Reed-Solomon coding for free-space optical communications through turbulent atmosphere

The average power loss and the channel power fading are among the major challenges of the free-space optical (FSO) communications through the turbulent atmosphere, which result in severe bit-error-rate (BER) performance degradation, or even communication link outage. For the same average received optical signal power, the degradation in BER performance is strongly dependent on the fading statistics. This paper studies channel coding using the Reed-Solomon (RS) codes for improving the FSO communication system performance and communication distance. Large-scale numerical simulations have been conducted to obtain the channel fading statistics and average received power for different receiver aperture sizes and distances of focused laser beams along the horizontal paths. The avalanche photodiode (APD) photodetector is used for direct detection, and the accurate McIntyre-Conradi model is adopted for system performance evaluation. Under codeword symbol level channel interleaving, if the channel demodulator provides channel erasure information when the channel is in deep fades, the RS codes are able to achieve more than 13 dB coding gain at BER = 10^-5 and more than 17 dB coding gain at BER = 10^-6. It is also shown that for relatively weak fading channels, the channel coding using the RS codes can achieve the BER performance almost identical to that of the corresponding ideal fading-free channel. The optimal receiver aperture sizes under channel coding are also studied.