Description
The RSSI Measurement Timing Configuration (RMTC) is a crucial mechanism defined in 3GPP specifications for enabling Listen-Before-Talk (LBT) and channel occupancy measurements in unlicensed and shared spectrum. It is a configuration message sent by the network (via RRC signaling) to a User Equipment (UE), defining a specific measurement window for performing wideband RSSI measurements on one or more carrier frequencies. The primary goal is to sense the radio environment to determine if a channel is idle or busy before the network or UE transmits, which is a regulatory requirement in many unlicensed bands (e.g., 5 GHz).
An RMTC configuration includes several key parameters: the carrier frequency (or list of frequencies) to measure, the measurement bandwidth, the timing of the measurement window (specified as an offset and duration relative to a reference SFN), and the periodicity of these measurement occasions. The UE uses this configuration to activate its receiver at the specified times and frequencies, measures the total received power (RSSI) within the configured bandwidth, and processes the results. The measurement is typically an energy detection mechanism, not requiring synchronization to any specific signal. The UE then reports the measurement results back to the network, often in the form of channel occupancy statistics (e.g., the percentage of time the RSSI was above a certain threshold).
The network utilizes these reports for dynamic channel selection and channel access procedures. For example, in License-Assisted Access (LAA) and NR-U (NR in unlicensed spectrum), a gNB may configure UEs with RMTC to scout for clean channels in the unlicensed band. Based on collective reports from multiple UEs, the gNB can decide which carrier to use for secondary cell (SCell) activation, thereby improving overall system performance and coexistence with other systems like Wi-Fi. The RMTC mechanism allows the network to delegate the sensing task to UEs, providing a distributed view of the radio environment without requiring the base station itself to constantly monitor all potential channels.
Purpose & Motivation
RMTC was introduced in 3GPP Release 13 as part of the LTE-LAA (License-Assisted Access) work item to enable LTE operation in the unlicensed 5 GHz spectrum. The core problem it solved was how to efficiently meet regulatory Listen-Before-Talk (LBT) requirements while maintaining network control. Unlike Wi-Fi, which uses distributed contention (CSMA/CA), LAA adopted a more centralized approach where the network (eNodeB) coordinates channel access. RMTC provided the tool for this coordination by allowing the network to instruct UEs—devices already equipped with receivers—to perform the necessary channel sensing on its behalf.
This approach addressed several limitations. First, it provided the network with a broader and more accurate view of interference conditions across different locations within a cell, which a single base station measurement could not achieve. Second, it enabled efficient use of UE resources by configuring measurements only when needed and on specific frequencies of interest. Third, it was essential for fair coexistence with other technologies like Wi-Fi, as it allowed the LTE/5G system to accurately assess channel occupancy before transmitting. The concept was later carried forward into 5G NR-U (Release 16 and beyond), where it remains a foundational element for operating NR in shared and unlicensed spectrum, supporting use cases like enhanced mobile broadband and private network deployments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (359 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, the RMTC function was newly introduced as part of the enhancements for Minimization of Drive Tests (MDT), specifically enabling the collection of Bluetooth and WLAN measurements. This addition allowed for the configuration and reporting of radio measurements from these non-3GPP access technologies alongside traditional cellular data. The corrections and clarifications for Bluetooth and WLAN measurement collection in MDT were subsequently provided to ensure proper configuration and operation.
- Introduction of QoE Measurement Collection for LTE TS 36.300CR1073
- Introduction of QoE Measurement Collection for MTSI services TS 36.300CR1140
- Introduction of QoE Measurement Collection for LTE TS 36.331CR3144
- Introduction of Bluetooth and WLAN measurement collection in MDT TS 36.331CR3342
- Introduction of SFTD measurement to neighbour cells for NR SA TS 38.331CR1139
- Stage 2 CR on Measurement gap configuration scenarios TS 36.300CR1206
+ 135 more changes
In Release 16, the key new capability for RMTC was the introduction of the **NeedForGap capability for NR measurement**, which allowed the network to understand a UE's requirement for measurement gaps during RSSI Measurement Timing Configuration. Furthermore, this release enabled **inter-frequency measurement without gap** as an alternative operational mode, providing greater flexibility in measurement scheduling and resource utilization.
- Introduction of NeedForGap capability for NR measurement TS 36.331CR4197
- Introduction of downgraded configuration for SRS antenna switching TS 38.331CR1433
- Introduction of additional RACH configurations for TDD FR1 TS 38.331CR1486
- Introduction of NeedForGap capability for NR measurement TS 38.331CR1453
- Introduction of inter-frequency measurement without gap TS 38.331CR1673
- Aperiodic CSI-RS triggering with beam switching timing of 224 and 336 TS 38.331CR1716
+ 80 more changes
In Release 17, the RMTC (RSSI Measurement Timing Configuration) function was enhanced through the introduction of RRC signaling for measurement gap enhancement, which included clarifications and corrections for the feature's configurations. This was complemented by the new UE capability for inter-RAT NR FR2 measurements without a measurement gap, allowing for more flexible measurement procedures. Additionally, corrections and clarifications were made to the joint gap configuration to ensure proper alignment and operation.
- RRC configuration for R17 UL Tx switching enhancement TS 38.331CR2909
- Introduction of RRC signaling for measurement gap enhancement TS 38.331CR2913
- Introduction of QoE measurements in NR TS 38.331CR2958
- Early Measurements for EPS fallback [IdleMeaEPSFB] TS 38.331CR2872
- Introducing IAB MAC CE Configurations in RRC TS 38.331CR3194
- Support of CHO with SCG configuration - 36331 [CHOwithDCkept] TS 36.331CR4823
+ 58 more changes
In Release 18, the RMTC function was enhanced through the introduction of measurements without gap with interruption and further measurement gap enhancements, including early implementation support. These changes were complemented by specific procedures for measurement sequences and the configuration of cell individual offsets in report configurations, alongside continued corrections and refinements for URLLC, timing resiliency, and IoT NTN scenarios.
- Introduction of measurements without gap with interruption TS 36.331CR4929
- Introduction of further measurement gap enhancements TS 38.331CR4063
- Introduction of URLLC and Timing Resiliency TS 38.331CR4258
- Introducing procedure for measurement sequence for intra-RAT and inter-RAT measurement [MeasSequence] TS 38.331CR4439
- Introduction of enhancements of delay measurements upon MO updates [SONMDT-enh] TS 38.331CR4469
- Enhancements to measurement report [meas_report_enh] TS 38.331CR4803
+ 53 more changes
In Release 19, the RMTC function was enhanced with specific corrections and clarifications, including a correction on the validation of reported idle/inactive and reselection measurements. Furthermore, the release introduced clarifications on the notification configuration procedure and made corrections related to neighbour cell measurement for IOT NTN TDD.
- Introduction of Secondary Cell Measurement Skipping for NR ATG TS 38.331CR5477
- Introduction of extension ratio configuration for MPR reduction TS 38.331CR5482
- Corrections on neighbour cell measurement for IOT NTN TDD TS 36.300CR1443
- Clarification on Notification Configuration TS 36.331CR5186
- Correction on PC5 Relay RLC channel configuration TS 38.331CR5510
- Corrections on validation of reported idle/inactive and reselection measurements TS 38.331CR5564
+ 3 more changes
Explore further
Broader topics and technologies where RMTC plays a role.
Defining Specifications
3GPP specifications that define or reference RMTC, 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 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |