Description
Radio Link Monitoring (RLM) is a fundamental physical layer procedure performed by the User Equipment (UE) in both LTE and NR systems. Its primary function is to continuously evaluate the quality of the downlink radio link from the serving cell. The UE does this by measuring specific reference signals transmitted by the gNB (in NR) or eNB (in LTE). These measurements are compared against two configurable thresholds: the out-of-sync (OOS) threshold (Q_out) and the in-sync (IS) threshold (Q_in).
When the estimated radio link quality falls below the Q_out threshold, the physical layer in the UE declares an 'out-of-sync' indication to higher layers. Conversely, when the quality recovers above the Q_in threshold, an 'in-sync' indication is declared. The higher-layer protocol stack (typically the RRC layer) implements a state machine that counts these consecutive indications. If a certain number of consecutive 'out-of-sync' indications are received (N310), a timer (T310) is started. If the required number of consecutive 'in-sync' indications (N311) is not received before T310 expires, the UE declares a Radio Link Failure (RLF).
Upon declaring RLF, the UE initiates a connection re-establishment procedure. It stops transmitting on the uplink, selects a new cell (which could be the same cell or a different one), and attempts to re-synchronize and re-establish the RRC connection. This entire process, governed by RLM, is vital for handling scenarios like sudden shadowing, deep fading, or interference, ensuring that the UE can autonomously recover connectivity without unnecessary signaling overhead from the network side.
The configuration parameters for RLM, such as Q_out, Q_in, N310, T310, and N311, are provided to the UE via RRC signaling (e.g., in the RRCReconfiguration message). These parameters can be tailored based on the service type, mobility state, or network deployment scenario. For instance, a UE configured for ultra-reliable low-latency communication (URLLC) might have more stringent thresholds or shorter timers to enable faster failure detection and recovery. RLM operates independently in both the source cell during handover preparation and in the target cell after handover execution, ensuring seamless mobility management.
Purpose & Motivation
Radio Link Monitoring exists to provide a robust, UE-autonomous mechanism for detecting a deteriorating or lost radio connection. Before standardized procedures like RLM, networks relied more heavily on network-side detection of link failure, which could be slower and less efficient. The primary problem RLM solves is service discontinuity due to poor radio conditions. It allows the UE to quickly and independently determine when the link is no longer usable for reliable communication, triggering a controlled recovery process.
The historical motivation stems from the need for reliable mobility in packet-switched cellular systems like LTE and 5G NR, where maintaining an 'always-on' IP connection is crucial. In earlier cellular generations, connection failures often led to dropped calls or sessions with significant delay before reconnection. RLM provides a proactive and standardized method to detect failure, stop useless transmissions (conserving UE battery and reducing interference), and swiftly attempt reconnection to the best available cell. It addresses the limitations of purely network-controlled supervision, which might not react quickly enough to rapid changes in the UE's radio environment, especially at cell edges or in high-mobility scenarios.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (87 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-12, normative work from Rel-15.
In Release 15, the RLM (Radio Link Monitoring) function was clarified for SSB-based operation, and corrections were made to the configuration for the number of RadioLinkMonitoringRS resources. This release also included general corrections on radio link failure related actions.
- Introduction of New Radio Access Technology in TS 36.300 TS 36.300CR0998
- Radio Protocol Architecture figure clarification with SDAP TS 37.340CR0017
- PDCCH monitoring for overlapped CORESETs TS 38.213CR0018
- Correction on PDCCH monitoring TS 38.213CR0040
- CR on PDCCH Monitoring for NR-DC TS 38.213CR0048
- Clarification on SSB-based BM, RLM and BFD TS 38.300CR0102
+ 20 more changes
In Release 16, enhancements to Radio Link Monitoring (RLM) included a Change Request on the determination of the number of reference signals for RLM and another on maintenance regarding the sharing factor of RLM and link recovery specifically for Integrated Access and Backhaul Mobile Terminals (IAB-MT). These updates provided more defined procedures for RLM resource configuration and adaptation for new network architectures.
