Conditional PSCell Addition or Change

Description

Conditional PSCell Addition or Change (CPAC) is a sophisticated mobility procedure introduced in 3GPP Release 17 for scenarios involving Dual Connectivity (DC), specifically E-UTRA-NR Dual Connectivity (EN-DC) and NR-NR Dual Connectivity (NR-DC). It extends the principles of Conditional Handover (CHO) to the management of the Primary SCell (PSCell) within a secondary node (SN) in a DC configuration. The core concept is to allow the network to pre-configure the User Equipment (UE) with one or more candidate PSCells along with specific execution conditions, typically based on radio measurement thresholds (e.g., RSRP, RSRQ). The UE then autonomously monitors these conditions for the candidate cells while maintaining its current connection. When a condition for a candidate PSCell is met, the UE executes the PSCell addition or change without needing further signaling from the network at the moment of execution, thereby reducing latency and the risk of failure during the critical transition phase.

Architecturally, CPAC involves coordination between the Master Node (MN) and the Candidate Secondary Node (C-SN). The procedure is initiated by the MN, which requests the preparation of candidate PSCell resources from one or more C-SNs via the Xn interface (or X2 in the case of EN-DC). The C-SN provides the necessary configuration, including the new PSCell's cell identity, bearer configuration, and security parameters, which are encapsulated in a 'Conditional PSCell Addition/Change' command sent to the UE via RRC signaling from the MN. This command includes the execution condition(s) for each candidate. The UE stores this configuration and begins evaluating the conditions. Key RRC messages involved are the RRCReconfiguration message for delivering the CPAC configuration and the subsequent RRCReconfigurationComplete message upon successful execution.

The execution phase is UE-controlled. Upon fulfilling a condition, the UE performs a synchronization and random access procedure to the target PSCell, applies the stored configuration, and releases the resources of the old PSCell if applicable. It then informs the MN of the successful change via an RRCReconfigurationComplete message. The network then updates its context and may initiate data forwarding and path switch procedures. CPAC is particularly valuable in high-mobility or challenging radio environments where the radio conditions for the current PSCell can degrade rapidly. By preparing alternatives in advance, CPAC enhances mobility robustness, minimizes data interruption, and contributes to higher reliability and seamless user experience in advanced 5G networks utilizing carrier aggregation and dual connectivity.

Purpose & Motivation

CPAC was created to address specific mobility robustness challenges inherent in Dual Connectivity and Multi-RAT Dual Connectivity deployments. In traditional 'blind' or measurement-report-triggered PSCell addition/change procedures, the decision and execution are network-controlled and sequential, which can lead to failures if radio conditions deteriorate faster than the network can react. This is especially problematic for the PSCell, which often operates on higher frequencies (e.g., mmWave) with more pronounced signal fluctuations. The primary problem CPAC solves is the reduction of handover failure rates and radio link failures (RLFs) associated with the secondary node, thereby improving overall connection stability and service continuity.

The historical context stems from the successful application of Conditional Handover (CHO) for the primary cell, introduced in earlier releases. Observing CHO's benefits in reducing handover failures for the PCell, 3GPP recognized a similar need for the secondary node's primary cell (PSCell) in DC scenarios. Previous approaches relied on timely measurement reports and network processing, which introduced latency and a single point of failure. If the signaling path was delayed or the source PSCell link failed before completion, the procedure would fail, potentially causing a DC link drop. CPAC mitigates this by decentralizing the execution decision to the UE, which has the most immediate knowledge of its radio environment, allowing for a faster and more reliable transition when predefined conditions are met.

Furthermore, CPAC supports the evolution towards more autonomous and intelligent UE behavior in 5G-Advanced networks. It enables network load balancing and optimization by allowing the network to pre-configure multiple candidate cells, potentially in different nodes or frequency layers. This prepares the network for ultra-reliable low-latency communication (URLLC) use cases and high-mobility scenarios like vehicular communications, where predictable and robust mobility is non-negotiable. By solving the PSCell mobility robustness gap, CPAC is a key enabler for reliable multi-connectivity, which is fundamental for achieving the high data rates and consistent performance promised by 5G and beyond.