Radio Link Addition

Description

Radio link addition is a mobility management procedure used primarily in UMTS (UTRAN) networks to add a new radio link to the active set of a UE. The active set is the group of cells with which the UE maintains a radio link simultaneously during soft handover. When the UE measures a pilot signal from a neighboring cell that exceeds a certain threshold, it reports this to the network. The RRC layer in the serving RNC then decides to add a new radio link to that cell, provided resources are available. This involves setting up the necessary radio bearers and configuring the physical layer for the new link. Once added, the UE can receive and transmit data via both cells, and the network combines the signals to improve the overall signal-to-interference ratio. This is particularly beneficial at cell edges where signal strength is weak. The procedure is controlled by the RRC protocol and involves signalling between the UE, the serving RNC, and the target NodeB. Radio link addition is distinct from radio link removal and radio link replacement, which are other active set management procedures. In LTE and 5G, the concept of soft handover is not used in the same way; instead, carrier aggregation and dual connectivity allow multiple links, but the terminology differs. Nevertheless, the principle of adding a link to improve performance persists. Radio link addition is crucial for maintaining call continuity and quality during mobility, especially for voice services.

Purpose & Motivation

Radio link addition exists to support soft handover in CDMA-based systems like UMTS. In hard handover, the UE breaks the connection with the old cell before establishing a new one, which can cause momentary interruptions and call drops. Soft handover, enabled by radio link addition, allows the UE to maintain multiple connections simultaneously, providing macro diversity gain. This improves coverage, reduces interference, and enhances capacity. The procedure solves the problem of reliable communication at cell boundaries, where signals from a single cell may be too weak. By adding a new link, the network ensures that the UE always has at least one strong connection. It also allows for seamless mobility without service interruption. The concept was motivated by the need for high-quality voice and data services in 3G networks. Although later technologies like LTE use different mobility mechanisms, the fundamental idea of adding a link to improve robustness remains relevant in carrier aggregation and dual connectivity.