How RF Link Budget Works
A link budget accounts for all gains and losses in a wireless communication path, from transmitter to receiver. It determines whether a signal will arrive with enough power for the receiver to decode it.
P_rx = P_tx - L_tx_cable + G_tx - FSPL - L_misc + G_rx - L_rx_cable
Where FSPL (Free Space Path Loss) = 20*log10(d) + 20*log10(f) + 32.44, with d in km and f in MHz.
Fade Margin
The fade margin is the difference between the received signal power and the receiver sensitivity. A positive fade margin means the link can tolerate additional losses (rain, multipath, etc.). Typical designs target 10-20 dB of fade margin for reliable links.