Problem with I and Q Samples During the Reference Period of CTE Packets

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Mon, 07 Apr 2025 08:08:16 GMT

Hi Louis

This is simply plotting the IQ samples reported by the MCU without any modification. Print the IQ samples using UART2USB line and then store the result in a file. In a for loop, go over the IQ samples and print them:

printf("%i,%i, i_val, q_val)

Then open the file that you've stored the IQ log by a python script and plot the numbers:

import matplotlib.pyplot as plt

IQ_LOG_FILE = "iq_raw_log/iq.txt"


def read_file_to_lists(file_name):
    with open(file_name, 'r') as f:
        # Read each line, split the string by spaces, and convert to integers
        return [list(map(int, line.split())) for line in f]


def plot_iq(samples):
    i_sample = samples[::2]
    q_sample = samples[1::2]
    plt.plot(i_sample, label='I')
    plt.plot(q_sample, label='Q')
    plt.legend()
    plt.show()


if __name__ == '__main__':
    lists_from_file = read_file_to_lists(IQ_LOG_FILE)
    plot_iq(lists_from_file[2])

It assumes the IQ is stored into the file in a format like this:

i11, q11, i12, q12, ...., i1n, q1n
i21, q21, i22, q22, ...., i2n, q2n

Each line has the IQ samples of one single CTE capture, I comes first and then Q, and they are seperated by comma

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Louis Chen — Mon, 07 Apr 2025 03:26:09 GMT

Hi Saleh,

I am currently using 52833-DK to try to output IQ sample plot, but I cannot output the diagram like yours. Can you please provide the corresponding code including algorithm or filtering method?

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Simonr — Tue, 04 Jul 2023 08:48:00 GMT

Hi Allen

I see you have created a ticket of your own here. I'll reply to you there, as this ticket is ~2 years old at this point.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

AllenWang — Tue, 04 Jul 2023 08:42:06 GMT

Hello, Saleh:

have you got this question resolved. I met the same issue as you. hope get kindly guidance from you

thanks and best wishes

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Piotr — Fri, 06 Aug 2021 21:10:26 GMT

Thank you for the suggestion, we will consider possibility to go with 1M PHY.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Fri, 06 Aug 2021 13:47:29 GMT

Thank you for clarification, It helped too much.

To prevent any misunderstanding specially for beginners, I suggest to change the default physical bit rate of the "tx sample", from 2MB to 1MB, by adding the "BT_LE_ADV_OPT_NO_2M" parameter option.

Also as you can find in this document: https://www.bluetooth.com/bluetooth-resources/bluetooth-direction-finding/

Page 26, part "5.3 Choices of PHY", has declared that support of 1MB is mandatory and 2Mb is optional. so from this point of view, choosing the 1MB as the default bit-rate could be a better option.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Piotr — Fri, 06 Aug 2021 13:15:29 GMT

Referring to you comments.

2. The offset between samples may or may not be visible with PHY_2M. Take a look here:

The plot presents IQ samples where there is visible offset in selected I and Q. It was produces on IQ data collected with the same FW I've been using to prepare former plot. This one presents IQ data for different CTE.

In case of PHY_1M, BT Core spec samples spacing in ref. period 1us provides more information for a period of the RF wave, so there is more likely that there won't be the situation with not visible offset between I and Q samples.

3. TSAMPLEOFFSET offset is a fine-tunig mechanism implemented in DFE. It may introduce better results when applied but it is not necessary. Radio takes samples in appropriate moments at least 125ns after beginning of the sampling slot.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Fri, 06 Aug 2021 12:27:47 GMT

Thanks a lot Piotr,

I was working on this issue for three days, there are some comments that I want to share:

1- Yes, Also I found both transmitter and receiver are working as expected and there is a phase-shift between "I" and "Q".

2- The reason that we can not see this phase-shift in the samples, is the frequency offset of CTE signal. when PHY_2M is used, the offset is 500kHz and when PHY_1M is used, it will be 250kHz. By default "PHY_2M" has chosen in Zephyr examples, so the offset of CTE signal is 500kHz.

According to the sampling frequency during the "reference period" which is done every 1us, the result is not clear enough to see the phase-shift.

the offset frequency of different physical bitrate has documented in this white paper, page 13: https://infocenter.nordicsemi.com/pdf/nwp_036.pdf

3- Maybe it is not directly relevant, but as you can see in the above white paper (page 12), it is needed to set the "TSAMPLEOFFSET" in order to read the samples when A/D converter has settled. Now this value is not set and is zero by default.

4- I changed the physical bit rate to 1M by adding the "BT_LE_ADV_OPT_NO_2M" option to the advertiser parameter of the transmitter:

static struct bt_le_adv_param param =
		BT_LE_ADV_PARAM_INIT(BT_LE_ADV_OPT_EXT_ADV |
				     BT_LE_ADV_OPT_USE_NAME |
				     BT_LE_ADV_OPT_NO_2M,
				     BT_GAP_ADV_FAST_INT_MIN_2,
				     BT_GAP_ADV_FAST_INT_MAX_2,
				     NULL);

And now you can see the phase-shift obviously between "I" and "Q" samples.

Regards,

Saleh

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Piotr — Fri, 06 Aug 2021 11:16:45 GMT

Hello,

I've taken a look into the issue. I've made some small custom change to enable oversampling of received CTE. Samples spacing I've used is 125ns instead of 1us that is BT Core spec ref. period spacing.

Pay attention that samples were collected before preparation to transport through HCI layer. They have different amplitude than what you would observe in application layer.

Have a look on the plot below.

