Using channel state information for low-latency applications in free-space optical communication

Channel coding techniques are widely applied to improve the reliability of high-speed terrestrial free-space optical (FSO) communication links, which are subjected to the atmospheric turbulence-induced signal fading. Since the channel variation is slow, the system performance can be further improved by using the instantaneous channel state information (CSI) at the receiver. The performance of a coding scheme applied in such systems mainly depends on the latency and data rate requirements. In this work, we analyse the code performance bounds of latency-constrained FSO links employing intensity modulation/direct detection (IM/DD) techniques. Assuming perfect receiver-side channel state information (CSI), we propose two approaches to evaluate the lower bound of the word error probability of a coding scheme for a given latency and information rate. While the first method uses Monte Carlo simulations, the second method uses an approximation to derive these bounds. Finally, using LDPC codes, we show that even under these low-latency constrains, a code can approach the performance bound, if CSI can be used at the decoding.