSO) communications is addressed. It is assumed that the avalanced-photo-detection (APD) is utilized for the direct-detection (DD) of on-off-keying (OOK) signals impaired by optical turbulence. The average primary photon count due to the background radiation is assumed to be large enough to allow for a Gaussian operation for the APD receiver. In the presence of large signal fluctuations, it is shown that the optimum structure, motivated by the maximum a-posteriori (MAP) rule, is a dual-threshold architecture where the thresholds are functions of the channel state. To remedy the channel effects, the channel state is estimated and the estimates are used to set the receiver thresholds, thereby resulting in a channel-aided scenario. Performance of the proposed optimum receiver is assessed in terms of the overall bit error rate. To assess performance, it is assumed that the turbulent channel obeys a Gamma-Gamma (GG) distribution and that the channel estimator yields an unbiased and efficient estimate of the channel state. This study shows that channel-aided reception reduces the error floor. Further, channel-aided receivers have error-floor-free signal ranges that are larger than those of the standard receivers´.