I and Q lines are samples collected every 125ns. Selected I and selected Q are samples you would observe in application (selected every 1us)

As you can clearly see, the selected I and Q look like there is no PI/2 phase shift between actual I and Q. In reality the shift is there but undersampling makes is not visible.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Thu, 05 Aug 2021 13:05:43 GMT

Thanks for your time Simnor

I have attached the recorded file, the format is:

<i>, <q>, <i>, <q>, ......, <i>, <q>\n

devzone.nordicsemi.com/.../iq_2D00_locator_2D00_nrf52833.txt

every line contains the IQ data captured from a CTE signal. The sample time slot is set to 1us, so in total 82 IQ samples are available. to reproduce it at your site, you can simply run the "direction finding example" and print the IQ samples. for more information please take a look at this issue:

https://github.com/zephyrproject-rtos/zephyr/issues/37412

Regards,

Saleh

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Simonr — Thu, 05 Aug 2021 12:05:31 GMT

Sorry about the misunderstanding on my part, I interpreted the BLE spec. incorrectly. I discussed this with a few colleagues, and what you're expecting to see (the phase shift between I and Q) should indeed be the case. The I and Q samples are always measured at the same point in time so they should indeed be phase shifted. Can you provide the IQ data itself so we can take a look on our side?

Best regards,

Simon

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Wed, 04 Aug 2021 15:03:59 GMT

Thanks Dmitry

Exactly! you are right. It should be 2 cycle at reference period (each cycle takes 4us so two cycle during 8us reference period).

Also it is mentioned in this references that CTE signal carries a 250kHz signal, so Dmitry assumption is correct:

Ref1: https://infocenter.nordicsemi.com/index.jsp?topic=%2Fnwp_036%2FWP%2Fnwp_036%2Fconstant_tone_extension.html
Ref2: https://dev.ti.com/tirex/explore/node?node=AHYhhuDNTaRXzkOlahOlvA__pTTHBmu__LATEST
Ref3: https://www.silabs.com/documents/public/application-notes/an1297-custom-direction-finding-solutions-silicon-labs-bluetooth.pdf

It seems the transmitter is sending an incorrect signal, maybe it is a software problem Simonr

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Wed, 04 Aug 2021 14:29:04 GMT

Yes it is, and can you explain why there is no phase-shift between "I" and "Q" signal during the "reference period" while it should be? As you mentioned before 90 degree difference should be visible between these two signal.

I think something is wrong at the lower layers that control the BLE.

Regards,

Saleh

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Simonr — Wed, 04 Aug 2021 13:27:24 GMT

The requirement in the BLE specification is "the receiver shall take an IQ sample each microsecond during the reference period and an IQ sample each sample slot" and that samples are taken "at the same point within each IQ Sampling Window". This is a requirement for the IQ samples being reported by the Controller to the Host.

Best regards,

Simon

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Dmitry — Wed, 04 Aug 2021 11:47:37 GMT

[quote userid="106258" url="~/f/nordic-q-a/78189/problem-with-i-and-q-samples-during-the-reference-period-of-cte-packets/323248#323248"]While your point is correct but I don't think it is the source of the problem in this case.[/quote]

Maybe, but you can see that frequency in reference period is two times higher than at third party device. 250 kHz shoud give two full sine periods at 8 usec, you have four.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Wed, 04 Aug 2021 11:05:31 GMT

Thanks Simnor, but I think your first assumption was correct.

I know there is 90 degree phase-shift between I and Q samples, my question is why this phase-shift is not visible at the first 8 samples which corresponds to the "Reference Period" of CTE Signal.

Three first images of the main question, belongs to the samples generated by the Zephyr examples, and as you can see, "I" and "Q" have same phases at the first 8 samples in this images.

and

and this one

But fourth image is a third party implementation and you can see the phase-shift in this image.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Simonr — Wed, 04 Aug 2021 10:52:17 GMT

I'm sorry, I might have misunderstood your initial question. Are you asking why the I and Q are 90 degrees phase-shifted? If so, please check out section 2.4 in the whitepaper explaining the I and Q components.

Best regards,

Simon

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Wed, 04 Aug 2021 08:43:03 GMT

Thank you Dmitry for your answer,

The reference period is 8us, and in all cases the sampling rate during this period is 1us, so exactly 8 samples are captured during this period in all cases. While your point is correct but I don't think it is the source of the problem in this case.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Dmitry — Wed, 04 Aug 2021 08:30:42 GMT

Hi,

from the polts I can only see that you have reference period with 2-usec slots while third party product has 1-usec reference slot. If the signal is sampled at 1/2 of its frequency, you cannot see a 90-degree phase shift on the graph.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Saleh — Wed, 04 Aug 2021 08:16:50 GMT

Hi Simonr

As described in the this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_036.pdf , part 4, Table 4: Antenna switching settings, same antenna has been used during the "reference period" , so despite of using antenna array or pcb antenna, similar behavior is expected during this period.

Anyway the first two images are captured by pcb antenna, the third one has captured using the antenna array and the problem exist.

To ensure about this problem, I executed the third party implementation (which our algorithm can calculate the transmitter angle based on its results) with the pcb antenna on nRF52833-DK (same as what I have used to run Zephyr examples) ad you can see the results here:

Obviously the "I" and "Q" samples have different phases in the reference period of CTE signal.

RE: Problem with I and Q Samples During the Reference Period of CTE Packets

Simonr — Wed, 04 Aug 2021 07:40:15 GMT

Hi Saleh

I'm guessing that your locator device is not using an antenna array, as receiving this data with only one antenna will result in a stable phase because there is only one antenna receiving these samples.

Best regards,

Simon