NB-IOT UE Attach Call Flow Messaging

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When a NB-IOT UE accesses a cell, it follows the same principle as for LTE: It first searches a cell on an appropriate frequency. Decode NPSS and NSSS for NCellID. After decoding NCellID, UE decode the NB-MIB Information transmitted over NPBCH, but there it can get to know NB-LTE deployment mode i.e. In-band, Guard-band or Standalone, the schedulingInfoSIB1, SIB1-NB size and number of repetitions, and its starting position.

After getting information about NB-SIB1 UE get to know following Cell Access Parameter

  • PLMN ID, TA Code
  • Cell identity & Cell Status and cell selection information like minimum receiver level
  • Scheduling Information for other SIBs – SI message type & Periodicity,
  • SIB mapping Info, SI Window length.

After successful decode of NB-SIB1, UE decodes further NB-SIBs transmitted over NPDSCH. From SIB2 UEs get information about configuration about

  • Common logical channel, and Physical Channel
  • Most information is NB-SIB2 is RACH configuration which is need for uplink synchronization.

After getting RACH configuration UE sends RACH Preamble, the UE first calculates its RA-RNTI from the transmission time. It looks then in the NPDCCH for the DCI N1 scrambled with the RA-RNTI to get the Random Access Response. The UE expects this message within the Response Window, which starts 3 SFs after the last preamble SF.

If Random Access Response message is not received, the UE transmits another Preamble. This is done up to a maximum number of attempted depending on the CE level. If the total number of access attempts is reached, an associated failure is reported to the RRC Layer. If RACH is successful, the UE gets in a temporary C-RNTI, timing advance command in RAR. Further, the RAR provides the UL grant for msg3, containing all relevant data for msg3 transmission. The remaining procedure is done like in conventional LTE, i.e. the UE sends an identification and upon reception of the Contention Resolution random access procedure is completed.

UE sends RRCConnectionRequest indicating that it wants to connect to the network and stating establishment cause. In NB-LTE it is restricted to mobile originated signaling, mobile originated data, mobile terminated access and exceptional reports. There is no establishment cause for delay tolerant traffic, because in NB-LTE all traffic is assumed to be delay tolerant. In response eNodeB sends RRCConnectionSetup message providing configuration of the signaling radio bearer (SRB1), and the protocols.

If UE accept all the configuration provided by eNodeB, UE sends RRCConnectionSetupComplete message including selected PLMN and MME, and can piggyback the NAS messages including Attach Request + PDN Connectivity Request.

Then Authentication and NAS level security procedure happens. After successful security procedure eNodeB may does UE capability Enquiry if it does not receives this information from MME, UE does reply with UE capability information indicating UE category, list of supported NB IOT bands, UE different layer parameter support like supported ROHC profile for PDCP, multi -tone support for Physical layer etc.

When UE receives attach accept and activate default bearer context request indicating core network accept UE and created default bearer context, UE replies with accept complete message.

Below are snapshot of some  NB-IOT Call Message

Master Information Block
BCCH-BCH-Message-NB
         systemFrameNumber-MSB-r13: 00        
  hyperSFN-LSB-r13: 00
          schedulingInfoSIB1-r13: 4
  NPDSCH repetitions - TBS 208 bits (0)
          systemInfoValueTag-r13: 0
          .... ...0 ab-Enabled-r13: False
          operationModeInfo-r13: standalone-r13 (3)
              standalone-r13

System Information Block Type #1
systemInformationBlockType1-r13 : 
  hyperSFN-MSB-r13 '00000000'B,
          cellAccessRelatedInfo-r13 
            plmn-IdentityList-r13 
                  plmn-Identity-r13 
                  mcc 2, 3, 0
                  mnc 0,1
  cellReservedForOperatorUse-r13 notReserved,
  attachWithoutPDN-Connectivity-r13 true
  trackingAreaCode-r13 '00000000 00010101'B,
            cellIdentity-r13 '00000000 00000100 00010000 0010'B,
            cellBarred-r13 notBarred,
            intraFreqReselection-r13 notAllowed
  cellSelectionInfo-r13 
            q-RxLevMin-r13 -55,
            q-QualMin-r13 -20
          freqBandIndicator-r13 1,
          schedulingInfoList-r13 
  si-Periodicity-r13 rf64,
              si-RepetitionPattern-r13 every8thRF,
              sib-MappingInfo-r13 
  si-TB-r13 b552
  si-WindowLength-r13 ms160

