Ergodic Capacity Analysis of RIS-Aided Systems with Spatially Correlated Channels

Abstract

This paper investigates the ergodic capacity of reflecting intelligent surface (RIS)-aided single-input single-output communication systems with spatially correlated Rayleigh-fading channels. The ergodic capacity for such systems does not admit an exact closed-form expression. Therefore, we consider two alternative fading distributions to approximate the systems' statistical characterization to enable the derivation of closed-form expressions for the ergodic capacity. We further simplify the ergodic capacity by proposing novel and unified approximations in the form of a weighted sum of logarithmic functions with optimized coefficients. We validate the effectiveness and the high accuracy of the adopted schemes and the proposed approximations through numerical results. Performance analysis to study the impact of several system parameters on the ergodic capacity is also conducted. Deploying an RIS to the communication system can significantly increase the ergodic capacity which increases even further with increasing the number of reflecting elements equipped on the RIS, and this effect is best seen when the direct path is weak.