- Aperiodic CSI-RS triggering with beam switching timing of 224 and 336 TS 38.331CR1716
- Introduction of Inter-gNB CSI-RS Based Mobility TS 37.340CR0214
- Introduction of Inter-gNB CSI-RS Based Mobility TS 38.300CR0249
- CR to 38.331 on CSI-RS inter-node message TS 38.331CR1354
- 38331 CR for CSI-RS-ResourceConfigMobility TS 38.331CR2250
- Introducing UE Radio Capability Mapping procedure for EN-DC TS 36.300CR1314
+ 14 more changes
In Release 17, specific corrections and clarifications were introduced for the Radio Link Monitoring (RLM) function, primarily focusing on its applicability and related procedures. The enhancements included an update to the applicability for Reduced Capability (RedCap) UE RLM test cases and a correction to the general applicability of RLM itself. Furthermore, the release addressed the configuration of RLM relaxation reporting for Secondary Cell Groups (SCG) within the `otherConfig` field description.
- Correction on the tables for determining Type0 PDCCH monitoring occasions TS 38.213CR0337
- Correction on multi-slot PDCCH monitoring in NR-DC and CA scenarios with mixed capability types TS 38.213CR0342
- Corrections on PDCCH monitoring enhancement for 52-71GHz spectrum TS 38.213CR0346
- Correction for multi-slot PDCCH monitoring in FR2-2 TS 38.213CR0372
- CR on PDCCH monitoring when overlapping with the rate matching pattern to TS 38.213 TS 38.213CR0383
- CR on PDCCH monitoring for inter-cell beam management TS 38.213CR0413
+ 14 more changes
In Release 18, the RLM function was enhanced with clarifications and corrections for specific UE operations and scenarios. Key updates included clarifications for RLM/BFD relaxation procedures during short DRX cycles and corrections to PDCCH monitoring adaptation behavior for DRX groups. Furthermore, specific monitoring behaviors were defined for new features like paging during SDT (CG-SDT-Enh) and for RedCap UEs operating in HD-FDD mode.
- CR on PDCCH monitoring for dedicated spectrum less than 5 MHz TS 38.213CR0596
- Correction of network timing synchronization status monitoring TS 38.300CR0817
- Clarification RLM/BFD relaxation and short DRX TS 38.331CR4771
- Paging monitoring for extended CG period [CG-SDT-Enh] TS 38.331CR4788
- Correction for Paging monitoring during SDT [CG-SDT-Enh] TS 38.331CR4901
- Addition of missing need code in NZP-CSI-RS-Resource TS 38.331CR5274
+ 11 more changes
In Release 19, the RLM function was updated with corrections for monitoring Low-Power Wake-Up Signal (LP-WUS/WUR) occasions to ensure proper UE wake-up behavior. Additionally, a correction was applied to multi-modality, uplink rate control, and available bit rate monitoring procedures. The release also removed the request for CSI-RS resource configuration specifically for Early CSI acquisition.
- Correction on multi-modality, uplink rate control and available bit rate monitoring TS 37.340CR0430
- Correction on WUS monitoring occasions for LP-WUS/WUR TS 38.213CR0750
- Correction on WUS monitoring for LP-WUS/WUR TS 38.213CR0760
- Removal of Request for CSI-RS resource configuration for Early CSI acquisition TS 38.300CR1079
Explore further
Broader topics and technologies where RLM plays a role.
Defining Specifications
3GPP specifications that define or reference RLM, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.842 vc00 | Small Cell Enhancements for LTE Higher Layers | Rel-12 |
| TS 36.867 vd00 | LTE DL 4 Rx Antenna Port Study TR | Rel-13 |
| TS 36.878 vd00 | LTE Performance Enhancements for High Speed Scenarios | Rel-13 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | Rel-19 |
| TS 37.816 vg00 | RAN-centric Data Collection & Utilization Study | Rel-16 |
| TS 38.106 vj20 | NR Repeater Radio Transmission and Reception | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.174 vj10 | NR Integrated Access and Backhaul Radio Spec | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TR 38.864 vi10 | Technical Report on Network Energy Savings for NR | Rel-18 |
| TR 38.869 vi00 | Study on low-power wake up signal and receiver for NR | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |