Sidelink Positioning Reference Signal

Description

The Sidelink Positioning Reference Signal (SL-PRS) is a specific sequence of symbols transmitted on the physical sidelink shared channel (PSSCH) or sidelink control channel (PSCCH) to facilitate positioning measurements between devices. Unlike downlink or uplink PRS which involve base station (gNB) coordination, SL-PRS is designed for direct device-to-device (D2D) scenarios as part of the 3GPP sidelink (SL) interface, crucial for Vehicle-to-Everything (V2X), public safety, and commercial proximity services. Its physical layer structure involves specific resource element mapping within sidelink resource blocks, following patterns configured by higher layers to avoid collision with other sidelink signals and data.

The signal is generated using pseudo-random sequences, often Gold sequences, which are derived from parameters like the transmitting UE's identity, a slot index, and a cyclic prefix type. This design ensures good auto-correlation and cross-correlation properties, allowing a receiving UE to detect the signal accurately even in noisy environments and distinguish between SL-PRS from different transmitting UEs. The configuration of SL-PRS—including its time-domain periodicity, frequency-domain comb structure, bandwidth, and muting patterns—is signaled via sidelink control information (SCI) or pre-configured in the UEs, enabling flexible adaptation to different service requirements (e.g., high-accuracy positioning for autonomous driving vs. lower-accuracy for pedestrian proximity).

In operation, a UE transmits SL-PRS in its allocated sidelink resources. Neighboring UEs receive these signals and perform measurements such as Time of Arrival (TOA), Reference Signal Time Difference (RSTD), or Angle of Arrival (AoA). These raw measurements can be used directly for relative positioning between UEs or reported to a positioning server (e.g., Location Management Function - LMF) in the network for absolute positioning calculation using techniques like multilateration. The integration of SL-PRS with other positioning signals (e.g., GNSS, downlink PRS) enables hybrid positioning, enhancing accuracy and availability, especially in GNSS-denied environments like urban canyons or tunnels. This makes SL-PRS a cornerstone for advanced V2X applications requiring precise relative localization, such as platooning, collision avoidance, and cooperative perception.

Purpose & Motivation

The SL-PRS was introduced to address the growing need for accurate and reliable device-to-device positioning, driven primarily by the evolution of V2X services and new commercial D2D applications in 5G and beyond. Traditional cellular positioning methods (e.g., Observed Time Difference of Arrival - OTDOA using downlink PRS) rely on base station infrastructure, which may be unavailable, insufficiently dense, or geometrically poor for relative positioning between moving vehicles or devices in close proximity. This limitation becomes critical for safety-related V2X applications like autonomous driving, where knowing the precise relative location of nearby vehicles is essential.

SL-PRS solves this by enabling direct ranging and positioning between UEs without always requiring network infrastructure. It allows vehicles or devices to determine their relative positions independently, supporting decentralized and robust operation. Its creation was motivated by 3GPP's work on enhanced V2X (eV2X) in Release 16 and later, which defined requirements for sub-meter-level accuracy and high reliability. By providing a standardized, sidelink-specific reference signal for positioning, 3GPP enabled interoperable, high-performance D2D positioning that complements network-based and satellite-based methods, forming a key enabler for the safety and automation use cases envisioned for 5G Advanced and 6G.