When UAV Base Station Enabled System Meets RIS-UAV Relay Assisted System: Joint Trajectory and Resource Allocation Optimization

Owing to the advantages of good air-to-ground links, high maneuverability, and easy deployment of unmanned aerial vehicles (UAVs), they can act as base stations (BSs) or relays to provide reliable communications for ground users. This paper considers a dual-system model consisting of a UAV BS enabled communication system and a reconfigurable intelligent surface installed UAV (RIS-UAV) relay assisted communication system. The dual-system rate is maximized via jointly optimizing user scheduling, phase shifts of RIS, power allocations of UAV BS and ground BS, and dual-UAV trajectories. Since the original optimization problem is non-convex and hard to solve directly, it is divided into four sub-optimization problems, including user scheduling optimization, phase shift optimization, power allocation optimization and trajectory optimization, in which the successive convex approximation (SCA) method is employed to solve them. Then, a four-layer alternating iterative optimization algorithm is proposed to achieve the suboptimal solutions of the original optimization problem by alternately optimizing the four sub-optimization problems. The results of the simulation indicate that compared with other benchmarks, the proposed optimization algorithm can significantly improve the average dual-system rate.