8.2.2 EN-DC Mobility

38.4013GPPArchitecture descriptionNG-RANRelease 17TS

8.2.2.1 Inter-gNB-DU Mobility using MCG SRB

This procedure is used for the case the UE moves from one gNB-DU to another gNB-DU within the same gNB-CU when only MCG SRB is available during EN-DC operation. Figure 8.2.2.1-1 shows the inter-gNB-DU mobility procedure using MCG SRB in EN-DC.

Figure 8.2.2.1-1: Inter-gNB-DU Mobility using MCG SRB in EN-DC

1. The UE sends an ULInformationTransferMRDC message to the MeNB.

2. The MeNB sends RRC TRANSFER message to the gNB-CU.

3. The gNB-CU may send UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU to query the latest SCG configuration.

4. The source gNB-DU responds with an UE CONTEXT MODIFICATION RESPONSE message that includes full configuration information.

5. The gNB-CU sends an UE CONTEXT SETUP REQUEST message to the target gNB-DU to create an UE context and setup one or more data bearers. The UE CONTEXT SETUP REQUEST message includes a CG ConfigInfo.

6. The target gNB-DU responds the gNB-CU with an UE CONTEXT SETUP RESPONSE message.

7. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU indicating to stop the data transmission to the UE. The source gNB-DU also sends a Downlink Data Delivery Status frame to inform the gNB-CU about the unsuccessfully transmitted downlink data to the UE.

8. The source gNB-DU responds the gNB-CU with an UE CONTEXT MODIFICATION RESPONSE message.

9. The gNB-CU sends an SGNB MODIFICATION REQUIRED message to the MeNB.

10/11. The MeNB Initiated SgNB Modification procedure may be triggered by the SgNB Initiated SgNB Modification procedure (e.g. to provide information such as data forwarding addresses, new SN security key, measurement gap, etc…).

12. The MeNB and the UE perform RRC Connection Reconfiguration procedure.

13. The MeNB sends an SGNB MODIFICATION CONFIRM message to the gNB-CU.

14. Random Access procedure is performed at the target gNB-DU. The target gNB-DU sends a Downlink Data Delivery Status frame to inform the gNB-CU. Downlink packets, which may include PDCP PDUs not successfully transmitted in the source gNB-DU, are sent from the gNB-CU to the target gNB-DU. Downlink packets are sent to the UE. Also, uplink packets are sent from the UE, which are forwarded to the gNB-CU through the target gNB-DU.

NOTE: It is up to gNB-CU implementation whether to start sending DL User Data to gNB-DU before or after reception of the Downlink Data Delivery Status.

15. The gNB-CU sends an UE CONTEXT RELEASE COMMAND message to the source gNB-DU.

16. The source gNB-DU releases the UE context and responds the gNB-CU with an UE CONTEXT RELEASE COMPLETE message.

8.2.2.2 Inter-gNB-DU Mobility using SCG SRB (SRB3)

This procedure is used for the case the UE moves from one gNB-DU to another gNB-DU when SCG SRB (SRB3) is available during EN-DC operation. The procedure is the same as inter-gNB-DU Mobility for intra-NR as defined in clause 8.2.1.1 but the UE CONTEXT SETUP REQUEST message includes a CG-ConfigInfo.

8.2.2.3 Inter-gNB-DU Conditional PSCell Change using MCG SRB without MN negotiation

This procedure is used for the case where the UE moves from one gNB-DU to another gNB-DU within the same gNB-CU when only MCG SRB is available and the MN’s configuration is not changed during EN-DC operation for conditional PSCell change. Figure 8.2.2.3-1 shows the inter-gNB-DU conditional PSCell change procedure using MCG SRB in EN-DC.

Figure 8.2.2.3-1: Inter-gNB-DU Conditional PSCell Change using MCG SRB without MN negotiation in EN-DC

1-4. The steps 1-4 are as defined in clause 8.2.2.1.

5. The gNB-CU sends an UE CONTEXT SETUP REQUEST message to the candidate gNB-DU to create an UE context and setup one or more data bearers. The UE CONTEXT SETUP REQUEST message is sent for each candidate cell and includes a CG-ConfigInfo.

6. The candidate gNB-DU responds the gNB-CU with an UE CONTEXT SETUP RESPONSE message including the target cell ID that was requested from the gNB-CU. The response message is sent for each requested candidate cell.

7. The gNB-CU sends an SGNB MODIFICATION REQUIRED message to the MeNB, which includes a generated RRCReconfiguration message.

8. The MeNB and the UE perform RRC Connection Reconfiguration/Complete procedure.

9. The MeNB sends an SGNB MODIFICATION CONFIRM message to the gNB-CU, to convey the received RRCReconfigurationComplete message at step 8.

10. An execution condition to trigger initiation of conditional PSCell change is fulfilled.

11. Random Access procedure is performed at the candidate gNB-DU, which becomes the target gNB-DU if successful. The target gNB-DU sends a Downlink Data Delivery Status frame to inform the gNB-CU. The target gNB-DU also sends an ACCESS SUCCESS message to inform the gNB-CU of which cell the UE has successfully accessed.

12-13. The UE responds with an RRCReconfigurationComplete message (embedded in an ULInformationTransferMRDC message), which the MeNB forwards to the gNB-CU via an RRC TRANSFER message.

14. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU and indicates to stop the data transmission for the UE. The source gNB-DU sends a Downlink Data Delivery Status frame to inform the gNB-CU about the unsuccessfully transmitted downlink data to the UE. Downlink packets, which may include PDCP PDUs not successfully transmitted in the source gNB-DU, are sent from the gNB-CU to the target gNB-DU. Downlink packets are sent to the UE. Also, uplink packets are sent from the UE, which are forwarded to the gNB-CU through the target gNB-DU.

NOTE: The step 14 may happen before step 13, as soon as the gNB-CU knows which cell the UE has successfully accessed.

NOTE: The gNB-CU may initiate UE Context Release procedure toward the other signalling connections or other candidate target gNB-DUs, if any, to cancel conditional PSCell change for the UE.

15. The source gNB-DU responds to the gNB-CU with the UE CONTEXT MODIFICATION RESPONSE message.

16-17. The steps 16-17 are as defined in steps 11-12 in clause 8.2.1.1.