Robust Beamforming and Phase Shift Optimization for RIS-Assisted UAV Multiuser MISO Communication Networks

Unmanned aerial vehicles (UAVs) are emerging as promising platforms to provide flexible and low-cost services for ground users of hard-to-reach areas in the era of 6G. Nevertheless, the limited battery life of UAVs poses a significant challenge when it comes to establishing reliable air-ground connections. A potential solution to address this challenge is to exploit reconfigurable intelligent surfaces (RIS), wherein RIS planes are affixed on UAVs to enhance energy efficiency. However, this scheme is subject to unwanted UAV oscillations. This paper presents a robust optimization framework for RIS-assisted UAV multiuser multiple-input single-output (MISO) communication networks while considering the practical undesired oscillations for UAV. Specifically, we formulate a weighted sum rate (WSR) maximization problem by jointly optimizing the transmit beamforming of the base station (BS) and the phase shift of RIS under UAV's undesired oscillations. To effectively tackle the intricate non-convex nature of the problem, we propose a robust block coordinate Lagrangian dual transform (RoBl) scheme by leveraging the block coordinate descent (BCD) and Lagrangian dual transform. Simulation results demonstrate that the proposed scheme can achieve a superior WSR compared to the state-of-the-art benchmarks, and confirm the effectiveness and robustness of our proposed scheme.