Phase and 3-D Placement Optimization for Rate Enhancement in RIS-Assisted UAV Networks

In this letter, we aim at improving the system sum-rate performance of an assisted aerial base station (ABS) network by considering use of reconfigurable intelligent surface (RIS). We jointly optimize the RIS phase shifts and ABS altitudes using the proposed conjugate gradient and particle swarm optimization (CG-PSO) scheme. Moreover, we divide the problem into sub-problems and solve them using an alternative optimization method. First, an optimal RIS phase shift is computed using the conjugate gradient (CG) under the fixed ABS altitude and varying transmit power. Finally, we find the optimal ABS altitude using particle swarm optimization (PSO), which results in improved sum rate under the optimized RIS phase shift. The results show that the CG-PSO scheme significantly improves the system sum rate compared to the baseline schemes.