AoA IQ Data soundness

RE: AoA IQ Data soundness

swz_nordic — Fri, 18 Nov 2022 02:58:40 GMT

OK,I think that the pdu antenna only switch in the first time.I use the logic analyzer to test the switch of antenna pattern.I have the IQ samples as follow.There are 4 antennas and 3 antennas are switch.

The IQ sample as follow:

CTE[0]: samples count 82, cte type AOA, slot durations: 1 [us], packet status CRC OK, RSSI -610
IQ sample content :
sample[0] = I:10 Q:-4
sample[1] = I:-11 Q:3
sample[2] = I:10 Q:-3
sample[3] = I:-11 Q:2
sample[4] = I:10 Q:-4
sample[5] = I:-11 Q:2
sample[6] = I:11 Q:-1
sample[7] = I:-11 Q:2
sample[8] = I:-8 Q:1
sample[9] = I:-10 Q:-3
sample[10] = I:-13 Q:7
sample[11] = I:-8 Q:0
sample[12] = I:-10 Q:-4
sample[13] = I:-13 Q:5
sample[14] = I:-9 Q:0
sample[15] = I:-9 Q:-5
sample[16] = I:-14 Q:5
sample[17] = I:-8 Q:-1
sample[18] = I:-9 Q:-5
sample[19] = I:-14 Q:4
sample[20] = I:-8 Q:-2
sample[21] = I:-9 Q:-6
sample[22] = I:-14 Q:2
sample[23] = I:-8 Q:-2
sample[24] = I:-9 Q:-6
sample[25] = I:-15 Q:2
sample[26] = I:-8 Q:-2
sample[27] = I:-8 Q:-7
sample[28] = I:-14 Q:0
sample[29] = I:-9 Q:-3
sample[30] = I:-8 Q:-7
sample[31] = I:-14 Q:0
sample[32] = I:-8 Q:-4
sample[33] = I:-7 Q:-8
sample[34] = I:-15 Q:-2
sample[35] = I:-8 Q:-5
sample[36] = I:-7 Q:-8
sample[37] = I:-15 Q:-2
sample[38] = I:-7 Q:-5
sample[39] = I:-5 Q:-9
sample[40] = I:-15 Q:-3
sample[41] = I:-7 Q:-5
sample[42] = I:-6 Q:-9
sample[43] = I:-14 Q:-4
sample[44] = I:-6 Q:-6
sample[45] = I:-4 Q:-10
sample[46] = I:-14 Q:-5
sample[47] = I:-6 Q:-7
sample[48] = I:-4 Q:-10
sample[49] = I:-14 Q:-7
sample[50] = I:-6 Q:-7
sample[51] = I:-4 Q:-9
sample[52] = I:-13 Q:-8
sample[53] = I:-6 Q:-7
sample[54] = I:-2 Q:-11
sample[55] = I:-13 Q:-8
sample[56] = I:-5 Q:-8
sample[57] = I:-1 Q:-11
sample[58] = I:-12 Q:-10
sample[59] = I:-5 Q:-8
sample[60] = I:-1 Q:-11
sample[61] = I:-11 Q:-10
sample[62] = I:-4 Q:-8
sample[63] = I:0 Q:-11
sample[64] = I:-10 Q:-10
sample[65] = I:-3 Q:-9
sample[66] = I:0 Q:-11
sample[67] = I:-9 Q:-12
sample[68] = I:-2 Q:-9
sample[69] = I:1 Q:-11
sample[70] = I:-9 Q:-11
sample[71] = I:-2 Q:-8
sample[72] = I:1 Q:-11
sample[73] = I:-8 Q:-13
sample[74] = I:-2 Q:-9
sample[75] = I:2 Q:-10
sample[76] = I:-7 Q:-13
sample[77] = I:-1 Q:-9
sample[78] = I:4 Q:-10
sample[79] = I:-7 Q:-15
sample[80] = I:-1 Q:-9
sample[81] = I:4 Q:-9

RE: AoA IQ Data soundness

Lennonbfn — Thu, 17 Nov 2022 13:43:58 GMT

[quote userid="114049" url="~/f/nordic-q-a/92265/aoa-iq-data-soundness/396120"]I think that the antenna pattern should include the pdu antenna,the pdu antenna should be placed first.Besides this,I have the sreen photo of six antennas.[/quote]

The first antenna in 'ant_patterns' is used to guard period+reference period.

