Upfade

Upfade is the term used in radio communications to describe a situation where multipath conditions (in which signals travel from the transmitter to the receiving antenna by two or more paths) cause a radio signal to gain strength.

Some Multipath conditions cause a signal's amplitude to be increased in this way because signals travelling by different paths arrive at the receiver in phase and become additive to the main signal. Hence, the total signal that reaches the receiver will be stronger than the signal would otherwise have been without the multipath conditions.