Sidelink Relative Time of Arrival

Description

Sidelink Relative Time of Arrival (SL-RTOA) is a positioning technique defined within the 3GPP framework for New Radio (NR) and LTE sidelink communications. It operates within the PC5 interface, the direct communication link between user equipments (UEs). The method is fundamentally a time-difference-of-arrival (TDOA) technique applied to the sidelink domain. It requires at least three participating UEs: a reference UE, a target UE whose position is to be determined relative to the reference, and one or more assisting UEs. The reference UE transmits a positioning reference signal (PRS) via the sidelink. Both the target UE and the assisting UE(s) receive this signal and measure its time of arrival. The target UE also transmits its own PRS, which is received and measured by the assisting UE(s). The core measurement is the difference between the time the assisting UE receives the PRS from the target UE and the time it receives the PRS from the reference UE. This time difference, when measured by multiple assisting UEs at known relative positions, allows for the calculation of the target UE's position relative to the reference UE through hyperbolic multilateration. The architecture involves specific physical layer signals (SL-PRS) defined in the relevant specifications, measurement procedures managed by the UE's protocol stack (RRC and MAC layers), and reporting mechanisms to either a location management function in the network or for direct use in peer-to-peer applications. Its role is to provide a foundational, infrastructure-independent relative positioning capability for advanced V2X, public safety, and commercial D2D services.

Purpose & Motivation

SL-RTOA was introduced to address the growing need for precise relative positioning in scenarios where absolute global positioning (e.g., GNSS) is unavailable, unreliable, or insufficiently accurate. Applications like platooning, cooperative collision avoidance, and augmented reality gaming require devices to know their positions relative to each other with centimeter- to decimeter-level accuracy, often in GNSS-denied environments like tunnels, urban canyons, or indoors. Traditional network-based positioning methods like UTDOA or E-CID rely on cellular infrastructure, which may not be present or may introduce latency unsuitable for safety-critical direct communication. SL-RTOA leverages the existing sidelink communication link itself to perform positioning, creating a self-contained relative positioning system among a group of devices. This solves the problem of enabling high-accuracy, low-latency relative localization purely through device-to-device technology, a key enabler for autonomous coordination and enhanced situational awareness in 3GPP-based V2X and Proximity Services (ProSe).