Communication Systems/Fading Channels
Over large distances, signal quality degrades even without the presence of large quantities of AWGN. This degradation is known as fading, and channels that exhibit these properties are known as fading channels.
What Causes Fading
Fading can be caused due to natural weather disturbances, such as rainfall, snow, fog, hail and extremely cold air over a warm earth. Fading can also be created by man made disturbances, such as irrigation, or from multiple transmission paths, irregular earth surfaces, and varying terrains.
Fading Channel Models
Scales of Fading
Fading generally is a signal loss either in amplitude or phase due to sudden changes in Channel response. Large scale fading " Shadowing" concerns about large distances effect so, its affect appears clearly incase of the displacement of either the Transmitter or the Receiver. The most clear example for this case is the RF reception for a Car or a moving vehicle where the signal is influenced by Multipath phenomena where the transmitted signal is received from more than one path.
Small scale fading is concerned about very small changes in the position of Transmitter or receiver in order of the wavelength as this affect greatly the received frequency due to doppler effect so as the speed of the vehicle increases as the frequency is more rapidly changing and it can be described as fast or slow fading AND flat or slow fading.
Small-Scale Fading Types
In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all frequency components of the signal will experience the same magnitude of fading. a signal undergoes flat fading if:
Bs << Bc
Fast fading occurs when coherant time of the channel is small relative to the delay constraint of the channel. Amplitude and phase changes imposed by the channel varies considerably over period of use.