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Content for  TS 38.401  Word version:  18.0.0

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8.2 Intra-gNB-CU Mobility  8.2.1 Intra-NR Mobility  8.2.1.1 Inter-gNB-DU Mobility  8.2.1.2 Intra-gNB-DU handover  8.2.1.3 Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC 
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8.2  Intra-gNB-CU Mobility  p. 31

8.2.1  Intra-NR Mobility  p. 31

8.2.1.1  Inter-gNB-DU Mobility  p. 31

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. Figure 8.2.1.1-1 shows the inter-gNB-DU mobility procedure for intra-NR.
Reproduction of 3GPP TS 38.401, Fig. 8.2.1.1-1: Inter-gNB-DU Mobility for intra-NR
Figure 8.2.1.1-1: Inter-gNB-DU Mobility for intra-NR
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Step 1.
The UE sends a MeasurementReport message to the source gNB-DU.
Step 2.
The source gNB-DU sends an UL RRC MESSAGE TRANSFER message to the gNB-CU to convey the received MeasurementReport message.
Step 2a.
The gNB-CU may send an UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU to query the latest configuration.
Step 2b.
The source gNB-DU responds with an UE CONTEXT MODIFICATION RESPONSE message that includes full configuration information.
Step 3.
The gNB-CU sends an UE CONTEXT SETUP REQUEST message to the target gNB-DU to create an UE context and setup one or more data bearers. The UE CONTEXT SETUP REQUEST message includes a HandoverPreparationInformation. In case of NG-RAN sharing, the gNB-CU includes the serving PLMN ID (for SNPNs the serving SNPN ID).
Step 4.
The target gNB-DU responds to the gNB-CU with an UE CONTEXT SETUP RESPONSE message.
Step 5.
The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU, which includes a generated RRCReconfiguration message 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.
Step 6.
The source gNB-DU forwards the received RRCReconfiguration message to the UE.
Step 7.
The source gNB-DU responds to the gNB-CU with the UE CONTEXT MODIFICATION RESPONSE message.
Step 8.
A Random Access procedure is performed at the target gNB-DU. The target gNB-DU sends a Downlink Data Delivery Status frame to inform the gNB-CU. 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.
Step 9.
The UE responds to the target gNB-DU with an RRCReconfigurationComplete message.
Step 10.
The target gNB-DU sends an UL RRC MESSAGE TRANSFER message to the gNB-CU to convey the received RRCReconfigurationComplete message. Downlink packets are sent to the UE. Also, uplink packets are sent from the UE, which are forwarded to the gNB-CU through the target gNB-DU.
Step 11.
The gNB-CU sends an UE CONTEXT RELEASE COMMAND message to the source gNB-DU.
Step 12.
The source gNB-DU releases the UE context and responds the gNB-CU with an UE CONTEXT RELEASE COMPLETE message.
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8.2.1.2  Intra-gNB-DU handover  p. 32

This procedure is used for the case that the UE moves from one cell to another cell within the same gNB-DU or for the case that intra-cell handover is performed during NR operation, and supported by the UE Context Modification (gNB-CU initiated) procedure as specified in TS 38.473. When the intra-gNB-DU handover is performed (either inter-cell or intra-cell), the gNB-CU provides new UL GTP TEID to the gNB-DU and the gNB-DU provides new DL GTP TEID to the gNB-CU. The gNB-DU shall continue sending UL PDCP PDUs to the gNB-CU using the previous UL GTP TEID until it re-establishes the RLC, and after then start sending using the new UL GTP TEID. The gNB-CU shall continue sending DL PDCP PDUs to the gNB-DU using the previous DL GTP TEID until it performs PDCP re-establishment or PDCP data recovery, and after then start sending using the new DL GTP TEID.
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8.2.1.3  Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC |R16|  p. 33

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.
Reproduction of 3GPP TS 38.401, Fig. 8.2.1.3-1: Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC for intra-NR
Figure 8.2.1.3-1: Inter-gNB-DU Conditional Handover or Conditional PSCell Change or subsequent CPAC for intra-NR
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Step 1-2.
The steps 1-2 are as defined in clause 8.2.1.1.
Step 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).
Step 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.
Step 5.
The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the source gNB-DU, which includes a generated RRCReconfiguration message.
Step 6.
The step 6 is as defined in clause 8.2.1.1.
Step 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.
Step 9.
An execution condition to trigger initiation of conditional handover or conditional PSCell change or subsequent CPAC is fulfilled.
Step 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.
Step 11-12.
The steps 11-12 are as defined in steps 9-10 in clause 8.2.1.1.
Step 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.
Step 14.
The source gNB-DU responds to the gNB-CU with the UE CONTEXT MODIFICATION RESPONSE message.
Step 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.
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