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I have read about fading of signals , and I have notice that the major cause of fading is multipath , but I have ask myself , what about attenuation , noise and free space loss , all of these make the power of signal be smaller , so my question is , why we emphasize on multipath when we discuss about the causes of signal fading ?

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Ultimately, the only causes of fading are absorption and scattering. BTW noise doesn't make the signal smaller. That's why we worry about signal/noise ratio. – Ben Voigt Apr 13 '14 at 20:12
up vote 4 down vote accepted

In communication systems, fading refers to a specific kind of attenuation which is highly frequency and time dependent. Other types of attenuation such as loss on transmission lines, path loss in radio transmission etc. do not change rapidly with time or frequency, at least not within the bandwidth of interest. On the other hand, fading varies strongly with time and frequency and can - unlike other sources of attenuation - result in a very large frequency-dependent attenuation. And this is caused by multi-path propagation.

As a very simple example, consider two paths. One direct path and another path with attenuation \$a\$ and time delay \$T\$. The squared magnitude of the total transfer function is

$$|1+ae^{-j\omega T}|^2=1+a^2+2a\cos(\omega T)\tag{1}$$

From (1) you can see that the frequency dependent attenuation can be quite large if \$a\$ is near unity. This example is of course overly simplistic because it is time-invariant and because there are only two paths. Fading is an important phenomenon in mobile radio and can be mitigated by using diversity techniques, i.e. by combining several channels and hoping that not all channels are affected by fading at the same time and in the same frequency band.

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Noise, free space loss and most of attenuation stays constant over time. Multipath interference can cause the signal to change its amplitude very quickly and be as deep as to cause the signal to disappear completely.

It's enough for some wave-reflecting object to move one wavelength in some direction to cause the signal disappear and reappear in the same or the opposite phase several times. A large wave-absorbing object can also cause the signal to change its amplitude that many times, but the required movement of such object will be comparable to the object's size, not the wavelength.

So multipath is emphathised because it has the largest ratio between the consequences and the cause of signal level change.

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There are no free space losses. If you had a perfect directional antenna pumping out 1 watt and a perfectly aligned identical antenna a million miles away it would receive 1 watt. Alignment will be problematic because the beam angle is so so fine. Antenna losses at either end make reception more likely to be a couple of hundred milli watts but, in principle, there are no losses in free space.

Noise isn't a concern to the above scenario but a signal that interferes could be if it's frequency is close and it could overwhelm the desired signal.

In reality, the beam angle is not zero degrees. Typically a dipole emits in most directions so the power measured at some point distance gets thinned out. Think of a lightbulb and a photodiode; as you get further away the photodiode signal gets smaller.

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Fading occurs due the fluctuations in the received signal power mainly due to the variation in the signal power amplitude and signal phase because of the different object present on the radio signal path produce attenuation of its power as they can absorb or scatter part of the signal (due to different practical phenomena like reflections, refraction, diffraction, etc) and different scattered signal received will get added constructively or destructively.

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