Indoor localization Indoor positioning with Bluetooth

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With the "Bluetooth Direction Finding" extension introduced in 2019, the inexpensive radio standard is also suitable for precise indoor positioning applications - as a proof-of-concept demonstrates.

Location tracking is one of the key benefits of IoT technology for businesses and consumers. For a moderate cost, fleet managers can track their vehicles, logistics companies can monitor their shipments, and farmers can keep tabs on their livestock — all in real time. Connected IoT devices can also be utilized to monitor the whereabouts of elderly family members, pets, or valuable items like a car.

Thanks to global navigation satellite systems (GNSS), including GPS, GLONASS, BeiDou, and Galileo, location technology has permeated almost every aspect of our economy and daily lives. At the same time, GNSS technology has improved year after year, with achievable accuracy transitioning from meters to a few centimeters, the time to first fix from tens of seconds to seconds, and service availability increasing even in the densest urban areas.

However, to date, highly accurate positioning has a significant blind spot: large indoor areas. The weak GNSS signals do not penetrate most indoor spaces with sufficient strength. As a result, applications - and therefore efficiency gains - through continuous localization are denied to many potential users:

• Hospital managers who track medical equipment, patients, and staff;

• Airport operators who optimize baggage handling and quickly locate delayed passengers;

• Manufacturing companies automating production processes;

• Service and retail companies tracking customer behavior and backend operations, or

• Warehouse managers looking to improve operations with autonomous ground robots.

As satellite signals are largely unavailable in these environments, a number of alternative technologies have been proposed to address this limitation. Devices equipped with mobile modems, for example, can determine their position in relation to nearby mobile towers using mobile signals, employing techniques such as network fingerprinting or advanced signal time-of-flight methods. Devices equipped with Wi-Fi can utilize similar approaches to determine their location in the vicinity of Wi-Fi hotspots. Similarly, Bluetooth devices can use signal strength indication (RSSI) to provide an approximate distance estimate relative to Bluetooth beacons.

These technologies all suffer from shortcomings that limit their acceptance in the aforementioned use cases, where the accuracy, availability, user-friendliness, and affordability of GNSS technology have set a high standard. Mobile and Wi-Fi-based location technologies require relatively high hardware costs and often do not meet expectations in terms of accuracy. Despite its low accuracy, Bluetooth RSSI has gained ground in applications requiring room-level positioning due to its low cost, low energy consumption, and compatibility with the majority of connected devices in circulation.

Bluetooth Direction Finding improves indoor positioning

In 2019, the Bluetooth SIG introduced Bluetooth Direction Finding to improve indoor positioning. This approach, utilizing a new type of Bluetooth signal and multiple antenna arrays to measure the angle of propagation of a Bluetooth message between a mobile tag and one or more static anchor points, provides a new potential solution to the indoor location determination problem that likely meets all the criteria for the first time. High precision? Achieved. Easy deployment? Achieved. Low device costs? Achieved. Low energy consumption? All achieved.

Since their introduction, Bluetooth-based indoor positioning solutions have garnered significant interest. ABI Research forecasts an average annual growth rate (CAGR) of 28.3% for the shipment of Bluetooth tags from 2019 to 2025, with the largest increase (64.2%) expected in smart offices and the largest absolute number (over 163 million) in the warehouse and logistics industry.

Due to the complementarity of GNSS solutions for outdoor use, the extensive global ecosystem of companies developing solutions based on this technology, its low power consumption and cost, as well as the positioning accuracy provided within a range of less than one meter, u-blox has focused significant research and development activities on promoting the acceptance of this technology.

Higher accuracy in positioning with Bluetooth direction finding

Bluetooth indoor positioning is based on Bluetooth direction finding, available in two variants that enable the determination of the propagation direction of a Bluetooth signal between a mobile tag and a fixed anchor point. In the case of Angle of Arrival (AoA), the anchor point calculates the direction of the incoming signal sent by the tag. Conversely, in Angle of Departure (AoD), the roles are reversed, and the tag is tasked with calculating the angle at which the signal was transmitted by the anchor point. This article focuses on AoA, which is better suited for indoor positioning solutions, while AoD provides advantages for indoor navigation.

