Description
The SS/PBCH Block Measurement Timing Configuration (SMTC) is a critical concept in 5G New Radio (NR) for managing UE measurements. In NR, especially with wide bandwidths and beamforming, continuously monitoring all possible SS/PBCH blocks (SSBs) from serving and neighbor cells is power-intensive for the UE. The SMTC addresses this by defining a periodic time window, known as the SMTC window or occasion. The network configures the UE with SMTC parameters via RRC signaling, primarily within the MeasObjectNR information element. These parameters include the periodicity (e.g., 5, 10, 20, 40, 80, 160 ms), the duration (length) of the measurement window (typically 1-5 ms), and a timing offset. The UE is only required to perform SSB-based measurements (like Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ)) during these configured SMTC windows. The network aligns the transmission of SSBs from cells (both serving and neighbor) with these SMTC windows. This alignment is coordinated between gNBs, often via the Xn interface, to ensure the SSBs the UE needs to measure are actually transmitted within its SMTC window. For intra-frequency measurements, a single SMTC is typically configured. For inter-frequency measurements, a separate SMTC can be configured per frequency. In FR2 (mmWave), where beam management is crucial, SMTC is tightly coupled with the SSB-based Radio Resource Management (RRM) measurement framework, allowing the UE to measure beams transmitted in different spatial directions during the window. The configuration ensures measurement accuracy while allowing the UE's receiver to sleep during off-periods, significantly improving battery life.
Purpose & Motivation
SMTC was introduced in 3GPP Release 15 as part of the foundational 5G NR specification to solve key challenges in the new radio access technology. NR operates over much wider bandwidths and employs massive MIMO/beamforming, leading to a more complex cell and beam measurement landscape. Without a coordinated measurement timing scheme, UEs would need to constantly monitor for signals, causing excessive power consumption, which is antithetical to 5G's goals for enhanced mobile broadband and IoT efficiency. Furthermore, in TDD systems and with beam-swept transmissions, SSBs are not continuously transmitted. SMTC provides a predictable, network-controlled schedule for measurements. It addresses the problem of measurement inefficiency by allowing the network to inform the UE exactly when to look for SSBs, aligning UE activity with network transmission patterns. This was a significant evolution from LTE's more continuous measurement approach, motivated by the need for sophisticated power saving techniques (often called 'wake-up signal' or 'discontinuous reception' concepts extended to measurement cycles) and the requirement to support reliable mobility and beam management in dense, beam-centric 5G deployments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (159 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SMTC function was newly introduced to provide the UE with a configured timing window, known as the SMTC window, for performing SS/PBCH block measurements on an NR carrier frequency. This configuration is part of the measurement setup and is specifically detailed for intra-frequency, inter-frequency, and inter-RAT measurements involving NR. The introduction includes the configuration of the MO (Measurement Object) with the SSB subcarrier spacing (SCS) for a given SSB frequency, as indicated in the related CR title.
- Introduction of QoE Measurement Collection for LTE TS 36.331CR3144
- Introduction of Bluetooth and WLAN measurement collection in MDT TS 36.331CR3342
- Correction for sidelink measurement periodical triggering condition TS 36.331CR3544
- Clarification to no barring configuration for Implicit UAC TS 36.331CR3554
- Corrections on the IDLE state measurement TS 36.331CR3560
- Correction on SPS configuration for HRLLC TS 36.331CR3572
+ 68 more changes
In Release 16, the key new feature for the SMTC function was the introduction of a second SMTC specifically for inter-RAT cell reselection. This enhancement allowed for more flexible measurement timing configurations when a UE performs measurements on cells from a different radio access technology. Additionally, Release 16 introduced the NeedForGap capability for NR measurements, which informs the network whether a UE requires measurement gaps to perform certain NR measurements.
- Introduction of a second SMTC for inter-RAT cell reselection TS 36.331CR4114
- Introduction of NeedForGap capability for NR measurement TS 36.331CR4197
- Introduction of NeedForGap capability for NR measurement TS 38.300CR0191
- Correction on the configuration of subframe #0 and #5 for MCH in MBMS dedicated cell TS 36.331CR4259
- Correction on MCCH configuration for 0.37kHz SCS TS 36.331CR4335
- Relaxed serving cell measurement for UEs using WUS TS 36.331CR4344
+ 30 more changes
In Release 17, the SMTC function was enhanced to support Quality of Experience (QoE) measurements in NR, as introduced by the corresponding CR to TS 38.300. This release also provided clarifications and corrections to the configuration and release procedures for these RAN visible QoE measurements. Furthermore, it introduced a new UE capability for performing inter-RAT NR FR2 measurements without requiring a measurement gap.
- 38.300 CR for Introduction of QoE measurements in NR TS 38.300CR0424
- Support of CHO with SCG configuration - 36331 [CHOwithDCkept] TS 36.331CR4823
- Support of CHO with SCG configuration - 37340 [CHOwithDCkept] TS 37.340CR0329
- Addition of NR-U RSSI/CO measurement UE capability TS 36.331CR4729
- Clarification on NR sidelink relay related configuration TS 36.331CR4859
- Correction on QoE configuration release TS 36.331CR4935
+ 18 more changes
In Release 18, enhancements to the SMTC function introduced measurements without gap with interruption and improved timing resiliency for URLLC. These updates were complemented by corrections to UE capabilities and configurations, as well as specific refinements for location-based measurements in IoT NTN scenarios. The release also supported early implementation of measurement gap enhancements from the previous standard version.
- Introduction of measurements without gap with interruption TS 36.331CR4929
- Introduction of Timing Resiliency and URLLC enhancements TS 38.300CR0730
- Correction to UE capabilities and configuration TS 36.331CR5040
- Corrections on location based measurements and need code for IoT NTN TS 36.331CR5052
- Correction on QoE measurements release at successful handover from LTE/5GC to NR TS 36.331CR5062
- Supporting R17 early implementation of R18 measurement gap enhancements TS 36.331CR5064
+ 13 more changes
In Release 19, the enhancements for SMTC introduced the Secondary Cell Measurement Skipping procedure specifically for NR ATG (Air-to-Ground) scenarios. This new capability allows for more efficient measurement management in non-terrestrial networks. The release also included clarifications on the associated notification configuration for these procedures.
- CR to TS 38.176-2: Improvements on Annex I - TRP measurement procedures TS 38.176CR0100
- Introduction of Secondary Cell Measurement Skipping for NR ATG TS 38.300CR1027
- Clarification on Notification Configuration TS 36.331CR5186
- Removal of Request for CSI-RS resource configuration for Early CSI acquisition TS 38.300CR1079
- (NR_IAB-Core)CR for 38.174, Correction on out-of-band blocking co-location requirement TS 38.174CR0132
- (NR_IAB-Perf)CR for 38.176-2, Correction on out-of-band blocking co-location requirement TS 38.176CR0080
Explore further
Broader topics and technologies where SMTC plays a role.
Defining Specifications
3GPP specifications that define or reference SMTC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | 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 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.831 vg10 | UE RF Requirements for FR2 Enhancements | Rel-16 |