Radio Link Failure in LTETriggering points of RLF upon indication from RLC that the maximum number of re transmissions has been reached upon expiry of Timer T310 (this timer is started when physical layer problems are detected i.e. upon receiving N310 consecutive out-of-sync indications from lower layers) upon random access problem indication from MAC while neither T300, T301, T304 nor T311 is running

Actions when RLF is detected UE shall initiate RRC Connection Re-establishment procedure. (click here to know about the RRC CONNECTION RE-ESTABLISHMENT procedure in detail) if AS security has not been activated then inform upper layers about the release of RRC connection with release cause 'other'

In-sync   Indications (N311)   received: 1. UE will stop T3102. UE maintains the RRC connection (no configurations are changed)

N311 is triggered when in-sync indications are received. Once when the condition of N311 is fulfilled, UE won't go for RLF. Upon expiry of this constant, UE will UE maintains the entire radio resource configuration

Timers and constants used in RLF:

T301 Transmission of RRCConnectionReestabilshmentRequest

T310Upon detecting physical layer problems i.e. upon receiving N310 consecutive out-of-sync indications from lower layers

T311  Upon initiating the RRC connection re-establishment procedure

N310 (constant) Maximum number of consecutive "out-of-sync" indications received from lower layers

N311 (constant) Maximum number of consecutive "in-sync" indications received from lower layers

LTE Quick Reference                                  Go Back To Index    Home : www.sharetechnote.com   Radio Link Failure (RLF)  To be honest I don't think I saw any clear/explicit definition of this term. My personal definition of Radio Link Failure is "Physical Layer(especially low PHY) break" and in most case this failure is unintentional. When UE Report RLF ? Then the next question would be "How UE or eNodeB can detect this kind of Radio Link Failure ?". Unfortunately we don't have any clear answer to this question either. So detailed detection implementation is up to UE maker and eNodeB maker, but we can think of several guidelines. UE may assume that Radio Link is broken in the following setuation.

The measured RSRP is too low (under a certain limit) It failed to decode PDCCH due to power signal quality (e.g, low RSRP, RSRQ) It failed to decode PDSCH due to power signal quality (e.g, low RSRP, RSRQ)

However, detailed mechanism to RLF is upto the chipset implementation. So the individual RLF detection mechanism may vary chipset-to-chipset. eNodeB may assume that that Radio Link is broken in the following setuation.

SRS Power (SINR) from UE is much lower than what eNB configured for the UE eNodeB couldn't detect (see) any NACK nor ACK from UE for PDSCH.

 Does UE or eNodeB declare "Radio link Failure" whenever it sees the problem described above, even for only one subframe ?No, it is not. In most case, this kind of problem should happen for a certain period of time consecutively and a couple of timers and parameters are involved in the criteria setup. (See T311, n310)  What UE tries to do when RLF Happens ?

 Then the next question would be "What UE does when it detects Radio Link Failure ?" or "What eNodeB does when it detects Radio Link Failure ?".The most typical procedure is to go through RRC Connection Restablishement procedure. 

< Attemp to recover Radio Link while Out of SYNC (Radio Link Failure) - RRC Connection ReEstablishment > 

  

< When UL Data arrives from higher layer while Out of SYNC (Radio Link Failure) > 

 

 

 T310  

 Note 1 : one "Out of Sync Indication" in this diagram means "20 subframes of consecutive PDCCH decoding failure.Note 2 : one "In Sync Indication" in this diagram means "10 subframes of consecutive PDCCH decoding success. 

 

 

LTE Quick Reference                                     Go Back To Index    Home : www.sharetechnote.com   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) UE --> NW : RRCConnectionReestablishmentRequestii) UE <-- NW : RRCConnectionReestablishmentiii) UE --> NW :  RRCConnectionReestablishmentComplete

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 happenedCase 2 : when Handover failure happenedCase 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 layersCase 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 T301ii) consider the current cell to be the PCelliii) re-establish PDCP for SRB1iv) re-establish RLC for SRB1v) perform the radio resource configuration procedure in accordance with the receivedradioResourceConfigDedicatedvi)  resume SRB1