Measurement Event B1 is triggered when the quality of an inter-RAT neighbor becomes better than a predefined threshold. Event B1 event 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.
Example Trigger:
Scenario: The network sets a threshold of -100 dBm for LTE RSRP.
Condition: UE measures LTE neighbor RSRP as -98 dBm for the configured TTT (e.g., 100 ms).
Result: Event B1 is triggered, and the UE sends a measurement report to the gNB.
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 | Release 15 | Release 17 |
|---|---|---|
| Thresholds | RSRP, RSRQ | Added SINR-based thresholds |
| Inter-RAT Support | Primarily LTE | Expanded to include Wi-Fi |
| Conditional Configurations | Not available | Added for proactive handovers |
| Beam-Specific Measurements | Limited | Enhanced for mmWave scenarios |
Summary of changes for event B1 in 3GPP specifications from 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.
The table below captures the progressive improvements and increasing sophistication of Event B1 over the three releases.
| Feature/Aspect | Release 15 | Release 16 | Release 17 |
|---|---|---|---|
| Purpose | Designed for inter-RAT handovers, primarily between NR and LTE. | Expanded inter-RAT support with refinements for multi-RAT mobility. | Enhanced inter-RAT and inter-system mobility, including Wi-Fi and satellite. |
| Measurement Quantities | RSRP and RSRQ for LTE neighbors. | Introduced SINR for finer quality assessment. | Enhanced SINR support, including beam-level measurements for mmWave. |
| Threshold Configuration | Thresholds for RSRP/RSRQ were absolute. | Thresholds could be defined as relative offsets for flexibility. | Conditional thresholds added for advanced scenarios like conditional handover. |
| Support for Conditional Handover | Not available. | Introduced limited support for conditional handovers. | Fully integrated with conditional handover mechanisms for seamless mobility. |
| Inter-RAT RATs Supported | NR to LTE only. | NR to LTE and NR to Wi-Fi in certain configurations. | NR to LTE, Wi-Fi, and other RATs like satellite for advanced use cases. |
| Beam-Specific Measurements | Not supported. | Added limited support for beam-specific measurements. | Comprehensive beam-specific measurement capability for Event B1, especially in FR2/mmWave. |
| Time-to-Trigger (TTT) | Basic TTT configurations available. | Extended configurations for TTT to support varying mobility conditions. | Flexible TTT options with support for dynamic adjustment based on mobility state. |
| Energy Efficiency Enhancements | Not explicitly addressed. | Basic optimizations for measurement triggering. | Advanced optimizations to minimize UE energy consumption during frequent B1 triggering. |
| Reporting Mechanisms | Single reporting mechanism via RRC signaling. | Enhanced reporting flexibility with support for segmented reports. | Efficient beam-specific reporting mechanisms for dense deployments. |
| Integration with Load Balancing | Limited to basic inter-RAT mobility. | Partial integration with load-balancing mechanisms. | Fully supports inter-RAT load balancing, including non-3GPP access. |
| Use Cases | Primarily for handovers to LTE. | Handover and load balancing across LTE and Wi-Fi. | Expanded to include satellite, Wi-Fi, and future RATs in smart mobility scenarios. |
Reference :
- TS 38.331: Radio Resource Control (RRC) Protocol
- TS 38.214: Physical Layer Procedures for Data
- TS 36.331: E-UTRA RRC Protocol
