Recieved Signal Strenght Indicator (RSSI) with obstacles

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Discussion Overview

The discussion centers on the feasibility of measuring differences in Received Signal Strength Indicator (RSSI) when wireless signals propagate through obstacles, specifically the human body. Participants explore the implications of signal distortion and interference patterns in the context of designing a positioning system using RSSI, with a focus on the limitations of smartphone sensors.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether the difference in signal strength caused by the human body is measurable, considering the limitations of smartphone sensitivity.
  • Another participant asserts that the human body, due to its high salt water content, significantly affects signal propagation, leading to variations in RSSI that can be detected.
  • A participant speculates about the possibility of the signal strength doubling and inquires if the observed +3dB increase is due to wave superposition around the body.
  • It is noted that the interference patterns created by multiple signal paths around the body can lead to both constructive and destructive interference, resulting in varying RSSI readings.
  • Participants discuss the potential for significant amplitude variations in RSSI due to nearby objects and movements, which can complicate measurements.
  • Areas of Agreement / Disagreement

    Participants generally agree that the human body affects RSSI measurements, but there is no consensus on the extent of the effect or the conditions under which it can be reliably measured. Multiple competing views on the nature of signal interference and its implications remain present.

    Contextual Notes

    Limitations include the dependence on the specific conditions of the environment, such as the presence of other obstacles and the configuration of the transmitting and receiving devices. The discussion does not resolve the mathematical implications of the interference patterns or the exact conditions under which signal strength variations occur.

    Who May Find This Useful

    This discussion may be useful for individuals interested in wireless communication, signal processing, and the design of positioning systems that utilize RSSI, particularly in environments with obstacles.

Quinten
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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! ;)
 
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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.
 
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?)
 
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.
 

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