Satellite Access Node

Description

A Satellite Access Node (SAN) is a fundamental component of 3GPP-defined Non-Terrestrial Networks (NTNs), introduced to formalize the integration of satellite-based access into the 5G system. Conceptually, a SAN is the satellite-borne entity that implements the radio interface towards the User Equipment (UE). Its specific architecture and functional split can vary, leading to two primary implementations: the 'gNB-on-satellite' (regenerative) and the 'satellite as a relay' (transparent) models.

In the regenerative model, the SAN incorporates the full gNB (Next Generation NodeB) protocol stack, including the Radio Resource Control (RRC), Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers. In this architecture, the satellite acts as a full-fledged base station in space. It demodulates, decodes, processes, and re-encodes the uplink signals before transmitting them down to a ground-based gateway or core network. This reduces latency on the satellite-to-ground feeder link but requires more processing power on the satellite.

In the transparent (or bent-pipe) model, the SAN functions primarily as a radio frequency (RF) relay. It receives the RF signal from UEs on the uplink, performs frequency conversion and amplification, and retransmits it towards a ground-based gNB. In this case, the gNB is located on Earth, and the SAN does not process the baseband protocols. The SAN's characteristics, such as its orbit (Geostationary Earth Orbit - GEO, Medium Earth Orbit - MEO, Low Earth Orbit - LEO), beam footprint, and propagation delay, are critical parameters that the 5G NR air interface and protocols must adapt to. Key specifications like TS 38.108 define the radio transmission and reception requirements for SANs, while performance requirements are covered in specs like TS 38.101 and TS 38.521.

Purpose & Motivation

The standardization of the SAN addresses the growing need to extend 5G coverage to underserved and unserved areas where terrestrial infrastructure is economically or geographically challenging to deploy, such as oceans, deserts, and remote rural regions. Prior to 3GPP's work on NTN, satellite communication systems operated largely independently of cellular standards, requiring dual-mode devices and lacking seamless service integration. The SAN concept provides a standardized framework for satellite access to be a native part of the 3GPP ecosystem.

Its creation, prominently in Release 17, was driven by global initiatives for ubiquitous connectivity and disaster resilience. It solves the problem of access node definition in space by specifying the functional requirements and interfaces, enabling satellite operators to build compliant payloads and device manufacturers to support NTN connectivity. By defining SAN behaviors and the necessary adaptations in the NR protocol stack (e.g., for long delay, high Doppler), 3GPP ensures that a standard UE can connect to a satellite network with minimal modification, fostering a unified market for equipment and services.