Joint Beamforming and Trajectory Optimization for UAV Communication Assisted by Intelligent Reflecting Surface

Unmanned aerial vehicle (UAV) communication system assisted by intelligent reflecting surface (IRS) has promising applications in future wireless communication. Considering the realistic high-efficient IRS-UAVs transmission system in the three-dimensional (3D) space, we propose a joint beamforming and UAVs' trajectory optimization algorithm to maximize the secrecy energy efficiency (SEE) of the system, which is constrained by the mobility of UAVs and the number of access users. In the proposed algorithm, the beamforming vector, multi-IRS diagonal phase shift matrix, and UAVs' trajectory are optimized alternately using block coordinate descent (BCD). We not only derive the optimal closed-form expression of the multi-IRS diagonal phase shift matrix, but also solve the 3D trajectory optimization of UAVs under the given beamforming matrix. Simulation results show the effectiveness of the proposed algorithm and the performance improvement for IRS-UAVs communication.