Description
Subsequent Conditional PSCell Addition or Change (S-CPAC) is a mobility procedure defined for 5G New Radio (NR), specifically within the context of Multi-Radio Dual Connectivity (MR-DC) like EN-DC (E-UTRA-NR Dual Connectivity) and NR-DC (NR-NR Dual Connectivity). It operates as an extension to the Conditional Handover (CHO) and Conditional PSCell Change (CPC) mechanisms. The Primary SCG Cell (PSCell) is the primary cell of the secondary node in a dual connectivity setup. In standard conditional procedures, the network prepares one or more candidate target cells, and the UE executes the change to one of them when its radio conditions meet predefined criteria (e.g., signal strength thresholds).
S-CPAC addresses a specific scenario: what happens after a UE has already been configured with a conditional handover or PSCell change command but has not yet executed it? S-CPAC allows the network to subsequently prepare an *additional* conditional PSCell addition or change *on top of* the already prepared one. This means the UE can maintain multiple layers of conditional mobility commands. For example, the network might first configure a conditional PSCell change from Cell A to Cell B. Later, before the UE executes that change, the network can use S-CPAC to configure a further conditional change from the prospective Cell B to a Cell C. The UE manages these as subsequent conditions.
The procedure involves specific RRC signaling between the UE and the master node (e.g., gNB in NR-DC). The network sends an RRCReconfiguration message containing the subsequent conditional configuration (e.g., a 'CondReconfigToAddMod' for the new candidate). The UE stores this configuration in addition to any previously stored conditional configurations. The execution logic remains event-driven based on measurement reporting. This enhances mobility robustness in dense or rapidly changing radio environments, as the UE can seamlessly transition through a chain of pre-approved candidate cells without needing to go back to the network for a new command after each execution, thereby minimizing service interruption time and signaling overhead.
Purpose & Motivation
S-CPAC was introduced in Release 18 to enhance the robustness and efficiency of conditional mobility procedures in advanced 5G deployments, particularly for ultra-reliable low-latency communication (URLLC) and in high-frequency bands (e.g., mmWave) where radio links can be volatile. The basic Conditional Handover (CHO) and Conditional PSCell Change (CPC), introduced in earlier releases, significantly reduced handover failure rates compared to legacy handovers by preparing backup paths in advance. However, they were primarily designed for a single conditional transition.
The limitation addressed by S-CPAC is the potential for a 'ping-pong' effect or a failed connection after executing a single conditional change. In a dynamic environment, the target cell chosen by a conditional execution might itself degrade quickly. Without S-CPAC, the UE would need to complete the handover, reconnect, and then trigger a new measurement report and receive a new handover command—a process that takes time and could lead to a radio link failure. S-CPAC proactively prepares for this by allowing the network to 'look ahead' and set up a chain of conditional moves. This is especially critical for use cases like industrial IoT and vehicular communications, where uninterrupted connectivity is paramount. It solves the problem of sequential mobility in conditional scenarios, making the entire procedure more predictive and resilient.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (93 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Subsequent Conditional PSCell Addition or Change (S-CPAC) function was newly introduced, enabling the preparation of a conditional PSCell change or addition as part of MR-DC operations. This is supported by specific procedures like the Xn-U Address Indication, which can include a CPC Data Forwarding indicator to trigger data forwarding for a Conditional PSCell Change. The release also defined related handling for scenarios like conditional handover involving dual connectivity.
- Addition of MCG cell ID to solve the PCI confusion at SN TS 38.423CR0002
- Transfer of the PSCell information for LI purposes TS 38.423CR0017
- Data volume reporting for MR-DC with 5GC TS 38.423CR0024
- Introduce IMEISV to addition request to Xn TS 38.423CR0050
- CR38423 for Addition of MN (MeNB) cell ID to solve the PCI confusion in SN(SgNB) modification Request message TS 38.423CR0071
- RAN paging failure handling in SN in case of MR-DC TS 38.423CR0076
+ 15 more changes
In Release 16, the S-CPAC (Subsequent Conditional PSCell Addition or Change) function was newly introduced, enabling a conditional PSCell change to be prepared and executed while a UE is already operating under MR-DC with a previously configured conditional handover. This is detailed as a "Conditional PSCell Change" within the Xn-U ADDRESS INDICATION procedure, where a specific "CPC Data Forwarding indicator" is used to trigger the associated data forwarding. The release also defined related procedures like the Conditional Handover Cancel to manage these conditional reconfigurations.
- Addition of SON features TS 38.423CR0221
- CR38.423 on TDD pattern for NR-DC power control cordination for sol1 TS 38.423CR0344
- Introduction of Additional RRM Policy Index (ARPI) TS 38.473CR0314
- Addition of SON features TS 38.473CR0441
- Addition of MDT features TS 38.473CR0492
- CR38.473 on TDD pattern for NR-DC power control cordination for sol1 TS 38.473CR0537
+ 14 more changes
In Release 17, the S-CPAC function was enhanced with specific corrections and clarifications to improve reliability. This included corrections to the handling of conditional MCG configurations within CPAC and clarifications on the interaction with SN-initiated SCG activation/deactivation during the procedure. Furthermore, an additional indicator was introduced to improve coordination between Conditional Handover (CHO) and Conditional PSCell Change/Addition (CPC).
- Inter MN resume without SN change [InterMNResume] TS 38.423CR0596
- Addition of SON features enhancement TS 38.473CR0710
- BLCR to TS 38.473 for Conditional PScell Change/Addition TS 38.473CR0795
- Addition of NR Timing Advance reporting for NR UL E-CID [NRTADV] TS 38.473CR0817
- MRO for SN change failure correction TS 38.423CR0814
- Rel-17 Correction for XnAP on the interaction with SN-intiated SCG (de)activation and SN Addition procedure TS 38.423CR0851
+ 13 more changes
In Release 18, the S-CPAC (Subsequent Conditional PSCell Addition or Change) function was introduced as a new procedure, enabling a further conditional mobility step after an initial Conditional Handover or Conditional PSCell Addition/Change. The release specified its integration within MR-DC operations, detailing support for both MN-initiated and SN-initiated scenarios, and introduced specific indicators like the S-CPAC Complete Configuration Indicator. Furthermore, corrections and enhancements were made to ensure proper interaction with existing features like CHO (Conditional Handover) and to manage data forwarding for SN-terminated bearers during the procedure.
- Addition of SON features enhancement TS 38.423CR0934
- Introduction of Subsequent CPAC TS 38.423CR1051
- Additions for L1/L2 triggered mobility TS 38.473CR1037
- Addition of SON features enhancement TS 38.473CR1105
- On Subsequent CPAC TS 38.473CR1227
- Review of the description of the S-CPAC solution TS 38.423CR1116
+ 23 more changes
In Release 19, the new S-CPAC (Subsequent Conditional PSCell Addition or Change) function was introduced, enabling a conditional PSCell change to be triggered after a primary conditional handover is executed. This is detailed as part of the Conditional Handover procedures for MR-DC operations, where specific indicators like the "CPC Data Forwarding indicator IE" are used to manage the associated data forwarding. The release also formalized the interaction of this procedure with other conditional reconfiguration mechanisms, such as the Conditional Handover Cancel procedure.
Explore further
Broader topics and technologies where S-CPAC plays a role.
Defining Specifications
3GPP specifications that define or reference S-CPAC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |