To improve the RACH Completion Success Rate (RACH SR) in LTE, focus on optimizing parameters related to PRACH false alarm detection, RSI (Root Sequence Index) planning, and uplink interference mitigation. Below is a structured approach:
1. Principle Mechanism
RACH SR degradation is often caused by:
PRACH false alarms: eNodeB falsely detects preambles from UEs, leading to unnecessary RARs and resource waste.
RSI conflicts: Overlapping root sequences in neighboring cells cause preamble collisions.
Uplink interference: Strong interference on PRACH channels increases false detection of MSG1.
Cell coverage issues: Beyond-cell-radius access or weak uplink coverage.
2. Configuration Steps
A. Enable PRACH False Alarm Detection
Switch: Turn on PRACHFalseAlarmDetSwitch to detect false alarms.
MOD CELLALGOWITCH: Localcellid=xx, RACHALGOSWITCH PRACHFalseAlarmDetSwitch-1;Parameters: Adjust thresholds to reduce false alarms:
MOD CELLRACHALGO: Localcellid=xx, PRACHFALSEALARMDETRADTHD=10000, RACHTHDBOOSTRATIO=200;B. Optimize RSI Planning
Root Sequence Gap: Ensure RSI gap ≥ 8 between sectors to avoid collisions.
Formula: Cell Radius = [(Ncs -1)x 800/839 – Delay Spread] x Speed of Light/2 .
Example: For 10 km radius, require 8 unique RSI per cell.
Tool: Use Huawei Genex Cloud for automated RSI planning 13.
C. Mitigate Uplink Interference
Enable PRACH IRC: Inhibit interference using PrachIrcSwitch (requires license):
MOD CELLALGOSWITCH: LocalCellId=xxx, IRCSWITCH=PrachlrcSwitch-1; # For non-LBBPcAdjust RAR Code Rate: Reduce RAR code rate to improve success rate:
MOD CELLDLSCHALGO: LocalCellId=xxx, RARANDPAGINGCR=35;D. Address Coverage Issues
Beyond Cell Radius: Increase Cell Radius for edge users.
Uplink Weak Coverage: Adjust E-Downtilt to control coverage range.
3. Impact Analysis
| Root Cause | Proportion | Solution |
| PRACH False Alarm | 10% | Enable PRACHFalseeAlarmDetSwitch |
| RSI Conflict | 50% | Replan RSI with =>8 gap |
| Uplink Interference | 40% | Enable PRACH IRC and adjust RAR Code rate |
4. Verification & Results
Post-Optimization: RACH SR improved by 10.85% – 25% in case studies.
Key Metrics: Monitor L.RACH.Success Rate, L.UL.Interference. PRACH.Index10-15 for interference trends 6
Conclusion: Prioritize PRACH false alarm detection and RSI planning for immediate RACH SR improvement. For persistent issues, investigate uplink interference and coverage gaps. Always validate changes with KPI trends and trace analysis.
Note: Parameter values (e.g., PRACHFALSEALARMDETRADTHD=10000) may vary by scenario; adjust based on network conditions.