Active Set

Description

The Active Set is a fundamental concept in UMTS (UTRAN) radio resource management. It refers to the set of radio links that are simultaneously involved in a specific communication service between a User Equipment (UE) and the UTRAN access network. Each radio link in the Active Set corresponds to a connection between the UE and a specific cell (sector) of a Node B, and all links in the Active Set carry the same user data, allowing the UE to combine signals from multiple cells. This is the mechanism that enables soft handover in UMTS, where the UE communicates with multiple Node Bs simultaneously rather than performing a hard switch from one cell to another.

The Active Set is managed through a set of neighbouring cell lists and measurement reports. The UE continuously measures the received signal quality of cells in the Monitored Set and reports events to the RNC (Radio Network Controller). Based on these measurements, the RNC decides when to add a radio link to the Active Set (Radio Link Addition), when to remove one (Radio Link Deletion), or when to replace one (Combined Radio Link Addition and Deletion). The typical trigger events are 1A (a primary CPICH enters the reporting range), 1B (a primary CPICH leaves the reporting range), and 1C (a non-active primary CPICH becomes better than an active primary CPICH). The Active Set size is limited by the UE's capability and network configuration, typically up to 6 radio links for soft handover and up to 3 for softer handover (links within the same Node B but different sectors).

In the downlink, the same data is transmitted from multiple cells to the UE, and the UE uses a Rake receiver to combine the signals from different radio links. This provides macro diversity gain, which improves the received signal quality and reduces the required transmit power, thereby increasing capacity. In the uplink, the UE transmits to multiple Node Bs, and the RNC combines the received data from different Node Bs through the Iub interface. This uplink macro diversity also improves coverage and reduces interference.

The Active Set concept is specific to UTRAN (UMTS) and does not exist in the same form in LTE or NR, where the UE connects to a single cell at a time (though carrier aggregation and dual connectivity allow multiple cells to serve the UE, the concept is different). In UTRAN, the Active Set is a key parameter for mobility management, and its management is handled by the RRC (Radio Resource Control) protocol. The Active Set is also relevant for the serving HS-DSCH cell and the E-DCH serving cell in HSPA, where the serving cell is selected from the Active Set.

The Active Set is defined in 3GPP TS 21.905 as part of the vocabulary for 3GPP specifications. Its definition has remained essentially unchanged across releases from Rel-4 to Rel-19, reflecting the stability of the UTRAN architecture and the continued relevance of soft handover in UMTS networks. Even as 3GPP has evolved towards LTE and 5G, the Active Set remains a foundational concept for understanding UMTS radio resource management and macro diversity.

Purpose & Motivation

The Active Set exists to enable soft handover in UMTS, a mechanism that allows a UE to maintain simultaneous radio links with multiple cells during a call. Before soft handover, hard handover was used in earlier systems like GSM, where the UE breaks the connection with the old cell before establishing a new one. This 'break-before-make' approach can cause call drops and service interruptions, especially at cell edges where signal quality is poor. Soft handover, enabled by the Active Set, uses a 'make-before-break' approach, ensuring seamless connectivity and improved reliability.

The primary motivation for the Active Set is to provide macro diversity gain, which combats fading and shadowing in the radio channel. By combining signals from multiple cells, the UE can achieve a more stable connection and reduce the required transmit power, leading to lower interference and higher network capacity. This is particularly important in CDMA-based systems like UMTS, where interference is the main capacity-limiting factor. The Active Set also improves coverage at cell boundaries, where a single link might be too weak to maintain the connection.

Historically, the Active Set was introduced with UMTS in Release 99 (though the term is documented from Rel-4 onwards in 21.905). It addressed the limitations of hard handover in GSM and IS-95 by providing a standardized framework for managing multiple radio links. The concept has been refined over releases, but its core definition has remained stable, reflecting its fundamental role in UTRAN. While LTE and NR have moved away from soft handover due to their different multiple access schemes (OFDMA), the Active Set remains a key concept for understanding UMTS and the evolution of mobility management in cellular networks.