Study of Reconfigurable Intelligent Surface Assisted Communications over Frequency Selected Channels

Abstract

In this paper, we study a downlink multi-input single-output (MISO) system with the assist of reconfigurable intelligent surfaces (RIS) in the frequency selected channel environment. The transmission from a multi-antenna base station (BS) to the single antenna user equipment (UE) is realized via the multiple propagation paths among which one path is contributed by the RIS reflection. In addition, a practical restriction is considered where only a limited number of discrete phase shifts per RIS element can be provided corresponding to the limited control information from RIS controller. An optimizing problem aiming to maximize the achiveable bit rate is formulated jointly considering the continuous digital beamforming at BS and the discrete phase analog beamforming at RIS. With the help of the spatial propagation model between BS/UE and RIS, the approximate solutions are derived to solve this problem for the case of single subcarrier and multiple subcarriers. The simulation results valify the effectiveness of the proposed approximate solutions for the multiple subcarriers scheduling conditions, and provide a considerable performance gain of RIS assisted MISO system compared to that of the traditional wireless system without RIS assistance.