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![]() | The contents of the Selective fading page were merged into Fading. For the contribution history and old versions of the redirected page, please see its history; for the discussion at that location, see its talk page. (2013-02-15) |
Currently, the article page definition of 'coherence time' seems really confusing. It is currently described (on the article page) as: "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." KorgBoy ( talk) 05:17, 7 December 2018 (UTC)
The article defines slow and fast fading in a contradictive manner. The doppler spread section need to be improved. Mange01 11:57, 17 November 2006 (UTC)
Why the slow fading is mixed up with the shadow fading. These things are completely different. Large scale fading, shadow fading is not relative to coherence time of the channel. This is usual mix up in terms that people make also in literature as well —Preceding unsigned comment added by 192.100.116.143 ( talk) 07:03, 18 September 2008 (UTC)
I notice Mange01 has re-inserted the following addition to the list of diversity techniques in the Mitigation section of the article:
I have no objection to including a mention of equalization somewhere in the article, as it performs a useful receiver function in both fading and AWGN channels. However, I feel that the location of its current mention -- as a short bullet point in a list of methods to specifically improve fading performance through diversity -- is misleading.
Specifically, as currently written the "Mitigation" section is used to emphasize that fading channels fundamentally have much poorer error performance than AWGN channels due to the fact that the channel gain is random and subject to deep fading events. Performance of all receivers becomes limited by the probability of a fading event in a single channel pat h, independent of what receiver algorithms are used. This causes the probability of error to decay as 1/SNR rather than exp(SNR). (A good reference for the derivation is the chapter on diversity in Fundamentals of Wireless Communication by Tse and Viswanath.)
This is typically combated by using diversity to create multiple parallel and independently fading channels to reduce the chance of the whole channel experiencing a fade. If the probability of a deep fade on a single channel is Pf, then the probability of a deep fade affecting all N independent channels is reduced to Pf^N. The corresponding effect is to increase the error exponent from SNR^(-1) to SNR^(-N).
All the techniques currently listed in that section (OFDM, Rake, STC, MIMO) specifically address the random nature of the channel gain by using such diversity techniques. Equalization, however, does not increase the diversity of the channel, so error performance is still dominated by the probability of a fade in a single channel path. At best it improves the constant coefficient of the error probability, and not the error exponent.
Mange01, if you think a specific mention of equalization in the fading context would be beneficial to the article, then I suggest moving it from the current location in the list of diversity techniques and adding a clarification of the difference between equalization and the list of diversity techniques in that section would be very helpful. In its current state, the section is misleading because it suggests that equalizers can also provide robustness to deep fading events. Elch Yenn 14:39, 26 July 2007 (UTC)
This page is a redirection for shadowing, but it is not explained what shadowing is. In stead, this page refers to itself to explain shadowing. WKW 130.89.14.44 ( talk) 10:37, 14 May 2009 (UTC)
![]() | This article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | ||||||||||
|
![]() | The contents of the Selective fading page were merged into Fading. For the contribution history and old versions of the redirected page, please see its history; for the discussion at that location, see its talk page. (2013-02-15) |
Currently, the article page definition of 'coherence time' seems really confusing. It is currently described (on the article page) as: "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." KorgBoy ( talk) 05:17, 7 December 2018 (UTC)
The article defines slow and fast fading in a contradictive manner. The doppler spread section need to be improved. Mange01 11:57, 17 November 2006 (UTC)
Why the slow fading is mixed up with the shadow fading. These things are completely different. Large scale fading, shadow fading is not relative to coherence time of the channel. This is usual mix up in terms that people make also in literature as well —Preceding unsigned comment added by 192.100.116.143 ( talk) 07:03, 18 September 2008 (UTC)
I notice Mange01 has re-inserted the following addition to the list of diversity techniques in the Mitigation section of the article:
I have no objection to including a mention of equalization somewhere in the article, as it performs a useful receiver function in both fading and AWGN channels. However, I feel that the location of its current mention -- as a short bullet point in a list of methods to specifically improve fading performance through diversity -- is misleading.
Specifically, as currently written the "Mitigation" section is used to emphasize that fading channels fundamentally have much poorer error performance than AWGN channels due to the fact that the channel gain is random and subject to deep fading events. Performance of all receivers becomes limited by the probability of a fading event in a single channel pat h, independent of what receiver algorithms are used. This causes the probability of error to decay as 1/SNR rather than exp(SNR). (A good reference for the derivation is the chapter on diversity in Fundamentals of Wireless Communication by Tse and Viswanath.)
This is typically combated by using diversity to create multiple parallel and independently fading channels to reduce the chance of the whole channel experiencing a fade. If the probability of a deep fade on a single channel is Pf, then the probability of a deep fade affecting all N independent channels is reduced to Pf^N. The corresponding effect is to increase the error exponent from SNR^(-1) to SNR^(-N).
All the techniques currently listed in that section (OFDM, Rake, STC, MIMO) specifically address the random nature of the channel gain by using such diversity techniques. Equalization, however, does not increase the diversity of the channel, so error performance is still dominated by the probability of a fade in a single channel path. At best it improves the constant coefficient of the error probability, and not the error exponent.
Mange01, if you think a specific mention of equalization in the fading context would be beneficial to the article, then I suggest moving it from the current location in the list of diversity techniques and adding a clarification of the difference between equalization and the list of diversity techniques in that section would be very helpful. In its current state, the section is misleading because it suggests that equalizers can also provide robustness to deep fading events. Elch Yenn 14:39, 26 July 2007 (UTC)
This page is a redirection for shadowing, but it is not explained what shadowing is. In stead, this page refers to itself to explain shadowing. WKW 130.89.14.44 ( talk) 10:37, 14 May 2009 (UTC)