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RRC Connection ReEstablishment

 

< What is this >

 

It is a kind of mechanism to make a recovery when radio link got broken for some reason. (See "When it happens" section for these reason)

 

 

< How it work >

 

it works in three steps as shown below (refer to 36.331 5.3.7 RRC connection re-establishment for the details)

I think most of those who read this page may be pretty familiar with LTE protocol, so just by looking at the contents of these RRC messages, they would figure out a lot of details without much explanation.

 

RRCConnectionReestablishmentRequest

 

        c1: rrcConnectionReestablishmentRequest (0)

            rrcConnectionReestablishmentRequest

                criticalExtensions: rrcConnectionReestablishmentRequest-r8 (0)

                    rrcConnectionReestablishmentRequest-r8

                        ue-Identity

                            c-RNTI: 0000 [bit length 16, 0000 0000  0000 0000 decimal value 0]

                            physCellId: 0

                            shortMAC-I: 0000 [bit length 16, 0000 0000  0000 0000 decimal value 0]

                        reestablishmentCause: reconfigurationFailure /handoverFailure/otherFailure/ spare1   

                        spare: 00 [bit length 2, 6 LSB pad bits, 00.. .... decimal value 0]

 

 

RRCConnectionReestablishment (An example based on 36.508 - RRCConnectionReestablishment)

 

        c1: rrcConnectionReestablishment (0)

            rrcConnectionReestablishment

                rrc-TransactionIdentifier: 0

                criticalExtensions: c1 (0)

                    c1: rrcConnectionReestablishment-r8 (0)

                        rrcConnectionReestablishment-r8

                            radioResourceConfigDedicated

                                srb-ToAddModList: 1 item

                                    Item 0

                                        SRB-ToAddMod

                                            srb-Identity: 1

                                            rlc-Config: defaultValue (1)

                                                defaultValue: NULL

                                            logicalChannelConfig: defaultValue (1)

                                                defaultValue: NULL

                                mac-MainConfig: explicitValue (0)

                                    explicitValue

                                        ul-SCH-Config

                                            maxHARQ-Tx: n5 (4)

                                            periodicBSR-Timer: sf20 (3)

                                            retxBSR-Timer: sf320 (0)

                                            ..0. .... ttiBundling: False

                                        drx-Config: release (0)

                                            release: NULL

                                        timeAlignmentTimerDedicated: sf750 (1)

                                        phr-Config: setup (1)

                                            setup

                                                periodicPHR-Timer: sf500 (5)

                                                prohibitPHR-Timer: sf200 (5)

                                                dl-PathlossChange: dB3 (1)

                                physicalConfigDedicated

                                    pdsch-ConfigDedicated

                                        p-a: dB-3 (2)

                                    pucch-ConfigDedicated

                                        ackNackRepetition: release (0)

                                            release: NULL

                                    pusch-ConfigDedicated

                                        betaOffset-ACK-Index: 9

                                        betaOffset-RI-Index: 6

                                        betaOffset-CQI-Index: 6

                                    uplinkPowerControlDedicated

                                        p0-UE-PUSCH: 0dB

                                        deltaMCS-Enabled: en0 (0)

                                        ..1. .... accumulationEnabled: True

                                        p0-UE-PUCCH: 0dB

                                        pSRS-Offset: 3

                                        filterCoefficient: fc4 (4)

                                    cqi-ReportConfig

                                        cqi-ReportModeAperiodic: rm30 (3)

                                        nomPDSCH-RS-EPRE-Offset: 0dB (0)

                                    soundingRS-UL-ConfigDedicated: setup (1)

                                        setup

                                            srs-Bandwidth: bw0 (0)

                                            srs-HoppingBandwidth: hbw0 (0)

                                            freqDomainPosition: 0

                                            ..1. .... = duration: indefinite

                                            srs-ConfigIndex: 20

                                            transmissionComb: 0

                                            cyclicShift: cs0 (0)

                                    antennaInfo: explicitValue (0)

                                        explicitValue

                                            transmissionMode: tm3 (2)

                                            codebookSubsetRestriction: n2TxAntenna-tm3 (0)

                                                n2TxAntenna-tm3: c0 [bit length 2, 6 LSB pad bits, 11.. .... decimal value 3]

                                            ue-TransmitAntennaSelection: release (0)

                                                release: NULL

                                    schedulingRequestConfig: setup (1)

                                        setup

                                            sr-PUCCH-ResourceIndex: 60

                                            sr-ConfigIndex: 30

                                            dsr-TransMax: n4 (0)

                            nextHopChainingCount: 0

    HEX string : 00 12 9B 3E 86 03 B5 79 E8 96 6C 30 64 99 80 20 A0 28 68 3C 1E 00

 

 

< When it happens >

 

There are several cases where this process get triggered. According to 36.331 5.3.7.2, there are several cases as described below.

 

    Case 1 : When radio link failure happened

    Case 2 : when Handover failure happened

    Case 3 : when mobility from E-UTRA failure happened

      According to 36.331 5.4.3.5 Mobility from E-UTRA failure,

        revert back to the configuration used in the source PCell, excluding the configuration configured by the physicalConfigDedicated, mac-MainConfig and sps-Config;

        initiate the connection re-establishment procedure as described in How it works and Common UE side Process

    Case 4 : when integrity check failure indication was received from lower layers

    Case 5 : when RRC connection reconfiguration failure happened

 

< Common UE side Process >

 

Whatever the case is, overall process on UE side is similar as follows. (36.331 5.3.7.5)

    i) stop timer T301

    ii) consider the current cell to be the PCell

    iii) re-establish PDCP for SRB1

    iv) re-establish RLC for SRB1

    v) perform the radio resource configuration procedure in accordance with the received radioResourceConfigDedicated

    vi)  resume SRB1