Bluetooth Supplement to Support Direction Finding#

To be able to find the direction of the incoming signal from phase differences, an unmodulated signal must be transmitted by the transmitter. Transmitting a CW (continuous wave) signal for a long time is not recommended outside of testing environments, because it has a very sharp spectrum and can cause serious interference with other devices working in the 2.4 GHz frequency range. Therefore, a short CW must be used, and the transmitter and the receiver must be synchronized so that both of them know when the CW signal is sent. Additionally, in the AoD method the receiver must know the exact time instance when antenna switching occurs on the transmitter side.

Many solutions can be found to overcome this simple problem, and indeed there are many indoor positioning solutions on the market already using direction-finding algorithms. None of them is based on a well-known standard, however. The Bluetooth standard, in contrast, is widespread, and it solves the synchronization problem by its nature: Bluetooth packets are sent with very strict timing, and the peer devices resynchronize their clocks on each reception.

Bluetooth 5.1 introduced a new method to request and send short CW signals as an extension of a normal package. This extension is called Constant Tone Extension (CTE), and it is sent after the CRC of the package when requested.

CTEs can be sent both through a connection or in periodic advertisements which helps to scale the system. The CTE also has parameters communicated through the Bluetooth package, such as CTE type (AoA/AoD), duration, and switching slot duration.

For a more detailed description of Bluetooth-based direction finding, see Bluetooth SIG documentation https://www.bluetooth.com/bluetooth-resources/bluetooth-direction-finding/.