QoS-driven UAVs placement optimization and bandwidth allocation in mixed FSO/RF systems with OIRS integration

This paper addresses the problem of ensuring high-capacity, reliable wireless connectivity in temporary or disaster-stricken urban environments. We propose an optical intelligent reflecting surface (OIRS)-enhanced free-space optical (FSO) backhaul framework integrated with unmanned aerial vehicles (UAVs) to overcome the limitations of conventional FSO systems, such as line-of-sight (LOS) blockages. Specifically, OIRS deployed on high-rise buildings intelligently redirects optical beams, creating virtual LOS paths and stabilizing FSO links between the ground station and UAVs. To fully exploit this architecture, we develop a comprehensive channel model that accounts for path loss, turbulence-induced fading, and pointing errors, and introduce the OPT-UAV algorithm for joint UAVs 3D placement, user association, and bandwidth allocation under backhaul capacity constraints. Numerical results demonstrate that the proposed framework improves fairness and coverage by serving up to 67% of mobile users, compared to 62% with baseline methods, such as the TLA algorithm, while maintaining competitive throughput. These results confirm the potential of OIRS-assisted mixed FSO/RF systems as a scalable and resilient solution for smart city deployments, disaster recovery, and temporary high-traffic events.