Circuit Switched Fallback (CSFB) is crucial for enabling voice services in LTE networks by redirecting the UE to a 3G network (UTRAN). The CSFB UTRAN EventB1 Time To Trigger parameter controls how quickly the UE reports suitable UTRAN cells, impacting handover performance. Proper tuning of this parameter helps balance between rapid fallback and avoiding unnecessary handovers.
Parameter Overview
- Parameter ID: CsfbHoUtranTimeToTrig
- Network Element: eNodeB
- MML Command: MOD CSFALLBACKHO / LST CSFALLBACKHO
- Default Value: 40ms
- Recommended Value: 40ms
- Value Range: 0ms, 40ms, 64ms, 80ms, 100ms, 128ms, 160ms, 256ms, 320ms, 480ms, 512ms, 640ms, 1024ms, 1280ms, 2560ms, 5120ms
- Unit: Milliseconds (ms)
- Feature Specification: 3GPP TS 36.331
Function and Impact on Handover
This parameter defines the delay before the UE sends a measurement report to the eNodeB after detecting a UTRAN cell that meets the event B1 criteria (signal quality exceeds a predefined threshold). The delay prevents unnecessary handovers caused by short-lived signal fluctuations.
- Short Time to Trigger (e.g., 40ms): Faster fallback to 3G, suitable for high-mobility scenarios (e.g., highways) but may lead to false handovers.
- Long Time to Trigger (e.g., 256ms): Reduces unnecessary handovers, ideal for stable environments (e.g., urban areas), but may delay fallback.
Optimization Strategy
- Scenario Analysis:
- For high-mobility areas: Set a shorter Time To Trigger (e.g., 40-80ms) to ensure fast fallback.
- For low-mobility areas: Use a longer Time To Trigger (e.g., 256-640ms) to avoid false handovers.
- Measurement Report Evaluation:
- Monitor UE logs and event reports to assess how frequently event B1 is triggered.
- Handover Performance KPIs:
- Track Call Setup Success Rate (CSSR), Call Drop Rate (CDR), and Inter-RAT Handover Success Rate to validate the changes.
Practical Example
Scenario: A suburban cluster shows frequent CSFB failures.
- Current Setting: CsfbHoUtranTimeToTrig = 40ms
- Analysis: UE logs show frequent but brief event B1 triggers.
- Optimization: Increase Time To Trigger to 160ms.
- Result: CSFB failures dropped by 8%, and call setup success improved.
Conclusion
The CSFB UTRAN EventB1 Time To Trigger parameter is important for balancing handover speed and stability during CSFB procedures. Proper tuning, based on area-specific conditions and KPI monitoring, improves fallback performance, reduces failures, and enhances user experience.