Air to Ground Network

Description

The Air to Ground (ATG) network is a specialized NG-RAN deployment introduced in 3GPP Release 18 to provide direct cellular connectivity to aircraft in flight. Unlike traditional satellite-based or air-to-ground systems, the ATG network reuses the standard 5G NR architecture, with ground-based gNBs acting as cell towers that communicate directly with an onboard ATG terminal installed on the aircraft. The onboard terminal functions similarly to a UE, maintaining a radio link with the terrestrial network as the aircraft moves through the sky. The typical operational altitude is around 10,000 meters for cruise flight, with take-off and landing phases occurring at altitudes as low as 3,000 meters. This architecture allows airlines to offer broadband services to passengers and crew using the same 5G core network and radio protocols as terrestrial users, with appropriate adaptations for high-speed mobility and altitude.

The ATG network is defined in 3GPP specifications 38.300 (NR overall description) and 38.321 (MAC protocol). It leverages the existing NG-RAN architecture, where gNBs are connected to the 5G Core via the NG interface. The key difference lies in the radio propagation environment: signals travel over long distances with line-of-sight conditions, and the aircraft moves at speeds up to 1,000 km/h. To handle this, the ATG network employs beamforming, large antenna arrays, and advanced mobility management to maintain connectivity. The onboard ATG terminal may aggregate multiple links or use multiple antennas to improve reliability. The network must also manage interference with terrestrial users and ensure that handovers between ground cells occur seamlessly despite the high velocity.

From a protocol perspective, the ATG network reuses the NR physical layer and MAC procedures, but with modifications to support larger cell radii (up to 300 km) and Doppler shift compensation. The MAC layer, as specified in 38.321, handles scheduling and HARQ for the ATG link, while the RRC layer manages connection mobility. The network may also implement quality-of-service differentiation for passenger traffic versus crew or operational data. Security is handled via the standard 5G authentication and encryption mechanisms, ensuring that the air interface is protected.

The ATG network is part of the broader 3GPP effort to integrate non-terrestrial and aerial vehicles into the 5G ecosystem. It complements satellite-based NTN (non-terrestrial networks) by providing high-capacity, low-latency connectivity over landmasses where ground infrastructure is available. The architecture supports both commercial aviation and unmanned aerial vehicles (UAVs), although the initial focus is on commercial aircraft. The ground gNBs can be deployed along flight corridors, and the network can be densified to increase capacity. The ATG terminal on the aircraft acts as a relay or a UE, depending on the service model, and may support multiple simultaneous connections to different gNBs for redundancy.

In Release 19, the definition was refined to improve clarity, but the fundamental architecture remains the same. The ATG network is expected to evolve with enhancements for higher throughput, lower latency, and better support for dense airspace. It represents a key enabler for the connected aircraft, allowing seamless integration of airborne users into the 5G system without relying solely on satellites. As such, it is a critical component of the future aviation communications landscape, bridging the gap between terrestrial and aerial networks.

Purpose & Motivation

The Air to Ground network was created to address the growing demand for broadband connectivity on commercial aircraft. Traditional satellite-based systems often suffer from high latency, limited bandwidth, and high cost, while terrestrial networks cannot serve aircraft at altitude due to coverage and interference constraints. The ATG network solves this by extending the 5G NR architecture to the sky, using ground-based gNBs to provide direct, high-capacity links to aircraft. This enables airlines to offer passengers a experience comparable to home broadband, supports real-time operational data exchange, and opens new revenue streams for operators.

Historically, in-flight connectivity relied on satellites (L-band, Ku/Ka-band) or early air-to-ground systems like CDMA-based networks. These had limited capacity and were expensive. The ATG network leverages the scalability and efficiency of 5G NR, including massive MIMO, beamforming, and network slicing, to deliver better performance. It also allows integration with the 5G core, enabling unified management and policy control. The motivation was to provide a standardized, interoperable solution that can be deployed by multiple operators and serve various aircraft types, from commercial jets to UAVs.

The ATG network also addresses the need for reliable connectivity during take-off and landing, where altitude is lower and handover between ground cells is frequent. By defining specific altitude ranges (3,000 to 10,000 meters), 3GPP ensures that the network can handle the unique propagation and mobility challenges of the aviation environment. This standardization effort is part of the broader 5G ecosystem evolution, aiming to connect all types of vehicles and users, whether on the ground or in the air.