Editing Fading (section)

===Selective fading===<!--[[Selective fading]] redirects here-->
[[File:Coherence bandwidth.png|thumb|upright=2]]

'''Selective fading''' or '''frequency selective fading''' is a [[radio propagation]] anomaly caused by partial cancellation of a [[radio signal]] by itself &mdash; the signal arrives at the receiver by [[Multipath propagation|two different paths]], and at least one of the paths is changing (lengthening or shortening).  This typically happens in the early evening or early morning as the various layers in the [[ionosphere]] move, separate, and combine.  The two paths can both be [[skywave]] or one be [[groundwave]].

Selective fading manifests as a slow, cyclic disturbance; the cancellation effect, or "null", is deepest at one particular frequency, which changes constantly, sweeping through the received [[Sound|audio]].

As the [[carrier frequency]] of a signal is varied, the magnitude of the change in amplitude will vary.  The [[coherence bandwidth]] measures the separation in frequency after which two signals will experience uncorrelated 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.
* In '''frequency-selective fading''', the coherence bandwidth of the channel is smaller than the bandwidth of the signal. Different frequency components of the signal therefore experience uncorrelated fading.

Since different frequency components of the signal are affected independently, it is highly unlikely that all parts of the signal will be simultaneously affected by a deep fade.  Certain modulation schemes such as [[orthogonal frequency-division multiplexing]] (OFDM) and [[code-division multiple access]] (CDMA) are well-suited to employing frequency diversity to provide robustness to fading.  OFDM divides the wideband signal into many slowly modulated narrowband [[subcarrier]]s, each exposed to flat fading rather than frequency selective fading. This can be combated by means of [[error coding]], simple [[equalization (communications)|equalization]] or adaptive [[bit loading]]. Inter-symbol interference is avoided by introducing a guard interval between the symbols called a [[cyclic prefix]].  CDMA uses the [[rake receiver]] to deal with each echo separately.

Frequency-selective fading channels are also ''dispersive'', in that the signal energy associated with each symbol is spread out in time.  This causes transmitted symbols that are adjacent in time to interfere with each other. [[Equalization (communications)|Equalizers]] are often deployed in such channels to compensate for the effects of the [[intersymbol interference]].

The echoes may also be exposed to [[Doppler shift]], resulting in a time varying channel model.

The effect can be counteracted by applying some [[diversity scheme]], for example OFDM (with subcarrier [[forward error correction#Interleaving|interleaving]] and [[forward error correction]]), or by using two [[receiver (radio)|receivers]] with separate [[antenna (radio)|antenna]]s spaced a quarter-[[wavelength]] apart, or a specially designed [[diversity reception|diversity receiver]] with two antennas.  Such a receiver continuously compares the signals arriving at the two antennas and presents the better signal.