Section: 8.2.1.4 Intra-gNB-DU LTM
Content: This procedure is used for the case when the UE moves within the same gNB-DU during NR operation for LTM. Figure 8.2.1.4-1 shows the intra-gNB-DU LTM procedure for intra-NR. Figure 8.2.1.4-1: Intra-gNB-DU LTM 1. The UE sends a MeasurementReport message (L3 measurement result) to the gNB-DU containing measurements of neighbouring cells. The 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 MODIFICATION REQUEST message to the gNB-DU containing one target candidate cell ID, the LTM configuration ID of the candidate cell, and LTM configuration ID mapping list, the CSI resource configuration. The gNB-CU requests PRACH resources from the gNB-DU. The gNB-CU may request the gNB-DU to provide the lower layer configuration for the purpose of generating the reference configuration. 4. If the gNB-DU accepts the request of LTM configuration, it responds with a UE CONTEXT MODIFICATION 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: Steps 3 and 4 may be initiated multiple times for LTM candidate cell preparation of multiple cells including the source cell. 5. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU including the collected TCI state configurations and the gNB-CU may send the CSI report configuration for all the accepted target candidate cells. 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 of the source cell. NOTE 2: In case of subsequent LTM, the CU-initiated UE Context Modification procedure may be invoked per each candidate cell to transfer to the candidate gNB-DU the CSI report configuration, TCI state information, RACH configuration, and the LTM configuration IDs of the candidate cells. 7. The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the gNB-DU, which includes the generated RRCReconfiguration message with the LTM configuration. 8. The gNB-DU forwards the received RRCReconfiguration message to the UE. 9. The UE responds to the gNB-DU with an RRCReconfigurationComplete message. 10. The gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. 11. Early synchronization may be performed as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2]. 12. The UE sends the L1 measurement result to the gNB-DU. The gNB-DU decides to execute LTM. 13. The gNB-DU sends the Cell Switch command to the UE. 14. The gNB-DU sends the DU-CU CELL CHANGE NOTIFICATION message to the gNB-CU to indicate the initiation of the Cell Switch command to the UE including the target cell ID and the TCI state ID. 15. The gNB-DU detects the UE access as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2]. 16. The target gNB-DU sends the ACCESS SUCCESS message to the gNB-CU with the target cell ID. 17. The UE sends an RRCReconfigurationComplete message to the gNB-DU. 18. The gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. 19. The gNB-CU may send the UE CONTEXT MODIFICATION REQUEST message to the gNB-DU to release the resources of prepared cells. 20. The gNB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message.