Adaptive Channel Coding and Power Control for Practical FSO Communication Systems Under Channel Estimation Error

Adaptive channel coding and power control for practical free-space optical communication systems are proposed in this paper. Particularly, we first assume that the channel state information (CSI) is perfectly known at the transmitter, and propose adaptive transmission schemes in which coding rate is adjusted either independently or jointly with transmit power according to the channel conditions. Moreover, an optimization problem is developed to attain power consumption minimization under free space optical (FSO) practical constraints, i.e., target bit-error rate, target outage probability, and maximum transmit power. We then solve the optimization problem and derive closed-form expressions for throughput and average transmit power for both adaptive schemes over the Gamma-Gamma atmospheric turbulence channels. Our results demonstrate the superiority of the proposed adaptive schemes over non-adaptive schemes, especially under strong turbulence conditions. In the second part of this paper, we move toward a more realistic scenario where the channel is not perfectly known to Tx/Rx and is estimated by the sequence of received signals. Therefore, we investigate the effect of channel estimation inaccuracies on the performance of the proposed schemes and show that the accuracy of the channel estimator depends on the length of the sequence and, for a sufficiently large length of the observation window, it achieves performance close to the receiver with known CSI. Extensive analytical derivations are provided to investigate the performance of the proposed adaptive schemes under channel estimation error. Our analytical results can be used for calculating and tuning of the optimum length of the observation window without resorting to Monte Carlo simulations.