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Content for  TS 37.340  Word version:  18.1.0

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10.19  Conditional Handover with Secondary Node |R17|p. 109

10.19.1  EN-DCp. 109

The Conditional Handover with Secondary Node procedure is used for configuration and execution of CHO with SN. This procedure includes the cases where the SN is kept, changed or added. If the SN is kept, the UE context at the SN is kept. If the SN is changed, the UE context at the source SN is moved to the target SN.
Copy of original 3GPP image for 3GPP TS 37.340, Fig. 10.19.1-1: Conditional Handover with Secondary Node procedure
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Figure 10.19.1-1 shows an example signaling flow for Conditional Handover with Secondary Node.
Step 1.
The source MN starts the conditional handover procedure by initiating the X2 Handover Preparation procedure including MCG configuration and, if the UE is configured with an SCG, SCG configuration. The source MN may include the (source) SN UE X2AP ID, SN ID, the UE context in the (source) SN and the Conditional Handover Information Request IE in the Handover Request message.
Step 2.
If the candidate MN decides to keep the UE context in the SN, the candidate MN sends the SgNB Addition Request message to the SN including the SN UE X2AP ID as a reference to the UE context in the SN that was established by the source MN. If the candidate MN decides to change the SN allowing delta configuration, the candidate MN sends the SgNB Addition Request message to the candidate SN including the UE context in the source SN that was established by the source MN. Otherwise, the candidate MN may send the SgNB Addition Request message to the candidate SN including neither the SN UE X2AP ID nor the UE context in the source SN that was established by the source MN. Within the SgNB Addition Request message, the candidate MN also includes the CHO related information, i.e., the source MN ID and the MN UE X2AP ID in the source MN, in order to indicate that the SgNB Addition Preparation procedure is triggered in relation to a CHO and to enable the SN to identify requests related to the same UE.
Step 3.
The (candidate) SN replies with the SgNB Addition Request Acknowledge message. The (candidate) SN may include the indication of full or delta RRC configuration.
Step 4.
The candidate MN includes within the Handover Request Acknowledge message a transparent container to be sent to the UE as an RRC message to perform the conditional handover, and may also provide forwarding addresses to the source MN. The candidate MN indicates to the source MN that the UE context in the SN is kept if the candidate MN and the SN decided to keep the UE context in the SN in step 2 and step 3.
Step 4a.
The source MN sends the Data Forwarding Address Indication message to the (source) SN. This Data Forwarding Address Indication message notifies conditional handover to the (source) SN, which may decide to perform, if applicable, early data forwarding for SN-terminated bearers, together with the sending of an Early Status Transfer message to the source MN.
Step 5.
The source MN sends an RRCConnectionReconfiguration message to the UE, including the CHO configuration, i.e. a list of RRCConnectionReconfiguration* messages and associated execution conditions, in which each RRCConnectionReconfiguration* message contains an MCG configuration and possibly an SCG configuration in the RRCReconfiguration** message received from the candidate SN in step 3.
Step 6.
The UE applies the RRCConnectionReconfiguration message received in step 5, stores the CHO configuration and replies to the MN with an RRCConnectionReconfigurationComplete message.
Step 7/8.
The UE maintains connection with the source MN and, if the UE is configured with a PSCell, with the source PSCell, after receiving CHO configuration, and starts evaluating the CHO execution condition for the candidate cell(s). If at least one CHO candidate cell satisfies the corresponding CHO execution condition, the UE detaches from the source MN, applies the stored corresponding configuration for that selected candidate cell, synchronises to that candidate cell and completes the RRC handover procedure by sending RRCConnectionReconfigurationComplete* message to the target MN. If the stored configuration for the selected candidate cell includes an SCG configuration, the UE includes an embedded SN RRCReconfigurationComplete** message for the target SN. The UE releases stored CHO configurations after successful completion of RRC handover procedure.
Step 9.
If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN.
Step 10.
If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SgNB Reconfiguration Complete message.
Step 11.
The target MN sends the Handover Success message to the source MN to inform that the UE has successfully accessed the target cell.
Step 12a/b.
The source MN sends SgNB Release Request message to the (source) SN including a Cause indicating MCG mobility and, if applicable, data forwarding information. The source MN indicates to the (source) SN that the UE context in SN is kept, if it receives the indication from the target MN. The (source) SN acknowledges the release request.
Step 12c.
The source MN sends the Handover Cancel message toward the other signalling connections or other candidate MNs, if any, to cancel CHO for the UE.
Step 12d/e.
If the target MN is configured with other candidate PCell(s) associated with other candidate SN(s) than the target SN, the target MN sends the SgNB Release Request message(s) to the corresponding candidate SN(s). Other candidate MN(s) send(s) the SgNB Release Request message(s) to other candidate SN(s), if configured. The other candidate SN(s) acknowledges the release request.
Step 13a.
The (source) SN sends the Secondary RAT Data Usage Report message to the source MN and includes the data volumes delivered to and received from the UE over the NR radio for the related E-RABs.
Step 13b.
The source MN sends the Secondary RAT Data Usage Report message to MME to provide information on the used NR resource.
Step 14.
For bearers using RLC AM, the source MN sends the SN Status Transfer message, including, if needed, SN Status received from the source SN to the target MN. The target MN forwards the SN Status to the target SN, if needed.
Step 15.
If applicable, data forwarding takes place from the source side (i.e. source MN or source SN). If the SN is kept, data forwarding may be omitted for SN-terminated bearers kept in the SN.
Step 16-19.
The target MN initiates the S1 Path Switch procedure.
Step 20.
The target MN initiates the UE Context Release procedure towards the source MN.
Step 21.
Upon reception of the UE Context Release message, the (source) SN releases C-plane related resources associated to the UE context towards the source MN. Any ongoing data forwarding may continue. The SN shall not release the UE context associated with the target MN if the UE context kept indication was included in the SgNB Release Request message in step 12a.
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10.19.2  MR-DC with 5GCp. 112

