Enhancing Spectral Efficiency of Ground-to-HAP FSO System with Adaptive MASK in Presence of Beam-wander and AoA Fluctuation

High Altitude Platform Station or HAP is an indispensable component for the upcoming wireless communication technologies. This paper presents an evaluation of the performance of a Ground-to-HAP communication system using free-space optical (FSO) technology. The performance of the system is determined by three factors: channel state, pointing error, and angle-of-arrival (AoA) fluctuation. Accordingly, the modulated-Gamma distribution is used as a channel modeling of the Ground-to-HAP uplink communication to analyze the effect of turbulence and beam wandering on the channel state. In this paper, a combined PDF of the uplink channel is driven considering atmospheric turbulence, beam wandering, pointing error, and link interruption due to AoA fluctuations. Furthermore, the performance of the system is presented in terms of average symbol error rate (ASER) as well as the outage probability for a signal modulated by using a Multi-Level (M-ary) ASK scheme. Finally, an ASER adaption is performed to maximize the number of bits transmitted per symbol period for different SNR values, enhancing spectral efficiency. © 2022 IEEE.