User Equipment to Satellite to User Equipment Communication

Description

UE-Satellite-UE Communication refers to a mode of communication where two or more User Equipments (UEs) exchange data through one or more serving satellites, with the user traffic not passing through the ground segment (i.e., not traversing terrestrial gateways or core network nodes). This is a form of direct device-to-device communication via satellite, where the satellite acts as a relay or switching node in space. The concept is part of the broader Non-Terrestrial Network (NTN) integration in 3GPP, aiming to extend coverage to remote or underserved areas and provide resilient communication paths.

In this architecture, the satellite (or a constellation of satellites) provides the radio access link to the UEs and may also perform routing or forwarding of user data between UEs. The communication can be established between UEs under the coverage of the same satellite or different satellites, potentially involving inter-satellite links (ISLs). The key characteristic is that the user plane traffic does not need to be routed through a ground station, which reduces latency and backhaul requirements. This is particularly beneficial for scenarios where terrestrial infrastructure is unavailable or damaged, such as in disaster relief, maritime, or remote industrial operations.

The technical implementation involves the satellite acting as a base station (gNB) or relay node, with the UEs establishing radio links to the satellite using NR NTN radio interfaces. The satellite may be transparent (bent-pipe) or regenerative (processing onboard). For UE-Satellite-UE communication without ground segment transit, the satellite must have onboard processing and switching capabilities to route traffic between UEs. The 3GPP specifications 23.700 (study on NTN), 28.874 (study on satellite backhaul), and 28.894 (study on satellite access) provide the architectural and procedural foundations.

From a protocol perspective, the communication may use a direct radio link between UEs and the satellite, with the satellite forwarding packets at the link layer or higher. The UEs may be synchronized to the satellite's timing and use the satellite's broadcast channels for initial access. Mobility management, security, and QoS are handled by the network, potentially with distributed functions on the satellite or via a ground-based core network that is not in the user data path. The challenge is to ensure seamless service continuity as satellites move and UEs may handover between satellites.

UE-Satellite-UE communication is expected to enable new applications such as direct satellite messaging, emergency communications, and IoT connectivity in remote areas. It complements terrestrial networks by providing ubiquitous coverage and resilience. The evolution in Release 20 continues to refine the architecture and protocols, with ongoing studies and normative work to support commercial deployment.

Purpose & Motivation

The primary motivation for UE-Satellite-UE communication is to provide direct connectivity between users in areas where terrestrial network infrastructure is absent, unreliable, or congested. Traditional satellite communication typically involves a ground station that connects to the core network, which introduces latency and dependency on ground infrastructure. By allowing user traffic to bypass the ground segment, this technology reduces end-to-end latency, improves resilience, and enables communication in isolated environments.

This capability is particularly valuable for public safety, disaster recovery, and military applications, where ground infrastructure may be damaged or unavailable. It also supports maritime and aeronautical communications, as well as remote industrial and agricultural operations. The 3GPP has been studying NTN integration since Release 15, and UE-Satellite-UE communication is a natural extension of those efforts, leveraging the growing availability of LEO satellite constellations with onboard processing.

The introduction in Release 19 addresses the need for a standardized approach to direct satellite-to-UE communication, ensuring interoperability and efficient use of satellite resources. It builds on earlier NTN work but adds the specific capability of routing user traffic between UEs without ground segment involvement. This is a step towards a fully integrated space-terrestrial network where satellites act as access and switching nodes, providing seamless global coverage.