RIS-Aided MIMO Communications: Angle-Dependent Amplitude-Phase Response Model and Capacity Analysis

Most phase-shift models for reconfigurable intelligent surfaces (RIS) rely on ideal models of the reflection coefficient, i.e., assuming full-reflection amplitude and constant reflection coefficient for any incident angle. Since electromagnetic waves incident on RIS from varying directions exhibit different responses, ideal phase-shift models may overestimate the capacity gain of the RIS-aided multiple-input-multiple-output (MIMO) communications system. With varactor diode-based RIS, this letter performs full-wave simulation to verify the impact of incident angle on its reflection coefficient. Specifically, we first propose an angle-dependent amplitude-phase response model. The proposed model is then applied to a RIS-aided MIMO communications system, where the capacity maximization problem is formulated and solved via joint RIS reflection coefficients and transmit covariance matrix optimization. Simulation results quantify the capacity gain achieved by RIS with varying incident angles.