“How can I determine the specific BLE advertising channel currently being used for Bluetooth CTE signals in my AOA positioning setup?”

[Aihu]

Hello, the CTE signal for Bluetooth AoA (Angle of Arrival) positioning is primarily transmitted over BLE advertising channels (37, 38, 39), corresponding to frequencies of 2402 MHz, 2426 MHz, and 2480 MHz, respectively. 
Since calculating the angle of arrival is frequency-dependent, how can I determine which specific channel the Bluetooth CTE signal is currently using? 
(I am using the direction_finding_connectionless_rx and direction_finding_connectionless_tx examples from the NCS SDK 3.0.1.)


Thank you very much for your reply！

[tesc]

Hi,

The channel used for the continuous tone extension (CTE) is provided in the bt_df_per_adv_sync_iq_samples_report structure, in the .chan_idx field. For the Bluetooth: Direction finding connectionless locator sample, the structure is used in the cte_recv_cb() callback function, around line 143 of main.c.

You should also be able to find (or check) the frequency from the IQ samples from the reference period at the beginning of the CTE. During the reference period, the first antenna will send the signal for a longer period of time. Please see IQ sampling configuration, and the rest of the Direction Finding white paper, for further details on the workings of Direction Finding with our SoCs.

Regards,
Terje

[Aihu]

Hello，

Thank you very much for your response! I used the method you suggested, and as shown in the example below, I successfully obtained the channel ID. However, I am using a connectionless example (non-connected mode). Based on my understanding, RF signals with CTE should utilize the advertising channels 37, 38, and 39. Why is the actual channel ID randomly selected from the range 0 to 39 in practice?

Regards,

Ai Hu