Content for  TS 37.340  Word version:  17.3.0

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.

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., CHO Information SN Addition IE.

The (candidate) SN replies with the SN Addition Request Acknowledge message. The (candidate) SN may include the indication of full or delta RRC configuration.

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.

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.

The UE applies the RRCConnectionReconfiguration message received in step 5, stores the CHO configuration and replies to the MN with an RRCConnectionReconfigurationComplete message.

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. The UE releases stored CHO configurations after successful completion of RRC handover procedure.

If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN.

If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SgNB Reconfiguration Complete message.

The target MN sends the Handover Success message to the source MN to inform that the UE has successfully accessed the target cell.

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) SN acknowledges the release request. 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 MN sends the Handover Cancel message toward the other signalling connections or other candidate MNs, if any, to cancel CHO for the UE.

The target MN or/and other candidate MN(s) sends the SgNB Release Request message(s) to other candidate SN(s), if configured. The other candidate SN(s) acknowledges the release request.

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.

The source MN sends the Secondary RAT Data Usage Report message to MME to provide information on the used NR resource.

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.

If applicable, data forwarding takes place from the source side. If the SN is kept, data forwarding may be omitted for SN-terminated bearers kept in the SN.

The target MN initiates the S1 Path Switch procedure.

The target MN initiates the UE Context Release procedure towards the source MN.

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.

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.

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., CHO Information SN Addition IE.

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.

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.

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.

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.

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.

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 RRC reconfiguration complete* message to the target MN. The UE releases stored CHO configurations after successful completion of RRC handover procedure.

If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN.

If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SN Reconfiguration Complete message.

The target MN sends the Handover Success message to the source MN to inform that the UE has successfully accessed the target cell.

The source MN sends SN Release Request message to the (source) SN including a Cause indicating MCG mobility. The (source) SN acknowledges the release request. 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 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.

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.

The target MN or/and other candidate MN(s) sends the SN Release Request message(s) to other candidate SN(s), if configured. The other candidate SN(s) acknowledges the release request.

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.

The source MN sends the Secondary RAT Data Usage Report message to AMF to provide information on the used NR/E-UTRA resource.

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.

If applicable, data forwarding takes place from the source side. If the SN is kept, data forwarding may be omitted for the SN terminated bearers or QoS flows kept in the SN.

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.

The target MN initiates the UE Context Release procedure towards the source MN.

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.