Reference Configuration

Description

In 3GPP radio access networks, a Reference Configuration is a set of parameters provided by the network to the User Equipment (UE) that is common to a group of configured non-complete candidate configurations within the same cell group. This concept is primarily used in the context of mobility management, particularly for handover and multi-connectivity scenarios, where the UE may need to evaluate multiple candidate target cells or configurations. By providing a common reference configuration, the network reduces signaling overhead and simplifies UE processing, as the UE can apply the shared parameters to all candidate configurations without receiving them individually.

The Reference Configuration is defined in 3GPP specifications such as TS 38.331 for NR and TS 21.905 for vocabulary. It is especially relevant in procedures like conditional handover (CHO) and dual connectivity (DC), where the UE is configured with multiple candidate target cells or cell groups. Each candidate configuration may be non-complete, meaning it lacks certain parameters that are instead provided in the reference configuration. The UE combines the reference configuration with each candidate configuration to form a complete configuration for evaluation and potential execution.

Architecturally, the Reference Configuration is generated by the network node (e.g., gNB or eNB) and signaled to the UE via RRC signaling. It typically includes parameters such as measurement configurations, radio resource configurations, and security parameters that are common across the candidate set. The UE stores this reference configuration and applies it when evaluating or executing a candidate configuration. This approach is particularly beneficial in dense deployments where the UE may have many candidate cells, as it minimizes the amount of dedicated signaling required for each candidate.

The concept has evolved over releases. Initially, in Rel-4, the term referred to a combination of functional groups and reference points showing possible network arrangements, as defined in ITU-T I.112. This definition persisted through Rel-17. In Rel-18, the definition was updated to its current form, reflecting the shift towards RAN-specific usage in 5G and beyond. This change aligns with the introduction of advanced mobility features in NR, such as conditional handover and multi-connectivity, where the reference configuration plays a critical role.

In practice, the Reference Configuration enables efficient and flexible mobility management. For example, in conditional handover, the network can provide a reference configuration common to multiple candidate target cells, and then each candidate configuration only needs to include cell-specific parameters. This reduces the size of RRC messages and speeds up the handover preparation. Similarly, in dual connectivity, the reference configuration can be used to share common parameters between the master cell group (MCG) and secondary cell group (SCG), simplifying the configuration of the secondary node. Overall, the Reference Configuration is a key enabler for scalable and low-latency mobility in modern cellular networks.

Purpose & Motivation

The Reference Configuration exists to optimize signaling and processing in scenarios where a UE is configured with multiple candidate configurations, such as in conditional handover or dual connectivity. Without it, the network would need to send a complete set of parameters for each candidate, leading to large RRC messages, increased signaling overhead, and higher UE processing complexity. By introducing a common reference configuration, the network can share parameters that are identical across candidates, reducing the amount of data transmitted and simplifying UE implementation.

Historically, the term originated in the context of network architecture, referring to combinations of functional groups and reference points as defined by ITU-T. This definition was used in early 3GPP releases to describe possible network arrangements. However, as networks evolved towards 5G and advanced mobility features, the need for a more RAN-specific concept emerged. The Rel-18 redefinition reflects this shift, aligning the term with the practical needs of NR and LTE-Advanced Pro networks.

The Reference Configuration solves the problem of signaling efficiency in multi-candidate scenarios. It allows the network to provide a baseline configuration that applies to all candidates, while each candidate only specifies its unique parameters. This not only reduces signaling overhead but also enables faster configuration and reconfiguration, which is critical for low-latency applications and high-mobility scenarios. Furthermore, it simplifies UE implementation by providing a clear separation between common and candidate-specific parameters, reducing the risk of configuration errors.