Fading (or fading channels) are mathematical models for the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media. Short term fading is due to multipath propagation, and is also known as multipath induced fading. Fading results from the superposition of transmitted signals which have experienced differences in attenuation, delay and phase shift while travelling from the source to the receiver. PRIMERGY MultiPath PRIMERGY MultiPath supports redundant Fiber Channel paths, the configured connections between server and subsystem that are such an important component of disaster-tolerant servers and clusters. ... PRIMERGY MultiPath PRIMERGY MultiPath supports redundant Fiber Channel paths, the configured connections between server and subsystem that are such an important component of disaster-tolerant servers and clusters. ... The term superposition can have several meanings: Quantum superposition Law of superposition in geology and archaeology Superposition principle for vector fields Superposition Calculus is used for equational first-order reasoning This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... Attenuation is the decrease of the amount, force, magnitude, or value of something. ... Delay is: In sound effects, any of a class of effect that adds one or more delayed versions of the original signal, to create effects such as echo or flanger. ... Waves with the same phase Waves with different phases The phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). ...

There are as metioned above different types of fading. The most common fadings which are known as "slow fading" and "fast fading" are explained below: Before hand it might be necessary to remind ourselves of definition of fading:

Fading refers to the time variation of the received signal power caused by changes in transmission medium or path.

Slow Fading: Multipath fading or Small-Scale fading is a kind of fading which is caused by small movment of the portable.

Fast Fading: Shadowing or Large-Scale fading is a kind of fading which is caused by the movment of surrondig enviroment.

For example, consider the common experience of stopping at a traffic light and hearing a lot of static on your radio, which is immediately corrected if you move less than a meter. Cellular phones also exhibit similar momentary lapses. The reason for these losses of signal is the destructive interference that multiple reflected copies of the signal makes with itself. To understand how a signal can destructively interfere with itself, consider the sum of two sinusoidal waveforms (which are similar to carrier modulated signals) with different phases. In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ...

The best way to combat fading is to ensure that multiple versions of the same signal are transmitted, received, and coherently combined. This is usually termed diversity, and sometimes acquired through multiple antennas. Mathematically, the simplest model for the fading phenomenon is multiplication of the signal waveform with a time-dependent coefficient which is often modeled as a random variable, making the received signal to noise ratio a random quantity.

Fading channel models are often used to model electromagnetic transmission of information over wireless media such as cellular phones, and broadcast communications. However, even for underwater acoustic communications the notion of fading is useful in understanding the distortion caused by the medium.

Small-scale fading is usually divided into fading based on multipath time delay spread and based on Doppler spread.

There are two types of fading based on multipath time delay spread:

• Flat fading, where the bandwidth of the signal is less than the coherence bandwidth of the channel or the delay spread is less than the symbol period.
• Frequency selective fading, where the bandwidth of the signal is greater than the coherence bandwidth of the channel or the delay spread is greater than the symbol period.

There are two types of fading based on doppler spread: Definition: Coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered flat, or in other words the approximate maximum bandwidth or frequency interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude fading. ... Please wikify (format) this article as suggested in the Guide to layout and the Manual of Style. ... In digital communications, the symbol rate is the bit rate divided by the number of bits transmitted in each symbol. ... The Doppler effect, named after Christian Andreas Doppler, is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ...

• Fast fading, which has a high doppler spread, and the coherence time is less than the symbol period, and the channel variations are faster than baseband signal variations.
• Slow fading, which has a low doppler spread. The coherence time is greater than the symbol period and the channel variations are slower than the baseband signal variations.

In addition to the small scale fading that is described above, for which the change in the signal strength occurs on the order of a fraction of a meter, the signal can also undergo shadow fading, or shadowing. This is due to the presence of obstacles between the transmitter and the receiver, and the scale of distance required to experience shadowing is about an order of magnitude larger than that of multipath fading. For an electromagnetic wave, coherence time is the time over which a propagating wave may be considered coherent. ... All signals are comprised of a whole range of different frequencies added up together. ... Shadows on a pavement A shadow is a region of darkness where light is blocked. ...

Examples of fading with reference to the distribution of the attennuation are:

• Nakagami fading
• Weibull fading
• Rayleigh fading
• Rician fading

Techniques used to overcome signal fading: This article or section needs to be wikified. ... In telecommunication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. ... In telecommunication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. ...

• Diversity reception and transmission
• Space–time codes
• MIMO

In telecommunications, diversity reception refers to a method for improving reception of a transmitted signal, by receiving and processing multiple versions of the same transmitted signal. ... A space–time code (STC) is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas. ... See also systems analysis. ...

See also

• Fade margin
• Fading distribution
• Link budget
• Multipath