Located User Equipment

Description

A Located UE is a User Equipment (UE) that acts as a Sidelink (SL) Reference UE within the 5G positioning architecture. Its defining characteristic is that its own location is either already known or can be determined using Uu-based positioning methods—that is, positioning techniques that rely on the cellular radio interface between the UE and the Radio Access Network (gNB). Because its position is established with high confidence, the Located UE can serve as a spatial anchor for determining the location of other devices, specifically a Target UE, through Sidelink Positioning procedures.

Sidelink Positioning is a Rel-19 feature that leverages direct device-to-device (PC5) communication to perform ranging and positioning between UEs without necessarily involving the network in every measurement. In this framework, the Located UE transmits and/or receives sidelink positioning reference signals (SL-PRS) to and from the Target UE. By measuring the time of flight, angle of arrival, or other signal characteristics of these sidelink transmissions, and by knowing its own absolute position, the Located UE enables the computation of the Target UE's position. This can be done either at the Target UE itself, at the Located UE, or at a location management function (LMF) in the core network, depending on the positioning mode.

The concept addresses a fundamental requirement of any positioning system: the need for reference points with known coordinates. In conventional Uu-based positioning (e.g., OTDOA, NR positioning), the reference points are gNBs whose locations are known to the network. However, in scenarios where GNSS is unavailable (indoors, underground, urban canyons) and where Uu coverage is limited or insufficient for accurate positioning, sidelink-based positioning using Located UEs provides an alternative. A Located UE can be, for example, a device mounted at a known fixed point, a UE that has recently obtained a high-accuracy position fix via Uu or GNSS, or a UE whose position has been determined through a combination of Uu measurements and network assistance data.

The architecture supporting Located UEs involves several network functions. The LMF (Location Management Function) coordinates positioning procedures and may provide assistance data. The AMF (Access and Mobility Management Function) handles mobility and authorization aspects. The UE itself must support sidelink communication (PC5 interface) and the SL positioning protocol. Specifications 23.273 (5G System Location Services) and 23.586 (Architecture enhancements for sidelink positioning) define the procedures, signaling flows, and requirements for how a Located UE is identified, how its location is validated, and how it participates in sidelink positioning sessions.

Key operational aspects include the selection of an appropriate Located UE (one whose position accuracy and reliability meet the required service level), the exchange of positioning capability information, and the security and privacy considerations around revealing a UE's location to other devices. The Located UE concept is particularly relevant for public safety, industrial IoT, and vehicular use cases where precise relative and absolute positioning among devices is critical and where infrastructure-based positioning alone is insufficient.

Purpose & Motivation

The Located UE concept exists to solve a critical gap in 5G positioning: the need for reliable, high-accuracy reference points when GNSS is unavailable and Uu-based infrastructure positioning is degraded or insufficient. Traditional cellular positioning relies on base stations as anchors, but indoors, underground, and in dense urban environments, Uu signals may be too weak or too few to achieve the required accuracy. Sidelink positioning offers an alternative by using nearby devices as anchors, but those devices must themselves have known positions—hence the Located UE.

Before Rel-19, sidelink communication was primarily used for V2X (vehicle-to-everything) and public safety, with limited positioning support. Positioning was almost exclusively Uu-based, requiring network infrastructure. This created a dependency on base station density and GNSS availability. The Located UE concept, introduced in Rel-19 as part of the sidelink positioning architecture, removes this dependency by allowing any UE with a known position—whether obtained via Uu, GNSS, or a hybrid method—to act as a reference for others. This is especially valuable for emergency services (locating a person inside a building), industrial automation (tracking assets in a factory), and connected vehicles (relative positioning in tunnels or parking structures).

The motivation also includes regulatory requirements for emergency call positioning (E911/NG911) and the growing demand for high-accuracy positioning in commercial applications. By enabling device-to-device positioning with Located UEs, 3GPP provides a flexible, scalable solution that complements existing Uu-based methods and extends positioning coverage to environments where traditional approaches fail.