User Selection in Reconfigurable Intelligent Surface Assisted Communication Systems
Xu Gan, Caijun Zhong, Yongxu Zhu, Zhaoyang Zhong
Abstract
This letter presents a detailed investigation on the performance of reconfigurable intelligent surface (RIS)-assisted communication system with user scheduling. Depending on the availability of channel state information (CSI) at the RIS, two separate scenarios are considered, namely without CSI and with CSI. Closed-form expressions are derived for the ergodic capacity of the system in both scenarios. It is found that CSI has a significant impact on the performance of the system. Without CSI, the RIS provides an array gain of N, where N is the number of reflecting elements, and user scheduling provides an multi-user gain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\log \log \text {M}$ </tex-math></inline-formula> , where M is the number of users. With CSI, the RIS provides an array gain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {N}^{2}$ </tex-math></inline-formula> , while no multi-user diversity gain can be obtained.