Feeder Link

Description

The feeder link is a fundamental component of Non-Terrestrial Networks (NTN) in 3GPP, defined as the wireless transport link between the NTN Gateway and the NTN payload. The NTN payload refers to the space-borne or airborne platform, such as a satellite (LEO, MEO, GEO) or a High Altitude Platform Station (HAPS), that provides radio access to user equipment (UE). The NTN Gateway is a ground-based station that connects the NTN payload to the terrestrial core network. The feeder link carries the aggregated traffic from all UEs served by the NTN payload to the gateway, and vice versa, effectively acting as the backhaul for the satellite access network.

In the NTN architecture, the feeder link operates over a radio interface that is distinct from the service link, which connects the UE to the NTN payload. The feeder link typically uses high-gain directional antennas on both the gateway and the payload to overcome the large path loss associated with the long distance and atmospheric attenuation. The frequency bands allocated for feeder links are often in the Ka, Ku, or V bands, depending on the regulatory and operational requirements. The feeder link must support high capacity and availability, as it aggregates traffic from many users and is a single point of failure for the entire satellite beam.

The feeder link is specified in various 3GPP documents, including TS 23.401 (GPRS enhancements for E-UTRAN), TS 23.501 (5G system architecture), TS 33.401 (security architecture), TS 36.108 (satellite access in E-UTRAN), TS 36.300 (E-UTRA overall description), and TS 38.108 (satellite access in NR). These specifications define the protocols, procedures, and interfaces for establishing and managing the feeder link, including aspects such as link adaptation, handover between gateways, and security. The feeder link is transparent to the UE, which communicates with the NTN payload via the service link; the payload then relays the traffic to the gateway over the feeder link.

One of the key challenges in NTN is the management of the feeder link, especially for LEO satellites that move rapidly relative to the Earth. This requires frequent handovers between gateways to maintain continuous connectivity. 3GPP has defined procedures for feeder link switching, where the satellite establishes a new feeder link with a different gateway before the old link becomes unavailable. This ensures seamless service continuity. Additionally, the feeder link must handle Doppler shift and propagation delay, which are more pronounced in NTN than in terrestrial networks.

The feeder link is also important for network synchronization and timing. The NTN payload may rely on the feeder link to receive timing references from the terrestrial network, which are then used to synchronize the service link. This is crucial for maintaining frame timing and ensuring that UEs can access the network correctly. Overall, the feeder link is a critical enabler for extending 5G and future networks to areas without terrestrial coverage, such as remote regions, oceans, and skies.

Purpose & Motivation

The feeder link exists to provide connectivity between the non-terrestrial network payload (e.g., satellite) and the terrestrial core network. Without it, satellite-based access networks would be isolated and unable to integrate with global telecommunications infrastructure. The feeder link solves the problem of backhauling traffic from a moving or remote platform to the ground, enabling mobile operators to extend coverage to areas where deploying terrestrial base stations is impractical or cost-prohibitive.

Historically, satellite communication systems used proprietary feeder links that were not standardized for interoperability with cellular networks. With the advent of 5G NTN in 3GPP Release 17, the feeder link was formally defined and integrated into the 3GPP architecture. This allows satellite operators to use standardized interfaces and protocols, reducing costs and enabling seamless integration with 5G core networks. The feeder link is essential for providing ubiquitous coverage, disaster recovery, and maritime and aeronautical connectivity.

The motivation for standardizing the feeder link in 3GPP was to ensure that NTN can be treated as just another radio access technology within the 5G ecosystem. This required defining the feeder link in terms of its functions, interfaces, and performance requirements, so that it can support the same services and QoS as terrestrial networks. The feeder link continues to evolve, with ongoing work in Releases 18, 19, and 20 to improve capacity, reduce latency, and support new frequency bands.