5G NR NSA Network Access Issues and Possible Root Cause

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Many of the operator across the global has started providing 5G services using NSA mode with Option 3.x . In NSA mode the network topology consist of a LTE cell, NR cell both connected to 4G Core Network. To get access to the 5G services UE has to first connect to 4G LTE cell as LTE is the master cell and then later based on the measurement a suitable 5G NR cell is added to UE.

In this post, we have listed some of the most common field issue seen when a 5G UE want access to the 5G NR cell access. At high level this issues can be classified in following categories.

  • Access Issue with LTE Procedure
    • LTE cell access failures
    • B1 Event configuration to UE via LTE cell
    • UE not reporting B1 Measurement Reports (MR)
  • Access preparation phase
    • LTE cell does not initiate SgNB Add Request
    •  5G NR Cell rejects SgNB Add request
    • LTE cell does not respond with SgNB Reconfig Complete
  • 5G NR Cell Synchronization
    • The UE does not initiate random access
    • RAR Access over the air interface times out.
    • UE MSG3 Failure

Issue#0 UEs fail to access the LTE Cell

To get access to 5G NR in NSA mode UE first needs to attach to 4G LTE cell. In some case UE may not able to attach to LTE cell due to following reasons.

Possible Root Cause:

    • The UE does not initiate access on the LTE network. Check if cell is barred, eNB is not in crashed state, RF transmission with necessary Tx power
    • The UE initiates an attach procedure on the LTE network and is rejected by the core network. Could be an issue with IMSI configuration at Core HSS node.

Issue#1 : B1 measurement is not delivered after the UE accesses the LTE network

When an NSA UE accesses the LTE network and the 5G B1 measurement control message is normally delivered, the reconfiguration message after the UE.

  • Possible Root Cause:
    After a UE attached to LTE cell, the eNodeB can deliver 5G B1 measurement control messages only when the following conditions are met:

    • UE capability report contains the UE capability of the Release 15
    • NSA capability of the UE is not prohibited on the core network
    • The QCI of the default bearer of the UE does not occupy the dedicated QCI (QCI 1-5, QCI 65/66) of the LTE network
    • The NSA and NR neighboring frequencies are correctly configured on the LTE
    • LTE cell NSA capability is enabled

Issue#2: UE does not report the B1 measurement report

When the NSA UE reports the B1 measurement, the RRC MEAS RPRT message carries the measResultCell-r15 of the 5G to notify the LTE network.

  • Possible Root Cause:
    • The frequency (NR-ARFCNs) contained in the B1 measurement control message is incorrect.
    • 5G The cell status is abnormal (not operation/crashed state) or the RRU transmit power is abnormal (no transmission), as a result, UEs cannot find 5G cell
    • SSB decode failure due to high SSB interference

Issue#3: Upon Receiving the B1 event, LTE cell does not initiate the SgNB ADD Request

On the air interface, after receiving measurement report LTE cell should initiate SgNB ADD Request over the X2 interface toward the measurement 5G NR Cell.

  • Possible Root Cause:
    • Neighboring Relation Table (NRT) cell configuration is wrong (missing or PCI conflict)
    • The X2 link to the target 5G cell is not establishment or abnormal

Issue#4 SgNB ADD Request is rejected by 5G NR Cell 

After receiving the SgNB  Add Request message, NR Cell send SgNB Add Reject message over X2 interface.

Possible Root Cause:

  • The SgNB Add Reject message shall contains the cause value for reject. One can determine the possible cause based on the cause value and some are listed below.
    •  Cause values as  “Transport resource not available”: Issue at 5G NR side as transport resource not available, these transport resource includes availability of X2-U link between eNB and gNB and S1-U link availability between gNB and 4G Core
    • Cause values as “No radio resource available”: 5G The number of users in the cell is insufficient. The gNB ID is duplicate or other resources of the 5G are abnormal (for example, no available SRS resources are available).

Issue#5  LTE network does not reply with a SgNB Reconfiguration Complete message

This issue can be traced with verifying X2 signaling tracing on the NR  cell side. In such situation, it can found that the 5G does not receive the SgNB Reconfiguration Complete message after sending the SgNB ADD Request ACK message. As a result of this, the timer expires and the 5G initiates a release procedure.

Possible Root Cause:

  • While checking the X2 signaling trace (wire-shark or internal logs) on the NR side. It should be found that the 5G does not receive the SgNB Reconfiguration Complete message. It could be be due to following
    • UE receives the reconfiguration message to add 5G NR cell, but the decoding is a failure. The UE does not respond to the reconfiguration complete message. Generally, the UE decoding error occurs because the version does not match.
    • The eNodeB does not send a reconfiguration complete message.

Issue#6 UE does not initiate RACH over the air interface

In UE air interface logs,  we can find  it is sending SCG Fail  Info  message to the LTE Cell immediately after receiving the RRC reconfiguration message.

  • Possible Root Cause:
    • The 5G cell search fails because accessed cell is not the strongest cell or the high inter-cell interference
    • The parameters in the SCG reconfiguration message fail to be verified on the UE side.

Issue#7  MSG 2 (RAR Message) over the air interface times out

In air interface trace we can see, as soon as UE receives an RRC reconfiguration message carrying the 5G SCG configuration, and sends an SCG FAIL INFO message to the LTE cell at a specified interval ( T304 configuration). The cause value carried for SCG FAIL Info  message can be ScgAccessFailure or scg-ChangeFailure.

  • Possible Root Cause:
    • The accessed 5G cell is not the strongest cell or the inter-cell interference is severe.
    • The PRACH parameter is incorrectly configured or the access fails due to physical layer reasons.

Issue#8  Failed to access the MSG3 over the air interface

When we observe SgNB Release Required with cause “radio-connection-with-UE-lost” after the 5G sends the SgNB ADD Request Ack message for a period of time (related to the timer on the MSG3 base station side).

  • Possible Root Cause:
    • The uplink TA value is too high or the uplink interference causes the MSG3 decode failure
    • MSG3 Failure Due to Incorrect Parameter Configurations on the UE or Base Station Side

Issue#9 Call is released immediately after the access is successful

Check to be done at LTE cell whether the signaling on the LTE side is released after receiving the E-RAB Modification Configuration message from the core network. If the S1AP ERAB Modification Configuration message contains the Cause IE, the eRab Modification fails. The core network is faulty. If the eRab Mod fails, the CN and LTE continue to locate the problem.

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