What is Bluetooth Direction Finding?

With the release of version 5.1, Bluetooth got the feature of direction finding which enables Bluetooth devices to determine the position of other Bluetooth devices using the angles at which signals are transmitted or received. Direction finding works on the concepts of Angle of Arrival (AoA) and Angle of Departure (AoD). Angle of arrival and angle of departure are the angles at which Bluetooth signals are received or transmitted respectively by a Bluetooth device. Angle of arrival and angle of departure methods make it possible for a device to find the position of another Bluetooth device in 2 or 3 dimensions using Bluetooth’s beaconing capabilities.

For direction finding to be made possible, one of the devices must have multiple antennas (antenna array) instead of a single antenna for transmission and reception. Antenna arrays produce slightly different results from single antennas, from which data can be obtained for the calculation of positions.

With the angle of departure measurement, a device with multiple antennas (Device A) sends direction-finding signals using its multiple antennas (antenna array), and the receiving device (Device B) receives all of those separate signals through its single antenna (as shown in the diagram above). By analyzing the change in phase shift of the signals from each antenna in the array using existing data called IQ sample data, the relative direction at which the signals departed can be estimated.

In angle of arrival measurement, a device with a single antenna (Device B) sends a special direction-finding signal (as shown in the diagram above) to a device with multiple antennas (Device A). As the transmitted signal reaches the array, the device sees a different signal phase for each antenna in the array due to the difference in distance from each of those antennas to the transmitting antenna. Similar to the calculation of AoD, this phase difference data is processed using IQ samples to determine the direction which the signal came from. This gives an estimation of the angle that the signal originated from.

Although both methods perform the same task, whether AoD or AoA should be chosen for effective applications, depends on the use cases themselves.

AoA allows for use of very simple, low-cost antennas in the source device of which location is to be determined. This system would have the more complex locator units with antenna arrays fixed in one place to determine the location of the other devices.

Such scenarios might include:

• Asset tracking in warehouses
• Value asset tracking in hospitals, govt. establishments etc.
• ID location of people and staff

AoD requires more complex antennas than AoA from a hardware and software perspective. In this scenario, the locator device can be simpler in terms of hardware design as it only needs a single antenna but it requires more complex software to determine and calculate the direction/position of the other devices. AoD also requires that the devices whose location is to be determined have antenna arrays for the functioning of direction-finding capabilities.

Such scenarios might include:

• Wayfinding in large spaces such as airports, hospitals etc.
• Point of interest assistance in shopping malls, exhibitions etc.
• Search and finding of items such as keys, remote controls etc.

AoD-based direction finding has some strong use cases that include smartphones and wearables.