What should I pay attention to before using the Angle estimation algorithm?

RE: What should I pay attention to before using the Angle estimation algorithm?

Zihao — Mon, 27 May 2024 09:49:07 GMT

Hi Paka，

Yes I am using the 2M PHY:

PER_ADV_SYNC[0]: [DEVICE]: 12:63:96:6B:22:D1 (random) synced, Interval 0x0780 (2400 ms), PHY LE 2M
success. Periodic sync established.
Enable receiving of CTE...
success. CTE receive enable

In fact, I didn't change the code, except for adding some lines to print the IQ data.

Thanks! Now I got the angle!

Thanks again!

RE: What should I pay attention to before using the Angle estimation algorithm?

PaKa — Mon, 27 May 2024 07:49:28 GMT

Difficult to say as you have changed the code. One thing to check is if you are using the 2Mbps PHY as that is using 500KHz typical modulation.

RE: What should I pay attention to before using the Angle estimation algorithm?

Zihao — Sun, 26 May 2024 09:58:26 GMT

Hello PaKa,

I've been trying to compensate the phase drift caused by temperature, noise, or crystal oscillator using the 8 reference samples, but I've encountered an issue.

According to the whitepaper nWP036, the 8 reference points should be 1us/sample, totaling an 8us duration. However, the CTE is a 250kHz single tone signal, with a period of 1/250kHz = 4us. Therefore, my understanding is that the reference should only have a signal length of approximately 2 cycles. However, the signal I've collected (observing the four plots in this case) looks more like 4 cycles, giving the impression that the CTE signal is 500kHz.

The same impression in another case:

Source: RE: AOA: Antenna switch pattern and IQ sampling via nrf52833

I think it should be like:

So where did I go wrong in my understanding?

Thanks!

RE: What should I pay attention to before using the Angle estimation algorithm?

Zihao — Thu, 23 May 2024 08:11:11 GMT

Got it, I understand! Your explanation is very clear, thank you for your reply!

RE: What should I pay attention to before using the Angle estimation algorithm?

PaKa — Thu, 23 May 2024 08:06:18 GMT

So the single antenna plot shows that you have a frequency that is not 250KHz, as a 250KHz would yield a horisontal line for the reference period. This is to be expected as I stated, the TYPICAL value is 250KHz but it will always be a bit off.

Then for the non-reference period you have a fairly straight line, a little bit of overlayd noise, also to be expected. The slope of this line is also coming from the non-perfect 250KHz so as indicated by the reference period.

When you start changing the antennas as you have done, then the phase will jump for each antenna change. How much will depend on the distance between the antennas, the distance from the transmitter to the antenna array and the position relative to the antenna array. How much the phase will change is something you can model with normal trigonometry.

You will have to use the reference phase to set the baseline for the signal, if not you will not be able to calculate any angles at all. How to do this is outside the scope of what Nordic will support as it is part of the algorithm work.

RE: What should I pay attention to before using the Angle estimation algorithm?

Zihao — Wed, 22 May 2024 14:46:21 GMT

Hi, thank you for your reply.
So, is the reference sampling actually useful? I thought the reference was just to make the receiver more stable before receiving the CTE...
Could you please provide me with some links about using these 8 samples with short interval to calculate the actual frequency? I just did some searching and found that there are so few materials that it's hard to find any useful information...
In addition, just to confirm, the "phase shift" you mentioned is what I said in the 2), right? After I compensate the "phase shift", the phase plot of any antenna should be a straight line, right?

I have trid to calculate the average fluctuations in IQ (ΔI and ΔQ) for compensating the drift, but that doesn't seem right either... Is my method of compensation unreasonable? Or we have to use the first eight Samples?

Thank you!

RE: What should I pay attention to before using the Angle estimation algorithm?

Zihao — Wed, 22 May 2024 14:29:33 GMT

Hello, thank you for your reply.
Yes, I understand that MUSIC is beyond your scope, so the main purpose of this case is to confirm that there is nothing wrong with the IQ data I got.
I don't quite understand what you said about "the graph should be similar to what you see with one antenna". Are you saying "The graph should look like a straight line"?
I used a square antenna array designed by Nordic with 12 antennas. I only use a linear array of four of these antennas. My ant_patterns[] = {0x2, 0x0, 0x5, 0x6}, which correspond to a line on the array.

RE: What should I pay attention to before using the Angle estimation algorithm?

PaKa — Wed, 22 May 2024 11:57:24 GMT

Input data looks not too far of what to be expected in my opinion.

All the frequencies are "typical" so you can't use the data without compensating for drift. This is specifically why the reference sampling is 8 samples with short interval as these are to be used to find the actual frequency and establish the phase of the signal.

When you have this then you can extrapolate the "reference" signal for the length of the entire sequence. Then you can see the phase shift per antenna compared to the reference, this is what the algorithm should use to determine the angle.

How to use/preprocess the date before handing it over to the MUSIC algorithm will be outside what Nordic will be able to support but there may be non-nordicans in the forum that may help you out.

RE: What should I pay attention to before using the Angle estimation algorithm?

Simonr — Wed, 22 May 2024 11:43:56 GMT

Unfortunately supporting the MUSIC algorithm is outside the scope from our side, so it's limited how much we can help you here I'm afraid, but you are right, the graph should be similar to what you see with one antenna, so something seems to be wrong here. I can't comment on how you've handled the IQ data, as we don't focus on that, but how have you set up the antenna switching? What antenna array are you using here and what antenna switching pattern is used here?

Best regards,

Simon