# 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

### Rician fading

### Rayleigh Fading

### nakagami fading

## Scales of Fading

### *Large-Scale 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 in
case of the 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

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

### Frequency-Selective Fading

### Flat Fading

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

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.
