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In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath-induced fading, weather (particularly rain), or shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.

Key concepts

Types

The terms ''slow'' and ''fast'' fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes. The coherence time is a measure of the minimum time required for the magnitude change or phase change of the channel to become uncorrelated from its previous value. * Slow fading arises when the coherence time of the channel is large relative to the delay requirement of the application. In this regime, the amplitude and phase change imposed by the channel can be considered roughly constant over the period of use. Slow fading can be caused by events such as shadowing, where a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver. The received power change caused by shadowing is often modeled using a log-normal distribution with a standard deviation according to the log-distance path loss model. * Fast fading occurs when the coherence time of the channel is small relative to the delay requirement of the application. In this case, the amplitude and phase change imposed by the channel varies considerably over the period of use. In a fast-fading channel, the transmitter may take advantage of the variations in the channel conditions using time diversity to help increase robustness of the communication to a temporary deep fade. Although a deep fade may temporarily erase some of the information transmitted, use of an error-correcting code coupled with successfully transmitted bits during other time instances (interleaving) can allow for the erased bits to be recovered. In a slow-fading channel, it is not possible to use time diversity because the transmitter sees only a single realization of the channel within its delay constraint. A deep fade therefore lasts the entire duration of transmission and cannot be mitigated using coding. The coherence time of the channel is related to a quantity known as the Doppler spread of the channel. When a user (or reflectors in its environment) is moving, the user's velocity causes a shift in the frequency of the signal transmitted along each signal path. This phenomenon is known as the Doppler shift. Signals traveling along different paths can have different Doppler shifts, corresponding to different rates of change in phase. The difference in Doppler shifts between different signal components contributing to a signal fading channel tap is known as the Doppler spread. Channels with a large Doppler spread have signal components that are each changing independently in phase over time. Since fading depends on whether signal components add constructively or destructively, such channels have a very short coherence time. In general, coherence time is inversely related to Doppler spread, typically expressed as : $T_c \approx \frac$ where $T_c$ is the coherence time, $D_s$ is the Doppler spread. This equation is just an approximation, to be exact, see Coherence time.

Block fading is where the fading process is approximately constant for a number of symbol intervals. A channel can be 'doubly block-fading' when it is block fading in both the time and frequency domains.

Upfade is a special case of fading, used to describe constructive interference, in situations where a radio signal gains strength. Some multipath conditions cause a signal's amplitude to be increased in this way because signals travelling by different paths arrive at the receiver in phase and become additive to the main signal. Hence, the total signal that reaches the receiver will be stronger than the signal would otherwise have been without the multipath conditions. The effect is also noticeable in wireless LAN systems.Barry D. Lewis, Peter T. Davis ''Wireless networks for dummies'', For Dummies, 2004 , page 234

Models

Examples of fading models for the distribution of the attenuation are: * ''Dispersive fading'' models, with several echoes, each exposed to different delay, gain and phase shift, often constant. This results in frequency selective fading and inter-symbol interference. The gains may be Rayleigh or Rician distributed. The echoes may also be exposed to Doppler shift, resulting in a time varying channel model. * Nakagami fading * Log-normal shadow fading * Rayleigh fading * Rician fading * Two-wave with diffuse power (TWDP) fading * Weibull fading

Mitigation

* Attenuation distortion * Backhoe fade * Diversity schemes * Fade margin * Fading distribution * Frequency of optimum transmission * Link budget * Lowest usable high frequency * Maximum usable frequency * Multipath propagation * OFDM * Rain fade * Rayleigh fading * Thermal fade * Two-Wave with Diffuse Power (TWDP) fading * Ultra-wideband * Upfade

References

Literature

* T.S. Rappaport, ''Wireless Communications: Principles and practice'', Second Edition, Prentice Hall, 2002. * David Tse and Pramod Viswanath
''Fundamentals of Wireless Communication''
Cambridge University Press, 2005. * M. Awad, K. T. Won

& Z. Li, ''An Integrative Overview of the Open Literature's Empirical Data on the Indoor Radiowave Channel's Temporal Properties,

IEEE Transactions on Antennas & Propagation, vol. 56, no. 5, pp. 1451–1468, May 2008. * P. Barsocchi,
Channel models for terrestrial wireless communications: a survey
', CNR-ISTI technical report, April 2006.