In bandwidth becomes wider and may exceed

In wireless communication system, Signal is sent from asource that usually attenuates, reflected, refracted and scattered when it before traveling to thedestination. These actions modeled one or more values known as the channel response which is making convolution with the transmitted signal. The response varied by the time and the frequency. The assumption of bandwidth over which the channel response is fixed (flat) is well known as coherence bandwidth. For high bit transmission, the data symbol rate is transmitted with high data rates, so the bandwidth becomes wider and may exceed the channel coherence bandwidth and these will lead to the intersymbol interference and causes distortion 1.                                                       Intersymbol interference degrades the signal which can be described mainly in two ways, in the first way previously transmitted symbol may interfere with the present ongoing symbol. The second one, the fraction of the present symbol whose energy is lost, as it causes intersymbol interference with the subsequent symbols. To minimize intersymbol interference, equalization  employed.

                               In the process of equalization which uses an adaptive digital filter with a practical integer of taps. Moreover, these taps are designed in such a way that the combination of channel response and equalizer response is constant value but equalizer has a certain number of limitations. In the equalizer, need to discover the finest weight of each and every tap, which include complex calculation process as it increases exponentially while the numeral of taps increases and also these weights are calculated under noisy estimate channel, and hence error of the estimate will be very  high compared to single tap needed flat channel 2.furthermore,major drawback is the numeral of taps increases then performance will be decreased. To knockout such multi-tap equalizers, a proposition is made in 1960’s that gives data is part into some different data streams that are in parallel accordingly diminishing the symbol rate as well as bandwidth. In the event that the parallel data streams are huge in number, at that point viable bandwidth will be less and turned out to be less than the channel bandwidth consequently it encounters flat channel response and these data streams are modulated utilizing orthogonal carriers named as subcarriers. In addition, these sub-carriers must be practicable fit in the shielded bandwidth 

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