Free Space Path Loss (FSPL) depends on two items: First is the frequency of radio signal; 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
Here is a table to select K based on units of 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 calculated with the following equation:
Free Space Path Loss (dB)=Tx Power(dBm)-Tx Cable Loss(dB)+Tx Antenna Gain(dB)+Rx Antenna Gain(dB) - Rx Cable Loss(dB) - Rx Sensitivity(dBm) - Fade Margin(dB)
Assume a desired fade margin of for example 12 dB and use actual equipment specifications to determine whether reliable communications can take place. A surplus signal power above the Free Space Path Loss, taking the equipment specs into account, means at least the minimum threshold for reliable communications has been met.