Bluetooth® Direction Finding uses IQ sampling, an in-phase and quadrature sampling technique, to measure the phase of incoming signals.
In IQ sampling, the receiver mixes the incoming signal with the local oscillator (LO) at 0 and 90 degree offsets. This creates two orthogonal functions known as the in-phase (I) and quadrature (Q) components. These components are plotted as the axes in the following diagram where the amplitude and phase of the main signal can be calculated as the magnitude and angle of the vector made up from the I and Q components.
Thus, it is possible to estimate the phase of the signal by measuring the magnitude of the I and Q components. This is also the reason why the phase samples can be represented as I and Q values, or phase and magnitude, whichever is preferred for a given use case.