Optimization of Mixed FSO-RF Downlink Systems With SLIPT-Enabled UAV Base Station and HAP-Mounted Optical RIS

In this paper, we investigate a mixed free-space optics (FSO)-radio frequency (RF) downlink communication system enhanced by a simultaneous lightwave information and power transfer (SLIPT)-enabled unmanned aerial vehicle base station (UAV-BS) and a high-altitude platform (HAP)-mounted optical reconfigurable intelligent surface (ORIS). We develop overall optimization frameworks for end-to-end systems aimed at minimizing transmitted power at optical ground station (OGS) for backhaul link or maximizing the multiuser fairness rate in the access link. This is achieved by jointly optimizing the UAV-BS placement, beamforming strategy, OGS power allocation, and power-splitting (PS) ratio under energy efficiency and quality of service (QoS) constraints. The formulated optimization problems are highly non-linear and non-convex, posing significant challenges for conventional methods. To address this, we employ inner approximation techniques and propose an iterative algorithm to achieve efficient solutions. Simulation results demonstrate substantial performance gains compared to conventional benchmarks, highlighting the efficacy of the proposed approach.