SCG/MCG failure handling

SCG Failure 5G NR

RLF is declared separately for the MCG and for the SCG

If radio link failure is detected for MCG, the UE initiates the RRC connection re-establishment procedure

The following SCG failure cases are supported:

SCG RLF
SN change failure
For EN-DC, NGEN-DC, and NR-DC, SCG configuration failure (only for messages on SRB3)
For EN-DC, NGEN-DC, and NR-DC, SCG RRC integrity check failure (on SRB3).
Upon SCG failure the UE suspends SCG transmissions for all radio bearers and reports the SCG Failure Information to the MN, instead of triggering re-establishment.
In all SCG failure cases, the UE maintains the current measurement configurations from both the MN and the SN and the UE continues measurements based on the configuration from the MN and the SN if possible. The SN measurements configured to be routed via the MN will continue to be reported after the SCG failure
The UE includes in the SCG Failure Information message the measurement results available according to the current measurement configuration of both the MN and the SN. The MN handles the SCG Failure Information message and may decide to keep, change, or release the SN/SCG. In all the cases, the measurement results according to the SN configuration and the SCG failure type may be forwarded to the old SN and/or to the new SN.

SCG failure information

The purpose of this procedure is to inform E-UTRAN or NR MN 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 on SRB3 and SCG integrity check failure
If the RLF happens on the UE’s SCG, a so-called SCG-RLF is declared by the UE. Since the UE’s MCG, where its RRC connection is terminated, is still operational, the consequences are less severe as there is no need to perform an RRC Reestablishment procedure.

How we can identify SCGFailureInformation from 3GPP specs

SCGFailureInformation

The SCGFailureInformation message is used to provide information regarding NR SCG failures detected by the UE.

Signaling radio bearer: SRB1

RLC-SAP: AM

Logical channel: DCCH

Direction: UE to Network

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

]]

TYPE OF SCG Failure Reason

t310-Expiry
synchReconfigFailure-SCG
random access problem
rlc-MaxNumRetx
srb3-IntegrityFailure
scg-reconfigFailure

The UE suspends SCG transmission for all Signaling Radio Bearers (SRBs) and DRBs and resets its MAC entity in SCG.
Afterward, the UE sends SCGFailureInformation message on its primary leg, i.e. to its Master Node. Note that in case of UE operating in EN-DC mode, the SCGFailureInformation message will be sent to an eNB, i.e. to E-UTRAN. The contents of the message indicate:
a. Failure type, i.e. whether the SCG failure occurred due to physical layer problems, random access issues, or RLC failure.
b. Radio resource management (RRM) measurements were obtained by the UE up to the moment of failure occurrence for the cells which were configured as UE’s serving cells in SCG as well as its neighboring cells.
The SCGFailureInformation may be used by the UE’s Master Node to facilitate subsequent SCG configuration. The Master Node may alternatively decide not to add an SCG for the UE

SCG failure information procedure triggered upon SCG RLF

SCGFailureInformationNR

failureType-r15

Any allowed value

t310-Expiry, OR randomAccessProblem, OR rlc-MaxNumRetx, OR synchReconfigFailureSCG, OR scg-reconfigFailure, OR srb3-IntegrityFailure

Timer	start	stop	expiry
t304	Upon reception of RRCReconfiguration message including reconfigurationWithSync	Upon successful completion of random access on the corresponding SpCell For T304 of SCG, upon SCG release	For T304 of MCG, in case of the handover from NR or intra-NR handover, initiate the RRC reestablishment procedure; In case of handover to NR, perform the actions defined in the specifications applicable for the source RAT. For T304 of SCG, inform network about the reconfiguration with sync failure by initiating the SCG failure information procedure as specified in 5.7.3.
t310	Upon detecting physical layer problems for the SpCell i.e. upon receiving N310 consecutive out-ofsync indications from lower layers.	Upon receiving N311 consecutive in-sync indications from lower layers for the SpCell, upon receiving RRCReconfiguration with reconfigurationWithSync for that cell group, and upon initiating the connection re-establishment procedure. Upon SCG release, if the T310 is kept in SCG.	If the T310 is kept in MCG: If AS security is not activated: go to RRC_IDLE else: initiate the connection re-establishment procedure. If the T310 is kept in SCG, Inform E-UTRAN/NR about the SCG radio link failure by initiating the SCG failure information procedure as specified in 5.7.3.

How to improve such failure in the network.

RLF-TimersAndConstants (the IE RLF-TimersAndConstants is used to configure UE specific timers and constants)

RLF-TimersAndConstants ::= SEQUENCE {
t310 ENUMERATED {ms0, ms50, ms100, ms200, ms500, ms1000, ms2000, ms4000, ms6000}, change the timer value depend upon condition of network, Start from o to 6000ms
n310 ENUMERATED {n1, n2, n3, n4, n6, n8, n10, n20},
n311 ENUMERATED {n1, n2, n3, n4, n5, n6, n8, n10},
…,
[[
t311 ENUMERATED {ms1000, ms3000, ms5000, ms10000, ms15000, ms20000, ms30000}
]]
}

RLC-Config

The IE RLC-Config is used to specify the RLC configuration of SRBs and DRBs.

T-PollRetransmit ::= ENUMERATED {ms5, ms10, ms15, ms20, ms25, ms30, ms35,ms40, ms45, ms50, ms55, ms60, ms65, ms70,ms75, ms80, ms85, ms90, ms95, ms100, ms105,
ms110, ms115, ms120, ms125, ms130, ms135, ms140, ms145, ms150, ms155, ms160, ms165, ms170, ms175, ms180, ms185, ms190, ms195,
ms200, ms205, ms210, ms215, ms220, ms225, ms230, ms235, ms240, ms245, ms250, ms300, ms350, ms400, ms450, ms500, ms800, ms1000,
ms2000, ms4000, spare5, spare4, spare3, spare2, spare1}
By changing T-Pollretransmit value, this will help the scg failure rate, In the LTE older release version,the maximum value was 500 ms , but now its increased up to 400 ms, high value
will improve the scg failure rate.

t-PollRetransmit T-PollRetransmit,
pollPDU PollPDU,
pollByte PollByte,
maxRetxThreshold ENUMERATED { t1, t2, t3, t4, t6, t8, t16, t32 } Maximum number of retransmission, if it is t32 , it means 32 retransmission
a higher value will improve the drop rate but it would affect throughput
}
DL-AM-RLC ::= SEQUENCE {
sn-FieldLength SN-FieldLengthAM OPTIONAL, — Cond Reestab
t-Reassembly T-Reassembly,
t-StatusProhibit T-StatusProhibit

srb3-IntegrityFailure

UE initiates transmission of the SCGFailureInformationNR message due to SRB3 integrity check failure

scg-reconfigFailure

UE initiates the transmission of the SCGFailureInformationNR message due to the Reconfiguration failure of NR RRC reconfiguration message.

During the SCG failure, UE also reports the available measurement result under I(measResultFreqListNR-r15 and measResultSCG