Sidelink Round Trip Time

Description

Sidelink Round Trip Time (SL-RTT) is a distance measurement and positioning technique standardized for 3GPP sidelink communications. It is a two-way time-of-flight measurement performed directly between two user equipments (UEs) over the PC5 interface. Unlike time-difference-based methods, SL-RTT is a ranging technique that measures the absolute propagation delay between a pair of devices. The procedure involves an initiating UE and a responding UE. The initiating UE transmits a specific ranging signal, often a Sidelink Positioning Reference Signal (SL-PRS), and records the time of departure (T1). The responding UE receives this signal, records its time of arrival, and after a known, fixed processing delay, transmits a response signal back to the initiator. The initiating UE receives this response and records its time of arrival (T4). The round-trip time is calculated as (T4 - T1) minus the known processing delay at the responder. The distance between the two UEs is then derived as (RTT * speed of light) / 2. This method's key advantage is that it does not require tight time synchronization between the two UEs, as the measurement is self-contained within the initiator's clock. The architecture involves precise physical layer signal design for the ranging signals, medium access control (MAC) procedures to coordinate the measurement exchange, and reporting protocols. SL-RTT can be used as a standalone ranging technique between two devices or combined with angle-of-arrival measurements or multiple SL-RTT measurements among several devices to determine relative or absolute positions in a network.

Purpose & Motivation

SL-RTT was developed to provide a simple, robust, and accurate method for direct distance measurement between two sidelink devices. It addresses the need for peer-to-peer ranging in applications like proximity detection, fine-grained relative positioning for collision avoidance, and secure device pairing. Previous methods often required network assistance or highly synchronized clocks, which are not always available or practical in pure device-to-device scenarios, especially for public safety or V2X communications outside network coverage. SL-RTT solves the clock synchronization problem by using a two-way measurement that cancels out the time offset between the two devices' local clocks. This makes it highly suitable for ad-hoc and infrastructure-less deployments. Its creation was motivated by the requirements of advanced V2X services, such as cooperative perception where vehicles need to know the exact distance to nearby vehicles or vulnerable road users, and industrial IoT applications requiring precise asset tracking via direct device communication.

Key Features

• Provides direct distance measurement between two UEs without requiring time synchronization
• Based on two-way time-of-flight (ToF) measurement principle
• Utilizes Sidelink Positioning Reference Signals (SL-PRS) for the ranging exchange
• Inherently compensates for clock offsets between the initiating and responding UEs
• Supports both ranging and positioning when combined with other measurements
• Defined with specific message sequences and timing constraints to ensure accuracy

Evolution Across Releases

Defining Specifications