Medium-Earth Orbiting satellites

Description

Medium-Earth Orbiting (MEO) satellites are a critical element within 3GPP's Non-Terrestrial Network (NTN) architecture, designed to extend 5G and future cellular services globally. They operate in orbits ranging from approximately 2,000 kilometers to just below the geostationary belt at 35,786 kilometers. This orbital altitude represents a strategic compromise, offering a significantly wider coverage footprint per satellite compared to Low-Earth Orbit (LEO) constellations while providing much lower signal latency and reduced path loss compared to Geostationary Earth Orbit (GEO) satellites. Within the 3GPP system, MEO satellites are integrated as either transparent (bent-pipe) or regenerative (on-board processing) payloads. As transparent payloads, they simply amplify and convert frequencies between the gateway earth station (known as the NTN Gateway) and the User Equipment (UE). Regenerative payloads, which are more complex, can demodulate, decode, switch, and re-encode signals on-board, effectively functioning as base stations in the sky (gNodeBs).

The integration of MEO satellites into the 3GPP radio access network (RAN) requires specific adaptations to handle the unique characteristics of satellite links. Key technical challenges addressed in the specifications include very long propagation delays (ranging from tens to over a hundred milliseconds), high Doppler shifts due to satellite motion relative to the ground, and significant path loss. The 3GPP RAN and Core Network protocols are enhanced to support these conditions. For instance, timing advance procedures are modified, hybrid automatic repeat request (HARQ) processes are adapted or disabled for very long delay scenarios, and scheduling is adjusted. The core network, via the Access and Mobility Management Function (AMF) and Session Management Function (SMF), must also be aware of the serving satellite's characteristics for proper mobility and session management, especially as UEs are handed over between satellite beams or between satellites.

MEO satellites play a pivotal role in achieving true global and seamless connectivity. They are a foundational technology for 5G NTN, enabling service continuity for terrestrial networks and providing direct-to-device or backhaul services. Architecturally, they connect to ground-based NTN Gateways, which then interface with the standard 5G Core Network (5GC). This allows MEO constellations to provide 3GPP-defined services like enhanced Mobile Broadband (eMBB) and massive IoT (mIoT) in regions without terrestrial infrastructure. The specifications covering MEO span service requirements (22-series), system architecture (23-series), protocols (24-, 25-, 36-, 38-series), and management (28-, 32-series), ensuring a comprehensive framework for satellite-integrated cellular networks.

Purpose & Motivation

The standardization of MEO satellites within 3GPP was motivated by the need to extend high-quality mobile broadband and IoT services to unserved and underserved regions globally. Traditional terrestrial networks are economically challenging to deploy in remote areas, over oceans, and in airspace. Prior to 3GPP's formal integration, satellite communication existed as a separate, often proprietary ecosystem with limited interoperability with cellular devices and services. The purpose of defining MEO in 3GPP standards is to bridge this gap, creating a unified network architecture where satellites are a native, integrated component rather than an external add-on.

This integration solves critical problems of coverage, resilience, and service continuity. It addresses the coverage gap problem by providing a scalable solution for global connectivity, which is essential for applications like maritime and aeronautical communications, disaster response, and rural broadband. Furthermore, it enhances network resilience by providing an alternative backhaul or access layer during terrestrial network failures caused by natural disasters. From a service perspective, it enables seamless mobility for users traveling between terrestrial and satellite coverage areas, a concept fundamental to the 3GPP vision of ubiquitous connectivity.

The creation of MEO standards was historically driven by the convergence of the satellite and telecom industries and the launch of new MEO constellations (like O3b and its successors). 3GPP Release 15 began the serious study of Non-Terrestrial Networks, with subsequent releases refining the architecture and protocols. MEO's specific orbital characteristics offered a viable middle-ground for early 5G NTN deployments, balancing the constellation size (number of satellites needed) against latency and ground station complexity, making it a strategically important component of the overall NTN portfolio defined by 3GPP.