Reconfigurable Intelligent Surfaces in 6G: Reflective, Transmissive, or Both?

TL;DR

This paper compares reflective, transmissive, and hybrid reconfigurable intelligent surfaces (RISs) in 6G wireless networks, analyzing their performance and optimal use cases based on user distribution.

Contribution

It provides the first comprehensive analysis of all three RIS types in a multi-user downlink scenario, highlighting their relative advantages.

Findings

Reflective RISs perform best for users on the same side as the RIS.

Transmissive RISs are advantageous for users on the opposite side.

Hybrid RISs offer flexible performance depending on user distribution.

Abstract

Reconfigurable intelligent surfaces (RISs) have attracted wide interest from industry and academia since they can shape the wireless environment into a desirable form with a low cost. In practice, RISs have three types of implementations: 1) reflective, where signals can be reflected to the users on the same side of the base station (BS), 2) transmissive, where signals can penetrate the RIS to serve the users on the opposite side of the BS, and 3) hybrid, where the RISs have a dual function of reflection and transmission. However, existing works focus on the reflective type RISs, and the other two types of RISs are not well investigated. In this letter, a downlink multi-user RIS-assisted communication network is considered, where the RIS can be one of these types. We derive the system sum-rate, and discuss which type can yield the best performance under a specific user distribution. Numerical results verify our analysis.