Radio Access Technology

Description

A Radio Access Technology (RAT) is the complete set of technical specifications and implementations that define the air interface and associated network functions for wireless communication between user equipment (UE) and the core network. It encompasses the physical layer (modulation, coding, duplexing), the data link layer (medium access control, radio link control), and key radio resource management procedures. Each RAT, such as GSM (2G), UMTS (3G), LTE (4G), and NR (5G), has a distinct architecture: GSM uses TDMA/FDMA with Base Station Subsystems; UMTS employs W-CDMA with NodeBs and RNCs; LTE utilizes OFDMA/SC-FDMA with a simplified eNodeB; NR uses flexible OFDMA with gNBs. The RAT defines the specific protocols for random access, channel establishment, handover, power control, and scheduling. It works by converting digital data into radio waves transmitted over specific frequency bands, with the base station (eNodeB, gNB) managing radio resources for multiple UEs. Key components include the UE's radio transceiver, the base station's radio unit and baseband processor, and the defined channel structures (physical, transport, logical). Its role is to provide the wireless link that carries user data and control signaling, forming the essential bridge between mobile devices and the core network. Modern networks are multi-RAT, with UEs and networks capable of operating across several RATs (e.g., LTE and NR), managed through mechanisms like inter-RAT mobility and carrier aggregation. The RAT fundamentally determines key performance indicators like data rate, latency, coverage, and capacity.

Purpose & Motivation

RATs exist to standardize wireless communication, enabling interoperability between devices and networks globally. Each new RAT generation is created to solve the limitations of its predecessors, driven by increasing demands for higher data rates, lower latency, greater capacity, and new service types. GSM (2G) introduced digital voice and SMS, moving beyond analog. UMTS (3G) addressed the need for mobile broadband data. LTE (4G) was developed to provide all-IP, high-speed data with simplified architecture, overcoming the complexity and inefficiency of UMTS's dedicated channels. NR (5G) is designed for extreme broadband, ultra-reliable low latency, and massive IoT, tackling LTE's limitations in latency and flexibility for diverse use cases. The evolution of RATs is motivated by spectrum efficiency improvements, technological advancements (like MIMO, advanced coding), and changing user demands. Multi-RAT operation is crucial for seamless service continuity, allowing networks to leverage existing infrastructure (e.g., LTE coverage) while deploying new technologies (NR). Standards bodies like 3GPP define these RATs to ensure a cohesive ecosystem, preventing fragmentation and enabling economies of scale.

Key Features

• Defines the physical layer waveform, modulation (e.g., QPSK, 256QAM), and multiple access scheme (e.g., OFDMA, CDMA)
• Specifies the radio protocol stack including MAC, RLC, PDCP, and RRC layers
• Governs radio resource management procedures: scheduling, handover, power control
• Determines supported frequency bands and duplexing methods (FDD, TDD)
• Enables mobility management within the same RAT (intra-RAT) and between different RATs (inter-RAT)
• Provides the foundation for network features like carrier aggregation, dual connectivity, and network slicing

Evolution Across Releases

Defining Specifications