Section: 10.19.1 EN-DC
Content: 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. Figure 10.19.1-1: Conditional Handover with Secondary Node procedure Figure 10.19.1-1 shows an example signaling flow for Conditional Handover with Secondary Node. NOTE 1: For a CHO without SN change, the source SN and the target SN shown in Figure 10.19.1-1 are the same node. NOTE 2: For a CHO with SN addition, the source SN and steps involving the source SN in Figure 10.19.1-1 are ignored. 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. NOTE 3: In case of the CHO with/without SN change, the source MN may trigger the MN-initiated SN Modification procedure (to the source SN) to retrieve the current SCG configuration, if configured, before step 1. 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. NOTE 3a: The target MN and other potential target MNs may trigger the SgNB Addition Preparation procedure to the same (target) SN. NOTE 3b: The source MN may initiate additional X2 Handover Preparation procedures towards the same or other target MNs. Based on each X2 Handover Preparation procedure, each target MN may decide to trigger SgNB Addition Preparation procedure. 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. NOTE 4: In CHO with SCG configuration, it is up to the candidate MN implementation to make sure that the CG-Config provided from the (candidate) SN can be used in all CHO preparations. 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. NOTE 4a0: Steps 1-4 may be produced in several instances, each instance initiated with a separate Handover Preparation procedure (step 1). The order of messages belonging to separate instances is not defined. 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. NOTE 4a: Separate Data Forwarding Address Indication procedures may be initiated to provide different forwarding addresses of the prepared conditional handovers. In this case, it is up to the source MN and SN implementations to make sure that the Early Status Transfer message(s) from the source SN, if any, is forwarded to the right target MN. The Data Forwarding Address Indication procedure may further be initiated to indicate to the (source) SN to stop already initiated early data forwarding for some SN-terminated bearers, if they are no longer subject to data forwarding due to the modification or cancellation of the prepared conditional handovers. 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. 6. The UE applies the RRCConnectionReconfiguration message received in step 5, stores the CHO configuration and replies to the MN with an RRCConnectionReconfigurationComplete message. 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. NOTE 5: In case the target SN includes the indication of full RRC configuration, the MN performs release of the SN terminated radio bearer configuration and release and add of the NR SCG configuration part towards the UE. 9. If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN. NOTE 6: The order the UE performs Random Access towards the MN (step 7) and performs the Random Access procedure towards the (target) SN (step 9) is not defined. 10. If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SgNB Reconfiguration Complete message. 11. The target MN sends the Handover Success message to the source MN to inform that the UE has successfully accessed the target cell. 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. 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. 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. 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. NOTE 7: The order the source SN sends the Secondary RAT Data Usage Report message and performs data forwarding with MN/target SN is not defined. The SgNB may send the report when the transmission of the related bearer is stopped. 13b. The source MN sends the Secondary RAT Data Usage Report message to MME to provide information on the used NR resource. 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. 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. 16-19. The target MN initiates the S1 Path Switch procedure. NOTE 8: If new UL TEIDs of the S-GW are included, the target MN performs the MN initiated SN Modification procedure to provide them to the SN. 20. The target MN initiates the UE Context Release procedure towards the source MN. 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.