I'll add one idea to the comments already made. The environment
where intersymbol interference occurs is typically linear (lossy
transmission lines, sometimes with reflections). Hence, a single
step response theoretically tells the whole story. From this,
an eye diagram can be constructed by time-shifting the step response
by multiples of the bit time (1/data rate) and adding. (You have to
use both the positive and negative step response, of course.)
This is more practical than it might sound. It takes only a few
time-shifted waveforms to get a pretty clear picture, typically.
And, along the way, what "intersymbol interference" means is
clarified. For example, reflected noise from previous data edges
getting time shifted into the data eye.
Hope that helps.
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