Satellite

Description

In 3GPP terminology, a satellite is defined as a space-borne vehicle that embarks either a bent pipe payload (also known as transparent payload) or a regenerative payload telecommunication transmitter. The satellite is placed into various orbits: Low-Earth Orbit (LEO) typically at altitudes between 500 km and 2000 km, Medium-Earth Orbit (MEO) typically between 8000 km and 20000 km, or Geostationary-satellite Earth Orbit (GEO) at approximately 35,786 km altitude. This definition, first introduced in Release 14 as part of the study on NTN, has evolved in wording but remains conceptually consistent. The satellite serves as the radio access point for User Equipment (UE) in NTN, communicating with UEs on the ground and with satellite-gateways that connect to the terrestrial core network.

The satellite's payload can be either transparent or regenerative. A transparent payload (bent pipe) simply amplifies and frequency-converts the received signal before retransmitting it, without any demodulation or decoding. This is the simpler and more common approach, especially for LEO satellites. A regenerative payload, on the other hand, includes on-board processing capabilities such as demodulation, decoding, and possibly routing, which can improve performance by reducing latency and enabling inter-satellite links. The choice of payload affects the overall architecture and the requirements on the satellite and ground segment.

The orbit determines the satellite's coverage pattern and latency. LEO satellites have low latency (a few milliseconds) but move quickly relative to the Earth, requiring a constellation of many satellites for continuous coverage. MEO satellites offer a compromise between latency and coverage, while GEO satellites appear stationary from the ground, providing continuous coverage over a fixed area but with higher latency (around 250 ms round-trip). The 3GPP NTN specifications support all these orbit types, with different parameter sets for each.

In the NTN architecture, the satellite communicates with the satellite-gateway via a feeder link and with UEs via a service link. The gateway connects to the core network (e.g., 5GC or EPC) through standard interfaces. The satellite may also support inter-satellite links (ISL) for routing traffic between satellites, especially in constellations with regenerative payloads. The 3GPP specifications define the radio protocols and procedures for NTN, including adaptations for the long propagation delays and Doppler shifts inherent in satellite communication.

The definition of satellite has evolved in 3GPP specifications. In Release 14, it was described as 'a platform integrating a transparent repeater or a telecommunication transmitter' placed in LEO, MEO, or GEO. In Release 15, the wording changed to 'a space-borne vehicle embarking a bent pipe payload or a regenerative payload telecommunication transmitter,' and the altitude for GEO was specified as 35,786 km. In Release 17, the definition was further refined to 'A space-borne vehicle embarking a transparent payload, or a regenerative payload telecommunication transmitter, placed into Low-Earth Orbit (LEO) or Geostationary Earth Orbit (GEO),' notably omitting MEO and changing 'bent pipe' to 'transparent.' This change reflects the focus of NTN specifications on LEO and GEO for initial deployments. Releases 18 and 19 maintained this wording. The evolution shows a progression towards more precise terminology and a narrowing of supported orbits in the normative text, although MEO may still be considered in other contexts.

Purpose & Motivation

The satellite concept in 3GPP exists to extend cellular network coverage to areas where terrestrial infrastructure is impractical or unavailable, such as remote regions, oceans, and during disasters. Satellites provide wide-area coverage and can complement terrestrial networks, ensuring ubiquitous connectivity. The definition and standardization of satellite characteristics enable interoperability and consistent performance across different satellite systems and operators.

Before the introduction of NTN in 3GPP, satellite communication was handled by separate standards bodies and proprietary systems. The integration of satellites into 3GPP networks allows for seamless roaming and service continuity between terrestrial and satellite access. The satellite definition, along with other NTN elements, addresses the need for a unified framework that accommodates the unique challenges of satellite links, such as high latency, Doppler shift, and large propagation delays.

The motivation for specifying different orbit types is to support various use cases and deployment scenarios. LEO satellites are favored for low-latency services, while GEO satellites are suitable for broadcast and fixed services. By defining satellites in 3GPP terms, the industry can develop standardized equipment and procedures, reducing costs and accelerating the adoption of NTN.