RSSI switching values

[Srikant]

Hi, (I'm a computer scientist, please be kind)

In the below attached image, I have collected the data outdoors (so no signal reflection or signal fading) from a single BLE beacon at two different pre-set power levels. I see that RSSI values fluctuate in a systematic way over time between two modes. Can someone please explain this phenomenon? Why this happens? How can we reduce / remove this?

RSSI vs time

[endnode]

What is the receiver and what is transmitter? Are both nRF5x chips with Nordic stack and custom FW? From the first look I would say: this is how typical "static" RSSI plot looks like. Even you think that both transmitter and receiver are static and that nothing is changing in the environment the signal strength and radio stages are so sensitive that even small changes and noise (e.g. from power stage on the board or inducted from any other source in the system) cause +-10dBm fluctuations. This is the main reason why most of these "location tracking" systems based on RSSI are failing (or take big effort to "smooth" the curve and cross check the data from many receivers and patterns).

[Matt]

Outdoor as in a soccer field?

[Srikant]

Hi @endnode,

Thanks for the lightning fast reply. The transmitter is nRF51822 chip but the receiver is a smartphone device. A noise would cause random fluctuations but this looks (very)periodic. Don't you think? I need to model this data for localization problem. Just wondering why this happens so that I get a better understanding about the measurement process.

[Srikant]

In the lawn, no user presence here. The beacon and mobile device (mounted on tripod) were half a meter apart.

[endnode]

I can only say that this is what most of (cheap) Android phones do and I've seen such periodic fluctuations many times. It's very hard to say what is actual root cause (and even radio engineers might not be able to tell without spending weeks in the lab with particular electronics and isolating the things on different layers one by one) but my theory is that systems with volume of 150x150x8mm3 packed with several antennas + 4-band GSM/3G/LTE + Wifi (sometimes dual-band) + Bluetooth + GPS(Glonass...) + bunch of more or less unshielded chips and all that powered from battery, it's even mystery for me how all of that can work so well. Having +-10dBm noise on some of these radios when you are +30/50dBm from sensitivity thresholds is probably peanuts for handset makers.