Section: 8.2.1.5 Inter-gNB-DU LTM
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 LTM. Figure 8.2.1.5-1 shows the inter-gNB-DU LTM procedure for intra-NR. Figure 8.2.1.5-1: Inter-gNB-DU LTM 1. The UE sends a MeasurementReport message (L3 measurement result) to the source gNB-DU containing measurements of neighbouring cells. The source gNB-DU sends an UL RRC MESSAGE TRANSFER message conveying the received MeasurementReport message to the gNB-CU. 2. The gNB-CU determines to initiate LTM configuration. 3. The gNB-CU sends a UE CONTEXT SETUP REQUEST message to the candidate gNB-DU(s), containing one target candidate cell ID, the LTM configuration ID of the candidate cell, LTM configuration ID mapping list, and the CSI resource configuration. The gNB-CU indicates the source gNB-DU ID, and requests PRACH resources from the candidate gNB-DU. The gNB-CU may request the candidate gNB-DU to provide the lower layer configuration for the purpose of generating the reference configuration. 4. If the candidate gNB-DU accepts the request of LTM configuration, it responds with a UE CONTEXT SETUP RESPONSE message including the generated lower layer RRC configurations (e.g., TCI state configuration, RACH configuration, and the CSI report configuration) for the accepted target candidate cell. NOTE 1: The CU-initiated UE Context Modification procedure may be initiated for preparing candidate cells in the source gNB-DU as specified in step 3 and 4 in 8.2.1.4 Intra-gNB-DU LTM. 5. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU including the collected CSI report configuration, RACH configuration and the TCI state configuration for the accepted target candidate cell(s) in other gNB-DU(s). 6. The source gNB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message which includes an updated lower layer configuration, e.g., containing the CSI report configuration. 7. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the candidate gNB-DU(s) containing the CSI report configuration, TCI state information, RACH Configuration, and the LTM configuration IDs of the candidate cells in other candidate gNB-DU(s). The gNB-CU may also provide the lower layer part of the reference configuration to the candidate gNB-DU(s). The gNB-CU may also provide an updated CSI resource configuration to the candidate gNB-DU(s). NOTE 2: The candidate cell may be the same cell as source cell. 8. The candidate gNB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message including the updated lower layer configuration. The candidate gNB-DU may also respond the updated CSI report configuration. 9. The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the source gNB-DU, which includes the generated RRCReconfiguration message with the LTM configuration. 10. The source gNB-DU forwards the received RRCReconfiguration message to the UE. 11. The UE responds to the source gNB-DU with an RRCReconfigurationComplete message. 12. The source gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. 13. Early synchronization may be performed as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2]. 14 - 15. The candidate gNB-DU sends the TA value, the associated CFRA resource information, the candidate cell ID and the source gNB-DU ID to the source gNB-DU via the DU-CU TA INFORMATION TRANSFER and CU-DU TA INFORMATION TRANSFER messages, for which the source gNB-DU ID is omitted in the CU-DU TA INFORMATION TRANSFER message. 16. The UE sends the L1 measurement result to the source gNB-DU. 17. The source gNB-DU decides to execute LTM to a candidate target cell. 18. The source gNB-DU sends the Cell Switch command to the UE. 19. The source gNB-DU sends the DU-CU CELL SWITCH NOTIFICATION message to the gNB-CU to indicate the initiation of the Cell Switch command to the UE, for which the message includes the target cell ID and the TCI state ID. 20. The gNB-CU forwards the target cell ID and the TCI state ID to the target gNB-DU in the CU-DU CELL SWITCH NOTIFICATION message. 21. The target gNB-DU detects the UE access as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2]. 22. The target gNB-DU sends the ACCESS SUCCESS message to the gNB-CU with the target cell ID. 23. The UE sends an RRCReconfigurationComplete message to the target gNB-DU. 24. The target gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. 25. The gNB-CU may send the UE CONTEXT RELEASE COMMAND message to the source gNB-DU to release the resources of prepared cells. 26. The source gNB-DU responds with a UE CONTEXT RELEASE COMPLETE message.