Radio System (Radio Access Technology Generation)

Description

In 3GPP terminology, a Radio System refers to the specific generation of radio access technology (RAT) that a mobile network operator has deployed to provide wireless connectivity between user equipment (UE) and the core network. The term is deliberately generic, allowing specifications to refer to the radio access component without being tied to a particular technology generation. Over the years, the definition has evolved from covering only 2nd and 3rd generation systems (UTRAN and GERAN) to encompassing all generations including 4G E-UTRAN and 5G NG-RAN.

The radio system is a fundamental architectural element because it determines the physical layer procedures, protocol stack, and interface characteristics that govern how devices communicate over the air. For example, GERAN (GSM/EDGE Radio Access Network) uses TDMA-based air interface with GMSK and 8-PSK modulation, while UTRAN (UMTS Terrestrial Radio Access Network) employs WCDMA, and E-UTRAN (Evolved UTRAN) uses OFDMA in the downlink and SC-FDMA in the uplink. NG-RAN (Next Generation RAN) introduces flexible numerology and massive MIMO for 5G. Each radio system has its own set of specifications covering physical layer, layer 2, and layer 3 protocols.

The concept of a radio system is closely tied to the overall network architecture. In 3GPP specifications, the radio system interacts with the core network through standardized interfaces: GERAN connects via Gb/Iu-CS/Iu-PS, UTRAN via Iu, E-UTRAN via S1, and NG-RAN via NG. These interfaces allow the core network to be largely agnostic to the specific radio technology, enabling multi-RAT deployments and interworking. The radio system also encompasses the radio resource management functions, including handover, power control, and scheduling, which are essential for maintaining service quality and capacity.

From a deployment perspective, the radio system includes the base stations (e.g., NodeB, eNodeB, gNB) and the associated radio network controllers or centralized units. The choice of radio system affects coverage, capacity, latency, and supported services. For instance, NG-RAN supports ultra-reliable low-latency communication (URLLC) and massive machine-type communication (mMTC), which are not possible with earlier generations. The term is used throughout 3GPP specifications to abstract the radio access part when describing procedures that are common across technologies, such as mobility management or session management.

In summary, the radio system is the radio access segment of a mobile network, defined by its generation and technology. It provides the wireless link and the necessary protocols to connect users to the core network, and its evolution from 2G to 5G reflects the continuous advancement in spectral efficiency, latency, and service capabilities.

Purpose & Motivation

The term 'Radio System' exists to provide a technology-agnostic way to refer to the radio access portion of a mobile network in 3GPP specifications. Early specifications needed to distinguish between 2nd generation (GERAN) and 3rd generation (UTRAN) systems when describing interworking, mobility, and core network procedures. As technology evolved, the definition was expanded to include 4G E-UTRAN and 5G NG-RAN, ensuring that specifications could remain applicable across multiple generations without requiring constant rewording.

The primary problem it solves is the need for a common abstraction that allows core network specifications to define procedures that work regardless of the underlying radio technology. For example, a specification on mobility management can refer to 'the radio system' when describing how a UE moves between cells, without specifying whether it is a GERAN, UTRAN, E-UTRAN, or NG-RAN cell. This abstraction simplifies the specification structure and promotes interoperability.

Historically, before the term was introduced, specifications often listed specific technologies, which became unwieldy as new generations were added. The generic term allows backward compatibility and forward extension. It also facilitates multi-RAT operation, where a network may deploy multiple radio systems simultaneously, and the core network must handle inter-system handovers and service continuity. Thus, the term is essential for maintaining a modular and extensible standard framework.