A fading simulator or radio channel emulator is a tool used to reproduce the actual signal propagation channel environment in a lab for testing in wireless communications. 

In wireless applications, the quality of signals degrades due to various fading effects. For example, when a signal transmitted from a transmitter to the receiver, the signal reaches the receiver after traveling through various propagation paths. The propagation path consists of a number of man-made and natural objects like air, buildings, and trees etc. The transmitted signal gets absorbed by air and gets reflected from buildings and trees, resulting in fluctuations in its amplitude and phase as it reaches the receiver. This phenomenon is called fading.

These factors result in degradation of the signal quality. While designing a wireless communication system, all these factors need to be taken into consideration. The fading simulator is used to generate or simulate these conditions in the lab so we can optimize receiver performance.