Content for  TS 37.340  Word version:  18.2.0

The MN initiates the inter-SN subsequent CPAC by requesting the candidate SN(s) to allocate resources for the UE by means of the SN Addition procedure, indicating that the request is for subsequent CPAC. The MN also provides the candidate cells recommended by MN via the latest measurement results for the candidate SN(s) to choose and configure the SCG cell(s), provides the upper limit for the number of PSCells that can be prepared by each candidate SN, and provides a list of KSN and associated sk-Counter values for each candidate SN. In the SN Addition procedure, the MN also includes information of other candidate SN(s), and for each candidate SN, a list of cells recommended by the MN via the latest measurement results for the candidate SN to select the PSCell(s) for the following execution of subsequent CPAC. Within the list of cells as indicated within the measurement results indicated by the MN, the candidate SN decides the list of PSCell(s) to prepare (considering the maximum number indicated by the MN) and, for each prepared PSCell, the candidate SN decides other SCG SCells and provides the new corresponding SCG radio resource configuration to the MN in an NR RRCReconfiguration** message contained in the SN Addition Request Acknowledge message with the prepared PSCell ID(s). For each prepared PSCell, the candidate SN also decides the list of PSCell(s) and associated execution conditions proposed for the following execution of subsequent CPAC. If data forwarding is needed, the candidate SN provides data forwarding addresses to the MN. The candidate SN may also propose data forwarding to the MN or other candidate SN(s) for subsequent CPAC. The candidate SN may include an indication that the SCG radio resource configuration of a prepared PSCell is a complete configuration, i.e. that it is not a delta configuration with respect to the SCG reference configuration. For the prepared PSCell(s) and the proposed PSCell(s) for the following execution of subsequent CPAC, the candidate SN can either accept or reject each of the candidate cells listed within the measurement results indicated by the MN, i.e. it cannot configure any alternative candidates. The MN may select one of the candidate SN(s) and requests providing the SCG reference configuration as part of the SN Addition procedure. Once obtained, the MN provides the SCG reference configuration to other candidate SN(s).

For SN terminated bearers using MCG resources, the MN provides Xn-U DL TNL address information in the Xn-U Address Indication message to the candidate SN(s).

For each candidate SN, the MN may initiate the SN Modification procedure towards the candidate SN to inform the prepared PSCells in other candidate SN(s), e.g., when not all proposed PSCells by this candidate SN for the following execution of subsequent CPAC were prepared by the candidate SN(s). If requested, the candidate SN sends an SN Modification Request Acknowledge message and if needed, provides the updated candidate SCG configuration(s) and/or the execution conditions for the following execution of subsequent CPAC to the MN.

The MN sends to the UE an RRCReconfiguration message including the subsequent CPAC configuration, i.e. a list of RRCReconfiguration* messages and associated execution conditions for the initial execution of subsequent CPAC and execution conditions for the following execution of subsequent CPAC, in which each RRCReconfiguration* message contains the SCG configuration in the RRCReconfiguration** message received from one of the candidate SN(s) in steps 2 and 4, and possibly an MCG configuration. Besides, the RRCReconfiguration message can also include an updated source MCG configuration, e.g., to configure the required conditional measurements. The RRCReconfiguration message also includes a security update configuration and may also include a reference configuration.

The UE applies the RRCReconfiguration message received in step 8, stores the subsequent CPAC configuration and replies to the MN with an RRCReconfigurationComplete message. In case the UE is unable to comply with (part of) the configuration included in the RRCReconfiguration message, it performs the reconfiguration failure procedure.

In case of SN terminated bearers, early data forwarding may take place. For the early data forwarding of SN terminated bearers, the MN forwards the PDCP SDU to the candidate SN(s). For the early transmission of MN terminated split/SCG bearers, the MN forwards the PDCP PDU to the candidate SN(s).

The UE starts evaluating the execution conditions for the initial execution of subsequent CPAC. If the execution condition of one candidate PSCell is satisfied, the UE applies RRCReconfiguration* message corresponding to the selected candidate PSCell, and sends an MN RRCReconfigurationComplete* message, including an RRCReconfigurationComplete** message for the selected candidate PSCell, and information enabling the MN to identify the SN of the selected candidate PSCell. The RRCReconfigurationComplete* message may also include the sk-Counter value associated with the selected candidate PSCell if a new sk-Counter value is selected.

The MN informs the SN of the selected candidate PSCell (i.e. the selected candidate SN) that the UE has completed the reconfiguration procedure successfully via SN Reconfiguration Complete message, including the RRCReconfigurationComplete** message. If the sk-Counter value is received by the RRCReconfigurationComplete* message, the MN also indicates the received sk-Counter value to the SN.

