Ranging (Sidelink Distance and Direction Determination)

Description

Ranging is a Rel-19 feature that enables User Equipment (UE) to determine the distance between two or more UEs and/or the direction of one UE relative to another using the sidelink interface (PC5). Unlike traditional cellular positioning, which relies on base station measurements and core network location services, ranging operates directly between devices, making it a device-to-device (D2D) proximity and positioning capability. It is specified in the context of 5G service requirements (TS 22.261) and the overall positioning architecture (TS 38.305).

The fundamental principle of ranging is to exploit the characteristics of sidelink radio signals exchanged between UEs. By measuring parameters such as time of arrival, round-trip time, angle of arrival, or signal strength of sidelink transmissions, a UE can estimate the geometric relationship to another UE. The distance determination typically uses timing-based methods: the initiating UE sends a ranging signal, the responding UE replies, and the initiator measures the round-trip time to compute distance. Direction determination may use angle-of-arrival estimation with antenna arrays or beamforming information available at the sidelink receiver.

Ranging is closely related to sidelink positioning, which 3GPP has been developing under the umbrella of NR positioning enhancements. The sidelink interface (PC5) supports both unicast and groupcast communication, and ranging can be performed in either mode. In unicast, two UEs establish a direct link and exchange ranging signals. In groupcast, multiple UEs can participate, allowing one UE to determine its distance and direction relative to several peers simultaneously. This is particularly useful for applications such as public safety, where first responders need to know the relative positions of team members in environments without reliable GNSS or cellular coverage.

The service requirements for ranging are captured in TS 22.261, which defines use cases including proximity-based services, asset tracking, and relative positioning for industrial and vehicular applications. The accuracy requirements vary by use case, ranging from coarse proximity detection (a few metres) to fine-grained distance and direction estimation (sub-metre). Ranging can operate with or without network assistance: in network-assisted mode, the gNB provides configuration and scheduling for sidelink resources; in standalone mode, UEs autonomously discover each other and perform ranging.

Ranging is a key enabler for advanced 5G and beyond applications. It supports relative positioning in scenarios where absolute positioning (e.g., GNSS) is unavailable or insufficient, such as indoors, underground, or in dense urban canyons. It also underpins new services like augmented reality collaboration, where devices need to know their spatial relationship to each other, and vehicle-to-everything (V2X) platooning, where vehicles must maintain precise inter-vehicle distances. As 3GPP moves toward 6G, ranging is expected to become a foundational capability for cooperative and context-aware applications.

Purpose & Motivation

Ranging was introduced to address the growing need for direct device-to-device distance and direction determination that does not depend on cellular infrastructure or GNSS. Traditional positioning methods in 3GPP networks rely on base stations (e.g., OTDOA, E-CID) or satellite systems (GNSS), both of which have limitations: cellular positioning requires network coverage and may not work indoors or in isolated areas, while GNSS is unavailable in many indoor, underground, and obstructed environments. Ranging fills this gap by using the sidelink interface to measure relative position directly between UEs.

The motivation for ranging comes from new use cases in 5G-Advanced and beyond. Public safety and emergency response require knowing the relative positions of team members in buildings or tunnels where GNSS fails. Industrial IoT needs precise relative positioning of robots, tools, and assets on a factory floor. Vehicular applications such as platooning and cooperative manoeuvring require vehicles to know their distance and direction relative to each other with high accuracy and low latency. Consumer applications like augmented reality and social proximity services also benefit from device-to-device ranging.

By standardising ranging in Rel-19, 3GPP provides a common framework that ensures interoperability across vendors and devices. It leverages the existing sidelink (PC5) interface, which was already developed for V2X and proximity services, thereby reusing spectrum, protocols, and hardware. This makes ranging a cost-effective enhancement rather than a completely new radio technology. The specification work in TS 22.261 and TS 38.305 ensures that ranging requirements are aligned with overall 5G service and positioning architectures.