Subsequent Conditional PSCell Addition or Change

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.