RIS-Assisted Massive MIMO Systems: Performance Limits and Realizations

This paper considers a multi-antenna communication system that employs reconfigurable intelligent surfaces (RISs) to assist its downlink transmissions from a base station (BS) to a user. It aims to thoroughly investigate the fundamental performance limits of a massive multiple-input and multiple-output (MIMO) communication system and how to realize them. We first devise an equivalent channel model for RIS-assisted multi-antenna communication and analyze its controllable statistical properties that characterize how the RIS-assisted channel gain can be manipulated by controlling an RIS in order to achieve non-coherent and coherent signal combining at the user. The outage probabilities of the RIS-assisted channel model for the non-coherent and coherent combining situations are derived, and their asymptotic upper and lower limits for RIS-assisted massive MIMO systems are also found. Our analyses crucially show why coherent combining may only sometimes outperform non-coherent combining and how much the performance limits of channel outage can be achieved for a large number of antennas at a BS and reflecting elements on an RIS.