Beyond Cell-Free MIMO: Energy Efficient Reconfigurable Intelligent Surface Aided Cell-Free MIMO Communications

Cell-free systems can effectively eliminate the inter-cell interference by enabling multiple base stations (BSs) to cooperatively serve users without cell boundaries at expense of high costs hardware and power sources due large-scale deployment BSs. To tackle this issue, low-cost reconfigurable intelligent surface (RIS) as a promising technique improve energy efficiency cell-free systems. In article, we consider an RIS aided MIMO system where RISs are deployed around BSs create favorable propagation conditions via reflections in way, thereby enhancing communications. maximize efficiency, hybrid beamforming (HBF) scheme consisting digital RIS-based analog is proposed. The maximization problem formulated iterative algorithm designed solve problem. impact transmit power, number RIS, size on investigated. Both theoretical analysis simulation results reveal that optimal depends numbers size. Numerical evaluations also show proposed achieve higher than conventional ones.