Service Link

Description

Service Link is a term introduced in 3GPP Release 17 to describe the wireless connection between a Non-Terrestrial Network (NTN) payload and a User Equipment (UE). In the context of NTN, the payload refers to the spaceborne or airborne platform that carries the communication equipment, such as a satellite (LEO, MEO, GEO) or a High Altitude Platform Station (HAPS). The Service Link is the radio interface that allows the UE to connect directly to the NTN payload, which then relays the signal to the ground-based gateway via a Feeder Link. Together, the Service Link and Feeder Link form the complete NTN communication path.

The Service Link operates over a radio interface that is based on the 3GPP New Radio (NR) or LTE specifications, adapted for the unique characteristics of NTN. These adaptations include handling large propagation delays (especially for GEO satellites), Doppler shifts due to satellite movement, and the need for precise timing advance and frequency compensation. The Service Link can operate in various frequency bands, including those traditionally used for terrestrial cellular (e.g., sub-6 GHz, mmWave) as well as dedicated satellite bands (e.g., L-band, S-band, Ka-band).

In the NTN architecture, the Service Link is established between the UE and the NTN payload, which may be a transparent payload (bent-pipe) or a regenerative payload. In a transparent payload, the satellite simply amplifies and frequency-converts the signal, and the Service Link is essentially an extension of the terrestrial gNB's radio interface. In a regenerative payload, the satellite carries a gNB (or parts of it), and the Service Link terminates at the satellite, which then processes the signal and forwards it to the core network via the Feeder Link. This distinction affects the protocol stack and the handling of the Service Link.

The Service Link is characterized by several unique challenges. The distance between the UE and the satellite can range from hundreds to tens of thousands of kilometers, resulting in propagation delays of several milliseconds to hundreds of milliseconds. For example, a GEO satellite at 35,786 km has a one-way delay of about 120 ms, which is significant compared to terrestrial networks. The Service Link must therefore support extended timing advance and HARQ (Hybrid Automatic Repeat Request) mechanisms to cope with these delays. Additionally, the high relative velocity of LEO satellites (about 7.5 km/s) causes Doppler shifts that can be several tens of kHz, requiring frequency pre-compensation at the UE or the network.

3GPP specifications for NTN, such as TS 38.300 (NR overall description), TS 23.501 (5G system architecture), and TS 36.300 (E-UTRA overall description), define the Service Link and its integration with the terrestrial network. The Service Link enables direct UE-to-satellite communication, which is essential for extending coverage to remote, rural, and maritime areas, as well as for providing resilience in disaster scenarios. It also supports new use cases such as satellite-based IoT, mobile broadband via satellite, and hybrid terrestrial-satellite networks.

The Service Link is a key enabler for the vision of ubiquitous connectivity, where users can stay connected anywhere on Earth. By standardizing the Service Link, 3GPP ensures interoperability between different satellite operators and terrestrial networks, paving the way for seamless integration of NTN into 5G and beyond.

Purpose & Motivation

The Service Link exists to enable direct communication between User Equipment and Non-Terrestrial Network payloads, such as satellites or high-altitude platforms. Before the introduction of NTN in 3GPP, satellite communication systems were largely proprietary and separate from terrestrial cellular networks. Users needed specialized satellite phones and separate subscriptions, and there was no seamless integration with mobile networks. The Service Link concept was created to standardize the radio interface between UEs and NTN payloads, allowing ordinary 3GPP-compliant devices to connect to satellites using the same protocols and procedures as terrestrial networks.

The motivation for the Service Link stems from the need to extend mobile coverage to areas where terrestrial infrastructure is unavailable or uneconomical to deploy. These include remote rural areas, oceans, deserts, and disaster zones. Traditional terrestrial networks cover only about 20% of the Earth's landmass and a small fraction of the oceans. NTN, with the Service Link, can provide coverage to the remaining areas, enabling ubiquitous connectivity. Additionally, the Service Link supports new business models, such as satellite-based IoT for agriculture, environmental monitoring, and logistics, as well as emergency communications when terrestrial networks are damaged.

The Service Link also addresses the limitations of previous satellite systems, which were often incompatible with cellular standards and required dedicated hardware. By integrating NTN into 3GPP specifications, the Service Link allows the reuse of existing cellular chipsets and protocols, reducing costs and accelerating adoption. It also enables advanced features like dual connectivity between terrestrial and satellite networks, ensuring service continuity and improved reliability. The Service Link is thus a cornerstone of the 3GPP's vision for a unified, space-terrestrial network.