Measuring an angle with an antenna array
Radio direction finding is not new. The Bluetooth SIG notes it has been practised since the early twentieth century, in systems for aviation and nautical navigation and for wildlife tracking. Bluetooth Core 5.1 added an optional direction finding capability to Bluetooth Low Energy, so one Bluetooth device can determine the direction of a signal sent by another.
The method relies on geometry. When a signal crosses an array of antennas, it travels a slightly different distance to each antenna, so it arrives at each one with a different phase. The receiver takes IQ samples of the signal while switching between the antennas in the array, and from that sample data it calculates the relative direction of the signal.
Angle of Arrival and Angle of Departure
Direction finding has two methods, depending on which side holds the antenna array.
| Angle of Arrival (AoA) | Angle of Departure (AoD) | |
|---|---|---|
| Transmitter | A single antenna, such as an asset tag | An antenna array, such as a fixed locator |
| Receiver | An antenna array, such as a fixed locator | A single antenna, such as a smartphone |
| Who calculates the direction | The fixed locator | The device carried by the person |
| Intended for | Real-time locating, item finding, point-of-interest information | Indoor positioning, such as wayfinding |
With AoA, the building learns where things are. The SIG’s use cases include factories tracking materials and expensive mobile equipment, sites keeping workers out of unsafe areas, and sports teams tracking athletes.
With AoD, the person’s device learns where it is. The SIG says AoD is often described as “Indoor GPS”, because it works in a similar way: fixed transmitters send, and the receiver works out its own position. Its examples include indoor navigation in airports, museums and convention centres, and automated guided vehicles that need to know where they are.
What it adds over signal strength
The Bluetooth SIG groups Bluetooth LE positioning into three features: advertising for presence, direction finding for direction, and RSSI and Channel Sounding for distance.
Before direction finding, Bluetooth location services estimated the distance between two devices from received signal strength, or RSSI, and combined distances to fixed points by trilateration. The SIG’s 2019 paper puts those systems at metre-level accuracy, about 1 to 10 metres, depending on the floor plan and the number of locators. With direction finding, the same systems can also use the direction of another device and combine angles by triangulation. The SIG says this enables positioning systems that can achieve centimetre-level location accuracy; its blog adds the condition “in the right deployment environment”.
Direction also changes what an app can do with a single device. The SIG’s examples include an item finder that shows which way a lost item is, not just how close, and a museum app where a visitor points the phone at an exhibit to get information about it. Indoor positioning covers how direction finding sits beside Wi-Fi and UWB methods.
What it asks of a deployment
Direction finding is not free to add. A Bluetooth SIG blog post on industrial sites says it needs antenna arrays, more deliberate infrastructure planning and better calibration discipline, and that it should not be the default answer to every facility problem. The same post suggests it pays off where wrong-zone errors are expensive, or where geofencing needs to be tighter and more trustworthy.
Because the feature is optional in the specification, support depends on the hardware and the software stack. The SIG’s 2019 blog anticipated item finding with AoA once smartphone vendors build it into their phones, so check the documentation for the specific phones, chips and operating system versions you plan to use rather than assuming support.
Direction finding and Channel Sounding
Direction finding answers “which way”. Bluetooth Core 6.0 added Channel Sounding to answer “how far”, using phase-based ranging and round-trip timing between two connected devices. The Core 6.0 feature overview contrasts it with the older path loss method based on RSSI, which it describes as vulnerable to interference and environmental factors and open to distance spoofing.
The features are designed to work together. The SIG says Channel Sounding can be used in conjunction with RSSI and direction finding, and the industrial post sums up the progression as presence first, then direction, then precise ranging only where the decision needs it. If you are comparing Bluetooth with other short-range radios for this job, BLE vs NFC vs UWB sets out the differences.