Iterative Beamforming Algorithm for Active RIS-Aided MIMO Systems

Abstract

The reconfigurable intelligent surface (RIS) is a promising technology for the beyond-5G and 6G wireless communication systems. It can improve the wireless communication performance, facilitate interference cancellation between users and other signals. In this paper, we present an active RIS-aided multiple-input multiple-output (MIMO) system, in which we configure an active RIS to assist point-to-point communication between a multiple-antenna base station (BS) and multiple-antenna user equipment (UE). Specifically, the transmit precoder at the BS and the reflecting coefficient matrix at the RIS are optimized to maximize the spectral efficiency (SE) at the UE based on an alternating optimization (AO) algorithm, where the precoder and reflecting coefficient are iteratively updated. First, the precoder at the BS is optimized using the singular value decomposition (SVD) method while keeping the RIS reflecting coefficient matrix fixed. Second, for the fixed transmit precoder, the RIS reflecting coefficient matrix is updated using the semidefinite relaxation (SDR) method. We evaluate the proposed algorithm against conventional passive and active RIS schemes and show that the proposed scheme outperforms the compared architectures in terms of SE for single-user MIMO systems.