The UE performs synchronisation towards the PSCell indicated in the RRCReconfiguration* message applied in step 11. The order the UE sends the MN RRCReconfigurationComplete* message and performs the Random Access procedure towards the SCG is not defined. The successful RA procedure towards the SCG is not required for a successful completion of the RRC Reconfiguration procedure.

If PDCP termination point is changed to the SN for bearers using RLC AM, the MN sends the SN Status Transfer message.

For SN terminated bearers or QoS flows moved from the MN, dependent on the characteristics of the respective bearer or QoS flow, the MN may take actions to minimise service interruption due to activation of MR-DC (Data forwarding).

If applicable, a PDU Session path update procedure is triggered by the MN.

If data forwarding is needed, the MN may send the Xn-U Address Indication message to the selected candidate SN. The SN 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 MN.

The UE starts evaluating the execution conditions for the following execution of subsequent CPAC. If the execution condition of one candidate PSCell is satisfied, the UE applies RRCReconfiguration* message corresponding to the selected candidate PSCell, and sends an MN RRCReconfigurationComplete* message, including an RRCReconfigurationComplete** message for the selected candidate PSCell, and information enabling the MN to identify the SN of the selected candidate PSCell. The RRCReconfigurationComplete* message may also include a sk-Counter value associated with the selected candidate PSCell if a new sk-Counter value is selected.

The MN informs the SN of the selected candidate PSCell that the UE has completed the reconfiguration procedure successfully via SN Reconfiguration Complete message, including the RRCReconfigurationComplete** message. If the sk-Counter value is received by the RRCReconfigurationComplete* message, the MN also indicates the received sk-Counter value to the SN.

The UE performs synchronisation towards the PSCell indicated in the RRCReconfiguration* message applied in step 22. The order the UE sends the MN RRCReconfigurationComplete* message and performs the Random Access procedure towards the SCG is not defined. The successful RA procedure towards the SCG is not required for a successful completion of the RRC Reconfiguration procedure.

The MN triggers the MN initiated SN Modification procedure to inform the last serving SN to stop providing user data to the UE, to switch to the prepared state, and if applicable, to allow provisioning of new data forwarding addresses based on the data forwarding proposals of the MN and the selected candidate SN. If applicable, the MN triggers the Xn-U Address Indication procedure to inform the last serving SN the address of the SN of the selected candidate PSCell, to start late data forwarding.

If PDCP termination point is changed for bearers using RLC AM, the SN sends the SN Status Transfer message to MN, which the MN sends then to the SN of the selected candidate PSCell, if needed.

If applicable, data forwarding from the last serving SN takes place. It may be initiated as early as the the last serving SN receives the early data forwarding address in step 21.

The source SN sends the Secondary RAT Data Usage Report message to the MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2.

If applicable, a PDU Session path update procedure is triggered by the MN.

If data forwarding is needed, the MN may send the Xn-U Address Indication message to the selected candidate SN. The SN 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 MN.

The source SN (i.e. SN-1) initiates the inter-SN subsequent CPAC procedure by sending the SN Change Required message, which contains a subsequent CPAC initiation indication. The message also contains candidate node ID(s) and may include an SCG reference configuration (to support delta configuration), and contains the measurements results which may include cells that are not subsequent CPAC candidates. The message also includes a list of proposed PSCell candidates recommended by the source SN, including execution conditions for the initial evaluation, the upper limit for the number of PSCells that can be prepared by each candidate SN, and may also include the SCG measurement configurations for subsequent CPAC (e.g. measurement ID(s) to be used for subsequent CPAC). The source SN may also propose data forwarding to the MN or other candidate SN(s) for subsequent CPAC.

The MN requests each candidate SN(s) to allocate resources for the UE by means of the SN Addition procedure(s), indicating the request is for subsequent CPAC, and the measurements results which may include cells that are not subsequent CPAC candidates received from the source SN to the candidate SN, and indicating a list of proposed PSCell candidates to the candidate SN(s) received from the source SN, but not including execution conditions. The MN also includes information of other candidate SN(s), and for each candidate SN, a list of proposed PSCell candidates recommended by the source SN for the candidate SN to select the PSCell(s) for the following execution of subsequent CPAC. The MN also provides the upper limit for the number of PSCells that can be prepared by each candidate SN and provides a list of KSN and associated sk-Counter values for each candidate SN. Within the list of PSCells suggested by the source SN, the candidate SN decides the list of PSCell(s) to prepare (considering the maximum number indicated by the MN) and, for each prepared PSCell, the candidate SN decides other SCG SCells and provides the new corresponding SCG radio resource configuration to the MN in an NR RRCReconfiguration** message contained in the SN Addition Request Acknowledge message with the prepared PSCell ID(s). For each prepared PSCell, the candidate SN also decides the list of PSCell(s) and associated execution conditions proposed for the following execution of subsequent CPAC. If data forwarding is needed, the candidate SN provides data forwarding addresses to the MN. The candidate SN may also propose data forwarding to the MN or other candidate SN(s) for subsequent CPAC. The candidate SN may include an indication that the SCG radio resource configuration of a prepared PSCell is a complete configuration, i.e. that it is not a delta configuration with respect to the SCG reference configuration. For the prepared PSCell(s) and the proposed PSCell(s) for the following execution of subsequent CPAC, the candidate SN can either accept or reject each of the candidate cells suggested by the source SN, i.e. it cannot configure any alternative candidates. The MN may select one of the candidate SN(s) and requests providing the reference SCG configuration as part of the SN Addition procedure. Once obtained, the MN provides the reference configuration to other candidate SN(s).

