Outage Probability and Average BER of UAV-Assisted RF/FSO System for Space-Air-Ground Integrated Networks under Angle-of-Arrival Fluctuations

Introducing free-space optical (FSO) communication into space-air-ground integrated networks (SAGINs) can enable the realization of high data rates to achieve next-generation wireless communication. However, investigating the performance of the unmanned-aerial-vehicle (UAV)-assisted dual-hop radio-frequency (RF)/FSO systems for SAGINs remains challenging owing to severe channel fading. This article proposes a UAV-assisted RF/FSO relay system with a decode-and-forward relay protocol. A unified statistical channel model that considers the influences of attenuation loss, atmospheric turbulence, pointing errors, and angle-of-arrival fluctuations on the FSO link is developed, and the Málaga distribution is employed to characterize atmospheric turbulence. Closed-form expressions of the outage probability and the average bit error rate are derived for the pure FSO link and overall RF/FSO relay system. We also derive expressions of the system metrics under atmospheric turbulence characterized by Gamma-Gamma and Log-normal distributions, leveraging the broader coverage of our proposed channel model. The effects of the system and channel parameters on the performance of the pure FSO link and the overall RF/FSO relay system are investigated. Finally, our analytical expressions agree well with the Monte Carlo simulation results, demonstrating the validity of the expressions.