Direction Finding Phase Shift Between Reference Period And First Slot

[wvfan]

Hi,

When I tried to use BLE5.1 phase data to achieve direction finding, I found a phase shift between reference period and the first slot, as the below graph shown:

The range of y-axis is 2 Pi. The antenna used for the reference period and the first slot is the same, but there is a phase shift of around 1 Pi ( a half of period ). I have tried for several different angles and all phase shifts are similar. I saw a paper that uses different antennas in reference period and the first slot to calculate the angle, so the extra phase shift should not happen. Will it affect the calculation of subsequent data? If it will, how can I solve this or put it into calculation?

Best Regards,

Fan

[Øyvind]

Hello Fan, 

Can you please elaborate on your setup? What is the reference point? Have a look at our white paper of direction finding. 

Unfortunately, this is all we have. 

Kind regards,
Øyvind

[wvfan]

Hi Øyvind,

Thank you for replying. Here is my setup of radio:

NRF_RADIO->MODE = RADIO_MODE_MODE_Ble_1Mbit;
NRF_RADIO->CTEINLINECONF = RADIO_CTEINLINECONF_CTEINLINECTRLEN_Disabled;
NRF_RADIO->DFEPACKET.MAXCNT = 1250;
NRF_RADIO->PSEL.DFEGPIO[0] = ANT1;
NRF_RADIO->PSEL.DFEGPIO[1] = ANT0;
NRF_RADIO->CLEARPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 3;
NRF_RADIO->SWITCHPATTERN = 1;
NRF_RADIO->SWITCHPATTERN = 1;
NRF_RADIO->SWITCHPATTERN = 1;
NRF_RADIO->SWITCHPATTERN = 1;
NRF_RADIO->PACKETPTR = (uint32_t)&packet;
NRF_RADIO->DFEMODE = RADIO_DFEMODE_DFEOPMODE_AoA;
NRF_RADIO->DFECTRL1 = 20 << RADIO_DFECTRL1_NUMBEROF8US_Pos
| 1 << RADIO_DFECTRL1_DFEINEXTENSION_Pos
| RADIO_DFECTRL1_SAMPLETYPE_MagPhase << RADIO_DFECTRL1_SAMPLETYPE_Pos
| RADIO_DFECTRL1_TSWITCHSPACING_4us << RADIO_DFECTRL1_TSWITCHSPACING_Pos
| RADIO_DFECTRL1_TSAMPLESPACINGREF_125ns << RADIO_DFECTRL1_TSAMPLESPACINGREF_Pos
| RADIO_DFECTRL1_TSAMPLESPACING_125ns << RADIO_DFECTRL1_TSAMPLESPACING_Pos;
NRF_RADIO->DFEPACKET.PTR = (uint32_t)&rfRaws;

The phase graph is after processed. I made the gap between all adjacent phases less than half the period so the curve will be continuous. And I used the linear regression line as the reference, and got the final curve as the above image shows.

Could you please provide a typical phase and magnitude graph as a reference? Currently I can get the IQ sample data, but it's not stable. There are too many possible issues that can cause this, like RF switch, antenna or programming, so I need some way to debug with it.

Best Regards,

Fan

[Dmitry]

Hi,

take a look at this section of document provided by Øyvind, the key phrase is "The hardware always aligns the first IQ sample in the first sample slot". After a reference period, there is a gap about 1/2 of TSWITCHSPACING (first switching slot) - 2 usec in your case. It has no impact on accuracy and need not be accounted in calculations.

wvfan said:

I saw a paper that uses different antennas in reference period and the first slot to calculate the angle

Even if it works, it's not the way the hardware is intended to use - Bluetooth Core specification puts a requirement to take samples at same points of all sample windows but nothing is said about reference period.

wvfan said:

Currently I can get the IQ sample data, but it's not stable.

What do you mean exactly? Are IQ data differ from sample to sample in a static environment, or you can't get a  stable relation between angle and phase shift? When testing indoors, multipath reflections are the main issue. Also it's crucial to have a well-designed array to get at least useful results, but your graph looks quite accurate.

[wvfan]

Thank you Dmitry. From your answer and the document, the reference period is correct and not a issue.

The unstable means the phase shifting and angle will change from time to time. I just did a test and get these phase differences: 93, 93, 90, 94, 79, 95, 103, 103, 103, 99, 95, 110, 104, 98, 106, 87, 100, 94. The maximum is 110 and the minimum is 79, for two antennas with 6.25cm distance, it's 33 degree and 23 degree ( but actually my angle is set to 75 degree ). I don't know how to calibrate with the data, if I just add a fixed delta to make the average angle to 75 degree ( phase diff 194 ), the 110 and 79 will become around 210 and 179, and now the angle is around 90 degree and 63 degree, which cannot be used.

Thank you for the information about the multipath reflection, can it be detected by the magnitude data? I will have a try. And also for the antenna array, I will reconsider it to avoid degree near 90.

[Øyvind]

What antenna are you using? The figure you have attached does not give much information I'm afraid. 

You need to measure the phase on all antennas against the reference slot. The reference phase should not be static, as the frequency on transmitter and receiver are not the same. The phase will drift in a linear fashion. 

Kind regards,
Øyvind