  Thanks to the line-of-sight (LoS) transmission and flexibility, unmanned
aerial vehicles (UAVs) effectively improve the throughput of wireless networks.
Nevertheless, the LoS links are prone to severe deterioration by complex
propagation environments, especially in urban areas. Reconfigurable intelligent
surfaces (RISs), as a promising technique, can significantly improve the
propagation environment and enhance communication quality by intelligently
reflecting the received signals. Motivated by this, the joint UAV trajectory
and RIS's passive beamforming design for a novel RIS-assisted UAV communication
system is investigated to maximize the average achievable rate in this letter.
To tackle the formulated non-convex problem, we divide it into two subproblems,
namely, passive beamforming and trajectory optimization. We first derive a
closed-form phase-shift solution for any given UAV trajectory to achieve the
phase alignment of the received signals from different transmission paths.
Then, with the optimal phase-shift solution, we obtain a suboptimal trajectory
solution by using the successive convex approximation (SCA) method. Numerical
results demonstrate that the proposed algorithm can considerably improve the
average achievable rate of the system.
