Transmission Design for STAR-RIS Assisted Dual-User Uplink MIMO Based on Statistical CSI

In this paper, a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is employed to assist uplink dual-user multiple-input multiple-output (MIMO) communications, which can create virtual links from users in both transmission and reflection region to the base station throughout the entire space, thereby enhancing coverage and reducing energy consumption. We aim to jointly design the transmit covariance matrix for the two users and the phase shift matrices for the STAR-RIS, based solely on statistical channel state information. The implemented alternating optimization algorithm for joint design is achieved by utilizing the derived closed-form expression of the ergodic sum rate. Specifically, the optimal transmit covariance matrices for the two users are determined using a water-filling algorithm, meanwhile the suboptimal solutions for the phase shift matrices of the STAR-RIS are obtained via the projection gradient ascent method. Simulation results demonstrate that the proposed algorithm achieves superior system performance.