The Conditional Handover with Secondary Node procedure is used for configuration and execution of CHO with SN or CHO with candidate SCG(s). This procedure includes the cases where the SN is kept, changed or added. If the SN is kept, the UE context at the SN is kept. If the SN is changed, the UE context at the source SN is moved to the target SN.
CHO with candidate SCG(s) is not supported for NE-DC and NGEN-DC.
Copy of original 3GPP image for 3GPP TS 37.340, Fig. 10.19.2-1: Conditional Handover with Secondary Node procedure
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Figure 10.19.2-1 shows an example signaling flow for Conditional Handover with Secondary Node.
Step 1.
The source MN starts the conditional handover procedure by initiating the Xn Handover Preparation procedure including MCG configuration and, if the UE is configured with an SCG, SCG configuration. The source MN includes the (source) SN UE XnAP ID, SN ID, the UE context in the (source) SN and the Conditional Handover Information Request IE in the Handover Request message. In case of CHO with candidate SCG(s), the source MN also provides the maximum number of conditional reconfigurations that the candidate MN can prepare for the UE in the Handover Request message.
Step 2.
If the candidate MN decides to keep the UE context in the SN, the candidate MN sends the SN Addition Request message to the SN including the SN UE XnAP ID as a reference to the UE context in the SN that was established by the source MN. If the candidate MN decides to change the SN allowing delta configuration, the candidate MN sends the SN Addition Request message to the candidate SN including the UE context in the source SN that was established by the source MN. Otherwise, the candidate MN may send the SN Addition Request message to the candidate SN including neither the SN UE XnAP ID nor the UE context in the source SN that was established by the source MN. Within the SN Addition Request message, the candidate MN also includes the CHO related information, i.e., the source MN ID and the MN UE XnAP ID in the source MN, in order to indicate that the SN Addition Preparation procedure is triggered in relation to a CHO and to enable the SN to identify requests related to the same UE. In case of CHO with candidate SCG(s), the candidate MN also provides the candidate PSCells recommended by the candidate MN via the latest measurement results for the candidate SN(s) to choose and configure the candidate SCG cell(s), and provides the maximum number of PSCells that the candidate SN can prepare for the UE in the SN Addition Request message.
Step 3.
The (candidate) SN replies with the SN Addition Request Acknowledge message. The (candidate) SN may include the indication of the full or delta RRC configuration. In case of CHO with candidate SCG(s), within the list of cells as indicated within the measurement results indicated by the candidate MN, the candidate SN decides the list of PSCell(s) to prepare (considering the maximum number indicated by the candidate MN) and, for each prepared PSCell, the candidate SN decides other SCG SCells and provides the new corresponding SCG radio resource configuration to the candidate MN in an NR RRC reconfiguration** message contained in the SN Addition Request Acknowledge message with the prepared PSCell ID(s).
Step 3a.
For the SN terminated bearers using MCG resources, the candidate MN provides Xn-U DL TNL address information in the Xn-U Address Indication message.
Step 4.
The candidate MN includes within the Handover Request Acknowledge message the MN RRC reconfiguration message to be sent to the UE in order to perform the conditional handover, and may also provide forwarding addresses to the source MN. If PDU session split is performed in the target side during handover procedure, more than one data forwarding addresses corresponding to each node are included in the Handover Request Acknowledge message. The candidate MN indicates to the source MN that the UE context in the SN is kept if the candidate MN and the SN decided to keep the UE context in the SN in step 2 and step 3. In case of CHO with candidate SCG(s), the candidate MN includes a list of one or more (candidate) SNs in Handover Request Acknowledge message with the PDU Session admission results, data forwarding addresses and list of prepared PSCells for each prepared (candidate) SN. The candidate MN also indicates to the source MN the parameters of the execution condition of each prepared candidate PSCell.
Step 4a.
The source MN sends the Xn-U Address Indication message to the (source) SN. This Xn-U Address Indication message notifies conditional handover to the (source) SN, which may decide to perform, if applicable, early data forwarding for SN-terminated bearers, together with the sending of an Early Status Transfer message to the source MN.
Step 5.
The source MN sends an RRC reconfiguration message to the UE, including the CHO configuration, i.e. a list of RRC reconfiguration* messages and associated execution conditions, in which each RRC reconfiguration* message contains an MCG configuration and possibly an SCG configuration in the RRC reconfiguration** message received from the candidate SN in step 3. For each configuration of CHO with candidate SCG(s), the source MN provides an execution condition for the candidate PCell and an execution condition for the candidate PSCell. Besides, each RRC reconfiguration* message contains an MCG configuration and an SCG configuration in the RRC reconfiguration** message received from the candidate SN in step 3.
Step 6.
