Design and performance of UAV-employed hybrid FSO/RF systems for weather-resilient 6G and beyond networks

Abstract

This study investigates the performance and optimization of a UAV-assisted hybrid Free Space Optics (FSO) and Radio Frequency (RF) communication system within a Space-Air-Ground Integrated Network (SAGIN) framework designed for 6G. The paper aims to enhance system reliability, energy efficiency, and capacity in dynamic environments affected by weather variability and the mobility of UAVs. The proposed hybrid architecture combines the high bandwidth of FSO with the robustness of RF, utilizing UAVs as relay nodes to counteract atmospheric attenuation, turbulence, pointing errors, and changing link conditions. FSO channels are modeled using Gamma-Gamma and Exponential Weibull distributions, while RF links consider Nakagami-m fading to reflect real-world impairments. Five advanced UAV relay selection strategies, Adaptive Threshold-Based Selection, Weighted SNR Optimization, Energy-Efficient Relay Selection, Multi-Objective Optimization, and Dynamic Multi-Hop Relay, are introduced to enhance SNR, energy efficiency, and reliability. A novel algorithm, Weather-Resilient Multi-Objective Swarm and Proximal Optimization with Dynamic Relay Selection (WRMSPO-DRS), is developed to optimize UAV trajectories and relay selection through weather-adaptive step sizes, proximal regularization, and energy harvesting via Simultaneous Lightwave Information and Power Transfer (SLIPT). Simulation results demonstrate strong performance under various conditions: in clear air, the system achieves a BER of 10⁻⁷ and an outage probability of 10⁻⁶ at 0 dBm (m = 3), three-hop relay configurations reach outage probabilities as low as 10^-30. The system's robustness further improves with higher RF quality and larger receiver apertures. These findings highlight the effectiveness of the hybrid FSO/RF system and intelligent relay selection in ensuring resilient 6G communications in challenging, dynamic environments.