Given that the Nyquist filter response must apply to the whole transmission link including TX and RX sections to achieve
zero ISI performance, it is necessary to consider where best to place the filtering within
the transmission system.
Filtering is almost always mandatory in the transmission unit, particularly in the case of
wireless communications, to constrain the 'on-air
bandwidth' of the signal to that dictated by regulation
or by the practical necessity to co-habit with users on adjacent channel frequencies. Good
receiver filtering is also vital (again especially in wireless applications), in order to
remove strong signal interferers from overloading the demodulator circuitry, and also to
reject as much noise as possible that does not fall within the modulation passband. For
these reasons, it is necessary for the Nyquist filter function to be shared between TX and
RX units (assuming that the channel response itself is flat or has been equalized in some way).
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