Small range of IQ in Bluetooth direction finding

RE: Small range of IQ in Bluetooth direction finding

Marcous — Mon, 27 Dec 2021 04:39:23 GMT

Hi, Shuang!

I have met the same phenomenon with you, somehow you may need to connect the chip with an antenna to get a larger IQ value from my experience.

And it also seemed that there is no obvious relationship between the degree of the absolute value of IQ value and RSSI value.

But I do believe with a larger IQ value, I can calculate the angle of arrival precisely.

RE: Small range of IQ in Bluetooth direction finding

shuang — Sun, 26 Dec 2021 09:23:45 GMT

First of all, thank you for writing such a thorough answer in your busy schedule. Both the explanation of the number of antennas and the detailed explanation of I/Q are very clear. I will follow the instructions to modify some settings in the future.

But for now, I still have some curiosity about why the I/Q changes are not too obvious. In another Q&A thread, its I/Q can reach 30 with an RSSI of -63 (sample [18]). I believe the nRF52833DK is capable of this. But on my side, no matter what I try, the I/Q varies within a range of 10. I think maybe there is a slight difference in the phase obtained from the (-2, -8) and (-7, -32). I've posted a few sets of IQ samples that I collected below.

RSSI: -60 [45 samples]:
(-6, -8), (5, 7), (-6, -9), (4, 8), (-5, -9), (5, 8), (-5, -9), (4, 9), (-4, -9), (-3, -10), (-3, -10), (-4, -10), (-2, -10), (-2, -10), (0, -10), (0, -10), (0, -10), (1, -10), (2, -10), (3, -10), (2, -10), (4, -9), (5, -9), (5, -8), (6, -9), (6, -8), (6, -8), (6, -8), (8, -6), (8, -5), (8, -7), (8, -5), (9, -4), (9, -4), (9, -3), (9, -1), (9, -1), (9, -2), (10, 0), (9, 0), (9, 0), (9, 1), (9, 2), (9, 2), (8, 4)

RSSI: -40 [45 samples]:
(-9, -6), (8, 5), (-8, -6), (7, 6), (-8, -7), (7, 6), (-8, -7), (7, 7), (-7, -8), (-7, -8), (-6, -8), (-6, -9), (-5, -9), (-4, -10), (-5, -9), (-3, -10), (-2, -10), (-2, -10), (-1, -10), (-1, -10), (0, -10), (1, -10), (1, -10), (2, -10), (3, -10), (3, -10), (4, -9), (5, -9), (6, -9), (6, -8), (6, -8), (7, -7), (7, -7), (7, -6), (9, -5), (9, -4), (9, -5), (9, -3), (9, -3), (9, -2), (9, -1), (9, 0), (9, -1), (9, 1), (9, 2)

RSSI: -37 [45 samples]:
(-10, -2), (9, 1), (-10, -3), (9, 2), (-10, -3), (9, 2), (-10, -3), (9, 2), (-10, -4), (-10, -4), (-9, -5), (-9, -5), (-8, -6), (-9, -6), (-8, -7), (-6, -8), (-7, -8), (-6, -9), (-6, -9), (-6, -9), (-4, -10), (-3, -10), (-3, -10), (-2, -10), (-2, -10), (-1, -10), (-1, -10), (0, -10), (1, -10), (2, -10), (3, -10), (3, -10), (3, -10), (4, -9), (5, -9), (5, -8), (6, -8), (6, -7), (7, -7), (8, -5), (8, -6), (8, -5), (8, -5), (9, -4), (9, -3)

I think I may have missed some points. I would appreciate any help you can give me.

RE: Small range of IQ in Bluetooth direction finding

Simonr — Tue, 21 Dec 2021 11:40:13 GMT

Unfortunately you won't get a very exact result with an array of only 3 antennae on the receiver end. I would recommend a 2x3 antenna array at the very least to get usable results, and would suggest going up to a 3x3 or 3x4 array for good accuracy.

Please note that you will need an AoA algorithm to handle the received sample data and output it so it makes sense. There is no point in looking at raw IQ data when received, as you should always convert the data to a vector in order to review it You can check out this page for information on the formulas used to translate between polar and rectangular form.

Best regards,

Simon