Reference Signal for Radio Link Monitoring

Description

Reference Signal for Radio Link Monitoring (RLM-RS) is a concept introduced in 5G New Radio (NR) starting from 3GPP Release 15. It refers to the specific downlink reference signal(s) that a UE is configured to measure for the purpose of Radio Link Monitoring (RLM). In NR, unlike LTE where Cell-specific Reference Signals (CRS) were always present and used for RLM, the network has the flexibility to configure which signal the UE should monitor. The primary candidates for RLM-RS are the Synchronization Signal Block (SSB) and the Channel State Information Reference Signal (CSI-RS).

The configuration of RLM-RS is provided to the UE via RRC signaling within the RadioLinkMonitoringRS information element. This configuration specifies the resource set (e.g., a specific CSI-RS resource set or SSB index) and the associated quasi-co-location (QCL) assumptions. The UE uses the configured RLM-RS to estimate the downlink radio link quality, comparing it to the configured thresholds Q_out and Q_in. This measurement process is continuous during RRC_CONNECTED mode whenever the UE is not in a measurement gap or a DRX sleep period.

The use of configurable RLM-RS is particularly important for NR's beam-based operation. A gNB might transmit multiple beams (via SSB or CSI-RS beams), and the UE's serving beam might change due to beam management procedures. The network can configure the RLM-RS to correspond to the UE's active Transmission Configuration Indication (TCI) state, ensuring the UE monitors the quality of the specific beam used for data transmission. If that beam degrades, the RLM procedure can trigger recovery before a full radio link failure occurs.

Furthermore, RLM-RS enables network energy saving. In LTE, CRS had to be transmitted continuously in all cells to support legacy functions like RLM, leading to constant power consumption. In NR, if the network configures RLM based on periodic CSI-RS, it can potentially mute or reduce the transmission of SSB in certain periods, saving energy. The network must ensure the configured RLM-RS is transmitted with sufficient periodicity and density to allow the UE to meet the required RLM evaluation accuracy and timeliness.

Purpose & Motivation

RLM-RS was created to address the limitations of the LTE RLM model and to support the new architectural principles of 5G NR. In LTE, RLM relied solely on the ever-present Cell-specific Reference Signal (CRS). This forced continuous CRS transmission from all cells, contributing to constant network energy consumption and limiting flexibility. NR aimed to be more efficient and beam-centric.

The primary problem RLM-RS solves is providing a flexible and efficient mechanism for radio link monitoring in a network where always-on, cell-wide reference signals may not be transmitted. It allows the network to tailor the monitoring signal to the specific operational context of the UE, such as its active beam. This is crucial for maintaining reliable connectivity in high-frequency bands (e.g., mmWave) where beamforming is essential. It also enables advanced network energy saving techniques by decoupling the necessity of always transmitting synchronization signals from the requirement for continuous link monitoring.