nRF51822 beacon kit: big notches in RSSI vs distance plot

[classicman]

Hello everybody,

I'm designing an app to track the position of the smartphone using six beacons and, in order to do that, I made some measures both indoors and outdoors and plotted a graph of RSSI vs distance using the data acquired from nRF Connect app (exported to .csv and fed to Matlab). The first set of measures was taken in a corridor about 3m wide and 35m long at different transmission power level, the second set was taken outdoors in an open field about 30x25 m at 0 dBm transmission power. Both the plots show serious notches, as you can see from the pictures, causing great troubles with the accuracy of the tracking.

I'd like to know if this behaviour is to be expected or there is something wrong in the setup of the measures.

Thanks.

[endnode]

Hi Classicman,

As Roger and Daniel pointed out, scalar RSSI is pretty much unusable for distance measurement in real environment unless you will start to do thing such as correlation of measurements at multiple points, filtering, fingerprinting of particular topology (= calibration of your algorithms to known properties of given room/corridor/building) etc. You can find many questions and answers which are pointing to this known property of 2.4GHz radio on this forum.

You can still use RSSI as "rough" indication of relative distance or movement direction but you need to understand that even thing like "I have now mobile phone in back pocket of my jeans" can change the value by 10 or 20dBm and you can only guess if the source suddenly moved 10 meters away or if some shielding/reflection happened.

Cheers Jan

[endnode]

Btw. I've found one older study (Thesis) showing how doomed RSSI is for indoor location positioning unless you invest years and millions of dollars in effort and SW to it.