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coherence bandwidth and carrier frequency

Started by weilun731 6 years ago4 replieslatest reply 6 years ago302 views

Hi,

I studied some materials regarding coherence bandwidth of the channel. These materials all mentioned that when the signal's bandwidth (baseband) is less than the coherence bandwidth of the channel it will not get distorted, but there is no mentioning of the effect of carrier frequency. 

I am wondering if my thinking is true:

Even if the bandwidth of the signal is less than the coherence bandwidth of the channel, if we shift the center frequency of the signal to a carrier frequency that is outside of the coherence bandwidth of the channel, the signal will still get distorted. 

Yes or No?

Thanks,

William

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Reply by Fred MarshallApril 4, 2017

Startibartfast gave you a good answer. 

I would only add:

Envision that the coherence bandwidth is a *measure* and not a *band*.  Then, if you shift the center frequency of the signal to a new carrier frequency, that won't move it "outside" because there is no band to move *outside of*.  The measure will still exist.

Just saying it a slightly different way....

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Reply by hpatdsprelatedApril 5, 2017

You should look at the RF spectrum and the RF bandwidth of your signal. If frequencies differ for much more than the coherence bandwidth then their transmission characteristics (channel model) become independent. For differences lower then the coherent bandwidth the dependency rises and you get a roughly flat fading depending on the channel model as Staribartfast already described. 

If you move the center frequency of your signal by the amount of the coherence bandwidth then a part of the spectrum can still be described by your channel model and may suffer from fading. The fequencies shifted outside the coherence bandwidth are subjected to another channel model.

With AM the carrier frequency plays an important role as it carries the main part of the signal power. If it is subjected to frequency selectve fading severe distortions of the signal will occur.


HP 

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Reply by SlartibartfastApril 4, 2017

No, the coherence bandwidth has to do with the dynamic fading of the channel.   It is a measure of how wide the fading regions are in the channel spectrum.   They're still dynamic, though, so if you move the frequency within the region where the channel model holds you'll still have the same fading with the same coherence bandwidth.

The coherence bandwidth metric is telling you that the channel behaves like flat fading, but only within that much bandwidth.   If your signal is wider than that then it will start to look like frequency-selective fading.    So move your signal in frequency by the amount of the coherence bandwidth, and you still have flat fading but it might be faded differently than at the other frequency.

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Reply by weilun731April 7, 2017

Thanks all for the answer. Now I have more clear understanding on this.