Optimum detection threshold for free-space optical communication with atmospheric scintillation

Scintillation caused by atmospheric turbulence impairs the performance of the free-space optical (FSO) communication system. Bit error rate (BER) of a FSO communication system can be improved by using an adaptive detection threshold, which is difficult to implement in real time. As a sub-optimum method, optimizing the threshold to a constant value according to statistics of parameters is proposed. Assuming multiplicative Gaussian noise and lognormal scintillation, the optimum constant threshold of a FSO communication system operating in an atmospheric turbulence channel is analyzed. The result of numerical analysis is compared with experimental data obtained in weak turbulence. Results of experiment and numerical analysis show that optimizing constant threshold is necessary and the performance degradation compared to adaptive thresholding is acceptable. The dependence of the optimum constant threshold on log amplitude variance and average received signal power is analyzed and the influence of other parameters is proved to be negligible. Based on previous analysis, a non-linear model is proposed to estimate the optimum constant threshold in practice.