Low-Earth Orbiting satellites

Description

Low-Earth Orbiting (LEO) satellites refer to artificial satellites that orbit the Earth at altitudes typically ranging from 500 to 2,000 kilometers. Within the 3GPP framework, starting from Release 14 study items and concretely in Release 15 onwards, LEO satellites are a primary focus for Non-Terrestrial Networks (NTN). They are integrated as aerial access nodes or relay nodes to provide seamless 5G (and beyond) service continuity and global coverage. Unlike Geostationary Earth Orbit (GEO) satellites, LEO satellites move rapidly relative to the Earth's surface, resulting in shorter orbital periods (approximately 90-120 minutes) and smaller coverage areas (cells) on the ground that are in constant motion.

From a network architecture perspective, a LEO satellite in a 3GPP NTN can act as a radio access node (effectively a cell tower in space), a transparent payload (bent-pipe relay), or a regenerative payload (with onboard base station functions). When acting as a transparent payload, the satellite simply amplifies and converts the frequency of the signal between the ground-based gateway (Next Generation NodeB - gNB) and the User Equipment (UE). The gateway connects to the 5G core network. In a regenerative architecture, the satellite contains a full gNB, processing the signal in orbit and connecting directly to the core network via an inter-satellite link or a dedicated ground gateway. The key technical challenge is managing the high Doppler shift due to the satellite's high velocity, large propagation delays (though significantly lower than GEO), and the continuous handovers required as beams move across the Earth.

The integration involves significant enhancements to the 5G New Radio (NR) and core network protocols. The physical layer (covered in specs like 38.101 and 38.108) is adapted to handle larger timing advance values, specific reference signals for tracking, and compensation for Doppler frequency shift. The Radio Resource Control (RRC) layer and mobility management procedures are enhanced to support predictable satellite movement, long cell dwell times, and efficient handover between moving beams or between satellite and terrestrial networks. The core network supports service continuity for UEs moving in and out of satellite coverage. The ultimate role of LEO satellites in 3GPP is to extend the reach of 5G services to airplanes, ships, and remote land areas, creating a truly global network fabric.

Purpose & Motivation

The integration of LEO satellites into 3GPP standards is driven by the imperative to provide ubiquitous, seamless connectivity beyond the reach of traditional terrestrial networks. Terrestrial cellular coverage is economically and geographically limited, leaving vast oceanic, aerial, and remote rural areas without service. Previous satellite communication systems operated in proprietary silos, with high latency (especially GEO systems) and no integration with mainstream consumer mobile devices or core networks. This created a coverage gap for critical services like maritime communications, in-flight connectivity, and disaster response.

3GPP's work on NTN, with LEO as a cornerstone, aims to solve this by making satellite access a native component of the 5G system. This allows a standard 5G smartphone, with some enhancements, to potentially connect directly to a LEO satellite network without specialized hardware, enabling global roaming and service continuity. The motivation includes supporting United Nations sustainable development goals for connectivity, enabling Internet of Things (IoT) services in agriculture and logistics across remote regions, and providing resilient back-up for terrestrial networks during failures or disasters.

Technically, LEO satellites were chosen over GEO because their lower altitude (500-2000 km vs 36,000 km) results in much lower propagation latency (20-40 ms vs 500+ ms), making them suitable for latency-sensitive 5G services. The proliferation of mega-constellations (like SpaceX's Starlink) demonstrated the commercial viability of dense LEO networks, prompting 3GPP to standardize interfaces and procedures to leverage this new infrastructure. The standardization ensures interoperability between different satellite operators and terrestrial network operators, fostering a competitive ecosystem and preventing vendor lock-in.