Ranging and Sidelink Positioning Protocol

Description

The Ranging and Sidelink Positioning Protocol (RSPP) is a protocol layer specified in 3GPP for performing ranging and positioning operations over the sidelink interface. Sidelink refers to direct communication between User Equipments (UEs) without routing data through the network infrastructure (gNB or eNB). RSPP operates within the sidelink protocol stack, interacting with layers like the Sidelink Radio Link Control (SL-RLC) and Sidelink Medium Access Control (SL-MAC). Its primary function is to facilitate the exchange of signals and messages between UEs to estimate the distance (ranging) and determine relative or absolute positions (positioning).

RSPP works by defining procedures for measurement and reporting. UEs exchange specific reference signals or packets designed for time-of-arrival (ToA), time-difference-of-arrival (TDoA), or angle-of-arrival (AoA) measurements. The protocol manages the scheduling of these signals, the collection of measurement data, and the calculation of range or position. It supports both one-way and two-way ranging techniques. In two-way ranging, for example, UE A sends a ranging request, UE B responds, and UE A calculates the round-trip time to estimate distance, with RSPP handling the message sequence and timing. The protocol may also support multi-lateration where multiple UEs collaborate to determine a target UE's location.

Key components of RSPP include the definition of sidelink positioning reference signals (SL-PRS), control messages for initiating and coordinating ranging sessions, and measurement reports. It interfaces with higher-layer positioning services and applications. RSPP's role is critical in scenarios where GNSS signals are unavailable or unreliable, such as indoors, in urban canyons, or during emergencies. By leveraging direct UE-to-UE communication, it enables decentralized positioning solutions that are resilient, low-latency, and can operate with partial or no network coverage, enhancing services like vehicle platooning, pedestrian safety, and asset tracking.

Purpose & Motivation

RSPP was introduced in 3GPP Release 18 to address the need for accurate, infrastructure-independent positioning in sidelink communication scenarios. Prior positioning methods in cellular networks primarily relied on uplink/downlink signals between UEs and base stations (e.g., OTDOA, UTDOA) or GNSS. These methods have limitations in environments without network coverage or where GNSS is denied. The proliferation of V2X (Vehicle-to-Everything) and advanced sidelink applications created a demand for relative positioning between nearby devices to support collision avoidance, cooperative driving, and formation keeping.

The protocol solves the problem of standardizing how UEs directly measure distances and positions between each other over the PC5 interface. Before RSPP, proprietary or non-standardized methods could lead to interoperability issues. RSPP provides a unified, 3GPP-specified framework, ensuring that UEs from different manufacturers can perform ranging and positioning reliably. It was motivated by use cases in autonomous vehicles, public safety (e.g., locating first responders in a building), and commercial applications like social networking or augmented reality games requiring proximity detection. By integrating positioning capabilities directly into the sidelink protocol stack, RSPP enhances the autonomy and functionality of device-to-device communication systems.