What is Fading

Fading in a communication system is the fluctuations in the amplitude and phase of the signal at the receiver side. It is a common phenomenon in cellular based systems. Basically when you are using a mobile phone, electromagnetic waves travel back and forth between the Base station and your mobile unit. Now the transmission from the base station to your mobile unit consists of a number of electromagnetic components that travel different reflective paths to reach your unit. These components then combine to give the final signal which is received by your unit.

Now when you are moving, the building structures around you change which therefore alters the nature of these multipath components in terms of amplitude, phase, time delay etc. This results in fluctuations in the amplitude and phase of the final received signal and fading occurs. Fading can be slow or fast depending upon the rate of fluctuations in the signal. It can also be flat (i.e. same throughout the whole spectrum) or frequency selective.

Fading presents a big problem when the various components happen to combine with each other in a destructive manner, therefore degrading the final signal quality all together. Such a situation is known as a deep fade and leads to the loss of the signal reception altogether. Therefore a reliable communication system requires some sort of technology for the control of fading levels. The techniques which are employed in today’s cellular system for the purpose are known as diversity combining techniques.

In diversity combining techniques, a number of fading channels are combined at the receiver side to achieve an optimum signal to noise ratio at all times. These multiple channels are achieved by:-

Using multiple sectorized antennas to obtain a number of non coherent fading channels. Using different frequency components that are non-coherent with each other. Using different time slots or time delay components. Again the condition of non-coherence should be satisfied.

The basic principle is that when a number of fading channels are used which are non-coherent in nature (i.e. the effect of fading on each of them is independent of the others); deep fades can be avoided by combining these channels to get the final signal.This is because the chances of a deep fade occurring in all these channels at the same time are very less and therefore a good quality of reception can be maintained at all times.

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