To evaluate the technology, u-blox set up an AoA-based demonstration setup for Bluetooth direction finding in its offices in Malmö, Sweden. A servo device attached to an anchor point for direction determination is programmed to track a mobile Bluetooth tag based on the real-time calculated angle of arrival.

Two extensions optimize the direction finding

Bluetooth direction finding utilizes two enhancements. Firstly, a new Bluetooth direction finding signal that contains additional data: the so-called Constant Tone Extension (CTE). While the rest of the Bluetooth message is modulated for data transmission purposes, the CTE consists solely of a series of ones. Therefore, the receiver can use this part of the message to precisely measure the phase differences between the signals.

Secondly, each anchor point contains not just a single antenna, but a multiple-antenna array. The image above shows how the direction-finding signal emitted by the mobile tag reaches the individual antennas within the antenna array of the static anchor point. Due to the varying distances traveled, each antenna receives the signal with a slight phase shift compared to the others, which is measurable thanks to the CTE.

Accurately calculate signal arrival angles

Algorithms running on a microcontroller embedded in the anchor point can then analyze this data to calculate the arrival angle of the signal with an accuracy of a few degrees. By using multiple anchor points instead of a single one, the approximate location of the direction-finding transmitter can be more precisely triangulated through the combined calculation of arrival angles.

For this, the exact positions and orientations of the anchor points need to be input into the location system, which then runs another algorithm to calculate the location of the tagged object—either in 2D or 3D—based on the arrival angles calculated by each anchor point. For example, in a simple 8 by 6 meter office with four anchor points positioned in the corners, it is possible to achieve an average accuracy of under 1 meter with a 95% probability.

Testing the technology in an industrial warehouse

U-Blox tested its indoor Bluetooth positioning solution in a real industrial warehouse—a typical use case for asset tracking applications. The 30 by 50 meter warehouse had metal shelves for storing devices and containers. While the lower layers for processing RF raw data are defined in the Bluetooth specification, the algorithm for calculating the actual arrival angle is not specified.

For the experiment, u-blox developed an efficient algorithm that runs on the embedded microcontroller in the Bluetooth chip, achieving high accuracy and update rates. Specifically, u-blox optimized the RF frontend, antennas, embedded algorithms running in the anchor point Bluetooth modules, as well as the wireless connectivity backbone connecting the anchor points to a network.

For the experiment, ten anchor points were used to cover a six-meter-high volume with a footprint of approximately 1,000 square meters. After careful planning and preparation, the installation of the positioning system took only about two hours.

To maximize line of sight between the tracker tags and the multi-antenna arrays, the anchor points were mounted three to five meters above the ground. Traxmate, a third-party solution, was used as the tracking software. Within Traxmate, the positions and orientations of the anchor points were input, and the positioning unit was configured through an integrated API. A Wi-Fi communication backbone between each anchor point and the localization system completed the installation.

Ideal: Unobstructed "view" of at least three anchor points

The setup aimed to mirror a real-world use case, providing reliable performance even in intricate indoor environments with typical obstacles like shelves. Initially, u-blox technicians strategically placed the anchor points to maximize the likelihood of line of sight between all likely tag positions and at least three anchor points.

Additionally, one had to take into account Doppler effects that occur, for example, when radio signals are reflected off walls. The algorithms running in the anchor points to calculate the angles incorporate a mitigation of these multipath interferences, delivering a reliable performance even in the challenging radio environment of a warehouse.

Bluetooth direction finding is a convincing solution

The experiences with this proof-of-concept have shown that the highly accurate Bluetooth indoor positioning delivers as promised. On the one hand, if well implemented, it can provide accuracies in the sub-meter range. As is typical for Bluetooth devices, the costs for the necessary hardware are significantly lower than those of competing technologies.

The same applies to power consumption. Using positioning solutions indoors has long been a particular challenge. The integration of the hardware used by u-blox with web interfaces has greatly simplified deployment.

With the release of the Bluetooth direction finding feature, the Bluetooth SIG has provided a compelling solution to the indoor positioning problem, addressing many of the shortcomings of currently available solutions on the market.

This article was originally published on our partner portal Elektronikpraxis.