System Information Block #2
sib2-r13 : 
                  radioResourceConfigCommon-r13 
                      rach-ConfigCommon-r13 
                        preambleTransMax-CE-r13 n6,
                        powerRampingParameters-r13 
                          powerRampingStep dB2,
                          preambleInitialReceivedTargetPower dBm-104
                        rach-InfoList-r13 
                            ra-ResponseWindowSize-r13 pp10,
                            mac-ContentionResolutionTimer-r13 pp8
                      bcch-Config-r13 
                          modificationPeriodCoeff-r13 n32
                     pcch-Config-r13 
                        defaultPagingCycle-r13 rf128,
                        nB-r13 oneT,
                        npdcch-NumRepetitionPaging-r13 r8
                      nprach-Config-r13 
                        nprach-CP-Length-r13 us66dot7,
                        nprach-ParametersList-r13 
                            nprach-Periodicity-r13 ms80,
                            nprach-StartTime-r13 ms32,
                            nprach-SubcarrierOffset-r13 n12,
                            nprach-NumSubcarriers-r13 n12,
                            nprach-SubcarrierMSG3-RangeStart-r13 oneThird,
                            maxNumPreambleAttemptCE-r13 n6,
                            numRepetitionsPerPreambleAttempt-r13 n1,
                            npdcch-NumRepetitions-RA-r13 r16,
                            npdcch-StartSF-CSS-RA-r13 v4,
                            npdcch-Offset-RA-r13 zero
                     npdsch-ConfigCommon-r13 
                        nrs-Power-r13 44
                      npusch-ConfigCommon-r13 
                        ack-NACK-NumRepetitions-Msg4-r13 
                          r8
                        dmrs-Config-r13 
                          threeTone-CyclicShift-r13 0,
                          sixTone-CyclicShift-r13 0
                        ul-ReferenceSignalsNPUSCH-r13 
                          groupHoppingEnabled-r13 FALSE,
                          groupAssignmentNPUSCH-r13 0  uplinkPowerControlCommon-r13 
                        p0-NominalNPUSCH-r13 -85,
                        alpha-r13 al1,
                        deltaPreambleMsg3-r13 4
                    ue-TimersAndConstants-r13 
                      t300-r13 ms10000,
                      t301-r13 ms10000,
                      t310-r13 ms1000,
                      n310-r13 n2,
                      t311-r13 ms1000,
                      n311-r13 n1
                    freqInfo-r13 
                      additionalSpectrumEmission-r13 1
                    timeAlignmentTimerCommon-r13 infinity
RRC Connection Request
rrcConnectionRequest-r13 :
            ue-Identity-r13 randomValue : '11010100 00100001 11101000 111110010011111'B,
            establishmentCause-r13 mo-Signalling,
            multiToneSupport-r13 true,
            spare '00000000 00000000 000000'B
RRC Connection Setup
rrcConnectionSetup-r13 :
              radioResourceConfigDedicated-r13
                  srb-ToAddModList-r13
                      rlc-Config-r13 defaultValue : NULL,
                      logicalChannelConfig-r13 defaultValue : NULL
                  mac-MainConfig-r13 explicitValue-r13 :
                      ul-SCH-Config-r13
                          periodicBSR-Timer-r13 pp8,
                        retxBSR-Timer-r13 infinity
                      drx-Config-r13 release : NULL,
                      timeAlignmentTimerDedicated-r13 infinity,
                      logicalChannelSR-Config-r13 setup :
                          logicalChannelSR-ProhibitTimer-r13 pp2048
                  physicalConfigDedicated-r13
                    npdcch-ConfigDedicated-r13
                      npdcch-NumRepetitions-r13 r16,
                      npdcch-StartSF-USS-r13 v4,
                      npdcch-Offset-USS-r13 zero
                         npusch-ConfigDedicated-r13
                      ack-NACK-NumRepetitions-r13 r8,
                      npusch-AllSymbols-r13 TRUE
                                     uplinkPowerControlDedicated-r13
                      p0-UE-NPUSCH-r13 0
RRC Connection Setup Complete
rrcConnectionSetupComplete-r13 :
  rrc-TransactionIdentifier 0,
          criticalExtensions rrcConnectionSetupComplete-r13 :
                selectedPLMN-Identity-r13 1,
              registeredMME-r13
                mmegi '10000000 00000001'B,
                mmec '00000001'B
         dedicatedInfoNAS-r13  xxxxxxxxxxxxxxxx’H

UE Capability Enquiry
ueCapabilityEnquiry-r13 :        
   rrc-TransactionIdentifier 0,        
   criticalExtensions c1 :  ueCapabilityEnquiry-r13

UE Capability Information
ueCapabilityInformation-r13 : 
  rrc-TransactionIdentifier 0,
          criticalExtensions ueCapabilityInformation-r13 : 
              ue-Capability-Container-r13 
                accessStratumRelease-r13 rel13,
                ue-Category-NB-r13 nb1,
                multipleDRB-r13 supported,
                pdcp-Parameters-r13 
                  supportedROHC-Profiles-r13 
                    profile0x0002 TRUE,
                    profile0x0003 FALSE,
                    profile0x0004 FALSE,
                    profile0x0006 FALSE,
                    profile0x0102 FALSE,
                    profile0x0103 FALSE,
                    profile0x0104 FALSE
                phyLayerParameters-r13 
                  multiTone-r13 supported
                rf-Parameters-r13 
                  supportedBandList-r13 
                      band-r13 1
                      band-r13 4
                      band-r13 12
              ue-RadioPagingInfo-r13 
                ue-Category-NB-r13 nb1

References:

  • R&S Narrow Band IOT White Paper
  • Slideshare

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