Rate Region of RIS-Aided URLLC Broadcast Channels: Diagonal versus Beyond Diagonal Globally Passive RIS

TL;DR

This paper investigates the rate region of RIS-aided URLLC broadcast channels, comparing different RIS architectures, and finds that globally passive beyond diagonal RISs offer performance gains, especially under strict reliability and latency constraints.

Contribution

It introduces and analyzes the finite-block-length rate regions for three RIS architectures, highlighting the advantages of beyond diagonal globally passive RISs over diagonal and locally passive designs.

Findings

GP BD-RIS outperforms LP and GP D-RIS in enlarging feasible solutions.

Performance gains decrease as the number of RIS elements increases.

RIS deployment yields higher gains under stricter reliability and latency requirements.

Abstract

We analyze the finite-block-length rate region of wireless systems aided by reconfigurable intelligent surfaces (RISs), employing treating interference as noise. We consider three nearly passive RIS architectures, including locally passive (LP) diagonal (D), globally passive (GP) D, and GP beyond diagonal (BD) RISs. In a GP RIS, the power constraint is applied globally to the whole surface, while some elements may amplify the incident signal locally. The considered RIS architectures provide substantial performance gains compared with systems operating without RIS. GP BD-RIS outperforms, at the price of increasing the complexity, LP and GP D-RIS as it enlarges the feasible set of allowed solutions. However, the gain provided by BD-RIS decreases with the number of RIS elements. Additionally, deploying RISs provides higher gains as the reliability/latency requirement becomes more stringent.