The Angle of Arrival (AoA) method uses a transmitter and receiver, where the receiver does the antenna switching to determine the direction to the transmitter.
This is enabled by the transmitter adding a Constant Tone Extension (CTE) to the data packet causing a specified part of the packet to have a fixed and constant frequency. Knowing this, the receiver can sample the waveform IQ components and determine the phase of the waveform. And doing this for multiple antennas, the receiver can calculate from which angle the transmitter signal comes from.
According to the Bluetooth Core Specification, the CTE of the packet contains a guard period of 4 µs, a reference period of 8 µs, and a number of antenna samples. In particular, Vol 6, Part B, section 2.5.4 of the Bluetooth® Core Specification states:
When receiving a packet that contains an AoA Constant Tone Extension, the receiver shall perform antenna switching at the rate and according to the switching pattern configured by the Host. In both cases, the receiver shall take an IQ sample each microsecond during the reference period and an IQ sample each sample slot (thus there will be 8 reference IQ samples, 1 to 37 IQ samples with 2 μs slots, and 2 to 74 IQ samples with 1 μs slots, meaning 9 to 82 samples in total).
This is illustrated in the following diagram.
The duration for Guard Period and Reference Period is fixed and cannot be changed. The number of samples taken during the Reference Period is determined by 8 µs/TSAMPLESPACINGREF. However, the sample slot and switch slot duration can be changed using the settings in the following table.
| Field | Value for 1 µs sample slots | Value for 2 µs sample slots | Description |
|---|---|---|---|
| TSAMPLESPACINGREF | 1 µs | 1 µs | Interval between samples in the reference period |
| TSAMPLESPACING | 2 µs | 4 µs | Interval between samples in the switching period |
| TSWITCHSPACING | 2 µs | 4 µs | Interval between every time the antenna is changed in the switching state |
The following diagrams illustrate AoA configuration for 1 µs and 2 µs slots.
