Common Causes of MSG2 RACH Failure in 5G NR

MSG2 RACH Failure Common Causes in 5G NR:

The Random Access Channel (RACH) procedure in 5G NR is a critical part of establishing a communication link between the (UE) and the gNB. MSG2, which is the Random Access Response sent by the gNB, is a vital step in the process. However, there are several circumstances in which MSG2 failure may occur, interrupting the RACH procedure. This post disucses the most common causes of MSG2 failures during the RACH procedure and discuss their potential solutions. The most common causes of RACH failures are disuccsed here

1. UE Fails to Receive DL DCI Format 1_0 with CRC Scrambled by Corresponding RA-RNTI Before Timer Expiry

  • Cause: The UE does not receive the Downlink Control Information (DCI) Format 1_0, scrambled by the RA-RNTI (Random Access Radio Network Temporary Identifier), before the _ra-ResponseWindow timer expires.
  • Effect: Without the MSG2 response from the gNB, the UE cannot proceed with the next steps of the RACH procedure, leading to failure.
  • Solution: Ensure that the UE is synchronized correctly with the gNB and that there is no significant delay in the transmission of the DCI.

2. Mismatched SFN (System Frame Number) in DCI

  • Cause: The UE detects the DCI Format 1_0 with the correct CRC scrambled by the RA-RNTI, but the Least Significant Bits (LSBs) of the SFN in the DCI are different from the corresponding LSBs of the SFN used by the UE to transmit the PRACH.
  • Effect: This mismatch prevents the UE from correctly associating the MSG2 with the PRACH transmission, leading to failure.
  • Solution: Ensure that both the gNB and the UE are in sync regarding the SFN to avoid this mismatch.

3. PDSCH Decode Failure

  • Cause: The UE fails to correctly decode the transport block in the corresponding PDSCH (Physical Downlink Shared Channel) within the allocated window.
  • Effect: If the PDSCH is not decoded successfully, the UE cannot retrieve the MSG2 from the gNB, resulting in failure.
  • Solution: Improve channel conditions or increase the power of the transmission to ensure the UE can decode the PDSCH properly.

4. RAPID Mismatch

  • Cause: The RAPID (Random Access Preamble Identifier) associated with the PRACH transmission does not match the RAPID received by the UE in the MAC PDU (Medium Access Control Protocol Data Unit).
  • Effect: A mismatch in the RAPID leads to confusion in the identification of the UE during the RACH process, preventing a successful MSG2 reception.
  • Solution: Ensure that the RAPID value is consistent and properly transmitted by the gNB. Ensure that UE is decoding the correct MAC payload

5. Collision in Random Access Preamble

  • Cause: If multiple UEs transmit the same Random Access Preamble at the same time, a collision occurs.
  • Effect: The gNB may not be able to distinguish between the different UEs, leading to MSG2 failure for all involved UEs.
  • Solution: UEs need to select different preambles based on the configured RACH parameters. Backoff mechanisms can be employed to resolve such collisions.

6. Insufficient or Incorrect Timing Advance

  • Cause: The UE’s transmission may not be correctly synchronized with the gNB’s timing window. An incorrect Timing Advance (TA) calculation can lead to a misalignment in the RACH procedure.
  • Effect: The gNB may fail to properly receive the PRACH transmission or may not be able to send the MSG2 in time.
  • Solution: Correct the UE’s timing by ensuring accurate TA synchronization between the gNB and UE.

7. Poor Channel Conditions or Interference

  • Cause: High interference or low Signal-to-Noise Ratio (SNR) may cause the PDCCH (Physical Downlink Control Channel) or PDSCH decode failures.
  • Effect: The gNB may not be able to properly decode or transmit the DCI and MSG2 due to poor channel conditions, leading to failure.
  • Solution: Ensure optimal radio conditions and reduce interference in the communication link.

8. Resource Allocation Failure at gNB

  • Cause: The gNB may fail to allocate necessary resources (e.g., PUSCH resources) for the UE due to PDCCH blocking or other resource allocation issues.
  • Effect: Without the required resources, the gNB cannot respond with the MSG2, leading to RACH procedure failure.
  • Solution: Check gNB load during RACH Failures and ensure that the gNB can allocate sufficient resources for all UEs attempting the RACH procedure.

9. Incorrect System Information

  • Cause: Although rare, incorrect CORESET or SearchSpace settings for the RACH procedure might lead to the UE checking the wrong search space for the DCI.
  • Effect: The UE may fail to detect the DCI, resulting in MSG2 failure.
  • Solution: Ensure that the system information is correctly configured and shared with all UEs for accurate RACH processing.

The 5G NR RACH procedure is a complex and critical process for establishing a communication link between the UE and gNB. Understanding the common causes of MSG2 failure can help address these issues and improve the reliability and efficiency of the procedure. By addressing issues related to synchronization, preamble collisions, resource allocation, and channel conditions, the chances of successful MSG2 reception can be significantly improved.

MSG2 RACH Failure Causes, Effects, and Solutions
Cause Effect Possible Solution
UE Fails to Receive DL DCI Format 1_0 UE cannot proceed with the next RACH steps due to missed MSG2. Ensure proper synchronization and timely transmission of DCI.
Mismatched SFN in DCI UE cannot correctly associate MSG2 with PRACH transmission. Synchronize SFN between gNB and UE to avoid mismatches.
PDSCH Decode Failure UE fails to decode MSG2 from PDSCH, leading to failure. Improve channel conditions and increase transmission power for better PDSCH decoding.
RAPID Mismatch UE cannot associate RAPID with the correct PRACH transmission, causing failure. Ensure consistent RAPID value transmission and reception by gNB and UE.
Collision in Random Access Preamble gNB cannot distinguish between multiple UEs using the same preamble. Use backoff mechanisms to reduce collision and ensure unique preambles.
Incorrect Timing Advance Misalignment in the RACH procedure, leading to transmission failure. Ensure accurate timing advance synchronization between UE and gNB.
Poor Channel Conditions or Interference PDCCH or PDSCH decode failure due to interference or low SNR. Optimize radio conditions and reduce interference to improve signal quality.
Resource Allocation Failure at gNB gNB fails to allocate necessary resources, causing MSG2 reception failure. Ensure proper resource allocation and avoid PDCCH blocking at gNB.
Incorrect System Information UE checks wrong search space for DCI, causing failure to detect MSG2. Verify correct CORESET/SearchSpace configuration and ensure synchronization.
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