Non-Terrestrial Networks

Description

Non-Terrestrial Networks (NTN) refer to a comprehensive 3GPP architecture where the access network is provided by non-terrestrial platforms, seamlessly integrated with the terrestrial 5G Core network. The primary platforms include Geostationary Earth Orbit (GEO), Medium Earth Orbit (MEO), and Low Earth Orbit (LEO) satellites, as well as High-Altitude Platform Stations (HAPS) like balloons or drones acting as quasi-stationary base stations. In this architecture, the satellite or HAPS carries a payload that functions as a 3GPP gNB (5G base station) or ng-eNB (LTE base station connected to 5GC), often referred to as a 'satellite node' or 'non-terrestrial node'. This node communicates with User Equipment (UE) via a service link (e.g., using adapted 5G NR waveforms in specific frequency bands like S-band or Ka-band) and connects to ground-based gateways, known as Earth Stations or Gateways, via a feeder link. The gateway then interfaces with the 5G Core Network over standard N2/N3 interfaces.

How it works involves significant adaptations to standard 5G procedures to cope with the unique characteristics of satellite links. The most critical challenge is the very long propagation delay, which can range from several milliseconds for LEO to hundreds of milliseconds for GEO. To handle this, 3GPP has introduced enhancements to timing advance procedures, hybrid automatic repeat request (HARQ) timelines, and random access channel (RACH) procedures. For mobility, NTN supports both Earth-fixed cell coverage (where the cell footprint is fixed on the ground, and the satellite beam moves) and Earth-moving cell coverage (where the beam is steered to keep the cell footprint stationary), requiring new mobility management schemes. The architecture also defines transparent payloads (bent-pipe) that simply amplify and forward signals, and regenerative payloads (on-board processing) that can decode, switch, and re-encode signals, impacting latency and complexity.

Key components include the NTN Terminal (UE with enhanced capabilities for satellite links), the Non-Terrestrial Network Node (satellite/HAPS payload), the Gateway (Earth Station with Network Data Forwarding Function), and the 5G Core Network. Its role is to provide service continuity, ubiquitous coverage, and broadcast/multicast services. It enables use cases like direct-to-device satellite connectivity for smartphones, massive IoT sensor monitoring in remote areas, backhaul for terrestrial networks, and reliable communications for maritime and aeronautical services. By integrating NTN, 5G systems truly become a unified global network, ensuring connectivity everywhere and enhancing resilience by providing an alternative when terrestrial networks fail due to disasters.

Purpose & Motivation

NTN was developed to address the fundamental limitation of terrestrial cellular networks: their inability to provide cost-effective, seamless coverage over the entire Earth's surface, including oceans, deserts, polar regions, and remote rural areas. Traditional cellular networks are economically viable only in areas with sufficient population density, leaving vast geographic regions unserved. This gap hindered the vision of truly global connectivity for Internet of Things (IoT) applications, aviation, maritime, and emergency services. Furthermore, terrestrial networks are vulnerable to natural disasters that can destroy infrastructure.

The motivation for standardizing NTN within 3GPP, starting in Release 15 as a study item, was to leverage the rapid advancements in satellite technology, particularly the emergence of large LEO constellations (like Starlink), and the growing demand for global broadband and IoT services. By creating a unified standard, 3GPP aimed to foster an ecosystem of low-cost, mass-produced devices that can access both terrestrial and non-terrestrial networks without requiring proprietary technologies. This solves the problem of fragmentation and enables economies of scale. NTN addresses the need for network resilience by providing a backup or complementary path, supports regulatory requirements for emergency communications (e.g., EU eCall), and unlocks new business models for connectivity in transportation, agriculture, and energy sectors across the globe.