Bluetooth® Direction Finding is based on the two key concepts of Angle of Arrival (AoA) and Angle of Departure (AoD). This makes use of the angular phase-shifts that occur between antennas as they receive (AoA) or transmit (AoD) RF signals.
With the use of antenna arrays at either side of the communication link, phase shift data can be determined and from this the location can be calculated.
An AoA system features the antenna array on the receiver side, so that by measuring the phase-shift of the incoming signal, the receiver can determine the direction of the incoming signal.
AoD, as the name implies, uses the antenna array to direct the transmitted signal in a given angle.
The transmitter in AoA and the receiver in AoD does not need an antenna array, but the radio must still be able to append or sample the Constant Tone Extension (CTE) as described in Constant Tone Extension.