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Fading is mostly caused by multiple paths from the transmitter to the receiver. The multiple path links change with time and add at some times and subtract at others.
In rare cases it is caused by atmospheric situations that allow transmission beyond the usual range. These situations come and go while you listen to the signal.
One such example is temperature inversions that cause a wave guide effect. Another is weather conditions which form a reflector high in the air.
As far as I know "fading" cannot be a good phenomenon , and the RAKE receiver utilizes the fact that in a diversity system the fading process in each diversity path differs from the other paths. Hence,if the probability of a path being faded is p, the probability of all paths being totally faded will be p to power L ,where L is the number of diversity paths.
If all paths go to a deep fading , the RAKE receiver simply can do nothing!
fading divided into two major types
flat fading and frequency selective fading
the first type of fading it is just a kind of attenuation in signal.
but in frequency selective fading the change happens in both amplitude and phase which causes ISI.
let's first define the fading to answer your question, Fading means the variation of the signal strength(the signal power), which means that the signal strength can be varied, which means it can be increased or decreased,but in all books and papers you read you'll find all of them speak about the fading as a problem,because if the signal stregth increased it isn't a problem,all of us wish that to happen but not every thing you wish happens so the signal strength may decrease which is a problem that we should solve,by many ways such as frequency diversity,antenna diversity,polarization diversity or just simply by increasing the transmitted power in GSM while in CDMA Rake Rx will be used.
But,note that the signal strength increase means that the signal power incresaed but of course no more than the original transmitted power,it just increased to be more than the expected received level
RAKE receiver or MIMO systems take advantage of "multipaths" and not the fading. So the important point is multipath not the fading.
As for as I understand it is the multipath and signal processing techniques that revoloutinzed the MIMO comunication systems and not the fading. The best MIMO system will be with rich scattering environment i.e. many multipaths and no fading at all. Making long story short, fading may be considered as pitfall rather than a sweet phenomenon.
Fading can indeed be good (under certain circumstance, of course).
One most well-known example is multiuser diversity in wireless communication systems. The key idea is to capture the peaks of the fading and only communicate at these peaks. To get an idea of this, take Rayleigh fading for example. Suppose the average SNR of a Rayleigh fading link is 1, then by exploiting the fading peaks, the effective SNR is given by log(N) where N is the number of users.
In short, in this example, under the assumption that there are many users in the systems, we can in fact exploiting the channel fading. (the reason why we need many users is that we do not want to waste any time in case no peaks are captured. With many users, any time there will be a good peak among them.)
By the way, this idea is not just for academic; it is widely applied in commercial products, like cdma2000 system. If you are a data user of Verizon, probably you are using this technique.
The academic side of the story comes from here
**broken link removed**
I'm really interested to the topic; just a couple of questions:
- Multipath loss: is correct to consider it in the budget as a -20, -40 dB Loss? How can I understand if the Receiver is multipath rejecting? Which characteristics do I have to check in the datasheet?
- Polarization Loss: in case I'm trasmitting a CP (Axial Ratio 1) signal and receiving with a CP (Axial Ratio 2) antenna, can anyone give me a table / formula about the consequent loss due to the different Axial Ratios?
Fading is not always problem. The new celluar system has taken the advantages of the fading. Specially MIMO sysem. In which different antenna are used to capture the signal from the multipath. So the signal strength at the receiver will be more stronger.
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