SCG Failure in 5G | Radio Link Failure

When a UE is connected to the network, ensuring seamless connectivity between UE and the network is vital for a reliable cellular communication service. One critical procedure of it is handling of radio link failure and when UE gNB is connected a secondary node handling of SCG Failure Information is vital. This process is vital in notifying the Master Node (E-UTRAN or NR) about failures experienced by the UE in the SCG

What is SCG Failure Information?

SCG Failure Information is a procedure in 3GPP standards designed to inform the Master Node about specific failures in the SCG.

The purpose of this procedure is to inform E-UTRAN or NR Master Node about an SCG failure the UE has experienced i.e. SCG radio link failure, failure of SCG reconfiguration with sync, SCG configuration failure for RRC message onSRB3, SCG integrity check failure, and consistent uplink LBT failures on PSCell for operation with shared spectrum channel access.

This mechanism ensures that the Master Node is promptly notified of any issues, enabling it to take corrective actions to maintain or restore connectivity.

SCGFailureInformation Call flow

When does UE initiate SCG Failure Information?

The UE initiates the SCG Failure Information procedure to report SCG failures when neither MCG nor SCG transmission is suspended and when one of the following conditions is met:

  • SCG Radio Link Failure: When the SCG experiences a radio link failure
  • Reconfiguration with Sync Failure: If the UE fails during SCG reconfiguration with synchronization.
  • SCG Configuration Failure: When an SCG configuration for an RRC message on SRB3 is unsuccessful.
  • Integrity Check Failure: If the SCG lower layers indicate an integrity check failure concerning SRB3.
  • Uplink LBT Failures: Persistent failures in uplink transmissions in a shared spectrum environment.

Importantly, the procedure is initiated only when both MCG and SCG transmissions are active and not suspended.

SCG Failure Information cause codes

Several reasons might cause the UE to trigger SCG failure, Below is the list of various failure causes as listed below

Failure Type Description
t310-Expiry UE initiates transmission of the SCGFailureInformationNR message due to T310 expiry. This is likely caused by poor RF conditions. UE starts the T310 timer when it detects problem at PHY and after conscutive N310 “out-of-sync” indications from PHY. T310 timer is started
synchReconfigFailure-SCG UE initiates transmission of the SCGFailureInformationNR message to provide reconfiguration with sync failure information for an SCG. This is likely the cause during SCG addition, If UE is not able to read MIB of the NR cell then it triggers synchReconfigFailure-SCG
randomAccessProblem UE initiates transmission of the SCGFailureInformationNR message to provide random access problem indication from SCG MAC. There are a Number of reasons why RACH might fail on SCG. Please check further RACH Failure reasons in 5G-NR
rlc-MaxNumRetx To provide indication from SCG RLC that the maximum number of retransmissions has been reached. In this scenario if there no RLC ACK’s to UL data transmistted by UE from gNB, UE initiates Max RLC Re-transmission:
srb3-IntegrityFailure UE initiates transmission of the SCGFailureInformation message due to SRB3 integrity check failure
scg-reconfigFailure UE initiates transmission of the SCGFailureInformation message due to Reconfiguration failure of NR. This usually happens when does not accept the Reconfiguration message and triggers this message. This is similar to reconfiguration failure on LTE and NR SA mode.
scg-lbtFailure-r16 [Release-16 IE] UE initiates transmission of the SCGFailureInformation message due to consistent uplink LBT
beamFailureRecoveryFailure-r16 [Release-16 IE] UE initiates transmission of the SCGFailureInformation message due to the failed beam failure recovery procedure indication from the SCG MAC
t312-Expiry-r16 [Release-16 IE] UE initiates transmission of the SCGFailureInformation message due to T312 expiry
bh-RLF-r16 [Release-16 IE] Applicable to IAB-node and the SCGFailureInformation is initiated due to the reception of a BH RLF indication on BAP entity from the SCG
SCG failure information illustration
SCG failure information illustration

Actions UE takes upon detection of SCG Failure

Upon detecting an SCG failure, the UE executes the following steps to handle the situation:

  1. Suspend SCG Transmission: All SRBs (Signaling Radio Bearers) and DRBs (Data Radio Bearers) on the SCG are suspended.
  2. Reset SCG MAC: The Medium Access Control (MAC) layer of the SCG is reset to ensure proper reinitialization.
  3. Stop T304 Timer: If the T304 timer for SCG is running, it is stopped immediately.
  4. Halt Conditional Reconfiguration Evaluation: Conditional reconfiguration evaluations for Conditional Primary Cell (CPC), if configured, are ceased.

Depending on the network configuration, the UE then initiates one of the following message transmissions:

  • SCGFailureInformationNR Message: If the UE is operating in (NG)EN-DC mode
  • SCGFailureInformation Message: For other configurations when UE is camped on NR-DC mode.

ASN.1 Notation

-- ASN1START
-- TAG-SCGFAILUREINFORMATION-START

SCGFailureInformation ::=                   SEQUENCE {
    criticalExtensions                      CHOICE {
        scgFailureInformation                 SCGFailureInformation-IEs,
        criticalExtensionsFuture                    SEQUENCE {}
    }
}

SCGFailureInformation-IEs ::=            SEQUENCE {
    failureReportSCG                FailureReportSCG       OPTIONAL,
    nonCriticalExtension            SCGFailureInformation-v1590-IEs  OPTIONAL
}

SCGFailureInformation-v1590-IEs ::=       SEQUENCE {
    lateNonCriticalExtension                OCTET STRING    OPTIONAL,
    nonCriticalExtension                    SEQUENCE {}     OPTIONAL
}

FailureReportSCG ::=               SEQUENCE {
    failureType                    ENUMERATED {
t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, synchReconfigFailureSCG, scg-ReconfigFailure, srb3-IntegrityFailure, other-r16, spare1},


measResultFreqList             MeasResultFreqList      OPTIONAL,
    measResultSCG-Failure      OCTET STRING (CONTAINING MeasResultSCG-Failure)                OPTIONAL,
    ...,
    [[
    locationInfo-r16          LocationInfo-r16            OPTIONAL,
   failureType-v1610         ENUMERATED {scg-lbtFailure-r16, beamFailureRecoveryFailure-r16,t312-Expiry-r16, bh-RLF-r16, spare4, spare3, spare2, spare1} OPTIONAL
    ]]
}

MeasResultFreqList ::=                   SEQUENCE (SIZE (1..maxFreq)) OF MeasResult2NR


-- TAG-SCGFAILUREINFORMATION-STOP
-- ASN1STOP

References

  • 3GPP TS 38.331: NR; Radio Resource Control (RRC); Protocol specification (Release 16)

Further reading