NCR vs. Passive/Active RIS: How Much NCR Amplification is Required to Beat RIS?

TL;DR

This paper compares RIS and NCR technologies in wireless communication, deriving SNR expressions and analyzing their performance tradeoffs, revealing conditions under which NCRs outperform RISs.

Contribution

It provides a comprehensive analysis of the fundamental tradeoff between RIS and NCR, including closed-form SNR expressions and practical deployment guidelines.

Findings

RIS SNR grows unbounded with elements, NCR SNR limited by noise

NCRs outperform RISs when deployed near the user with sufficient amplification

Quantifies amplification needed for NCRs to surpass RIS performance

Abstract

This paper investigates the fundamental tradeoff between reconfigurable intelligent surfaces (RISs) and network-controlled repeaters (NCRs) in terms of achievable signal-to-noise ratio (SNR). Considering an uplink system with a multi-antenna base station (BS) and a single-antenna user equipment (UE), we derive closed-form SNR expressions for passive RIS-, active RIS-, and NCR-assisted communication under line-of-sight propagation between the BS-RIS/NCR and RIS/NCR-UE. Both narrowband and wideband transmissions are analyzed, with and without the presence of a direct BS--UE link. Our analysis reveals a key structural difference: while the SNR achieved with RISs grows unboundedly with the number of RIS elements, the SNR provided by an NCR is fundamentally limited by the UE--repeater channel due to noise amplification. Nevertheless, we show that NCRs can outperform both passive and active RISs when deployed close to the UE, provided that sufficient amplification is available. Numerical results based on realistic path loss models quantify the amplification levels required for NCRs to outperform RISs across different deployment geometries and system dimensions. These findings provide clear design guidelines for the practical integration of RISs and NCRs in future wireless networks.