inter symbol interference vs digital modulation I am a little confused  after my lecture today. My professor started off with explaining what inter symbol interference (ISI) is and he ended the lecture talking about the different types of digital modulations out there.

I am now confused with how these two actually come together, or is it two different topics that have no connection?

Is ISI about sending digital signals in a wire? Does ISI come into play when doing lets say QPSK? I thought the symbol signal sent in QPSK is just a sin signal. or it uses this ISI?
ISI is an (typically undersired) effect of the channel – that can be a cable, an optical fiber, free space or the air you use to transport your wave.

The different kinds of constellations are a method of putting data in a waveform.

So, they are connected: The modulations define which symbols your system uses. ISI is what happens to the symbols when you rapidly send symbols one after the other.

Hence, your receiver needs to deal with ISI, and it needs to understand how to get the data back from the symbols it received.