Satellite Access Node Type 2-O

Description

SAN type 2-O is a classification of Satellite Access Node (SAN) defined in 3GPP Release 19 for 5G non-terrestrial networks (NTN). A SAN is the satellite-borne equivalent of a terrestrial gNB, providing NR radio access to UEs. SAN type 2-O specifically operates at FR2-NTN (Frequency Range 2 for NTN, typically millimeter-wave bands) and is characterized by a requirement set consisting only of OTA (Over-The-Air) requirements defined at the RIB (Radiated Interface Boundary).

The RIB is a conceptual reference point used in 3GPP specifications to define radiated requirements for active antenna systems (AAS). Unlike conducted requirements, which are defined at a physical connector, OTA requirements are defined at the RIB, which represents the boundary between the antenna and free space. This means that SAN type 2-O performance is specified in terms of radiated power, radiated sensitivity, and other over-the-air parameters, rather than conducted parameters. This is particularly important for satellite systems, where the antenna is integrated with the payload and cannot be tested via conducted connections.

Architecturally, SAN type 2-O is part of the NTN radio access network (RAN). It communicates with UEs via the service link (the radio interface between satellite and UE) and with the ground network via the feeder link (the radio interface between satellite and ground station) or inter-satellite links (ISL). The SAN may be transparent (amplify-and-forward) or regenerative (decode-and-forward), but type 2-O specifically refers to a regenerative payload with OTA requirements. The FR2-NTN designation indicates operation in frequency bands above 24 GHz, which offer wider bandwidth but suffer from higher atmospheric attenuation and require precise beam pointing.

The classification of SAN types (e.g., type 1-O, type 2-O) is used in 3GPP specifications to group requirements for different satellite architectures. Type 2-O is distinguished by its exclusive use of OTA requirements at the RIB, which simplifies testing and ensures that performance is characterized in a realistic operational environment. This is in contrast to types that may include conducted requirements or a mix of both. The "O" in type 2-O likely stands for "OTA" or "Over-the-air," while "2" may denote the second category or FR2 operation.

SAN type 2-O is specified in 3GPP TS 38.108 (NR Satellite Access Node radio transmission and reception) and TS 38.181 (Satellite Access Node conformance testing). These specifications define the RF requirements, including output power, frequency stability, unwanted emissions, and receiver sensitivity, all expressed as OTA requirements at the RIB. The specifications also cover the test methods and procedures for verifying compliance, which involve radiated measurements in an anechoic chamber or compact antenna test range.

In the broader 5G NTN context, SAN type 2-O enables high-capacity satellite links for broadband services, backhaul, and fixed wireless access. Its OTA-only requirement set reflects the trend towards integrated active antenna systems in both terrestrial and non-terrestrial networks, where the antenna is an inseparable part of the radio unit and conducted testing is impractical. This approach ensures that satellite manufacturers and operators can demonstrate compliance based on radiated performance, which is the ultimate measure of system capability.

Purpose & Motivation

SAN type 2-O was created to standardize the radio performance requirements for a specific class of satellite access nodes operating in FR2-NTN bands. Prior to Release 19, 3GPP NTN specifications primarily focused on FR1 (sub-6 GHz) bands and transparent payloads, with limited provisions for millimeter-wave satellite operation. As satellite constellations began to explore FR2 bands for higher capacity, the need arose for a clear definition of requirements that could be applied consistently across different satellite designs.

The key problem it solves is the lack of a standardized framework for OTA-only requirements in satellite systems. Terrestrial base stations often have conducted requirements defined at antenna ports, but satellite payloads integrate the antenna with the radio, making conducted testing impossible. By defining SAN type 2-O with OTA requirements at the RIB, 3GPP provides a testable and unambiguous specification that accounts for the integrated nature of satellite antennas. This facilitates interoperability, reduces development costs, and enables fair comparison of different satellite systems.

Historically, satellite communication standards (e.g., DVB-S2X, 5G NTN in 3GPP Release 17) have used OTA requirements for some parameters, but Release 19 is the first to define a complete requirement set exclusively at the RIB for FR2-NTN. This reflects the maturation of millimeter-wave satellite technology and the need for a dedicated SAN type that addresses the unique challenges of high-frequency satellite links, such as beam pointing accuracy, phase noise, and atmospheric effects. The motivation was to support emerging NGSO (non-geostationary orbit) constellations in FR2 bands and to ensure that 5G NTN can deliver fiber-like speeds from space.