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)).

As you see here, in case of data plane both Master Node(LTE) and Secondary Node(gNB) has direct interface with LTE corenetwork(S-GW), but in case of control plane only Master Node(LTE) has direct interface with LTE core network(MME).

 

 

 

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

 

 

Following is based on 37.340 - Figure 4.2.2-3 and Figure 4.2.2-1. As mentioned before, there are roughly two options when LTE and NR interplay. One option is to make LTE as a master and NR as a slave. The other option is to make NR as a master and LTE as a slave. In real deployment, especially at the early deployment, the first option (i.e, LTE Master and NR Slave) would be the major deployment option. This illustration is also to show the overal radio stack structure of LTE Master and NR Slave.

 

 

 

 

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 SSBlock(PSS, SSS and PBCH) of NR gNB.  For the details of SSBlock, see SSBlock page.

 

Step 6  : Once it successfully detects PSS, SSH, PBCH of NR gNB, it performs RACH procedure to PSCell of the SN (NR gNB). UE aquire all the information required for RACH procedure from RRC Connection Reconfiguration message instead of SIB, this process is described in RACH Process for LTE-Interworking(SNDC) .

 

 

 

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,

    nonCriticalExtension            RRCConnectionReconfiguration-v15x0-IEs      OPTIONAL

}

 

 

RRCConnectionReconfiguration-v1510-IEs ::= SEQUENCE {

    nr-Config-r15                   CHOICE {

        release                         NULL,

        setup                           SEQUENCE {

            endc-ReleaseAndAdd-r15  BOOLEAN,

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

            p-MaxEUTRA-r15                  P-Max               OPTIONAL    -- Need ON

        }

    }                                                           OPTIONAL,   -- Need ON

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

    nr-RadioBearerConfig1-r15       OCTET STRING                OPTIONAL,   -- Need ON

    nr-RadioBearerConfig2-r15       OCTET STRING                OPTIONAL,   -- Need ON

    tdm-PatternConfig-r15           CHOICE {

        release                         NULL,

        setup                           SEQUENCE {

            subframeAssignment-r15          SubframeAssignment-r15,

            harq-Offset-r15                 INTEGER (0.. 9)

        }

    }                                                           OPTIONAL,   -- Need ON

    nonCriticalExtension            SEQUENCE {}                 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-SecondaryCellGroupConfig1-r15,nr-SecondaryCellGroupConfig2-r15 : Includes NR RRCReconfiguration message.

                                             The field includes the configuration of RBs configured with NR PDCP.

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

}

 

 

 

Example >

 

 

As mentioned above, RRC Connection Reconfiguration in LTE for Adding NR Cell carries only a couple of container that carries a huge tree of NR RRC message.  Due to the complicated RRC structure in NR, I found it difficult to put the whole structure and description of any NR RRC message in a single page. In this example, you will see a couple of starting points of NR RRC part as shown below and you should follow the link until you reach the final destination.

 

rrcConnectionReconfiguration

    measConfig

    mobilityControlInfo

    dedicatedInfoNASList

    radioResourceConfigDedicated

    nonCriticalExtension

        laterNonCriticalExtension

        nonCriticalExtension

            otherConfig-r9

            fullConfig-r9

            nonCriticalExtension

                sCellToReleaseList-r10

                sCellToAddModList-r10

                nonCriticalExtension

                    systemInformationBlockType1Dedicated-r11

                    nonCriticalExtension

                        wlan-OffloadInfo-r12

                        scg-Configuration-r12

                        sl-SyncTxConfrol-r12

                        sl-DiscConfig-r12

                        sl-CommonConfig-r12

                        nonCriticalExtension

                            sCellToReleaseListExt-r13

                            sCellToAddModListExt-r13

                            lwa-Configuration-r13

                            lwip-Configuration-r13

                            rdwi-Configuration-r13

                            nonCriticalExtension

                                sl-V2X-ConfigDedicated-r14

                                sCellToAddModListExt-v1430

                                perCC-GapIndicationRequest-r14

                                systemInformationBlockType2Dedicated-r14

                                nonCriticalExtension

                                    nr-Config-r15

                                        endc-ReleaseAndAdd-r15

                                        nr_SecondaryCellGroupConfig-r15

                                            cellGroupID

                                            rlc-BearerToAddModList

                                            mac-CellGroupConfig

                                            physicalCellGroupConfig

                                            reconfigurationWithSync

                                            rlf-TimersAndConstants

                                            rlmInSyncOutOfSyncThreshold

                                            spCellConfigDedicated

                                        p_MaxEUTRA-r15

                                    sk-Counter-r15

                                    nr-RadioBearerConfig1-r15

                                    nr-RadioBearerConfig2-r15

                                    tdm-PatternConfig-r15

                                    nonCriticalExtension

 

 

 

 

Reference

 

[1] LTE-NR tight-interworking and the first steps to 5G (Errisson Research Blog)  

[2] 4G-5G Interworking (SamSung)