Reconfigurable Intelligent Surface Aided Cell-Free MIMO Communications

Abstract

Cell-free system is an efficient solution to eliminate inter-cell interference by serving users without cell boundaries. To further improve the spectrum efficiency, in this letter, we consider a reconfigurable intelligent surface (RIS) aided cell-free MIMO system. Multiple RISs are employed to create favorable propagation conditions via configurable reflection from the BS to users. Benefited from multiple transmission paths the spatial resources can be better utilized. We develop a hybrid beamforming (HBF) scheme consisting of the digital beamforming at the BS and the RIS-based beamforming. A sum rate optimization (SRO) problem is formulated and an iterative algorithm is designed to solve this problem. Simulation results show that the proposed system can achieve a significant performance gain compared to conventional single cell systems without RISs and distributed antenna systems.