One of the more intriguing aspects of wireless channels is fading. Unlike pathloss or shadowing, which are large-scale attenuation effects owing to distance or obstacles, fading is caused by the reception of multiple versions of the same signal.
The multiple received versions are caused by reflections that are referred to as multipath.
Because of multiple paths between transmitter and reciever, there will be phase difference between arriving signal, and the intereference can be either constructive or distructive, which causes a very large observed difference in the amplitude of the received signal even over very short distances.
In other words, moving the transmitter or the receiver even a very short distance can have a dramatic effect on the received amplitude.This happens because each frequency harmonic of transmitted signal goes through different level of attenuation and fading.Which ultimatly results in delay spread and in turn ISI (Inter symbol Interference). It is very complex to implement equilizer for time varying wireless channel.
OFDMA is the technique which can help a lot to mitigate Inter Symbol Intereference (ISI).
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