Performance analysis of RIS-assisted dual-hop mixed FSO-RF UAV communication systems

In this paper, we investigate a Reconfigurable Intelligent Surface (RIS)-assisted Free-Space Optics–Radio Frequency (FSO–RF) mixed dual-hop communication system for Unmanned Aerial Vehicles (UAVs). In the first hop, a source UAV transmits data to a relay UAV using the FSO technique. In the second hop, the relay UAV forwards data to a destination Mobile Station (MS) via an RF channel, with the RIS enhancing coverage and performance. The relay UAV operates in a Decode-and-Forward (DF) mode. As the main contribution, we provide a mathematical performance analysis of the RIS-assisted FSO–RF mixed dual-hop UAV system, evaluating outage probability, Bit-Error Rate (BER), and average capacity. The analysis accounts for factors such as atmospheric attenuation, turbulence, geometric losses, and link interruptions caused by UAV hovering behaviors. To the best of our knowledge, this is the first theoretical investigation of RIS-assisted FSO–RF mixed dual-hop UAV communication systems. Our analytical results show strong agreement with Monte Carlo simulation outcomes. Furthermore, simulation results demonstrate that RIS significantly enhances the performance of UAV-aided mixed RF/FSO systems, although performance saturation is observed due to uncertainties stemming from UAV hovering behavior.