Frequency-selective time-varying fading causes a cloudy pattern to appear on a spectrogram. Time is shown on the horizontal axis, frequency on the vertical axis and signal strength as grey-scale intensity.

In wireless communications, fading is deviation of the attenuation affecting a signal over certain propagation media. The fading may vary with time, geographical position or radio frequency, and is often modeled as a random process. A fading channel is a communication channel comprising fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.

Key concepts[link]

The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio.

A common example of multipath fading is the experience of stopping at a traffic light and hearing an FM broadcast degenerate into static, while the signal is re-acquired if the vehicle moves only a fraction of a meter. The loss of the broadcast is caused by the vehicle stopping at a point where the signal experienced severe destructive interference. Cellular phones can also exhibit similar momentary fades.

Fading channel models are often used to model the effects of electromagnetic transmission of information over the air in cellular networks and broadcast communication. Fading channel models are also used in underwater acoustic communications to model the distortion caused by the water. Mathematically, fading is usually modeled as a time-varying random change in the amplitude and phase of the transmitted signal.

Slow versus fast fading[link]

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 of the channel to become uncorrelated from its previous value. Alternatively, it may be defined as the maximum time for which the magnitude change of channel is correlated to its previous value.

• Slow fading arises when the coherence time of the channel is large relative to the delay constraint of the channel. 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 amplitude 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 constraint of the channel. In this regime, 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 travelling 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 single 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

Failed to parse (Missing texvc executable; please see math/README to configure.): T_c = \frac{1}{D_s}

where Failed to parse (Missing texvc executable; please see math/README to configure.): T_c

is the coherence time, Failed to parse (Missing texvc executable; please see math/README to configure.): D_s
is the Doppler spread (Doppler shift). This equation is just an approximation[1], to be exact, see Coherence time.

Flat versus frequency-selective fading[link]

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 decorrelated 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 OFDM and CDMA are well-suited to employing frequency diversity to provide robustness to fading. OFDM divides the wideband signal into many slowly modulated narrowband subcarriers, each exposed to flat fading rather than frequency selective fading. This can be combated by means of error coding, simple equalization or adaptive bit loading. Inter-symbol interference is avoided by introducing a guard interval between the symbols. 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. 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.

Fading models[link]

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
• Weibull fading

Mitigation[link]

Fading can cause poor performance in a communication system because it can result in a loss of signal power without reducing the power of the noise. This signal loss can be over some or all of the signal bandwidth. Fading can also be a problem as it changes over time: communication systems are often designed to adapt to such impairments, but the fading can change faster than the adaptations can be made. In such cases, the probability of experiencing a fade (and associated bit errors as the signal-to-noise ratio drops) on the channel becomes the limiting factor in the link's performance.

The effects of fading can be combated by using diversity to transmit the signal over multiple channels that experience independent fading and coherently combining them at the receiver. The probability of experiencing a fade in this composite channel is then proportional to the probability that all the component channels simultaneously experience a fade, a much more unlikely event.

Diversity can be achieved in time, frequency, or space. Common techniques used to overcome signal fading include:

• Diversity reception and transmission
• MIMO
• OFDM
• Rake receivers
• Space–time codes

See also[link]

• 2-Ray Ground Reflection Model
• Backhoe fade
• Fade margin
• Fading distribution
• Frequency of optimum transmission
• Link budget
• Lowest usable high frequency
• Maximum usable frequency
• Multipath propagation
• Rain fade
• Thermal Fade
• Ultra-wideband

References[link]

Literature[link]

• 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. Wong [1] & Z. Li, An Integrative Overview of the Open Literature's Empirical Data on the Indoor Radiowave Channel's Temporal Properties,[2] 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.

External links[link]

• Fading due to multipath effect

Decyfer Down
Decyfer Down performing in Jacksonville, Florida on March 29, 2011.
Background information
Origin	Morehead City, North Carolina, United States
Genres	Christian rock Hard rock[1] Post-grunge Alternative metal
Years active	1999–Present
Labels	Fairtrade Services, formerly known as (INO Records)
Website	http://www.decyferdown.com/
Members
TJ Harris Brandon Mills Josh Oliver Chris Clonts
Past members
Caleb Oliver

Decyfer Down is a rock band formed in 1999 and based in Morehead City, North Carolina. Until 2002 – the year that Chris Clonts joined the band – they went by the moniker Allysonhymn (pronounced All-eyes-on-Him). Following Clonts' arrival they switched to the current name, Decyfer Down. Decyfer Down is made up of Christians, but the band is known for their mainstream success having toured with bands like Puddle of Mudd, Breaking Benjamin, and Theory of a Deadman. They have also toured with Christian rock bands like Skillet and RED.

