Content for  TS 38.401  Word version:  18.0.0

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.

The gNB-CU determines to initiate LTM configuration.

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.

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.

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.

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.

The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the gNB-DU, which includes the generated RRCReconfiguration message with the LTM configuration.

The gNB-DU forwards the received RRCReconfiguration message to the UE.

The UE responds to the gNB-DU with an RRCReconfigurationComplete message.

The gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message.

Early synchronization may be performed as specified in TS 38.300.

The UE sends the L1 measurement result to the gNB-DU. The gNB-DU decides to execute LTM.

The gNB-DU sends the Cell Switch command to the UE.

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.

The gNB-DU detects the UE access as specified in TS 38.300.

The target gNB-DU sends the ACCESS SUCCESS message to the gNB-CU with the target cell ID.

The UE sends an RRCReconfigurationComplete message to the gNB-DU.

The gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message.

The gNB-CU may send the UE CONTEXT MODIFICATION REQUEST message to the gNB-DU to release the resources of prepared cells.

The gNB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message.

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.

The gNB-CU determines to initiate LTM configuration.

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.

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.

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).

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.

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).

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.

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.

The source gNB-DU forwards the received RRCReconfiguration message to the UE.

The UE responds to the source gNB-DU with an RRCReconfigurationComplete message.

The source gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message.

Early synchronization may be performed as specified in TS 38.300.

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.

The UE sends the L1 measurement result to the source gNB-DU.

The source gNB-DU decides to execute LTM to a candidate target cell.

The source gNB-DU sends the Cell Switch command to the UE.

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.

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.

The target gNB-DU detects the UE access as specified in TS 38.300.

The target gNB-DU sends the ACCESS SUCCESS message to the gNB-CU with the target cell ID.

The UE sends an RRCReconfigurationComplete message to the target gNB-DU.

The target gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message.

The gNB-CU may send the UE CONTEXT RELEASE COMMAND message to the source gNB-DU to release the resources of prepared cells.

The source gNB-DU responds with a UE CONTEXT RELEASE COMPLETE message.

The source gNB-DU forwards the Measurement Report to the gNB-CU-CP.

The gNB-CU-CP decides to initiate LTM configuration.

The gNB-CU-CP sends a BEARER CONTEXT SETUP REQUEST message containing UL TNL address information for NG-U to setup the bearer context in the target gNB-CU-UP.

The target gNB-CU-UP responds with a BEARER CONTEXT SETUP RESPONSE message containing the UL TNL address information for F1-U, DL TNL address information for NG-U, and the TNL address information for data forwarding to the target gNB-CU-UP.

LTM configuration procedures are performed between gNB-CU and candidate gNB-DUs, and between gNB-CU and source gNB-DU as specified from step 3 to step 8 in clause 8.2.1.5.

The gNB-CU-CP sends the RRC Reconfiguration message to the UE.

The UE sends the lower layer measurement result to the source gNB-DU, and the source gNB-DU decides to execute LTM to a candidate target cell.

The source gNB-DU sends the DU-CU CELL SWITCH NOTIFICATION message to the gNB-CU-CP with the selected target cell ID.

The gNB-CU-CP performs the Bearer Context Modification procedure to retrieve the PDCP UL/DL status and to exchange the TNL address information for data forwarding for the bearers.

The gNB-CU-CP performs the Bearer Context Modification procedure to send the DL TNL address information for F1-U and the PDCP UP/DL status to the target gNB-CU-UP.

Data Forwarding may be performed from the source gNB-CU-UP to the target gNB-CU-UP.

The target gNB-DU detects the UE in the target cell.

The target gNB-DU sends an ACCESS SUCCESS message to the gNB-CU-CP.

Path Switch procedure is performed to update the DL TNL address information for the NG-U towards the core network.

Bearer Context Release procedure may be performed to release the UE context in the source gNB-DU.