Radio Link

Description

A radio link is a fundamental concept in cellular networks, representing the logical connection between a UE and a specific access point (e.g., NodeB, eNodeB, gNB). Unlike the radio interface, which is a broad term for the entire wireless connection, a radio link is a specific instance tied to a particular cell and UE. It is the entity that the network manages for mobility, resource allocation, and handover. Physically, a radio link is realized by one or more radio bearers, each carrying user data or control signalling with specific QoS characteristics. The radio link is established during initial access (e.g., RRC connection setup) and can be modified or released as the UE moves or service requirements change. In soft handover scenarios (UMTS), multiple radio links can be active simultaneously between the UE and different cells, forming an active set. The network combines signals from these links to improve reliability. In LTE and 5G, the concept persists but with different terminology (e.g., serving cell, secondary cell). The radio link is managed by the RRC layer, which handles configuration, reconfiguration, and release. It also involves lower layers for synchronization, power control, and feedback. The quality of a radio link is continuously monitored through measurements (e.g., RSRP, RSRQ, SINR) to trigger handovers or link adaptation. Thus, the radio link is the workhorse of radio resource management, enabling seamless connectivity and mobility.

Purpose & Motivation

The radio link concept was introduced to provide a manageable, logical unit for controlling the connection between a UE and a cell. In early systems, the physical channel was the primary focus, but as networks became more complex with multiple cells and frequencies, a higher-level abstraction was needed. The radio link solves the problem of mobility: by treating each connection as a distinct link, the network can add, remove, or modify links as the UE moves, ensuring service continuity. It also enables soft handover in CDMA systems, where multiple links are combined for gain. Furthermore, the radio link provides a framework for resource allocation and QoS management, as each link can be associated with specific radio bearers and their parameters. Without this concept, managing interference, power control, and handover would be chaotic. The radio link also facilitates security by allowing ciphering and integrity protection to be applied per link. Its definition has evolved to encompass multiple radio access technologies, reflecting the heterogeneous nature of modern networks.