Target User Equipment

Description

The Target UE is a fundamental concept in 3GPP location services and sidelink positioning. It represents the user equipment (UE) whose location, distance, or direction is being determined by the network or by other UEs. In traditional cellular positioning, the Target UE is the device being located by the network using techniques such as OTDOA, E-CID, or GNSS. With the introduction of sidelink (PC5) communication in later releases, the Target UE can also be positioned using sidelink signals exchanged with other UEs, known as SL Reference UEs. This enables positioning even when network coverage is limited or unavailable.

The architecture for positioning a Target UE involves several network elements. The Location Management Function (LMF) in 5GC or the Evolved Serving Mobile Location Centre (E-SMLC) in EPC coordinates the positioning procedure. For sidelink positioning, the Target UE and SL Reference UEs exchange sidelink positioning reference signals (SL-PRS) to measure time of arrival, angle of arrival, or round-trip time. These measurements are then used to compute the Target UE's location. The Target UE may also perform measurements on downlink signals from gNBs or eNBs, depending on the positioning method.

Key procedures include the positioning capability transfer, assistance data delivery, and location information transfer. The Target UE may initiate a positioning request (mobile originated location request, MO-LR) or the network may initiate it (mobile terminated location request, MT-LR). In sidelink positioning, the Target UE can act as a transmitter, receiver, or both, depending on the configuration. The SL Reference UEs provide known positions or act as anchors to enable relative or absolute positioning.

The Target UE concept has evolved from simple network-based positioning to include UE-based and UE-assisted positioning, and now sidelink-based positioning. This evolution reflects the growing demand for accurate positioning in diverse environments, including indoor, underground, and emergency scenarios. The Target UE is also a key enabler for new services such as ranging, which measures the distance between UEs, and for advanced applications like autonomous vehicles, industrial IoT, and public safety.

In 3GPP specifications, the Target UE is referenced in numerous documents, including 22.071 (Location Services), 23.271 (Functional stage 2 description of LCS), and 23.586 (Sidelink positioning). The term's definition has been refined over releases to encompass sidelink-based positioning, highlighting its adaptability to new technologies.

Purpose & Motivation

The Target UE concept exists to identify the specific device whose location is being sought, enabling a clear separation between the entity being positioned and the supporting infrastructure. In early releases, positioning was primarily network-centric, with the network measuring signals from the UE. However, as location-based services expanded, the need for more flexible and accurate positioning methods grew. The Target UE designation allows the network to apply appropriate positioning procedures, manage privacy and security, and deliver location information to authorized clients.

With the advent of sidelink communication, the limitations of network-only positioning became apparent, especially in coverage holes or indoor environments. The Target UE concept was extended to support sidelink positioning, where other UEs (SL Reference UEs) assist in determining the Target UE's location. This addresses scenarios where GNSS is unavailable or unreliable, and where network infrastructure is insufficient. The Target UE thus becomes a central element in a cooperative positioning ecosystem, improving accuracy, availability, and latency.

Historically, the term evolved from "the UE being positioned" in Rel-4 to a more precise definition in Rel-19 that explicitly includes sidelink and ranging. This reflects the ongoing innovation in positioning technologies and the need to standardize terminology across diverse use cases.