Joint UAV Trajectory and Beamforming Designs for RIS-Assisted MIMO System

This paper considers a wireless downlink multiple-input multiple-output (MIMO) system, where communication between the base station and multiple users is facilitated by a reconfigurable intelligent surface (RIS) deployed on an unmanned aerial vehicle (UAV). Under the $\ell _{0}$ -norm-based capacity-limited backhaul, the average weighted sum rate is maximized over the flying period by jointly designing the user scheduling, UAV's trajectory, active beamforming and passive beamforming. Given the highly coupled variables and non-convex combinatorial constraints, the underlying problem is decomposed into three subproblems. Relaxing the mixed-integer constraint as a weighted $\ell _{1}$ -norm, the transmit precoding and user scheduling can be jointly obtained by the conic relaxation. The RIS phase shift with unit-modulus constraint is obtained via the alternating direction method of multipliers. The UAV's trajectory is optimized by the successive convex approximation approach. Numerical results demonstrate the effectiveness of the proposed algorithm in improving the average achievable rate within the constraints of a finite-capacity backhaul.