Calculating STL Fade Margin
The net path loss previously derived is then subtracted from this number resulting in the received power level at the far end of the link. The difference between that value and the receiver sensitivity is the resulting fade margin. If the receiver sensitivity is listed in dBm then the conversion is simple. If it is listed in terms of dBmV, dBuV, µV, or some other similar unit, additional conversion must occur before the fade margin drops out of the equation.
The units dBmV, dBuV, and µV are voltage units while dBm (sometimes written as dBmW) is a power measurement. The voltage units are related to power units via the impedance of the equipment under consideration. STL systems are mostly 50 ohm systems, so only that case will be considered here, with the resulting relationships as the following:
Note also that dBmV, dBuV, and µV are interrelated by the following:
So as an example let us consider a 950MHz system that has total fixed loss of 8.5dB, a total antenna gain of 36.0dB, and a path length of 23.0 kilometers. This system also has a transmitter power of 10W and a receiver sensitivity of 4µV.
First the free space attenuation is given by:
The total system gain is then determined and subtracted from the free space attenuation to get the total path attenuation or net path loss.
--Continued on page 3
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