fastfading

Fast fading, or fast fading, refers to rapid fluctuations in the amplitude and phase of a radio signal over short time or space intervals due to multipath propagation. The received signal is the sum of many copies of the transmitted signal that take different paths to the receiver, causing constructive and destructive interference that changes as the transmitter, receiver, or environment moves. Fast fading occurs on time scales comparable to or shorter than the symbol duration and is driven by Doppler shifts from relative motion and the richness of the multipath environment.

In urban and suburban channels with many reflectors but no dominant line-of-sight component, the envelope of the received signal often follows a Rayleigh distribution; if a strong line-of-sight component exists, the distribution is Rician. The rate of fading is related to the Doppler spread, which increases with speed and carrier frequency. The coherence time, a measure of how quickly the channel changes, is inversely related to the Doppler spread. Fast fading can cause rapid degradation of signal quality, especially for wideband signals, but its effects may average out over time for some systems with sufficient diversity or coding.

Fast fading is distinct from slow fading, or shadowing, which involves variations caused by large-scale obstacles and changes more slowly with time or position. Mitigation techniques include diversity (time, frequency, spatial), interleaving and error-control coding, adaptive modulation and coding, equalization, and channel estimation in multi-carrier systems such as OFDM, often combined with rake receivers in CDMA systems.