If you use the PDU antenna in first of 'ant_patterns', this antenna first receive the PDU and after this is used to guard period+reference period. I think that it can work too.

The difference is that you can't see the 12us of guard period+reference period in GPIOs Signals, because from the PDU packet to guard period+reference period there is not an antenna switch as you can see below.

Before the red line, is PDU antenna (0xC)(0b0011). Between the red lines 12us (0x0)(0b0000) that is the first antenna in 'ant_patterns'. After the red lines we have antenna (0x2)(0b0010) and antenna (0x1)(0b0001) alternating.

RE: AoA IQ Data soundness

swz_nordic — Thu, 17 Nov 2022 06:21:49 GMT

I think that the antenna pattern should include the pdu antenna,the pdu antenna should be placed first.Besides this,I have the sreen photo of six antennas.

RE: AoA IQ Data soundness

Lennonbfn — Thu, 17 Nov 2022 03:04:34 GMT

I was going through the same difficulty. I used a oscilloscope to measure the gpios that control the switch pattern. I bring an image of GPIOs measurements and the variable configuration to contribute.

Upper left side are the antenna array configuration variables.

The antenna used as PDU, is activated by default when outside the switching period as we can see "ant 9" in the figure.

The first value in 'ant_patterns' is the antenna used to sample during the guard period+reference period as we can see 12us in the figure.

The IQ sampling will occur in the other two antennas "0x2,0x1" of 'ant_patterns' for the example mentioned.

In short, for this configuration, we have.

RE: AoA IQ Data soundness

ReubeyGalleos — Mon, 03 Oct 2022 16:07:20 GMT

I've aquired a scope to be able to probe the GPIO pins and have been able to answer my questions about how the ncs sample uses the ant_patterns array. The first pattern is held for 12us, which corrolates with the Guard + Reference periods from the AoA spec. The antenna is then switched to the next pattern in the array, where the first (non-reference, 8 reference samples are taken in the aforementioned reference period) IQ sample is taken. The patterns array is then traversed by the software, and if the number of samples is greater than the length of the patterns array, the software will loop back to the first element in the patterns array, the same pattern used for the guard and reference periods.

RE: AoA IQ Data soundness

Simonr — Wed, 28 Sep 2022 08:06:06 GMT

Hi Reubey

1. Sorry about the "basic" information first. It seems I mistook your experience with Direction Finding initially. The switch pattern is set by the ant_patterns array in the main.c file. I'm not sure what "PDU" antenna pattern you refer to in the .overlay file, as the only things defined there by default are the number of antennas, the setting to set which antenna to enable for RX PDU, and the GPIOs used for antenna switching.

2. The range of "good IQ values" is hard to define, as it depends on the use case, environment, antennas in the array, and number of antenna arrays in your application. From testing on our end with our 12-antenna array we've seen good IQ values up to 7-8 meters at trade shows where we have multiple other demos running and generally lots of interference from phones and computers, etc. In less noisy environments (if my memory serves me right) I think we've seen good IQ data at up to ~20 meters.

I've also seen use cases where multiple antenna arrays are used, to give a cm accuracy in rooms up to 5x5 meters.

Best regards,

Simon

RE: AoA IQ Data soundness

ReubeyGalleos — Tue, 27 Sep 2022 12:28:04 GMT

Hi Simon, thank for your reply.