For SN terminated bearers using MCG resources, the MN provides Xn-U DL TNL address information in the Xn-U Address Indication message to the candidate SN(s).

The MN may indicate the candidate PSCells accepted by each candidate SN to the source SN via SN Modification Request message before it configures the UE, e.g., when not all candidate PSCells were accepted by the candidate SN(s). If requested, the source SN sends an SN Modification Request Acknowledge message and if needed, provides an updated measurement configuration and/or the execution conditions for the initial execution of subsequent CPAC to the MN. For each candidate SN, the MN may initiate the SN Modification procedures towards the candidate SN to inform the prepared PSCells in other candidate SN(s), e.g., when not all proposed PSCells by this candidate SN for the following execution of subsequent CPAC were prepared by the candidate SN(s). If requested, the candidate SN sends an SN Modification Request Acknowledge message and if needed, provides the updated candidate SCG configuration(s) and/or the associated execution conditions for the following execution of subsequent CPAC of the list of PSCell(s) to the MN.

The MN sends to the UE an RRCReconfiguration message including the subsequent CPAC configuration, i.e. a list of RRCReconfiguration* messages and associated execution conditions for the initial execution of subsequent CPAC and execution conditions for the following execution of subsequent CPAC, in which each RRCReconfiguration* message contains the SCG configuration in the RRCReconfiguration** message received from one of the candidate SN(s) in steps 2 and 3, and possibly an MCG configuration. Besides, the RRCReconfiguration message can also include an updated MCG configuration, as well as the NR RRCReconfiguration*** message generated by the source SN, e.g., to configure the required conditional measurements. The RRCReconfiguration message also includes a security update configuration and may also include a reference configuration.

The UE applies the RRCReconfiguration message received in step 7, stores the subsequent CPAC configuration and replies to the MN with an RRCReconfigurationComplete message, which can include an NR RRCReconfigurationComplete*** message. In case the UE is unable to comply with (part of) the configuration included in the RRCReconfiguration message, it performs the reconfiguration failure procedure.

If an SN RRC response message is included, the MN informs the source SN with the SN RRCReconfigurationComplete*** message via SN Change Confirm message. If step 5 and 6 towards the source SN are skipped, the MN will indicate the candidate PSCells accepted by each candidate SN to the source SN in the SN Change Confirm message. The MN sends the SN Change Confirm message towards the source SN to indicate that subsequent CPAC is prepared, and in such case the source SN continues providing user data to the UE. If early data forwarding is applied, the MN informs the source SN the data forwarding address(es), the source SN, if applicable, together with the Early Status Transfer procedure, starts early data forwarding. The PDCP SDU forwarding may take place during early data forwarding. In case multiple candidate SNs are prepared, the MN includes a list of Target SN ID and list of data forwarding addresses to the source SN.

The UE starts evaluating the execution conditions for the initial execution of subsequent CPAC. If the execution condition of one candidate PSCell is satisfied, the UE applies RRCReconfiguration* message corresponding to the selected candidate PSCell, and sends an MN RRCReconfigurationComplete* message, including an RRCReconfigurationComplete** message for the selected candidate PSCell, and information enabling the MN to identify the SN of the selected candidate PSCell. The RRCReconfigurationComplete* message may also include the sk-Counter value associated with the selected candidate PSCell if a new sk-Counter value is selected.

The MN informs the SN of the selected candidate PSCell (i.e. the selected candidate SN) that the UE has completed the reconfiguration procedure successfully via SN Reconfiguration Complete message, including the RRCReconfigurationComplete** message. If the sk-Counter value is received by the RRCReconfigurationComplete* message, the MN also indicates the received sk-Counter value to the SN.

