Recieved Signal Strenght Indicator (RSSI) with obstacles

[Quinten]

Hi there,

I'm trying to design a positioning system that makes use of RSSI.
When a wireless signal would propagate through a human body, the signal strength would be weaker than when it wouldn't have to overcome any obstacles. My question is: would this difference in signal strength be measurable? (Keep in mind, the sensor that measures the RSSI is a smartphone, so sensitivity is limited)
If anybody has any information on the subject I would really appreciate it! ;)

 

[Baluncore]

Welcome to PF.
The human body contains so much salt water that a smartphone signal will not easily pass all the way through it. The presence of a body moving nearby will distort the signal received by the phone. The field will tend to move in multiple paths around the body. Sometimes there will be +3dB more signal, other-times there will be deep nulls with much less, probably –30dB. That is more than sufficient to sense using RSSI.

 

[Quinten]

So it is possible that the signal strenght doubles? If it would be -3dB it would be perfect...
Is the +3dB the result of superposition of several waves passing around the body? (If so, would it occur a lot? Or is it more an exception?)

 

[Baluncore]

Yes, all the significant effects are due to the constructive and destructive interference pattern created by the multiple paths around the moving body. The body dimensions are less than 60 wavelengths, so the body will be in the near field of the receive antenna.

There will be a few signal points with more than +3dB. The deep nulls will occur where two similar amplitude signals via different paths cancel due to opposite phase.

You will see very little effect from signals passing through the body.

There is also the problem of the transmitters field. Any change in the illumination due to changes in the propagation path to the body will produce local variation. Aircraft, motor vehicles or other people nearby will give the RSSI a fluttering level. That flutter may easily have amplitude variations greater than 3dB.