Section: 8.2.1.3 Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC
Content: This procedure is used for the case when the UE moves from one gNB-DU to another gNB-DU within the same gNB-CU during NR operation for conditional handover or conditional PSCell change or subsequent CPAC. Figure 8.2.1.3-1 shows the inter-gNB-DU conditional mobility procedure for intra-NR. Figure 8.2.1.3-1: Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC for intra-NR 1-2. The steps 1-2 are as defined in clause 8.2.1.1. 3. 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 HandoverPreparationInformation (conditional handover) or a CG-ConfigInfo (conditional PSCell change or subsequent CPAC). 4. The candidate gNB-DU responds to 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. 5. The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the source gNB-DU, which includes a generated RRCReconfiguration message. 6. The step 6 is as defined in clause 8.2.1.1. 7-8. The UE responds to the source gNB-DU with an RRCReconfigurationComplete message, for which the source gNB-DU forwards to the gNB-CU via an UL RRC MESSAGE TRANSFER message. 9. An execution condition to trigger initiation of conditional handover or conditional PSCell change or subsequent CPAC is fulfilled. 10. A 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. 11-12. The steps 11-12 are as defined in steps 9-10 in clause 8.2.1.1. 13. 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 also 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. NOTE: The step 13 may happen before step 12, 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 handover or conditional PSCell change for the UE. 14. The source gNB-DU responds to the gNB-CU with the UE CONTEXT MODIFICATION RESPONSE message. 15 -16. The steps 15-16 are as defined in steps 11-12 in clause 8.2.1.1. The step 15~16 shall not happen if all of the PSCells in the source gNB-DU remain configured as the candidate PSCell(s) in the subsequent CPAC.