Orientation Optimization With Low-Bit Discrete Phase Shifts for RIS-Aided Communication

Reconfigurable intelligent surface (RIS) is a promising technology for enhancing spectral efficiency (SE) in communication networks. Although discrete phase shifting is cost-effective and preferred for RIS, it cannot always provide the optimal phase shift required to steer the reflected signal in the desired direction, resulting in a performance reduction. Given that the orientation of RIS significantly impacts the amplitude and phase of incident signals, this letter proposes to utilize the RIS orientation as an additional degree of freedom to mitigate the performance degradation caused by discrete phase shifting. To handle the non-convex constraint of the SE upper bound maximization problem, we propose an alternating optimization (AO) algorithm to design RIS phase shift and optimize orientation iteratively. Specifically, the RIS orientation is represented using a rotation matrix, and the optimization principle on the manifold is applied to address the inherent overdetermination of rotation matrices. Simulation results confirm the effectiveness of the proposed algorithm and demonstrate a significant improvement in system performance by incorporating RIS orientation information.