Outage Probability Analysis and Joint Optimization for UAV-Aided FSO/RF Systems With Nonlinear Power Amplifiers

The unmanned aerial vehicle (UAV)-aided free-space optical/radio frequency (FSO/RF) technique has recently attracted significant attention, which offers new possibilities thanks to its dynamic deployment capability. However, many researchers have not considered the fading characteristic of both RF and FSO links and the limited UAVs' size, weight, and power (SWAP) simultaneously. This paper focuses on a mixed FSO/RF system that facilitates the communication between a base station and mobile stations via a hovering UAV acting as a decode-and-forward relay. Considering the fading effects and the power amplifier (PA) non-linearity caused by the SWAP constraint, we mathematically analyze the closed-form and asymptotic outage probabilities. Especially in high transmit power regions, we present the destructive impact of nonlinear PAs on the outage probability and investigate modulation schemes to mitigate this degradation. The derived outage expressions match well with numerical results. Moreover, we propose a joint placement and transmit power optimization algorithm that minimizes the end-to-end outage probability with round-robin and absolute signal-to-noise ratio-based scheduling schemes. Finally, simulation results show that the performance of the proposed algorithm is comparable to that of the brute-force method.