Measurement Report Event B1 in 5G NR

Measurement Report Event B1 in 5G NR

Measurement Event B1 is triggered when the quality of an inter-RAT neighbor becomes better than a predefined threshold. Event B1 is primarily used for mobility between 5G NR and other RATs like LTE.

• Trigger Condition: Event B1 occurs when the measured quality (e.g., RSRP, RSRQ, SINR) of an inter-RAT neighbor surpasses a threshold set by the network.

Purpose

Event B1 is triggered to facilitate:

• Seamless handover between NR and LTE.
• Mobility to other RATs for optimized connectivity.
• Inter-RAT load balancing in multi-RAT environments.

How is Event B1 Triggered?

The network configures the UE with the following parameters in the RRCReconfiguration message:

1. Threshold Value:
   • Defines the minimum quality required to trigger the event.
   • Specified as an absolute value or relative offset.
2. Measurement Quantity:
   • Quality metrics such as RSRP, RSRQ, or SINR are used to evaluate the condition.
3. Hysteresis and Time-to-Trigger (TTT):
   • Hysteresis: Avoids frequent triggering due to minor fluctuations.
   • TTT: The condition must persist for a configured duration.

Entering Condition (B1-1)

• Condition: The entering condition for Event B1 is satisfied when the inter-RAT neighboring cell signal strength (Mn) minus the hysteresis value (Hys) is greater than a defined threshold value (Thresh).
• Formula:

$$M_n + O_{fn} + O_{cn} - Hys > Thresh$$

Leaving Condition (B1-2)

• Condition: The leaving condition for Event B1 is satisfied when the inter-RAT neighboring cell signal strength (Mn) plus the hysteresis value (Hys) is less than the threshold value (Thresh).
• Formula:

$$M_n + O_{fn} + O_{cn} + Hys < Thresh$$

Scenarios Where Event B1 is Used

• Inter-RAT Handover: Moving from NR to LTE when NR coverage weakens (e.g., in rural areas).
• Network Optimization: Redirecting UEs to less congested RATs (e.g., Wi-Fi) for load balancing.
• Service Continuity: Maintaining connectivity during service outages or planned maintenance on NR.

Changes from Release 15 to Release 17

Enhanced Metrics for Thresholds

• Introduction of SINR-based thresholds in addition to RSRP and RSRQ.
• Support for multi-RAT configurations, including NR-to-Wi-Fi transitions.

Conditional Measurements

• In Release 17, Event B1 can be linked to conditional handover configurations, allowing proactive handover preparation.

Beam-Based Measurements

• Enhanced support for beam-specific measurements to improve accuracy in millimeter-wave deployments.

Measurement Event B1 is a critical component of inter-RAT mobility in 5G NR, ensuring seamless transitions and optimized connectivity. With the enhancements introduced in Release 17, Event B1 is more robust, offering improved flexibility and accuracy in diverse deployment scenarios.

Feature Comparison: Release 15 vs Release 17

Summary of Changes for Event B1 (R15 to R17)

1. Release 15:
   • Focused on basic inter-RAT handovers, specifically between 5G NR and LTE.
   • Relied on RSRP and RSRQ as measurement quantities.
   • Limited flexibility in threshold configurations.
2. Release 16:
   • Introduced SINR as a new measurement quantity, enabling finer quality assessments.
   • Expanded use cases to support NR to Wi-Fi transitions in specific scenarios.
   • Enhanced TTT and reporting flexibility for varying mobility conditions.
3. Release 17:
   • Enhanced support for beam-specific measurements to improve performance in mmWave bands.
   • Fully integrated Event B1 with conditional handovers and inter-RAT load balancing.
   • Added support for advanced use cases, including satellite mobility and comprehensive inter-RAT scenarios.

Progressive Improvements Table

Reference

• TS 38.331: Radio Resource Control (RRC) Protocol
• TS 38.214: Physical Layer Procedures for Data
• TS 36.331: E-UTRA RRC Protocol

Cheatsheet

Further Reading

• 5G NR Measurement Report Event A1
• 5G NR Measurement Report Event A2
• 5G NR Measurement Report Event A3
• 5G NR Measurement Report Event A5
• 5G NR Timers and Constants