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Due to each of subcarriers in OFDM system that experince same flat fading channel. Then one tap equalizer is enough for compensate by used only one tap of delay and Mag from tapped-delay line channel .
If your channel is low-pass (integrating) then you can compensate or equalize it by a high-pass operation (differentiation).
In a discrete time system the derivative of a signal y is dy/dn = y-y(n-1), where the index n represents time. Clearly, y(n-1) represents delaying the signal by one sampling unit. If you pass the signal through a shift register (or memory), each 'tap' of the register reprsents a unit delay. To form the derivative you subtract the first tap from the current value. This is the simplest 'one-tap' equalizer.
You can create more complex equalizers by a linear combination of current and past taps. For example: y - 0.75*y(n-1) - 0.5*y(n-2) is a more fancy differentiator with different tap 'weights'. If you replace the negative weights with positive ones you'll get a low-pass integrator (instead of a high-pass differentiator). The one-tap equalizer is the easiest to implement, and hence most popular.
Note though that differentiation will emphasize receiver noise, so you can't play this game indefinitely without diminishing returns.