Diagnosing and Resolving “FAILURE_MSG4_CT_TIMER_EXPIRED” in 5G Standalone Networks

In the deployment and optimization of 5G Standalone (SA) networks, ensuring the robustness of the Random Access Channel (RACH) procedure is critical. DIGIS Squared identified and resolved a recurring RACH failure – “FAILURE_MSG4_CT_TIMER_EXPIRED” – during performance testing using our proprietary tools: INOS™ and Katana™. This white paper outlines the nature of the problem, diagnostic process, root cause analysis, and optimization strategies applied to restore optimal network performance.

Background

The 5G NR contention-based RACH procedure is essential for initial access, handover, and beam recovery. It involves a four-message handshake:

Msg1: RACH preamble from UE
Msg2: Random Access Response (RAR) from gNB
Msg3: MAC CE or RRC message from UE
Msg4: Contention resolution from gNB

The failure occurs when the UE sends Msg3 but does not receive Msg4 within the contention resolution timer window, resulting in an aborted RACH attempt.

Problem Identification

Failure Type: FAILURE_MSG4_CT_TIMER_EXPIRED
Detection Tool: INOS™ (field testing)
Confirmation Tool: Katana™ (OSS KPI analysis)

During extensive drive tests in both urban and suburban environments, INOS flagged multiple instances of RACH failure where Msg1, Msg2, and Msg3 were correctly transmitted, but Msg4 was not received. This was corroborated through Katana’s analysis of OSS counters, revealing high contention timer expiries in cells with:

Low SS-RSRP values (< -110 dBm)
High load and scheduling delays
Specific PRACH configurations

Root Cause Analysis

The following contributing factors were identified:

Msg3 Decoding Failures: UL signal degradation or beam misalignment prevented gNB from decoding Msg3.
Delayed Msg4 Scheduling: Resource contention at gNB delayed the contention resolution message.
Timer Misconfiguration: The default timer (sf64) was too short for specific TDD configurations.

Standards Reference:

3GPP TS 38.331: RRC Protocol for NR
3GPP TS 38.321: MAC Protocol for NR

Optimization Actions

Network-Side Adjustments

Increased ra-ContentionResolutionTimer from sf64 to sf128.
Reviewed and optimized PRACH Configuration Index and ZeroCorrelationZone settings.
Prioritized Msg4 scheduling at MAC layer in high-load scenarios.

Coverage Optimization

Fine-tuned beamforming and UL power control.
Extended PRACH monitoring duration in gNB firmware.

Post-Optimization Results

Verification was conducted using both INOS (field KPIs) and Katana (OSS trends), confirming significant improvement across all measured metrics.

Conclusion

This case study highlights the necessity of cross-layer observability in managing 5G SA network performance. By leveraging both real-time field data from INOS and OSS intelligence from Katana, DIGIS Squared successfully diagnosed and mitigated a complex RACH failure. The resolution not only improved RACH success rates but also enhanced user experience and access reliability.

Author: Obeidallah Ali, R&D Director at DIGIS Squared

Obeidallah Ali leads the Research & Development efforts at DIGIS Squared, driving innovation in AI-powered telecom solutions. With deep expertise in 5G Network Design, Optimization, and Automation, he focuses on developing tools like INOS™ and Katana™ that help operators diagnose, troubleshoot, and enhance network performance worldwide.

For inquiries, please contact:
Email: info@digis2.com