RSSI smoothing methods

[arsalanjun]

Hi,

I am planning to make a program so that when a beacon goes far away a specific distance (around 5m) then the led should light up.

I have taken some measurements and they are very unexpected. I would like to know if anyone has used BLE for this purpose or not and how did you implement it.

My measurements are as follows. Distance is in meters.

For the above example, at 5 meters sometimes I get RSSI -98, sometimes -100 and -101.

So I tried to tackle this with another method. Now this time the RSSI is fixed and the distance varies. Please view the image below.

If we look at the 9th measurement, we get -100 RSSI at 7m, 8m and 7.5m.

Do you guys have any suggestions?

[endnode]

Hello arsalan,

The suggestion is: don't use BT LE RSSI for anything related to distance measurement because results are very uncertain.

If you go deeper into how 2.4GHz radio works you will see that signal can be reflected and shielded by pretty much everything. The topology can be linked to RSSI only in very isolated (laboratory) conditions, otherwise you can maybe talk about some close/medium/distant zones and accuracy (relating RSSI to such "zones") of less then 100%. If you see all these "BT RSSI vs. distance" or "Wifi RSSI vs. distance" (which is very similar physics) solutions they usually come to only one or two thresholds which is making zone boarders and then they more or less say where the thing is. If you really want something more accurate then you need to work with multiple broadcasters and receivers model, known topology and calibration. Nothing for low cost solutions such as "beacon and mobile phone".

Btw. there are several posts on this topic (search for RSSI) on this forum and conclusions are basically the same.

Cheers Jan

[arsalanjun]

Thanks for your answer Jan. I a aware that RSSI and BLE are not meant to be used to measure distance but I just want to use it as a proximity purpose. Just like the advertisement of BLE proximity (here), I want to use it to turn on an LED after a specific distance.

How to implement this? Turn on a buzzer/led when link is lost? or turn it on at a specific RSSI dbm?

[Øyvind Karlsen]

If visuals are important you could PWM a LED when you enter the RSSI range that you want to trigger at, let the LED get more and more lit as you get lower and lower RSSI relative to your decision level. Otherwise just turn on a led when you have 10 samples under a threshold or something.

[arsalanjun]

PWN is a good idea Øyvind but the RSSI varies a lot. How do you smooth the values so that you do not get a lot of variations?

[Øyvind Karlsen]

To be honest, I am not sure. I'm guessing that some form of averaging will net you better results, but you will have to try this out. Due to the nature of the 2.4 GHz radio signals there will be a lot of fluctuations.