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I would like to calculate the exact location of a mobile device inside a building ( so no GPS access)
I want to do this using the signal strength(in dBm) of at least 3 fixed wifi signals(3 fixed routers of which I know the position)
Google already does that and I would like to know how they figure out the exact location based on the this data
Check this article for more details : http://www.codeproject.com/Articles/63747/Exploring-GoogleGears-Wi-Fi-Geo-Locator-Secrets
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In cases where you have no way of telling what the wireless standard your router is using, you may calculate its range by adding your router's transmit power (dBm), antenna gain (dBm), and a fixed value of 4,000 (square feet).
The researchers found that using WIO improved a device's speed and distance calculations dramatically. For example, devices using WIO calculated distance with a margin of error ranging from 5.9% to 10.5%. Without WIO, the devices calculated distance with a margin of error from 40% to 49%.
How to Measure Received Signal Strength. You can use a Wi-Fi scanner application to measure the received signal strength at a specific location and time. The number you are looking for is the RSSI, which means Received Signal Strength Indicator.
You can calculate the distance information from RSSI value with this formula: Distance = 10^((Measured Power - Instant RSSI)/10*N). N is the constant for the environmental factor.
FSPL depends on two parameters: First is the frequency of radio signals；Second is the wireless transmission distance. The following formula can reflect the relationship between them.
FSPL (dB) = 20log10(d) + 20log10(f) + K d = distance f = frequency K= constant that depends on the units used for d and f If d is measured in kilometers, f in MHz, the formula is: FSPL (dB) = 20log10(d)+ 20log10(f) + 32.44 From the Fade Margin equation, Free Space Path Loss can be computed with the following equation.
Free Space Path Loss=Tx Power-Tx Cable Loss+Tx Antenna Gain+Rx Antenna Gain - Rx Cable Loss - Rx Sensitivity - Fade Margin
With the above two Free Space Path Loss equations, we can find out the Distance in km.
Distance (km) = 10(Free Space Path Loss – 32.44 – 20log10(f))/20 The Fresnel Zone is the area around the visual line-of-sight that radio waves spread out into after they leave the antenna. You want a clear line of sight to maintain strength, especially for 2.4GHz wireless systems. This is because 2.4GHz waves are absorbed by water, like the water found in trees. The rule of thumb is that 60% of Fresnel Zone must be clear of obstacles. Typically, 20% Fresnel Zone blockage introduces little signal loss to the link. Beyond 40% blockage the signal loss will become significant.
FSPLr=17.32*√(d/4f) d = distance [km] f = frequency [GHz] r = radius [m] Source : http://www.tp-link.com/en/support/calculator/
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