In the discussions so far, we have assumed that the channel is able to
pass all frequency components of a data signal from 0 Hz up to a bandwidth of B Hz. There
are, however, many communications channels that are not able to pass low frequency
components, either because they need to be ac-coupled
for practical reasons (such as eliminating unwanted dc-offsets), or because the bandwidth
allocated to a given user is in a part of the spectrum well removed from 0 Hz, for
example, wireless communications.
The domestic telephone channel is a classic example of a channel which is bandpass in nature, that is, it has a low frequency and high
frequency cut-off in its gain response.
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