Section: 8.19.4.3 Inter-gNB-CU switch from direct to indirect path
Content: The signalling flow for U2N Remote UE switch from direct to indirect path with gNB-CU change is shown in Figure 8.19.4.3-1. Figure 8.19.4.3-1. U2N Remote UE Direct-to-indirect Path Switch with gNB-CU change procedure 1. The Uu measurement configuration and measurement report signalling are performed between U2N Remote UE and source gNB-CU to evaluate both relay link measurement and Uu link measurement. The U2N Remote UE may report one or multiple candidate U2N Relay UE(s) and Uu measurement results after it measures/discovers the candidate U2N Relay UE(s). 2. The source gNB-CU decides to switch the U2N Remote UE to one of the candidate U2N Relay UE(s). 3. The source gNB-CU sends the HANDOVER REQUEST message to the target gNB-CU. The HANDOVER REQUEST message may include a list of candidate U2N Relay UE(s) of same cell of the target gNB. 4. The target gNB-CU decides to accept the indirect path switching to the target U2N Relay UE among the candidate U2N Relay UE(s). 5. The reconfiguration to target U2N Relay UE is performed among target U2N Relay UE, the target gNB-DU and target gNB-CU, if the target U2N Relay UE is in RRC_CONNECTED state. The target gNB-CU allocates the local ID for the U2N Remote UE. The target gNB-CU sends an RRCReconfiguration message to the target U2N Relay UE. If the target U2N Relay UE is in RRC_IDLE/INACTIVE state, this step is skipped and the configuration to the target U2N Relay UE is performed in Step 15. 6. The target gNB-CU sends the UE CONTEXT SETUP REQUEST message for the U2N Remote UE to the target gNB-DU, which contains the path switch configuration at least. 7. The target gNB-DU responds with the UE CONTEXT SETUP RESPONSE message to the target gNB-CU. 8. The target gNB-CU responds the source gNB-CU with an HANDOVER REQUEST ACKNOWLEDGE message by including the RRCReconfiguration message. The contents in the RRCReconfiguration message may include at least path switch configuration, PC5 Relay RLC channel configuration for relay traffic, bearer mapping and the associated radio bearer(s). 9. The source gNB-CU sends to the source gNB-DU the UE CONTEXT MODIFICATION REQUEST message to send the handover command to the U2N Remote UE, and to indicate to stop the data transmission for the U2N Remote UE. 10. The source gNB-DU sends the RRCReconfiguration message to the U2N Remote UE. The U2N Remote UE stops UP and CP transmission over Uu after reception of RRCReconfiguration message from the source gNB. 11. The source gNB-DU sends the UE CONTEXT MODIFICATION RESPONSE message to the source gNB-CU. 12. The source gNB-CU sends an SN STATUS TRANSFER message to the target gNB-CU. 13. Data Forwarding may be performed from the source gNB-CU to the target gNB-CU. 14. The U2N Remote UE establishes PC5 connection with target U2N Relay UE. 15. The U2N Remote UE completes the path switch procedure by sending the RRCReconfigurationComplete message to the target gNB-DU via the target U2N Relay UE. In case the target U2N Relay UE is in RRC_IDLE/ INACTIVE state when receiving the RRCReconfigurationComplete message, the reception of the RRCReconfigurationComplete message will first trigger RRC setup/resume procedure for the target U2N Relay UE to enter RRC_CONNECTED state. 16. The target gNB-DU sends the UL RRC MESSAGE TRANSFER message to target gNB-CU by including the RRCReconfigurationComplete message. 17. Path Switch procedure is performed to switch the DL data path towards the target gNB-CU and to establish an NG-C interface instance towards the target gNB-CU. 18. The target gNB-CU sends an UE CONTEXT RELEASE message to the source gNB-CU. 19. The source gNB-CU initiates a F1 UE Context Release procedure to release the UE context of the U2N Remote UE in the source gNB-DU.