On the Beam Width Optimization for the Ergodic Capacity of FSO Channels With Misalignment Errors Modeled by Beckmann Distributions

Despite the growing interest in underwater and space applications, terrestrial freespace optical (FSO) links still remain of main priority, and the performance of such systems is deeply dependent not only on weather conditions but also on atmospheric scintillation and misalignment errors. In this paper, a thorough investigation of the capacity performance of FSO systems in the presence of generalized pointing errors is carried out. Novel approximate closed-form solutions to estimate the capacity from low to high signal-to-noise ratio (SNR) are for the first time presented and carefully analyzed when pointing errors follows the wellknown Beckmann distribution. By using the developed expressions, it is concluded that the capacity can be optimized according to a specific beam width, achieving an improvement on the order of ≈10 dB. The most striking thing is that the optimum beam width value depends on the SNR range, i.e., on atmospheric scintillation and misalignment errors, deriving different optimum values at low, medium, and high SNR. Analytical results are verified by Monte Carlo simulation results.