Joint Optimization of Phase Shift Matrices and Trajectory for AF Relay-Based Cooperation Communication With RIS-Enabled UAV System

In the complicated or hard-to-reach communication areas, sometimes much higher average achievable rate (AAR) or wider coverage is in urgent and temporary need. To address this challenge, this article proposes a novel amplify-and-forward (AF) relay-based cooperation communication scheme with reconfigurable intelligent surface (RIS)-enabled unmanned aerial vehicle (UAV) enhancing. The UAV acts as a moving RIS in the sky, which provides additional links to enhance the performance of the AF relay-based cooperation on the ground. Aiming to maximize the AAR of the proposed scheme, we jointly optimize the trajectory of UAV and the phase shift matrices of RIS over total time frames. At first, we decompose the original intractable problem into two subproblems of the phase shift matrices optimization and trajectory optimization. Subsequently, the optimal phase shift matrices are achieved by aligning the phases of received signals. Since the trajectory optimization subproblem is still complicated and nonconvex, we apply the successive convex approximation (SCA) method to reformulate it into a convex problem and further solve it by iterative method. Simulation results demonstrate the superiorities of the proposed scheme as compared with the benchmark schemes and also reveal the effects of various factors on its performance.