CODEWITHSUNDEEP
androidwifiandroid-wifi
Danis Fermi
Danis Fermi

Reputation: 611

Is it possible to get the RSSI values of reflected WiFi Signals?

Suppose I have a single WiFi AP. I want to obtain the RSSI values of all the signals being received from that access point (the signal that comes straight, the one that is reflected off the walls, the one that propagates through a wall etc.)

I am able to get only one RSSI value for one AP at the application layer but ideally, the WiFi NIC card should ideally receive all the signals and choose the signal with the highest RSSI value right?

Please correct me if my question is conceptually wrong. I intend to use this to get the location of an Android device in an indoor environment using WiFi.

Upvotes: 0

Views: 439

Answers (1)

notloc
notloc

Reputation: 136

There is no way for an AP to tell you that because there is no physical way for it to know what path the waves took, unfortunately.

Longer version: The signal sent from one device to another takes multiple paths, as your question correctly suggests, some portion of the signal passes in a straight line between the sender and the receiver, some portion reflects off objects, some through walls or other materials, etc. When those signals reach the antenna they merge back together in to one.

If the signals arrive "In phase" (meaning the peaks of the waves line up with each other) they add together and the signal appears stronger, if they arrive "out of phase" (with the peak of one wave arriving the same time as the valley of another) then they will cancel each other and the signal will appear weaker. This happens because each path is a different length, so the waves arrive at slightly different times.

I mention that because it is possible, for example, for a direct wave and a reflected wave to both be very strong, but then cancel almost completely at the receiving antenna. So signal strength at the receiver doesn't tell you much about the strength or position of the transmitter, all it really tells you is how the signals combine in space at that exact position.

This video shows a simulation of how the waves interact. You can see that even in the same room as the transmitter, there is a diagonal band where the signal is weak. https://www.youtube.com/watch?time_continue=9&v=6hcK9B4HHY8 All the receiver knows is the strength of the combined waves at the point where it is located.

A beam-forming AP might be able to give you signal strength as a function of direction, which would help, but I don't know if they make that information available.

Upvotes: 1

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