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.

翻译：本文研究了可重构智能表面（RIS）与网络控制中继器（NCR）在可达信噪比（SNR）方面的基本权衡。考虑一个具有多天线基站（BS）和单天线用户设备（UE）的上行链路系统，在BS-RIS/NCR与RIS/NCR-UE之间视距传播条件下，推导了无源RIS、有源RIS以及NCR辅助通信的闭式SNR表达式。分析了窄带和宽带传输场景，并考虑了有无直接BS-UE链路的情况。分析揭示了一个关键的结构性差异：RIS实现的SNR随其单元数量无限增长，而NCR提供的SNR从根本上受限于UE-中继器信道（由于噪声放大）。然而，研究表明，当NCR部署在UE附近且具备足够放大增益时，其性能可超越无源和有源RIS。基于实际路径损耗模型的数值结果量化了在不同部署几何形状和系统维度下NCR超越RIS所需的放大增益水平。这些发现为未来无线网络中RIS与NCR的实际集成提供了清晰的设计准则。