SMTC in 5G NR: Measurement Timing Configuration

The SS/PBCH Block Measurement Timing Configuration (SMTC) is a critical concept in 5G New Radio (NR) networks. It defines the specific measurement windows—or occasions—when the User Equipment (UE) is expected to measure Synchronization Signal Blocks (SSBs). This ensures the UE efficiently measures neighbor cells without draining its battery by constantly monitoring the entire radio frame.

What is SMTC?

In 5G NR, an SMTC window dictates exactly when and for how long a UE should tune its receiver to measure SS/PBCH blocks. By restricting SSB measurements to these configured windows, the network saves UE power and reduces processing overhead. Outside of the configured SMTC duration on the indicated frequency, the UE does not consider SS/PBCH block transmissions for Radio Resource Management (RRM) measurements.

The first subframe of each SMTC occasion occurs at a System Frame Number (SFN) and subframe of the NR SpCell that satisfies formulas based on periodicity and offset (defined in 3GPP TS 38.331).

Key SMTC Parameters

The SMTC configuration (often sent within MeasObjectNR) consists of three primary parameters:

• Periodicity: Defines how often the measurement window occurs (e.g., 5, 10, 20, 40, 80, or 160 ms).
• Offset: The time offset in subframes indicating when the measurement window starts within the period.
• Duration: The length of the measurement window (e.g., 1, 2, 3, 4, or 5 ms).

Additional Configurations

• smtc: The primary measurement timing configuration. If absent, the UE assumes SSB periodicity is 5ms for intra-frequency cells.
• smtc2: A secondary measurement timing configuration for specific physical cell identities (PCIs). Its periodicity is strictly shorter than the primary smtc.
• smtc2-LP: Used for intra-frequency neighbor cells with a Long Periodicity (e.g., if primary smtc is 20ms, smtc2-LP could be 40ms or 80ms).
• smtc3list: Used exclusively for Integrated Access and Backhaul (IAB) nodes for discovery purposes.
• ssb-ToMeasure: A bitmap indicating exactly which SS blocks within the SMTC duration should be measured. If absent, the UE measures all SS blocks.

Example SMTC Configurations

To help visualize how SMTC is configured in real-world scenarios, here are the standard SMTC patterns defined in 3GPP TS 38.133 for testing:

By using these parameters, network operators can tightly control when UEs measure SSBs, balancing measurement accuracy with battery conservation.