I understand the function of the guard and reference periods, my confusion lies with exactly how the NCS direction_finding_connectionless_rx example assigns antenna patterns for these periods. The demo is highly obfuscated as it makes heavy use of HCI RPMsg calls to do most of the interfacing with the radio registers. Because of that I can't determine what data it uses to perform the first two writes to the SWITCHPATTERN register, whether this comes from the first two entries in the ant_patterns array or whether it uses the 'PDU' antenna pattern provided in the .overlay file. If this is the wrong place to ask for specific technical details about the NCS sdk I apologise.

My other main question in the opening post was regarding the expected range of IQ values. While the technical documention provides information on what size of information can fit in the DMA registers used to store the IQ values, I haven't been able to find information on what range I should expect "good" IQ values to fit within. Any information you could provide on this would be invaluable.

RE: AoA IQ Data soundness

Simonr — Tue, 27 Sep 2022 09:12:06 GMT

From the Direction Finding technical overview from Bluetooth, you can see that the Guard and Reference period is done first, then you do switching between antennas and sampling. Here you need to switch between all (3 in your case) antennas until the sampling period is over.

Guard periods, used in many communications systems, are a technique designed to ensure there’s a gap between adjacent transmissions so that they do not interfere with each other.

During the reference period, 8 x IQ samples are acquired from the first antenna at 1µs intervals. No antenna switching occurs during the reference period. The host may be able to use the 8 reference samples to estimate the frequency of the signal and, from this, the wavelength. This allows more accurate angle calculations to be performed.

Sample and switch slots may either be 1µs or 2µs long. Support for 2µs slots is mandatory while support for 1µs is optional. HCI configuration indicates which slot length is to be used by the controller.

The full technical overview can be downloaded here.

Best regards,

Simon

RE: AoA IQ Data soundness

ReubeyGalleos — Mon, 26 Sep 2022 13:11:01 GMT

Apologies, I don't think I made my enquiry very clear. I'm familiar with the whitepaper, but as I'm using the NCS as that would be what we use in production, certain things are unclear. In the whitepaper, the switching sequence the first 2 writes to the SWITCHPATTERN register are used to set the antenna's used in the Guard then Reference periods. Subsequent writes are used to set the sequence that will be repeated if there are slots remaining after having being traversed. In NCS however, we set the "PDU" antenna in the .overlay file, and the README.rst says "If the number of switch-sample periods is greater than the number of stored switching patterns, then the radio loops back to the first pattern." This to me implies that the ant_patterns array shouldn't contain the Guard and Reference antenna patterns, however from your response I assume this is incorrect, and the format of the ant_patterns array should instead be [GUARD, REFERENCE, SWITCH1, SWITCH2...]. Can you confirm this is correct?

RE: AoA IQ Data soundness

Simonr — Mon, 26 Sep 2022 12:29:31 GMT

As mentioned already here, you need to add a switching pattern for the number of antennas you want to use which seem to be 3 antennas here. If you're new to Direction Finding we also have a whitepaper here with some more in-depth information on how Direction Finding works.

1. Indeed, 3 antennas are not likely to give a very accurate result as the triangulation will be very rough. I don't think you can expect better than ~1 meter worth of accuracy. As "how flakey" is very much a case of definition, and will vary very on the range from transmitter to receiver and the environment it operates in, I'm not sure what I can share on this.

2. The raw IQ data that is transmitted must be converted into vector form to be reviewed properly. We don't have have any specific sample code showing this, but you can check out this page for information on the various formulas used to translate IQ data between polar and rectangular form. Nordic will not provide a Direction Finding algorithm for you I'm afraid.

Best regards,

Simon

RE: AoA IQ Data soundness

swz_nordic — Sat, 24 Sep 2022 00:45:11 GMT

If you want to use the 3 antenna, you should configure the antenna switching parttern for 3 antennas not 2 antennas.As for reflection and noise,how do you distinguish it?I think that there are error factors,but i can't prove it.Besides that,my I/Q values are smaller than yours, they are in the range 0±30.i use the nrf52833dk,if you have the interest,we can make a talk.