Performance enhancement of mixed RF–FSO networks via UAV positioning and opportunistic user scheduling

Abstract This paper investigates the enhancement of communication performance in mixed radio frequency–free space optical (RF–FSO) wireless networks by employing unmanned aerial vehicles (UAVs) as mobile relays. A dual-hop transmission model is considered, where the UAV acts as an intermediary between a fixed ground source and multiple mobile users distributed along the boundary of a predefined geographical region. To improve the reliability and efficiency of the system, we propose a joint optimization framework that determines the UAV’s optimal altitude and horizontal location. Additionally, an opportunistic user scheduling mechanism is implemented, selecting the mobile user with the highest instantaneous signal-to-noise ratio (SNR) for communication. The RF link is modeled using Rayleigh fading statistics, and the FSO link is modeled using the Gamma–Gamma fading distribution, capturing the effects of atmospheric turbulence, pointing errors, and angle-of-arrival fluctuations. Closed-form expressions for outage probability and its asymptotic approximation at high SNR levels are derived to evaluate system performance analytically. The analytical results are further validated through Monte Carlo simulations. The outcomes demonstrate that the proposed UAV positioning strategy and user scheduling scheme effectively minimize outage probability and significantly enhance the robustness of the RF–FSO communication system. These results provide valuable insights for the deployment of UAV-assisted hybrid optical wireless networks in dynamic or infrastructure-constrained environments.