Description
The Positioning Reference Unit (PRU) is a defined network element within 3GPP's positioning architecture, introduced to enhance location accuracy and reliability. It functions as a terrestrial transmitter of positioning reference signals, analogous to a base station but dedicated or optimized for positioning purposes. PRUs can be deployed as standalone units or integrated into existing network infrastructure like gNBs or ng-eNBs. They broadcast specific downlink signals, such as Positioning Reference Signals (PRS) or other synchronization signals, which are measured by the target UE or by other network nodes (like Location Management Functions) to perform positioning calculations.
The operation of a PRU is managed by network functions like the Location Management Function (LMF) in the 5G Core network. The LMF can configure the PRU's transmission parameters, including signal timing, frequency, and power. The primary positioning methods leveraging PRUs include Downlink Time Difference of Arrival (DL-TDOA) and Round-Trip Time (RTT). In DL-TDOA, the UE measures the time difference of arrival of signals from multiple synchronized PRUs (and/or base stations) to calculate its position. For RTT-based methods, the PRU and UE exchange signals to measure the round-trip propagation time.
Architecturally, the PRU connects to the core network via interfaces defined for positioning services, such as the NLs interface to the LMF. This allows the LMF to control the PRU and collect measurement data. The PRU's role is critical in environments where satellite-based positioning (like GNSS) is unavailable or unreliable, such as indoors, in urban canyons, or underground. By providing a dense, controlled network of terrestrial reference points, PRUs significantly improve positioning availability and accuracy, meeting stringent requirements for emergency services, industrial IoT, and commercial location-based services.
Purpose & Motivation
The PRU was introduced to address the growing demand for highly accurate and reliable positioning services that cannot be met by Global Navigation Satellite Systems (GNSS) alone. GNSS signals are often weak or unavailable indoors, in dense urban environments, and in underground facilities, creating coverage gaps critical for applications like emergency caller location (E911/E112), asset tracking, and autonomous systems. Previous cellular positioning methods, such as Cell-ID or observed time difference of arrival (OTDOA) using standard base stations, often lacked the necessary accuracy or required dense base station deployments not optimized for positioning.
The creation of the PRU as a dedicated positioning node allows for a tailored deployment strategy. Network operators can strategically place PRUs to fill coverage holes and enhance signal geometry specifically for positioning, independent of the macro network coverage plan. This dedicated architecture, standardized in 3GPP Release 17 and enhanced in subsequent releases, provides a scalable and cost-effective way to achieve meter-level or even sub-meter-level positioning accuracy. It solves the problem of providing ubiquitous, high-accuracy positioning as a native network service, which is a fundamental requirement for 5G and beyond verticals such as industrial automation, augmented reality, and public safety.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (234 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the PRU (Positioning Reference Unit) function was introduced, defining it as a unit realized by a UE for positioning purposes. The release established procedures for PRU association and dis-association with an LMF and included the PRU within the architectural reference models for both non-roaming and roaming scenarios for Location Services.
- Correct the reference in ServingNetworkName in AuthenticationInfoRequest TS 29.503CR0158
- Corrections to descriptions, references and SUPI parameter in Discovery Request TS 29.510CR0004
- Correct reference for Event Report Information TS 29.518CR0051
- Correction of CorrelationId Reference in OpenAPI TS 29.518CR0055
- Correct the references TS 29.518CR64
- Altitude in Provide Positioning Information TS 29.518CR107
+ 9 more changes
In Release 16, the PRU (Positioning Reference Unit) function was formally introduced as a UE-based unit that can be associated with or dis-associated from an LMF for positioning purposes. This release also added support for new positioning methods like UL+DL positioning and LMF-based AI/ML positioning, alongside clarifications for procedures such as UE assisted and UE based positioning. Furthermore, the architecture was expanded to explicitly include the PRU within both the non-roaming and roaming service-based and reference-point representations for Location Services.
