Opportunistic Scheduling With Beam Focusing in Distributed RIS-Aided MU-MIMO Systems

Abstract

This paper introduces a novel cooperative framework for distributed Reconfigurable Intelligent Surfaces (RISs) in MU-MIMO systems, aimed at enhancing adaptability and performance. Multiple RISs are strategically deployed to ensure Line-of-Sight (LoS) connectivity with the Base Station (BS) and among each other, thereby improving coverage in traditionally weak zones. Each RIS can dynamically switch between acting as a main RIS (mRIS) to directly serve users or as an intermediate RIS (iRIS) to relay signals to another mRIS. A key contribution of this work is the optimization of mRIS phase shifts using a statistical RIS response function to enhance signal strength. Additionally, iRIS configurations utilize a beam focusing codebook for efficient signal transmission. An opportunistic scheduling scheme based on Lyapunov optimization adapts to user distribution and network conditions to optimize the use of the resulting spatial multiplexing gains, ensuring fairness and stability. Two scheduling policies are proposed: Slot-Based RIS Operational Mode Assignment (SB-ROMA) for high adaptability and Coherence-Time RIS Operational Mode Assignment (CT-ROMA) for reduced computational overhead. Analytical and simulation results validate the effectiveness of the proposed designs.