Positioning Method (Locating Method)

Description

A positioning method in 3GPP refers to a specific technical solution or algorithm employed to estimate the geographical location of a target mobile device. These methods are the building blocks of the overall positioning functionality and vary in accuracy, complexity, and applicability depending on the radio access technology and environment. The 3GPP specifications, particularly TS 23.271 (Location Services Stage 2) and TS 43.059 (GERAN positioning), define a range of positioning methods that have evolved from basic cell coverage-based techniques to advanced hybrid solutions combining terrestrial and satellite measurements.

Positioning methods can be broadly classified into several categories. Network-based methods rely on measurements made by the network infrastructure, such as Uplink Time Difference of Arrival (UTDOA), where multiple Location Measurement Units (LMUs) measure the time of arrival of a signal from the UE and compute the location based on time differences. UE-based methods involve the UE computing its own position using assistance data from the network, such as Assisted GNSS (A-GNSS), where the UE receives ephemeris and almanac data to quickly acquire satellite signals and calculate its location. UE-assisted methods have the UE collect measurements (e.g., reference signal time difference for OTDOA) and send them to the network (e.g., E-SMLC or LMF) for position calculation. Hybrid methods combine multiple techniques to improve accuracy and availability.

Key positioning methods include: Cell ID (using the serving cell's known location and optionally timing advance), Enhanced Cell ID (adding signal strength or timing measurements), Observed Time Difference of Arrival (OTDOA) for LTE and NR, where the UE measures the time difference of reference signals from multiple cells; Uplink Time Difference of Arrival (UTDOA) for LTE, where LMUs measure uplink signals; Assisted GNSS (A-GNSS), which leverages satellite systems like GPS, GLONASS, Galileo, and BeiDou with network assistance; and 5G NR positioning methods such as Round Trip Time (RTT), Angle of Arrival (AoA), and Angle of Departure (AoD) using beamforming. Each method has its own accuracy, latency, and availability characteristics, and the network may select the most appropriate method based on the requested Quality of Service (QoS) and UE capabilities.

The evolution of positioning methods has been driven by the need for higher accuracy, especially for emergency services and commercial applications. In R99, the primary methods were Cell ID, Time of Arrival (TOA), and Time Difference of Arrival (TDOA) based techniques like E-OTD for GSM. Rel-5 introduced a more comprehensive definition, including GPS and Assisted GPS methods. Rel-9 added E-UTRAN (LTE) positioning methods, including OTDOA and A-GNSS. Rel-11 introduced UTDOA for LTE. In 5G, NR positioning methods have been designed to achieve sub-meter accuracy in challenging environments, supporting use cases like industrial IoT and autonomous vehicles.

The selection and configuration of positioning methods are managed by the network's location server (e.g., E-SMLC in LTE, LMF in 5G), which considers factors such as UE capabilities, desired accuracy, latency constraints, and network topology. The methods are standardized to ensure interoperability across different vendors and networks, and they are continuously enhanced in each 3GPP release to meet new requirements.

Purpose & Motivation

Positioning methods exist to provide a standardized set of techniques for determining the location of a mobile device with varying levels of accuracy, latency, and availability. Before the introduction of standardized methods, location determination was proprietary and limited to basic cell ID, which offered poor accuracy and was insufficient for regulatory and commercial needs. The development of positioning methods was driven by regulatory mandates (e.g., E911) and the growing demand for location-based services, which required more precise and reliable location information.

Each positioning method addresses specific challenges. Cell ID is simple and always available but has low accuracy. OTDOA and UTDOA provide higher accuracy in urban and suburban environments but require network infrastructure and may be affected by multipath. A-GNSS offers high accuracy outdoors but is unreliable indoors or in urban canyons. By combining multiple methods, the network can achieve the best possible accuracy and availability for a given scenario. The evolution of positioning methods reflects the continuous effort to improve accuracy, reduce time-to-first-fix, and support new use cases across all radio access technologies.

In 5G, positioning methods are designed to meet the stringent requirements of vertical industries, such as industrial automation (sub-meter accuracy, low latency) and autonomous driving (high reliability, integrity). The standardization of these methods ensures that devices and networks from different vendors can interoperate, enabling a global ecosystem for location-based services.