Section: 10.20 Subsequent Conditional PSCell Addition or Change
Content: A Subsequent Conditional PSCell Addition or Change (subsequent CPAC) is defined as a conditional PSCell addition or change procedure that is executed after a PSCell addition, a PSCell change, a PCell change or an SCG release based on pre-configured subsequent CPAC configuration of candidate PSCell(s) without reconfiguration and re-initiation of CPC/CPA. The UE keeps the configured subsequent CPAC configuration (unless the network indicates to release it) and evaluates the execution conditions of candidate PSCells after completion of a PSCell addition, a PSCell change, a PCell change or an SCG release. Intra-SN subsequent CPAC initiated by the SN, inter-SN subsequent CPAC initiated by either MN or SN are supported. The following principles apply to subsequent CPAC: - For MN initiated subsequent CPAC, the MN initially triggers the candidate cell preparation of subsequent CPAC procedure and generates the execution conditions for the initial execution of subsequent CPAC (e.g. CPA or CPC). - For SN initiated subsequent CPAC, the source SN initially triggers the candidate cell preparation of subsequent CPAC procedure and generates the execution conditions for the initial execution of subsequent CPAC. - For both MN and SN initiated inter-SN subsequent CPAC, the candidate SN generates the execution conditions for the following execution of subsequent CPAC when the candidate SN prepares the candidate SCG configuration(s) for candidate PSCell(s). For SN initiated intra-SN subsequent CPAC, the source SN generates the execution conditions for the following execution of subsequent CPAC when the source SN prepares the candidate SCG configuration(s) for candidate PSCell(s). - The subsequent CPAC configuration contains candidate SCG configuration(s) of candidate PSCell(s), execution conditions, and may contain the MCG configuration (to be applied when subsequent CPAC execution is triggered), the reference configuration and the security update configuration. - The subsequent CPAC configuration for CPA or inter-SN CPC candidate PSCell(s) is provided in MN format. The subsequent CPAC configuration for intra-SN CPC candidate PSCell(s) is provided in MN format or SN format. It's up to the source SN to decide which format to be used for intra-SN subsequent CPAC. - For one UE, the subsequent CPAC configuration for all candidate PSCells (including inter-SN and/or intra-SN) is provided in the same format, i.e., either MN format, or SN format. If the configured candidate PSCell(s) includes at least one inter-SN CPC candidate PSCell, the subsequent CPAC configuration can only be provided in MN format. If only intra-SN CPC candidate PSCell(s) is configured, the subsequent CPAC configuration can be provided in either MN format or SN format. - Each candidate PSCell configuration is provided as a delta configuration on top of a reference configuration or a complete configuration. Only one reference configuration is supported. - The MN generates the MCG part of the reference configuration (if any), while the SN generates the SCG part of the reference configuration. The MN can request an SCG reference configuration from any one of the involved SNs. - The network explicitly configures a subsequent CPAC configuration for the current serving PSCell if the network wants to use that PSCell as a candidate PSCell for subsequent CPAC. - The network always explicitly releases the subsequent CPAC configuration for candidate PSCells after an inter-MN PCell change. - Upon the release of SCG, the UE releases the stored subsequent CPAC configuration in SN format. Upon the release of SCG, the UE releases or maintains the stored subsequent CPAC configuration in MN format according to the network indication. - The same candidate PSCell configuration can be used for CPA execution and CPC execution, but with different execution conditions of the candidate PSCell. - The subsequent CPAC configuration with CPA execution condition(s) maintained after SCG release can be used for the subsequent CPA execution. - Upon inter-SN subsequent CPAC execution, the UE uses the first unused sk-Counter value for S-KgNB generation, based on the per-SN pre-configured sk-Counter value list. - Upon PCell change, PSCell change or SCG release, if the subsequent CPAC configuration is maintained, the UE also maintains the unused sk-Counter values. MN initiated subsequent CPAC The subsequent CPAC procedure is initiated by the MN for inter-SN subsequent CPAC configuration and inter-SN subsequent CPAC execution. Figure 10.20-1: Inter-SN subsequent CPAC - MN initiated Figure 10.20-1 shows an example signalling flow for the inter-SN subsequent CPAC initiated by the MN: 1/2/3/4. 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 includes the indication of the complete or delta RRC 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). NOTE 1: If the UE was configured with SN-1 in Dual Connectivity operation (i.e. SN-1 is the source SN), then the MN starts the subsequent CPAC operation with SN-1 via the MN-initiated SN Modification procedure instead of the SN Addition procedure. NOTE 2: If the UE was configured with SN-1 in Dual Connectivity operation (i.e. SN-1 is the source SN), then the MN may trigger the MN-initiated SN Modification procedure to SN-1 to retrieve the current SCG configuration or request a SCG reference configuration for the subsequent CPAC, and to allow provision of data forwarding related information before step 1. NOTE 3: If applicable, the MN stores the data forwarding addresses and data forwarding proposals provided from all the candidate SN(s). 5. 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). 6/7. If the lists of prepared PSCells received from the candidate SN(s) in steps 2 and 4 are different than the lists of proposed PSCells, e.g., when not all proposed PSCells were accepted by the candidate SN(s), the MN may initiate the SN Modification procedures towards all the candidate SN(s) to inform them about the updated lists of prepared PSCells in other candidate SN(s). If requested, the candidate SN(s) sends an SN Modification Request Acknowledge message and if needed, provides the updated candidate SCG configurations and/or the execution conditions for the following execution of subsequent CPAC to the MN. 8. 