the frequency spectrum of ISI

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luckyvictor

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Hi all

I have a question on the effect of ISI. As far as my understanding, the cause of ISI is due to the fact that symbol in time domain spreads, so it interferences with its neighbours, therefore causing the receiver makes incorrect decision.

However, this is what I don't understand. According to another material, it says in the frequency domain, we need to guarantee that the fourier transform of the sampled response is flat, why? what does it really mean please?
 

The condition of no ISI is that the samples of the time response taken at t=nT are 0 for any integer n != 0.
The same condition expressed in frequency domain is that the Fourier transform of the discrete sequence is flat. Equivalently, the folded Fourier transform of the pulse has to be flat.
Regards

Z
 

can you please explain it in more detail?
 

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