Spectral-Energy Efficiency Tradeoff of Nearly-Passive RIS in MIMO URLLC Downlink: Diagonal vs. Beyond Diagonal

TL;DR

This paper analyzes the spectral and energy efficiency tradeoffs of nearly-passive RIS architectures in MIMO URLLC downlink, highlighting the advantages of beyond-diagonal designs and their sensitivity to circuit power consumption.

Contribution

It introduces a comprehensive comparison between diagonal and beyond-diagonal RIS architectures in URLLC, revealing their spectral and energy efficiency characteristics and limitations.

Findings

GNP BD-RIS achieves highest spectral efficiency gains.

EE of BD-RIS is sensitive to static circuit power consumption.

Benefits of BD-RIS diminish with more data streams per user.

Abstract

This paper investigates the spectral and energy efficiency (EE) tradeoff of nearly-passive (NP), both locally and globally NP (GNP), reconfigurable intelligent surface (RIS), considering diagonal and beyond-diagonal (BD) implementations in multi-user multiple-input multiple-output (MU-MIMO) broadcast channels designed for ultra-reliable low-latency communication (URLLC). We demonstrate that while all RIS architectures enhance the spectral efficiency, GNP BD-RIS achieves the highest gains. However, its EE is highly sensitive to the static circuit power consumption since BD-RIS has many more circuit elements than diagonal architectures. Furthermore, we demonstrate that the benefits of BD-RIS over diagonal RIS diminish as the number of data streams per user increases due to enhanced channel diversity in MIMO systems.