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 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. 9. 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. 11. The UE starts evaluating the execution conditions. 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. 12. 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. 13. 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. 14. If PDCP termination point is changed to the SN for bearers using RLC AM, and when RRC full configuration is not used, the MN sends the SN Status Transfer message. 15. 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). 16. 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 source MN. NOTE 4: Separate Xn-U Address Indication procedures may be initiated to provide different forwarding addresses of the prepared subsequent CPAC. In this case, it is up to the MN and the candidate SN implementations to make sure that the Early Status Transfer message(s) from the selected SN, if any, is forwarded to the right other candidate SN. 18. The UE starts evaluating the execution conditions. 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. 19. 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. 20. The UE performs synchronisation towards the PSCell indicated in the RRCReconfiguration* message applied in step 18. 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. 21/22/23. 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. 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. 24/25. If PDCP termination point is changed for bearers using RLC AM, and when RRC full configuration is not used, 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. 26. 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 17. 27. 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 source MN. NOTE 5: Separate Xn-U Address Indication procedures may be initiated to provide different forwarding addresses of the prepared subsequent CPAC. In this case, it is up to the MN and candidate SN implementations to make sure that the Early Status Transfer message(s) from the selected SN, if any, is forwarded to the right other candidate SN. SN initiated subsequent CPAC The subsequent CPAC procedure is initiated by the SN for inter-SN subsequent CPAC configuration and inter-SN subsequent CPAC execution. Figure 10.20-2: Inter-SN subsequent CPAC - SN initiated Figure 10.20-2 shows an example signalling flow for the inter-SN subsequent CPAC initiated by the source SN: 1. 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). 2/3/4/5. 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 includes the indication of the complete or delta RRC 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). NOTE 6: The MN may trigger the MN-initiated SN Modification procedure (to the source SN) to request a reference configuration for the subsequent CPAC before step 2. NOTE 7: If applicable, the MN stores the data forwarding addresses and data forwarding proposals provided from all the candidate SN(s) and the source SN. 6. 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). 7/8. If the lists of prepared PSCells received from the candidate SN(s) in steps 3 and 5 are different than the lists of proposed PSCells, e.g., when not all proposed PSCells were accepted by the candidate SN(s), the MN may initiate the SN Modification procedures towards the source SN and all the candidate SN(s) to inform them about the updated lists of prepared PSCells in other candidate SN(s). If requested, the source SN or the candidate SN(s) sends an SN Modification Request Acknowledge message and if needed, provides the updated candidate SCG configurations and/or the execution conditions for the following execution of subsequent CPAC for the prepared PSCell to the MN. 9. 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 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 3 and 5, 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. 10. The UE applies the RRCReconfiguration message received in step 9, 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. 11/12. 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 7 and 8 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 addresses as received from the candidate SN(s), 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. NOTE 8: The Xn-U Address Indication procedure may further be invoked to indicate to the source SN to stop already initiated early data forwarding for some PDCP SDUs if they are no longer subject to data forwarding due to the modification or cancellation of the prepared conditional PSCell change. NOTE 9: For the early transmission of MN terminated split/SCG bearers, the MN forwads the PDCP PDU to the candidate SN(s). 13. The UE starts evaluating the execution conditions. 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. 14. 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. 15. The UE performs synchronisation towards the PSCell indicated in the RRCReconfiguration* message applied in step 13. 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. 16/17/18. 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. 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. 19/20. If PDCP termination point is changed for bearers using RLC AM, and when RRC full configuration is not used, 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. 21. 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 12. 22. 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 source MN. NOTE 10: Separate Xn-U Address Indication procedures may be initiated to provide different forwarding addresses of the prepared subsequent CPAC. In this case, it is up to the MN and the candidate SN implementations to make sure that the Early Status Transfer message(s) from the selected SN, if any, is forwarded to the right other candidate SN. 24. In subsequent evaluation and execution phase, i.e., for the following execution of subsequent CPAC, the similar steps as steps 13~23 are performed.