Serving satellite

Description

In the context of 3GPP Non-Terrestrial Networks (NTN), a Serving satellite is the satellite that currently provides radio access to a User Equipment (UE). It plays the role of a base station (gNB) or a relay node in the radio access network, offering one or more serving cells to the UE. The serving satellite is responsible for maintaining the radio link, managing radio resources, and handling mobility procedures for the UE. Depending on the orbit type—Low Earth Orbit (LEO), Medium Earth Orbit (MEO), or Geostationary Earth Orbit (GEO)—the satellite may be moving relative to the Earth's surface, which means it can only serve a specific geographic area for a limited period. For LEO satellites, this period can be as short as a few minutes, necessitating frequent handovers to ensure service continuity.

Architecturally, the serving satellite is part of the NTN Radio Access Network (RAN). It communicates with the UE via the Uu interface (or a variant for NTN) and with the ground segment via a feeder link. The ground segment includes the NTN Gateway, which connects the satellite to the 5G core network. The serving satellite may be transparent (bent-pipe) or regenerative. In a transparent architecture, the satellite acts as a repeater, amplifying and frequency-converting the signal between the UE and the gateway. In a regenerative architecture, the satellite carries part of the gNB functionality, such as demodulation and scheduling, reducing latency and improving performance. The serving satellite is selected by the network based on factors like coverage, signal strength, and load. As the satellite moves, the network must perform handover to a new serving satellite to maintain connectivity. This handover can be triggered by the UE, the network, or based on ephemeris data that predicts satellite positions.

Key components of the serving satellite include the payload, which houses the antennas and transceivers; the onboard processing unit (for regenerative satellites); and the inter-satellite links (ISLs) that allow communication between satellites. The serving satellite also supports beamforming to create multiple spot beams, each covering a smaller area on the ground. These beams can be steered electronically or mechanically to track UEs or to compensate for satellite motion. The satellite's coverage area, often called a footprint, moves as the satellite orbits, and the network must manage the allocation of radio resources accordingly.

The role of the serving satellite in the network is to extend 5G coverage to areas where terrestrial infrastructure is unavailable, such as remote regions, oceans, and skies. It enables ubiquitous connectivity and supports use cases like disaster recovery, maritime communication, and aeronautical connectivity. The serving satellite also plays a crucial role in ensuring service continuity during handovers. 3GPP has defined procedures for NTN handover, including both hard and soft handover, to minimize service interruption. The serving satellite may also support features like location-based services, as the satellite can determine the UE's position using timing advance and Doppler shift measurements.

In summary, the serving satellite is a fundamental element of NTN, providing the radio access link to UEs. Its dynamic nature due to orbital motion introduces unique challenges for mobility management, resource allocation, and service continuity, which 3GPP addresses through specific NTN enhancements. As NTN evolves, serving satellites are expected to become more capable, with advanced onboard processing, inter-satellite networking, and integration with terrestrial networks.

Purpose & Motivation

The concept of a serving satellite was introduced to enable 5G New Radio (NR) to support non-terrestrial networks, thereby extending coverage to areas beyond the reach of terrestrial base stations. Before NTN, satellite communication systems were largely proprietary and not integrated with cellular standards, leading to fragmented ecosystems and limited interoperability. The serving satellite concept solves the problem of providing seamless 5G service via satellites by defining a clear role for the satellite as the access node, analogous to a gNB. This allows the reuse of 5G protocols and procedures, reducing complexity and cost.

The motivation for the serving satellite also stems from the need for ubiquitous connectivity in the 5G era. Terrestrial networks cover only about 20% of the Earth's landmass and a small fraction of the oceans. Satellites can fill these coverage gaps, enabling new use cases such as connected cars in remote areas, IoT devices in agriculture, and broadband access on airplanes and ships. The serving satellite is key to achieving the 5G vision of connecting everything, everywhere.

Historically, satellite communication was characterized by high latency, low bandwidth, and expensive terminals. With the advent of LEO constellations, latency has been reduced, and bandwidth has increased. 3GPP NTN aims to integrate these satellites into the 5G ecosystem, making them an integral part of the network. The serving satellite is a central concept in this integration, providing the radio interface to UEs and enabling mobility across satellites.