measurement of RSSI or transit time of a message via BLE with the Nordic 52840 and another slave board.

[Leonardo Balocchi]

I have this problem, with a Nordic nRF52840 board I have to transmit a byte of known length, which I have to select a priori, and transmit it to a second slave evaluation board and then measure the RSSI value and the time it takes for the data packet to arrive depending on where the board is located, thus also going back to the distance value at which the two boards are located.
I would then also like to do the same thing with my android smartphone used as a slave.

I have tried to follow the examples I can find of nRF connect for desktop a bit, and in particular the codes available in visual studio code, but the standard codes there do not seem to answer my question.
Could you perhaps help me by explaining where to find an example code to modify to achieve this, and how to program the second board via the nRF52840?

thanks in advance!

[Amanda Hsieh]

Hi, 

Using signal strength (RSSI) for estimating distance is not a reliable way for getting precise distance measurements. Since there are so many factors such as reflections and physical blocking affect the signal strength of the signal, hence the calculation of distance. I would suggest you consider this sample Bluetooth: nRF Distance Measurement with Bluetooth LE discovery which would also report RSSI value. Also, take a look at this post. 

Regards,
Amanda H.

[Leonardo Balocchi]

Thank you Amanda for your reply.

I have managed to get the two Nordic boards talking.
Now what I would like to be able to do is to save the various RSSI, IFFT and RTT values in a file, so that I can then graph the time trend of these values.
Can you please suggest a method for doing this?

If there is a possibility, it would also be useful to save this file inside the nRF52840, so that I don't have to have it connected to the PC during measurements. And then later transfer those data saved in the non-volatile memory to the PC.

Thank you again for your support!

[Amanda Hsieh]

Hi, 

Leonardo Balocchi said:

I have managed to get the two Nordic boards talking.

That's good news. 

I would suggest at least one DK connect PC to check the log and make sure these two DK work as expected. Then, you could also save those values by the terminal tools. 

-Amanda H.

[Leonardo Balocchi]

Hello Amanda, thanks for your reply.

The two boards communicate, but I have encountered a problem.
I'll explain the hw and sw setup: one board is the nRF52840 which was loaded with the nrf_dm application, the other is the evaluation board BC840E which has the default program with which it was pre-loaded (so I didn't program it).

I placed the two boards 1m apart, and for the 2.4 GHz frequency I should have a path loss of 40.04 dB.
On the BC840E board I connected a 5 dBi gain antenna.
Knowing that the nRF52840 in transmission I set it for 0 dBm and that the antenna gain on the nRF52840 is 0 dBi, I should see about -35 dBm of RSSI, the problem is that the measured RSSI is about -2 dBm.
(if I don't put the external antenna on the BC840E the count doesn't add up, in fact I see rssi_openspace=199.53, which isn't correct anyway).

I can't tell if I have misinterpreted the RSSI value, can you please clarify this for me?

Thanks in advance!

[Amanda Hsieh]

Hi, 

I would suggest programming both boards with the nrf_dm application and comparing the result of using two nRF52840DKs with the nrf_dm application.

-Amanda H.