Robust Hybrid Beamforming Design for Multi-RIS Assisted MIMO System With Imperfect CSI

Abstract

Reconfigurable intelligent surface (RIS) has been developed as a promising approach to enhance the performance of fifth-generation (5G) systems through intelligently reconfiguring the reflection elements. However, RIS-assisted beamforming design highly depends on the channel state information (CSI) and RIS's location, which could have a significant impact on system performance. In this paper, the robust beamforming design is investigated for a RIS-assisted multiuser millimeter wave system with imperfect CSI, where the weighted sum-rate maximization problem (WSM) is formulated to jointly optimize transmit beamforming of the BS, RIS placement and reflect beamforming of the RIS. The considered WSM maximization problem includes CSI error, phase shifts matrices, transmit beamforming as well as RIS placement variables, which results in a complicated nonconvex problem. To handle this problem, the original problem is divided into a series of subproblems, where the location of RIS, transmit/reflect beamforming and CSI error are optimized iteratively. Then, a multiobjective evolutionary algorithm is introduced to gradient projection-based alternating optimization, which can alleviate the performance loss caused by the effect of imperfect CSI. Simulation results reveal that the proposed scheme can potentially enhance the performance of existing wireless communication, especially considering a desirable trade-off among beamforming gain, user priority and error factor.

An Efficient Block Successive Upper-Bound Minimization Algorithm for Caching a Reconfigurable Intelligent Surface-Assisted Downlink Non-Orthogonal Multiple Access System

“…Additionally, Hybrid Beamforming was also examined and designed to accommodate the needs of sum rate maximization of multi-user for RIS-assisted SWIPT system [20]. To handle the non-ideal condition in practical condition, the imperfect channel state information was considered for RIS assisted communications, and robust hybrid beamforming vectors were designed to maximize the spectral efficiency for a typical system setting of a multi-antenna BS, multi-RIS, and multi-user [21]. In [22], a tractable analytical framework for RIS-assisted small-cell networks was introduced, based on which a cell-association method was proposed whose mathematical expressions of probabilities for user association and coverage, respectively, in general cases were also derived.…”