Capacity Characterization for Reconfigurable Intelligent Surface Aided MIMO Communication Under Practical Phase Shift Model

Recently, reconfigurable intelligent surface (RIS) is emerging as an enabling technology to further enhance the performance of wireless communication systems. RIS is actually a metasurface which is consist of a large number of reflection elements. Each of the elements is capable of re-scatter the incident wave by an independent reflection coefficient. Signals can be redirected to the receiver by delicately assigning reflection coefficients to the elements. Thus a better receiving condition can be realized, and system performance can be enhanced. Existing studies already proved the superiority of general RIS-aided systems, e.g., single-user and multi-user, unicast and multicast, orthogonal and non-orthogonal multiple access. However, most of them considered ideal reflection elements. They assumed that full reflection can be realized at any values of phase-shift, which however is unrealistic due to energy dissipation in reflection elements. To circumvent this problem, in this paper we consider a practical phase shift model where reflection amplitude and phase shift are coupled. Specifically, under this practical model we try to maximize the achievable rate of an RIS-aided MIMO system. An alternating optimization based algorithm is proposed to solve the corresponding optimization problem. Reflection coefficients at the RIS and transmit covariance matrix at the transmitter are optimized alternately with the others being fixed. Simulation results are presented to manifest the validity of the proposed scheme.