To enable Angle of Arrival (AoA) and Angle of Departure (AoD), Bluetooth® Direction Finding uses a Constant Tone Extension (CTE).
The CTE is appended to the packet, after the CRC, and appears as an offset unmodulated carrier, or an unwhitened sequence of modulated 1's. Because Bluetooth LE uses Gaussian Frequency Shift Keying (GFSK) modulation, the frequency is constantly changed throughout the packet to distinguish the different symbols, or 1’s and 0’s sent. Without the frequency being stable, the receiver will not be able to estimate the phase of the signal.
The exact frequency of the CTE also depends on the channel transmitted on at a given time. As the CTE is offset 250 kHz higher than the channel center frequency, this must be accounted for in order to calculate the correct difference in distance travelled.