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what is the bandwidth of a digital channel

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Square pulse has infinite bandwidth. You need to implement some filter on top of it. (this will introduce distortion on your square pulse)

like raised cosine filter etc for avoiding ISI b/w symbols which arise due
to spread in time because of finite BW

the bw of square pulse is =1/T where T is the high time of pulse (if base band)
if pass band signal multiply by 2
if raised cosine is used bw = Rb/2*(1+α) where α is the roll off factor of filter
sure 1/T = Rb
& second case is geneal case
if ideal nyquist channel is assumed therefore bw =Rb/2

even in digital system, you don't transmit the 0's & 1's. You do modulation there as well. PCM is mostly used.

yes, the bandwidth of a square pulse is infinity.
but there is no practical base band digital communication to transmit square pulse. it is always passband. normally we use a raised cosine which is like a sinc with few side bands, in the time domain.

ok i know that bandwidth of pulse os infinity...let me convince u by what im saying..lets draw the pulse in the frequency domain..let the pulse width in time is T therfore the pulse in frequency is equal to sinc with first null equal to 1/T &extends to infinity..& since most of power exists in the main lobe( fisrt null to null) therefore we can assume that the bandwidth of the pulse is equal to 1/T ..u can review haykin for this..&by the way im a student in faculty of engineering &we took this method this year,,...i didn't invent it..
good luck

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