Satellite Gateway

Description

A satellite-gateway, also referred to as an earth station or gateway, is a critical component of a Non-Terrestrial Network (NTN). It is located on the surface of the Earth and is responsible for providing the necessary radio frequency (RF) power and sensitivity to communicate with the satellite. The gateway acts as the interface between the satellite and the terrestrial core network (e.g., 5G Core or EPC). It transmits and receives signals to and from the satellite over the feeder link, which is the radio link between the gateway and the satellite. The satellite then relays these signals to and from User Equipment (UE) over the service link. This architecture allows UEs to access the core network services via the satellite, extending coverage to areas without terrestrial infrastructure.

The satellite-gateway typically consists of a large antenna (often a parabolic dish) with high gain, high-power amplifiers, and low-noise receivers to overcome the significant path loss and atmospheric attenuation associated with satellite links. The gateway must be able to track the satellite if it is in a non-geostationary orbit (e.g., LEO or MEO), which requires precise pointing and tracking mechanisms. For GEO satellites, the gateway can be fixed since the satellite appears stationary. The gateway also includes baseband processing equipment to interface with the core network, performing functions such as modulation/demodulation, coding/decoding, and protocol conversion.

In the 3GPP NTN architecture, the satellite-gateway is connected to the core network via standard interfaces, such as the N1/N2/N3 interfaces for 5G or S1 interfaces for LTE. The gateway may also host some network functions, such as the gNB (for NR NTN) or eNB (for LTE NTN), depending on the architecture. In a transparent payload scenario, the satellite acts as a repeater, and the gateway contains the full base station functionality. In a regenerative payload scenario, the satellite may host the base station, and the gateway serves as a relay to the core network. The 3GPP specifications define the requirements and procedures for the gateway to ensure interoperability and performance.

The definition of satellite-gateway was introduced in Release 17, the first release to specify NTN in detail. The relevant specifications are 36.108 (LTE NTN) and 38.108 (NR NTN). The definition states: 'An earth station or gateway is located at the surface of Earth, and providing sufficient RF power and RF sensitivity for accessing to the satellite.' This definition has remained unchanged in Releases 18 and 19, indicating stability in the concept. The gateway is essential for connecting the satellite network to the terrestrial network, and its performance directly affects the overall quality of service experienced by users.

Key aspects of the satellite-gateway include its RF capabilities, which must be sufficient to close the link with the satellite. This involves careful link budget analysis, considering factors such as satellite altitude, frequency band, antenna gain, and atmospheric losses. The gateway also needs to support the necessary protocols and interfaces to integrate with the core network, including security functions, mobility management, and QoS enforcement. In multi-gateway deployments, the network may support gateway diversity or handover between gateways to ensure continuous service as the satellite moves. The 3GPP specifications provide the framework for these operations, ensuring that NTN can be seamlessly integrated with terrestrial networks.

Purpose & Motivation

The satellite-gateway exists to bridge the satellite network and the terrestrial core network, enabling UEs to access cellular services via satellite. It solves the problem of connecting remote or mobile satellite terminals to the core network, which is essential for providing ubiquitous coverage. Without a gateway, the satellite would be isolated and unable to connect to terrestrial networks, limiting its usefulness to isolated communication.

Before the standardization of NTN in 3GPP, satellite systems used proprietary gateways and interfaces, which hindered interoperability and increased costs. The 3GPP definition of satellite-gateway provides a standardized functional entity that can be implemented by multiple vendors, promoting competition and reducing costs. It also ensures that the gateway meets specific RF performance requirements, which are critical for reliable communication over the challenging satellite channel.

The motivation for defining the satellite-gateway in 3GPP was to integrate satellite access into the existing cellular ecosystem, allowing operators to extend their coverage using satellites while leveraging their core network infrastructure. This integration enables new use cases such as rural broadband, maritime and aeronautical connectivity, and disaster recovery. The gateway is a key enabler for these applications, and its standardization ensures that different satellite systems can interoperate with 3GPP networks.