Spectral Efficiency vs Complexity in Downlink Algorithms for Reconfigurable Intelligent Surfaces

Abstract

Reconfigurable Intelligent Surfaces (RIS) are an emerging technology that can be used to reconfigure the propagation environment to improve cellular communication link rates. RIS, which are thin metasurfaces composed of discrete elements, passively manipulate incident electromagnetic waves through controlled reflective phase tuning. In this paper, we investigate co-design of the multiantenna basestation precoder and multielement RIS phase shifts. The downlink narrowband transmission uses sub-6 GHz frequency bands, and the user equipment is equipped with a single antenna. Subject to the non-convex constraints due to the RIS phase shifts, we maximize the spectral efficiency or its equivalent channel power proxy. Our contributions in improving RIS-aided links include (1) design of gradient ascent co-design algorithms, (2) asymptotic (Big O) computational complexity analysis of proposed and considered prior algorithms, and (3) comparison of seven co-design algorithms in spectral efficiency vs. computational complexity.