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An Efficient Precoding Algorithm for Reconfigurable Intelligent Surface-Based MIMO Communications

For beyond 5G communications, the reconfigurable intelligent surface (RIS) has emerged as a more promising technology to manufacture extreme massive multi-input multi-output (MIMO) array than the conventional antenna-based alternative. In this paper, we propose a MIMO precoder design that can maximize the spectral efficiency of the RIS-based massive MIMO via optimizing both the RIS reflection phases and the digital precoder. To gauge the optimality of the proposed design, we derive an upper bound to the RIS channel capacity using majorization theory. Our design can work for an RIS with either discretely tunable phases or continuously tunable phases, and it is computationally efficient. The simulation results corroborate the superior performance of the proposed algorithm over the state-of-the-art and show its (near)-optimality by comparing with the capacity upper bound.