Biography[link]

Decyfer Down was originally formed in 1999 as Allysonhymn by guitarist Brandon Mills and drummer Josh Oliver. Formerly an acoustic outfit, the group eventually switched to a more rock-oriented sound. Josh Oliver's brother Caleb later joined the band as a bass player, followed in 2002 by guitarist Chris Clonts. His arrival gave the band "a new sound and purpose"^[2] and they began to play with mainstream artists such as Puddle of Mudd, Breaking Benjamin, Crossfade and Adema. Around this time the band renamed themselves Decyfer Down.

The band has shared the stage with acts such as Pillar, Thousand Foot Krutch, Day of Fire, The Showdown, Skillet, Disciple, Hawk Nelson, The Letter Black and many more.^[3] In 2008, Caleb, appeared with Skillet on their "Comatose Comes Alive" tour. On the song "Those Nights" he played the bass.

[edit] End of Grey

A record deal with INO Records led to the band's debut album, End of Grey, which was released on June 6, 2006. The album had five singles "Fight Like This","Break Free", "Burn Back the Sun," "No Longer" and "Life Again" all of which reached #1 on the Christian rock charts. The album peaked at #43 on the Billboard Top Christian Albums.^[4] "Fight Like This" was featured in UFC promo which aired on September 7, 2007 for twelve hours on the front page of YouTube.

[edit] Crash

On July 7, 2008 Decyfer Down's released the lead single off their upcoming album; "Crash" to radio. "Crash" went on to peak at the #1 spot on the Christian Rock charts and #20 on the ChristianRock.net end of year charts.^[5] The song was also used to promote the Jeff Hardy vs. Matt Hardy match at WrestleMania XXV. In support of their upcoming album the band released a 3 song EP to iTunes on September 30. The EP contained "Crash" as well as two other new songs entitled "Best I Can" and "Now I'm Alive".

The band announced in August 2008, that original lead singer/bassist Caleb Oliver(brother of drummer Josh) had left the band to deal with some personal/ family issues. T.J. Harris formerly of Fighting Instinct joined a few months later. On February 3, the second single "Fading" was released. Fading peaked at #1 on the Christian Rock Charts, making it their second #1 single from their new album and sixth consecutive #1 single. It is their first official public studio recording TJ Harris. On February 17, the band released an updated EP with TJ's vocals, containing the songs "Crash", "Fading", and "Moving On".

On May 5, 2009 Decyfer Down's second studio album, "Crash" was released. It debuted at #66 on the Billboard 200 in its first week, selling nearly 9,000 copies. It stayed in the Billboard 200 for three weeks, dropping to #128 and then to #181 before falling off the chart.^[6] Drummer Josh Oliver stated in an interview in July that the record had sold upwards of 30,000 copies in 3 months.

On December 3, 2009 it was announced on their official site that Crash had been nominated for a Grammy for Best Rock or Rap Gospel album of the year. Decyfer Down did not win the Grammy in 2009 (award going to Third Day) but being nominated was a huge success to the band.^[7]

Third Studio Album[link]

In March 2010, the band announced that they had begun writing for a 3rd album. In September 2010, the band posted a short video of them in the studio, working on a new song. The new, untitled song sounded heavier than the sound of Crash and closer to the style on End of Grey. It has been confirmed that their third studio album will be released in late 2012. In March 2012, the band went into the studio to record the yet to be named third album.

Members[link]

• TJ Harris – Lead Vocals, Acoustic Guitars (2008–Present)
• Chris Clonts – Lead Guitars (2002–Present)
• Brandon Mills – Guitars/Backing Vocals (1999–Present)
• Josh Oliver – Drums, Percussion (1999–Present)

Touring Members[link]

• Glenn Hartzog – Bass (Toured a few tours during stretch of 2008-2009)

Former members[link]

• Caleb Oliver – Lead vocals, Bass Guitars (1999–2008) (Recorded bass guitar for most of "Crash" album).

Discography[link]

Studio albums[link]

Upcoming Decyfer Down Album (2012)

EPs[link]

Compilations[link]

Singles[link]

References[link]

External links[link]

• Decyfer Down Official website
• Decyfer Down on Facebook
• Decyfer Down on Twitter
• Decyfer Down on iTunes "Ping"
• Decyfer Down on "Amazon"
• Decyfer Down on MySpace
• Decyfer Down on Purevolume
• Decyfer Down on Songkick
• Decyfer Down Official Youtube
• Decyfer Down on Getglue
• Decyfer Down on Rdio
• Decyfer Down on Spotify
• Decyfer Down on Vevo
• TJ Harris of Decyfer Down on Facebook
• TJ Harris on Twitter
• Josh Oliver on on Twitter
• Brandon Mills on Twitter
• Chris Clonts on Twitter