- Introduction of NR positioning TS 37.355CR0250
- Introduction of Release-16 UE positioning capabilities TS 37.355CR0261
- Introduction of NR positioning TS 38.305CR0017
- Introduction of NR positioning TS 38.305CR0034
- Correction on UE Assisted and UE Based Positioning Procedure TS 23.273CR0001
- Clarification on positioning access selection in the 5GC-MO-LR procedure TS 23.273CR0008
+ 25 more changes
In Release 17, the PRU (Positioning Reference Unit) function was formally defined within the 5G system architecture, specifying procedures for PRU association and dis-association with an LMF. The release also introduced enhancements for storing UE positioning capabilities, including in failure cases, and integrated support for new positioning methods like LMF-based AI/ML positioning. Furthermore, the architecture was updated to support ranging-based services and sidelink positioning, as detailed in the associated technical specifications.
- Add definition and usage for 5G positioning area and 5G enhanced positioning area TS 23.273CR0166
- UE positioning capability storage TS 23.273CR0176
- Add DNNMF in Reference model TS 29.503CR0632
- Add UE Positioning Capabilities TS 29.518CR0580
- Introduction of R17 Positioning Enhancements in LPP TS 37.355CR0332
- Introduction of R17 Positioning Enhancements TS 38.305CR0086
+ 32 more changes
In Release 18, the PRU function was enhanced to support new procedures including PRU positioning over the user plane connection, AMF verification of the PRU during positioning, and support for both user-unaware positioning and ranging-based services. Furthermore, Release 18 introduced sidelink positioning into the LCS architecture, requiring updates to LMF selection and enabling new MO-LR and MT-LR procedures for ranging and sidelink positioning. These additions expanded the PRU's role within the positioning framework, integrating it with new protocols and network functions like the NWDAF for assisted positioning.
- Support of User Plane Positioning TS 23.273CR0246
- Support of UE Positioning without UE/User Awareness TS 23.273CR0251
- PRU verification by AMF during PRU positioning TS 23.273CR0265
- Support of User Unaware Positioning TS 23.273CR0276
- Introduce new feature: support of low power and high accuracy positioning TS 23.273CR0283
- introduce new feature: NWDAF assisted positioning TS 23.273CR0286
+ 93 more changes
In Release 19, the key enhancement for the PRU (Positioning Reference Unit) function was the formal introduction of **LMF-based AI/ML Positioning**, which utilizes a trained ML model available at the LMF. This was accompanied by specific clarifications and procedures for data collection from the NG-RAN to support this new AI/ML-based positioning method. The release also included updates to the general descriptions and procedures for AI/ML-based positioning within the overall architecture.
- KI#1 - LMF enhancements for UE positioning using a ML model TS 23.273CR0532
- Introduction of LMF enhancement for Direct AI/ML based Positioning TS 23.273CR0543
- General Support for MWAB involved positioning TS 23.273CR0547
- ML Model based UE positioning support TS 23.273CR0537
- LMF enhancements for LMF-based AI/ML Positioning TS 23.273CR0574
- KI#1 - Addressing Editors Note: Procedure for Data Collection for AI/ML based positioning from NG-RAN TS 23.273CR0597
+ 44 more changes
In Release 20, the PRU (Positioning Reference Unit) function was newly defined within the 5G LCS architecture, with its association and dis-association procedures to an LMF formally specified. The release also introduced a reference architecture for API exposure, as indicated by the corresponding Change Request title, integrating the PRU into both reference point and service-based interface (SBI) representations for non-roaming and roaming scenarios.
- API exposure Reference architecture TS 23.700CR0003
Explore further
Broader topics and technologies where PRU plays a role.
Defining Specifications
3GPP specifications that define or reference PRU, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.273 vj50 | 5G Location Services Stage 2 Architecture | Rel-19 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 24.571 vj20 | Control Plane LCS Procedures | Rel-19 |
| TS 29.503 vj50 | UDM Service Based Interface Stage 3 | Rel-19 |
| TS 29.510 vj50 | NRF Service Based Interface Protocol | Rel-19 |
| TS 29.518 vj50 | AMF Service Based Interface Protocol | Rel-19 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 38.305 vj00 | NG-RAN UE Positioning Stage 2 | Rel-19 |
| TS 38.843 vj00 | Study on AI/ML for NR Air Interface | Rel-19 |