According to National Dong Hwa University, frequency selective fading is the channel spectral response of any radio transmission, which has dips or fades in the response and is not flat. These dips or fades are due to the reflections, which cause the cancellation of certain frequencies at the receiver.
Multipath signals are caused by reflections off nearby objects, such as buildings, trees and the ground. These signals can have similar signal power to the direct signal, which can result in deep nulls in the received signal power, which is caused by destructive interference. Narrow bandwidth transmissions can experience a complete loss of signal if the null in frequency response occurs at the transmission frequency.