Using nRF52811 for direction finding

[greentree]

nRF52811 has only one receiver. Most accurate direction finding can be made based on estimation of phase difference between two signals received in two spaced antennas. Such phase difference can be accurately estimated if we take samples in two receiving channels at the same moment. So, two receiving channels shall have the same coherent LO source and samples shall be made synchronously. Correspondingly, I would like to ask have 3 questions:

1. Can we use two nRF52811 systems with the same coherent LO source?

2. Can we synchronize IQ sampling moments in both nRF52811 systems?

3. If question 1 and 2 are negative, what AOA estimation approach shall be used with nRF52811 ?

If my questions are incorrect, I would appreciate any clarifications regarding that.

[greentree]

Hi Simon,

Thank you. I think I got the idea of Direction Finding implemented in nRF52811. IQ sampling is made over demodulated signal having frequency of 250kHz or 500kHz. Since signal frequency is relatively low, samples taken in different antennas can be effectively synchronized. Yes, we can get an estimate of AOA. The accuracy of that estimation will be low. 

Many factors affects the accuracy of direction finding. SNR, multipath propagation and relative distance between antennas are some of the most powerful factors. Influence of SNR is quite understandable. Effect of multipath is more complicated and it may be much more powerful than effect of low SNR. All errors will be aggregated in error of estimated phase difference between signals received in two spaced antennas. After all, accuracy of angle estimation is mostly an accuracy of phase difference measurement multiplied by ratio of wavelength to a distance between antennas.

If signal frequency is 500kHz, the wavelength is 600 meters. If we have two antennas allocated only at 1 meter from each other, error of AOA estimation (at least when real AOA is 1 ... 3 degrees) is defined as 600 * phase difference error. Multipath may (and will) shift phase estimate to some value. Shift of AOA estimate due to that only reason will be very significant. That is why it is better to make measurements at the carrier signal. If carrier frequency is 2.4GHz, wavelength is 0.125m.

However, that is all a theory.

Did somebody make indoor phase measurements (for instance with SDR) when the signal frequency is 500kHz?

How the real life environment affects the measurements?

Best regards,

Vlad.

[Simonr]

Hi Vlad

We currently don't have these details/measurements in place as this is work in progress.

Best regards,

Simon

[greentree]

Thank you Simon,

Maybe somebody from DevZone tested that method using SDR? 

Unfortunately, theoretical estimation of the method doesn't look encouraging. Assume we have 1 meter distance between antennas  and measure phase difference between signals of 500kHz. If AOA will be changed from 0 to 30 degrees, corresponding phase difference will be changed from 0 to 0.3 degree... Real phase errors may be over 1 degree. Utilization of multiple baselines will slightly improve accuracy of measurements, but we need to increase frequency of the signal and distance between antennas significantly to achieve acceptable accuracy of the system. If we increase frequency, it goes out of Bluetooth conception. Even 1 meter of distance is a quite large size of antenna array for indoor applications.

I appreciate any comments. Maybe I missed something?

 

[greentree]

Hi Simon,

I would like to clarify one more question to understand how it may work in a good way.

Does nRF52811 have a two channels receiver? Only "yes" or "not" would be enough.

Best regards,

Vlad.

[Simonr]

No, it does not.

Best regards,
Simon