Switching-Based HAP Relays with Hybrid SAG-FSO/RF Transmission over Correlated Turbulence for Satellite Communications

Free-space optical (FSO) communications are gaining increased attention as a possible candidate for next-generation satellite communication (SatCom) systems. However, their performance is vulnerable to beam wandering and atmospheric turbulence effects. Hybrid FSO/radio frequency (RF) and space-air-ground (SAG) FSO transmission schemes are proposed to increase the reliability and improve the performance of SatCom systems. In this paper, we propose and study switching-based HAP relays with a hybrid SAG-FSO/RF transmission for Sat-Com. The transmission system will switch between HAPs based on the ground-HAP channel quality. Meanwhile, the ground-HAP FSO transmissions may encounter correlated turbulence when propagating through the atmosphere. To evaluate the performance of the transmission system, we analytically derive the expression for the average symbol error probability and validate it by Monte Carlo simulations. Numerical results show that the proposed system significantly outperforms single HAP and hybrid FSO/RF transmission systems, even under the adverse effect of correlated turbulence.