Conditional Primary Secondary Cell Addition

Description

Conditional PSCell Addition (CPA) is a mobility enhancement introduced in 3GPP Release 17 for 5G New Radio (NR) multi-connectivity, specifically for the addition of a Primary Secondary Cell (PSCell) in EN-DC, NR-DC, and NE-DC scenarios. In traditional PSCell addition, the UE reports a measurement event, the network decides to add a secondary node (SN), and then sends an RRC reconfiguration message with the PSCell configuration. This process involves signaling over the air and between network nodes, which can introduce latency and increase the risk of radio link failure, especially in high-speed mobility scenarios. CPA addresses this by allowing the network to configure the UE with a conditional configuration in advance, including a list of candidate PSCells and execution conditions. The UE then monitors the radio conditions and autonomously executes the PSCell addition when the condition is satisfied, without waiting for a new RRC reconfiguration from the network.

The CPA procedure involves several key steps. First, the master node (MN) decides to prepare one or more candidate secondary nodes (SNs) for conditional addition. The MN sends an SN Addition Request to each candidate SN, which includes the UE's capabilities and the requested configuration. Each candidate SN responds with an SN Addition Request Acknowledge containing the radio configuration for the candidate PSCell. The MN then generates an RRCReconfiguration message that includes a list of conditional configurations, each associated with a candidate PSCell and an execution condition. The execution condition is typically based on measurement events, such as A3, A4, or A5, and can be configured with thresholds and hysteresis. The UE receives this message, stores the conditional configurations, and starts monitoring the conditions. When the condition for a candidate PSCell is met, the UE applies the corresponding configuration, performs random access to the target PSCell, and sends an RRCReconfigurationComplete message to the MN. The MN then informs the selected SN that the UE has successfully added the PSCell, and the SN becomes the serving SN.

Architecturally, CPA is supported by enhancements in the NG-RAN interfaces, specifically the Xn interface between the MN and SN, and the Uu interface between the UE and the network. The 3GPP specifications 37.340 (NR; Multi-connectivity; Overall description), 38.401 (NG-RAN; Architecture description), and 38.473 (NG-RAN; F1AP) define the signaling flows and procedures. The MN is responsible for managing the conditional configurations and for coordinating with the candidate SNs. The UE must support the conditional addition feature, which is indicated by its capabilities. The candidate SNs must also support the feature and provide the necessary configuration parameters. The execution condition can be based on NR measurements, and the UE must be able to evaluate these conditions while maintaining the connection to the MN.

One of the main benefits of CPA is the reduction in signaling latency during PSCell addition. By pre-configuring the UE with candidate PSCells, the network avoids the need for a round-trip of signaling between the UE and the network at the time of addition. This is particularly beneficial in scenarios where the UE is moving quickly and the radio conditions change rapidly. CPA also improves mobility robustness by allowing the UE to autonomously select the best PSCell based on the configured conditions, reducing the risk of failures due to delayed or lost signaling. Additionally, CPA can help to reduce the load on the network by minimizing the signaling overhead associated with frequent PSCell additions and changes.

CPA is part of a broader set of conditional mobility enhancements in 5G, including Conditional Handover (CHO) and Conditional PSCell Change (CPC). These features share the common principle of pre-configuring the UE with target configurations and execution conditions, enabling faster and more reliable mobility. In Release 18 and beyond, further enhancements to CPA are being studied, such as support for multiple candidate PSCells, more flexible execution conditions, and integration with network slicing and other advanced features. As 5G networks evolve towards higher frequencies and denser deployments, CPA will play a crucial role in ensuring seamless and reliable multi-connectivity.

Purpose & Motivation

Conditional PSCell Addition was introduced to address the limitations of traditional PSCell addition procedures in 5G multi-connectivity. In traditional addition, the UE must first detect a suitable PSCell, report the measurement to the network, and then wait for the network to decide and send an RRC reconfiguration. This process involves multiple signaling messages over the air and between network nodes, which can introduce significant latency, especially in high-mobility scenarios. The delay can lead to radio link failures or suboptimal performance because the radio conditions may change before the addition is completed. CPA solves this problem by allowing the network to pre-configure the UE with candidate PSCells and execution conditions, so that the UE can autonomously execute the addition when the condition is met, without waiting for further signaling.

The motivation for CPA also comes from the need to improve mobility robustness in dense deployments with small cells and high-frequency bands, such as millimeter wave. In such deployments, the coverage of a single cell is limited, and frequent PSCell additions and changes are necessary to maintain connectivity. Traditional procedures may not be fast enough to keep up with rapid changes in radio conditions, leading to dropped connections and degraded user experience. CPA enables faster and more reliable addition by reducing the signaling latency and allowing the UE to make decisions based on real-time measurements. This is particularly important for applications that require ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB).

Historically, CPA builds on the concept of Conditional Handover (CHO) introduced in Release 16 for primary cell handovers. CHO demonstrated the benefits of pre-configured target cells and execution conditions for reducing handover failures. CPA extends this concept to the addition of a secondary node in multi-connectivity, which is a more complex scenario because it involves coordination between the master node and multiple candidate secondary nodes. The standardization of CPA in Release 17 required enhancements to the Xn interface and the RRC protocol to support the conditional configurations and the associated signaling. Overall, CPA is a key enabler for seamless mobility in 5G and beyond, and it will continue to evolve in future releases to support more advanced use cases.