Reference Source

Description

In 3GPP positioning architecture, a Reference Source is defined as a physical entity or part of a physical entity that provides signals (e.g., RF, acoustic, infra-red) that can be measured by a Target Device in order to obtain the location of the Target Device. This concept is central to location-based services, emergency call positioning, and regulatory requirements for E-911 and similar mandates. The Reference Source can be a terrestrial beacon, a satellite (e.g., GNSS), a base station (e.g., eNB or gNB), or even a part of the UE itself in some cases. The signals emitted by the Reference Source are used by the Target Device to perform measurements such as time of arrival (TOA), time difference of arrival (TDOA), angle of arrival (AOA), or received signal strength (RSS), which are then used to compute the location.

The Reference Source is a key component in various positioning methods standardized by 3GPP, including Observed Time Difference of Arrival (OTDOA), Uplink Time Difference of Arrival (UTDOA), Assisted GNSS (A-GNSS), and NR positioning methods like DL-TDOA, UL-TDOA, and multi-RTT. In OTDOA, for example, the Reference Source is typically a base station (eNB or gNB) that transmits positioning reference signals (PRS). The UE measures the time difference of arrival of PRS from multiple base stations and reports these measurements to the location server (E-SMLC or LMF), which then calculates the UE's position. In A-GNSS, the Reference Source is a satellite, and the UE uses assistance data from the network to acquire and measure GNSS signals.

The Reference Source is defined in 3GPP specifications such as TS 36.355 (LTE Positioning Protocol, LPP), TS 37.355 (NR Positioning Protocol, NRPPa), and TS 37.857 (Study on Indoor Positioning Enhancements). These specifications outline the signaling and procedures for requesting and providing location information, including the characteristics of Reference Sources. The location server may provide the UE with assistance data about Reference Sources, such as their identities, coordinates, and signal characteristics, to aid in measurement and positioning.

Architecturally, the Reference Source interacts with the Target Device (UE) and the network's positioning nodes. The UE measures signals from one or more Reference Sources and may also use assistance data from the network. The measurements are then used to compute the location either at the UE (UE-based) or at the network (UE-assisted). The Reference Source itself may be part of the network infrastructure (e.g., a base station) or external (e.g., a GNSS satellite). In some cases, the Reference Source can be another UE, such as in sidelink positioning, where UEs exchange signals to determine relative positions.

The concept of Reference Source has been present since Rel-9, when positioning enhancements were introduced in LTE. It has since been adopted in 5G NR positioning specifications. The definition has remained unchanged across releases, indicating its fundamental and stable nature. As positioning technologies evolve, the Reference Source remains a cornerstone, enabling accurate and reliable location determination in both indoor and outdoor environments.

Purpose & Motivation

The Reference Source concept exists to provide a standardized way to describe the entities that emit signals used for positioning. In cellular networks, accurate positioning is essential for emergency services, location-based applications, and regulatory compliance. Before the formal definition of Reference Source, positioning methods relied on various ad-hoc descriptions of signal sources, which led to interoperability issues and lack of clarity in specifications. The introduction of this term in Rel-9 provided a unified vocabulary for describing any entity that can be measured for positioning, whether it is a base station, satellite, or other beacon.

The Reference Source solves the problem of ambiguity in positioning architectures. By defining it as a physical entity or part of a physical entity that provides measurable signals, 3GPP enabled a clear separation between the source of signals and the device that measures them (the Target Device). This abstraction allows different positioning methods to be described in a consistent manner, facilitating the development of multi-method positioning systems that can combine measurements from multiple sources for improved accuracy.

Historically, the need for a precise definition arose with the proliferation of positioning methods in LTE. As networks moved towards 5G, the concept was extended to new signal types and higher frequency bands. The Reference Source remains method-agnostic, covering RF, acoustic, and infrared signals, which makes it applicable to a wide range of positioning technologies, including those used in indoor environments where GNSS signals may be unavailable. This flexibility ensures that the term remains relevant as positioning technologies continue to evolve.