Channel Aging-Aware Precoding for RIS-Aided Multi-User Communications

Abstract

Reconfigurable intelligent surfaces (RISs) have received intensive attention for their outstanding potential of reconstructing wireless propagation channels to significantly enhance the communication quality. In this article, we study the sum spectral efficiency (SE) of a multi-user communication system aided by an RIS. As opposed to previous works that assume independent and identically distributed block-fading channel models, we focus on a realistic scenario with the impact of channel aging due to user mobility and the spatial correlation among the reflecting elements of the RIS. Particularly, we first present a joint pragmatic model which encompasses the impact of channel estimation error and channel aging that facilitate the analytical derivations for the sum SE of the RIS-aided system. On this basis, we formulate the joint transmit beamforming and phase shift design as a nonconvex optimization problem to maximize the system sum SE. To address the design problem, we propose an efficient suboptimal channel aging-aware precoding (CAP) algorithm to obtain efficient suboptimal solutions for the considered problem capitalizing on the generalized fractional programming, the alternating direction method of multipliers method, and the alternating optimization method. Simulation results are provided to verify the effectiveness of the proposed algorithm and illustrate the impact of key system and channel parameters on the system performance. Our analysis unveils that a higher transmit power at the base station, a larger value of temporal correlation coefficients, a larger number and the size of RIS elements are beneficial to the system performance.