The UE performs synchronisation towards the PSCell indicated in the RRCReconfiguration* message applied in step 11. The order the UE sends the MN RRCReconfigurationComplete* message and performs the Random Access procedure towards the SCG is not defined. The successful RA procedure towards the SCG is not required for a successful completion of the RRC Reconfiguration procedure.

If the source SN is configured as a candidate SN, the MN triggers the MN initiated SN Modification procedure to inform the source SN to stop providing user data to the UE, to switch to the prepared state, and if applicable, to allow provisioning of new data forwarding addresses based on the data forwarding proposals of the MN and the selected candidate SN. If applicable, the MN triggers the Xn-U Address Indication procedure to inform the source SN the address of the SN of the selected candidate PSCell, to start late data forwarding. If the source SN is not configured as a candidate SN, the MN triggers the MN initiated SN Release procedure to inform the source SN to stop providing user data to the UE, and triggers the Xn-U Address Indication procedure to inform the source SN the address of the SN of the selected candidate PSCell and if applicable, starts late data forwarding.

If PDCP termination point is changed for bearers using RLC AM, the SN sends the SN Status Transfer message to MN, which the MN sends then to the SN of the selected candidate PSCell, if needed.

If applicable, data forwarding from the source SN takes place. It may be initiated as early as the the source SN receives the early data forwarding address in step 10.

The source SN sends the Secondary RAT Data Usage Report message to the MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2.

If applicable, a PDU Session path update procedure is triggered by the MN.

If data forwarding is needed, the MN may send the Xn-U Address Indication message to the selected candidate SN. The SN 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 MN.

The SN initiates the conditional SN modification procedure by sending the SN Modification Required message, which contains an intra-SN subsequent CPAC initiation indication. The message includes a list of PSCell(s) to prepare and associated execution conditions proposed for the initial execution of subsequent CPAC and execution conditions proposed for the following execution of subsequent CPAC, and for each prepared PSCell, the SN decides SCG SCells and provides the new corresponding SCG radio resource configuration to the MN in an NR RRCReconfiguration** message contained in the SN Modification Required message. The SN may include an indication that the SCG radio resource configuration of a prepared PSCell is a complete configuration, i.e. that it is not a delta configuration with respect to the reference SCG configuration.

The MN initiated SN Modification procedure may be triggered by SN Modification Required message, e.g. when an SN security key change needs to be applied.

The MN sends to the UE an RRCReconfiguration message including the subsequent CPAC configuration, i.e. a list of RRCReconfiguration* messages and associated execution conditions for the initial execution of subsequent CPAC and execution conditions for the following execution of subsequent CPAC, in which each RRCReconfiguration* message contains the SCG configuration in the RRCReconfiguration** message received from the SN in step 1 and possibly an MCG configuration. Besides, the RRCReconfiguration message can also include an updated MCG configuration, as well as the NR RRCReconfiguration*** message generated by the SN, e.g., to configure the required conditional measurements. The RRCReconfiguration message may also include a reference configuration and a security update configuration.

The UE applies the RRCReconfiguration message received in step 4, stores the subsequent CPAC configuration and replies to the MN with an RRCReconfigurationComplete message, which can include an NR RRCReconfigurationComplete*** message. In case the UE is unable to comply with (part of) the configuration included in the RRCReconfiguration message, it performs the reconfiguration failure procedure.

If an SN RRC response message is included, the MN informs the SN with the SN RRCReconfigurationComplete*** message via SN Modification Confirm message. The MN sends the SN Modification Confirm message towards the SN to indicate that subsequent CPAC is prepared.

The UE starts evaluating the execution conditions for the initial execution of subsequent CPAC. If the execution condition of one candidate PSCell is satisfied, the UE applies RRCReconfiguration* message corresponding to the selected candidate PSCell, and sends an RRCReconfigurationComplete* message, including an RRCReconfigurationComplete** message for the selected candidate PSCell, and information enabling the MN to identify the selected candidate PSCell. The UE keeps the configured subsequent CPAC configuration and evaluates the execution conditions of other candidate PSCells after completion of the subsequent CPAC execution.

If the RRC connection reconfiguration procedure was successful, the MN informs the SN of the selected candidate PSCell via SN Reconfiguration Complete message, including the SN RRCReconfigurationComplete** message.

The UE synchronizes to the PSCell indicated in the RRCReconfiguration* message applied in step 7.

If PDCP termination point is changed for bearers using RLC AM, the SN Status Transfer takes place between the MN and the SN (Figure 10.20-3 depicts the case where a bearer context is transferred from the SN to the MN).

If applicable, data forwarding between MN and the SN takes place (Figure 10.20-3 depicts the case where a user plane resource configuration related context is transferred from the SN to the MN).

The SN sends the Secondary RAT Data Usage Report message to the MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2.

If applicable, a PDU Session path update procedure is performed.