The UE applies the RRC reconfiguration message received in step 5, stores the CHO configuration and replies to the MN with an RRC reconfiguration complete message.
Step 7/8.
The UE maintains connection with the source MN and, if the UE is configured with a PSCell, with the source PSCell, after receiving CHO configuration, and starts evaluating the execution condition for the candidate PCell(s) and if any, the execution condition for the candidate PSCell(s):
  • If at least one candidate PCell satisfies the corresponding execution condition and the associated candidate PSCell satisfies the corresponding execution condition, the UE detaches from the source MN, applies the stored corresponding configuration for that selected candidate PCell and the associated candidate PSCell, synchronises to that candidate PCell, and completes the RRC handover procedure by sending RRC reconfiguration complete* message to the target MN. The UE includes an embedded SN RRCReconfigurationComplete** message for the target SN, and information enabling the target MN to identify the target SN of the selected candidate PSCell.
  • Else if at least one candidate PCell satisfies the corresponding execution condition and there is no associated execution condition for a candidate PSCell, the UE detaches from the source MN, applies the stored corresponding configuration for that selected candidate PCell and, if included, the associated PSCell, synchronises to that candidate PCell and completes the RRC handover procedure by sending RRC reconfiguration complete* message to the target MN. If the stored configuration for the selected candidate PCell includes an SCG configuration, the UE includes an embedded SN RRCReconfigurationComplete** message for the target SN.
  • The UE releases the stored CHO configurations after successful completion of the RRC handover procedure.
Step 9.
If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN.
Step 10.
If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SN Reconfiguration Complete message.
Step 11.
The target MN sends the Handover Success message to the source MN to inform that the UE has successfully accessed the target cell. In case of CHO with candidate SCG(s), the target PSCell ID may also be included in the Handover Success message.
Step 12a/b.
The source MN sends SN Release Request message to the (source) SN including a Cause indicating MCG mobility. The source MN indicates to the (source) SN that the UE context in SN is kept, if it receives the indication from the target MN. The (source) SN acknowledges the release request.
Step 12c.
The source MN sends XN-U Address Indication message to the (source) SN to transfer data forwarding information. More than one data forwarding addresses may be provided if the PDU session is split in the target side.
Step 12d.
The source MN sends the Handover Cancel message toward the other signalling connections or other candidate MNs, if any, to cancel CHO for the UE.
Step 12e/f.
If the target MN is configured with other candidate PCell(s) associated with other candidate SN(s) than the target SN, the target MN sends the SN Release Request message(s) to the corresponding candidate SN(s). Other candidate MN(s) send(s) the SN Release Request message(s) to all the associated candidate SN(s), if configured. The associated candidate SN(s) acknowledges the release request.
Step 13a.
The (source) SN sends the Secondary RAT Data Usage Report message to the source MN and includes the data volumes delivered to and received from the UE over the NR/E-UTRA radio as described in clause 10.11.2.
Step 13b.
The source MN sends the Secondary RAT Data Usage Report message to AMF to provide information on the used NR/E-UTRA resource.
Step 14.
For bearers using RLC AM, the source MN sends the SN Status Transfer message to the target MN, including, if needed, SN Status received from the source SN. The target MN forwards the SN Status to the target SN, if needed.
Step 15.
If applicable, data forwarding takes place from the source side (i.e. source MN or source SN). If the SN is kept, data forwarding may be omitted for the SN terminated bearers or QoS flows kept in the SN.
Step 16-19.
The target MN initiates the Path Switch procedure. If the target MN includes multiple DL TEIDs for one PDU session in the Path Switch Request message, multiple UL TEID of the UPF for the PDU session should be included in the Path Switch Ack message in case there is TEID update in UPF.
Step 20.
The target MN initiates the UE Context Release procedure towards the source MN.
Step 21.
Upon reception of the UE Context Release message from source MN, the (source) SN releases C-plane related resources associated to the UE context towards the source MN. Any ongoing data forwarding may continue. The SN shall not release the UE context associated with the target MN if the UE contest kept indication was included in the SN Release Request message in step 12a.
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10.19.3  CHO with candidate SCG(s) |R18|p. 116

A CHO with candidate SCG(s) is defined as a PCell change with PSCell addition/change that is executed by the UE when the execution conditions for both candidate PCell and the associated candidate PSCell are met. The UE starts evaluating the execution conditions for candidate PCell(s) and candidate PSCell(s) simultaneously upon receiving the CHO with candidate SCG(s) configuration, and stops evaluating the execution conditions once a PCell change or a PSCell change is triggered. The UE does not execute CHO with candidate SCG(s) until the execution conditions for both the candidate PCell and the associated candidate PSCell are met.
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