The Magic of Bluetooth AoA/AoD Direction Finding

Published on March 31, 2020

The Magic of Bluetooth AoA/AoD Direction Finding

A Compelling Application – Solved by Bluetooth

The problem of lost or misplaced things is a very old one with a number of high-tech, modern solutions. Just about anything can be tracked to a degree of precision by a GPS/GNSS module, which uses satellite positioning to locate a device within 3 to 4 meters. Further correction can be done to achieve a higher degree of accuracy. But this is a process that demands a bit more power and hardware than is necessarily practical for every IoT application.

If you simply wanted to be able to track down a key fob wirelessly for example, a GPS module might not be the most efficient way to keep track of it. GPS consumes a larger amount of power, in a larger form factor, and it just isn’t practical for this application.

This is where Bluetooth Low Energy (LE) makes such an exciting candidate for tracking services. After all, Bluetooth Low Energy already benefits from extremely low power consumption, sleep and doze modes that can make a coin cell last for years, and tiny form factor that is unobtrusive in most applications.

As of Bluetooth SIG specification version  5.1, Bluetooth direction finding is powered by a methodology known as Angle of Arrival (AoA) / Angle of Departure (AoD), and it’s this method that makes it possible for a device to find another more accurately than prior Bluetooth LE beaconing capabilities for location finding, that used RSSI to estimate distance... So how does it work?

Angle of Arrival (AoA) vs. Angle of Departure (AoD)

Put simply, the process of using one Bluetooth LE device (such as a smartphone) to find another Bluetooth LE device (such as a key fob) is made possible by ascertaining the angle that the signal is coming from. It’s something like a game of “Marco Polo,” in which one player closes their eyes and tries to find people by the sound of their voice. The seeker doesn’t know the actual location, but can only use relative angle and sound to identify the other players.

Angle of Arrival and Angle of Departure are the angles from which one device is receiving a signal, but they are calculate in different ways. They’re made possible by one of the devices having multiple antennas, which produce slightly different results from which meaningful insights are drawn.

Angle of Departure (AoD)

With Angle of Departure measurement, a device with multiple antennas (Device A) sends the signal using multiple antennas(an antenna array), and the receiving device (Device B) receives all of those separate signals through its single antenna, but is also able to analyze (using something called IQ sample data) the difference between these to estimate the relative signal direction from which they departed.


AoA and AoD graphic_AoD.png

Angle of Arrival (AoA)

With Angle of Arrival measurement, something similar is accomplished, but in reverse. A device with a single antenna (Device B) sends a special direction-finding signal to a device with multiple antennas (Device A). That device may use as many as 74 antennas, and then processes those samples to determine which direction the signal came from, based in part on phase differences between what is received at each antenna. This is used to measure the expected angle that the signal originated from.

AoA and AoD graphic_AoA.png

Applications – Direction Finding for A Massive New Array of Devices

There are many, many applications which can benefit from direction finding like this. Beacons used in retail can offer more precise and specific advertising data – and make meaningful observations about where devices tend to linger over time. Nearly anything can be fitted with a tracker for when it’s been lost or misplaced, such as tools on a job site or storage containers in a warehouse.

These are not new applications, but Bluetooth LE is a uniquely suited way to accomplish them. The combination of lower power usage, less maintenance, and Bluetooth ubiquity makes direction finding more practical, affordable, and achievable than it has been in the past.

What’s Next? BLE Direction Finding To Get Even Smarter

Already, there is work being done to further perfect this application and provide a higher degree of precision. By combining the Angle of Arrival / Departure with the time of flight (difference between broadcast time and receive time), a further degree of precision that can yield accuracy down to a few centimeters.

What this means is that it will no longer be a game of Marco Polo – the goal is to generate not just a direction, but a distance, which adds up to an actual position. This is still far on the horizon, with lots of work to be done in order to make it a practical reality. But it’s both practical and possible, and will someday mean the difference between taking your earbuds on that jog, or leaving them lost under the couch.

Bluetooth 5.2 Ready Modules – Provided by Ezurio (formerly Laird Connectivity)

Ezurio provides Bluetooth modules which are suited perfectly for your next-gen applications with ultimate design flexibility.

The upcoming BL653 is our latest Bluetooth 5.1 offering, which is Bluetooth 5.2 ready. Featuring the Nordic nRF52833 chipset, the BL653 is designed with the IoT in mind. Long range, highly secure, and ideal for inclusion in smart devices and industrial IoT applications. Visit the BL653 page and register for a chance to win a BL653 Development Kit.

For more on our line of Bluetooth 5.x modules, visit our Bluetooth 5 modules page.