Positioning Signal Measurement Function

Description

The Positioning Signal Measurement Function (PSMF) is a logical entity within the 3GPP network architecture, specifically part of the network-based positioning infrastructure. Its primary role is to collect and process radio signal measurements related to a target User Equipment (UE) from one or more network nodes, such as base stations (NodeBs, eNBs, gNBs) or Location Measurement Units (LMUs). These raw measurements form the basis for calculating the UE's geographical position. The PSMF does not typically perform the final position calculation itself; instead, it provides the processed measurement data to a Positioning Determination Function (PDF) or a Secure User Plane Location (SUPL) Location Platform (SLP).

Architecturally, the PSMF can be integrated within a Radio Network Controller (RNC) in UMTS, a base station in LTE and NR, or reside as a standalone node. It works by coordinating with the radio access network to instruct specific cells to take measurements on signals from the target UE. These measurements include, but are not limited to, Observed Time Difference of Arrival (OTDOA) in LTE/NR, where it measures the time difference of received pilot signals from multiple cells; Uplink Time Difference of Arrival (UTDOA), where it measures the time of arrival of the UE's uplink signal at multiple LMUs; and Enhanced Cell ID (E-CID) methods, which combine cell identity, timing advance, and received signal strength. The PSMF manages the measurement session, ensuring measurements are taken simultaneously or within a required time window for accuracy, and may perform initial filtering or quality assessment on the data.

Key components of the PSMF functionality include the measurement controller, which requests and collates measurements from distributed points; the measurement database, which may store calibration or reference signal data (e.g., positions of base stations and their timing characteristics); and the interface handlers for protocols like LTE Positioning Protocol (LPP) or LTE Positioning Protocol A (LPPa) to communicate with the UE and other network elements. Its role is critical in separating the measurement gathering, which is tightly coupled to the radio technology and network topology, from the position calculation algorithm, which can be more generic. This abstraction allows for the introduction of new positioning methods and enhancements without overhauling the entire location services architecture.

Purpose & Motivation

The PSMF was created to enable accurate, network-based positioning to meet regulatory, commercial, and safety requirements. A primary driver was (and remains) support for emergency caller location, mandated by regulations in many countries (e.g., E911 in the USA). Network-based methods are essential when the UE itself cannot determine its position (e.g., lacks GPS capability) or is not permitted to (e.g., privacy settings, or in a lawful interception scenario). The PSMF provides the network with the independent capability to locate a device.

Historically, early cellular systems had very coarse location capabilities, essentially limited to the serving cell identity. As location-based services (LBS) like navigation, fleet tracking, and geo-fencing emerged, more accuracy was demanded. The PSMF, introduced in the era of UMTS, formalized the network's role in performing sophisticated signal measurements. It addressed the limitations of purely UE-based methods (like A-GPS), which could be unavailable indoors or drain battery life, and hybrid methods, by providing a reliable, network-controlled alternative. It also solved the problem of locating legacy or low-complexity devices that lack advanced positioning hardware.

The evolution of the PSMF has been motivated by the continuous need for higher accuracy, lower latency, and support for new use cases. From the basic timing measurements in UMTS, it evolved to support OTDOA with positioning reference signals in LTE, and now encompasses multi-RAT (Radio Access Technology) measurements and NR positioning techniques like multi-cell Round Trip Time (RTT). Its development is closely tied to the expansion of location-critical applications, including IoT asset tracking, vehicular communication (V2X), and industrial automation, where precise and reliable device location is a fundamental service enabler.

Key Features

• Coordinates radio signal measurements from multiple network points for a target UE
• Supports multiple positioning methods (e.g., OTDOA, UTDOA, E-CID)
• Interfaces with base stations and Location Measurement Units (LMUs) to gather data
• May perform initial processing and quality assessment of raw measurements
• Provides measurement data to Positioning Determination Function (PDF) or SUPL platform
• Manages measurement sessions, including timing and resource allocation

Evolution Across Releases

Defining Specifications