5G/NR - Interworking with LTE                      Home : www.sharetechnote.com

 

 

 

 

This is about one of NR deployment options where LTE work as a master and NR work as a secondary cell (In 3GPP terms, this is about EN-DC (EUTRA-NR Dual Connectivity) / MR-DC with EPC as described in 37.340). In this configuration, UE get connected to LTE network first and then connected to NR via RRC Connection Reconfiguration process. Further details on lower layer process is yet to be studied, but just looking into the conentents of RRCConnectionReconfiguration would give you some general idea about the mechanism.

 

 

 

Overall Network Architecture

 

Overall RAN architecture of EN-DC can be illustrated as below (this is based on 37.340 - 4.3.1 and 4.3.2). As you see here, UE is communicating with both LTE eNB and NR gNB in Radio side, but all those communication (signaling and data) are going through LTE core network. Though not shown in this illustration, I would point out that LTE eNB and NR gNB are using their own PHY/MAC (i.e, independent MAC Scheduler)).

 

 

 

Now let's read the picture. If you take a close look at the illustration and just describe it in words, you would get the descrition as follows. If you don't like reading (like me :), just take 10 minutes (not 10 seconds) and look thorugh each and every part and lines of the picture.

 

  • For C-Plane
    • There is an interface between MN(Master Node : MeNB in this csae) and the SN (Secondary Node : En-gNB in this case). This interface is called X2-C.
    • There is an interface between MN and CN(Core Network : MME in this case). This interface is called S1-MME.
    • There is NO direct interface(connection) between SN and CN
  • For U-Plane
    • There is an interface between MN(Master Node : MeNB in this csae) and the SN (Secondary Node : En-gNB in this case). This interface is called X2-U.
    • There is an interface between MN and CN(Core Network : MME in this case).  This interface is called S1-U
    • There is an interface(connection) between SN and CN. This interface is called S1-U.

 

 

 

Overall Layer 2 Architecture

 

 

 

 

 

Overall Signaling Procedure

 

This section is about how to add NR cell(Secondary Node) to an existing LTE Cell.(Master Note). Overall signaling flow is illustrated below. This flow is based on 37.340 - 10.2.1 except the step 6-0 which is added by me. As you may notice,the process is iniated by MN (Master Node : LTE eNB in this case) and final confirmation is done by MME.

 

 

< 37.340 - Figure 10.2.1-1: Secondary Node Addition procedure >

 

 

 

Step 1 : MN (Master Node : LTE eNB) send SgNB Addition Request to SN (Secondary Node : NR gNB in this case). LTE eNB forward following informations to NR gNB.

  • E-RAB Characteristics (E-RAB Parameters, TNL address information)
  • The requested SCG configuration information including the entire UE capabilities and UE capability coordination result
  • The latest measurement result for SN to choose
  • Securiy Information to enable SRB3
  • In case of bearer option that requires X2-U between MN and SN
    • X2-U TNS address information
  • In case of SN terminated split bearers,
    • the maximum supportable QoS level

Step 2 : (If SN decided to accept the request), it sends SgNB Addition Request Acknowlege performing followings

  • Allocate the necessary radio resources transport network resources
  • decides Pscell and other SCG Scells and provide the new SCG radio resource configuration to MN
  • In case of bearer options that requires X2-U between MN and SN
    • provides X2-U TNS address informations
  • In case of SCG radio resources being requested
    • provide SCG radio resource configuration

Step 3 : If NR gNB accept the SN addition request and provides all the necessary information to LTE eNB, LTE eNB generate RRC Connection Reconfiguation message carrying all the necessary information and send it to UE. This message carries NR RRC Connection Cofiguration in it so that UE can figure out the necessary configuration information for NR gNB.

 

Step 4  : After UE received RRCConnectionReconfiguration, it checks if all the configurations in the message is doable in UE side, it sends RRCConnectionReconfigurationComplete message. This message includes NR RRC Response as well.

 

Step 5  : Once MN (LTE eNB) received RRCConnectionReconfigurationComplete from UE, the MN informs SN(NR gNB) that UE has completed the reconfiguration procedure.

 

Step 6-0  : Based on the information contained in NR RRCConnectionConfiguration within RRC Connection Reconfiguration message, UE detects PSS, SSS and PBCH of NR gNB.

 

Step 6  : Once it successfully detects PSS, SSH, PBCH of NR gNB, it performs RACH procedure to PSCell of the SN (NR gNB).

 

 

 

LTE RRC Configuration

 

 

RRCConnectionReconfiguration-v1430-IEs ::= SEQUENCE {

    sl-V2X-ConfigDedicated-r14      SL-V2X-ConfigDedicated-r14      OPTIONAL,   -- Need ON

    sCellToAddModListExt-v1430      SCellToAddModListExt-v1430      OPTIONAL,   -- Need ON

    perCC-GapIndicationRequest-r14  ENUMERATED{true}                OPTIONAL,   -- Need ON

    systemInformationBlockType2Dedicated-r14    OCTET STRING (CONTAINING SystemInformationBlockType2)                                      OPTIONAL,   -- Cond nonHO

    nonCriticalExtension            RRCConnectionReconfiguration-v15x0-IEs      OPTIONAL

}

 

 

RRCConnectionReconfiguration-v15x0-IEs ::= SEQUENCE {

    endc-Config-r15             SEQUENCE {

        scg-ConfigReleaseNR-r15             BOOLEAN,

        sk-Counter-r15                      INTEGER (0.. 65535)    OPTIONAL,   -- Need ON

        nr-SecondaryCellGroupConfig-r15     OCTET STRING           OPTIONAL,   -- Need ON

        nr-RadioBearerConfig-r15            OCTET STRING           OPTIONAL,   -- Need ON

        nr-RadioBearerConfigS-r15           OCTET STRING           OPTIONAL,   -- Need ON

        tdm-PatternSingle-Tx-r15            SEQUENCE {

            subframeAssignment-r15              SubframeAssignment-r15,

            harq-Offset-r15                     INTEGER (0.. 9)    OPTIONAL    -- Need ON

            }                       OPTIONAL    -- Need ON

    }                                                              OPTIONAL    -- Need ON  nonCriticalExtension            SEQUENCE {}                    OPTIONAL

}

 

nr-SecondaryCellGroupConfig : Includes NR RRCReconfiguration message.

                                             Mainly for secondaryCellGroupToAddModList and measConfig.

nr-RadioBearerConfig : Include NR RadioBearerConfig. Mainly for DRB, EPS, NR PDCP Configuration

 

 

 

RRCConnectionResume-v15x0-IEs ::= SEQUENCE {

    sk-Counter-r15                      INTEGER (0.. 65535)         OPTIONAL,   -- Need ON

    nr-RadioBearerConfig-r15            OCTET STRING                OPTIONAL,   -- Need ON

    nr-RadioBearerConfigS-r15           OCTET STRING                OPTIONAL,   -- Need ON

    nonCriticalExtension                SEQUENCE {}                 OPTIONAL

}

 

 

 

 

 

Reference

 

[1]