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?

  • $\begingroup$ QPSK is not just a sin signal-- it is a SYMBOL based on the phase of a Sine carrier. With QPSK we have 4 possible states: +45°,+135°,-45°,-135° that we can choose to send with each symbol based on the value of 2 bits (4 states allows us to send two bits at "once" per symbol, So if our data was 01111011 we would actually transmit 4 symbols 01 11 10 11 corresponding to the phases above depending on how we mapped it. Each symbol by the time it is received can have slowly decaying tails such that they run into subsequent symbols. This is ISI simplified and can occurs for all modulations. $\endgroup$ Commented Jan 21, 2020 at 4:54

1 Answer 1


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.

  • $\begingroup$ Thanks for your reply! There is still something I don't quite get. You said "effect" of a channel, but that's only when I use pulses to transmit a signal right? So when symbols are sent one after the other, how are they sent? In pulses or plain sin waves like all pictures show when one types "Digital Modulation Methods" in google? Or are those pictures already showing the effect of the channel? $\endgroup$ Commented Jan 20, 2020 at 13:51
  • $\begingroup$ pulse shaping is the answer. It feels like you're trying to read an introductory textbook on digital communications backwards. $\endgroup$ Commented Jan 20, 2020 at 14:20
  • $\begingroup$ Well the topic just started and my professor introduced these in the same lecture. So just to sum it up, pulse shaping is the technique used to send digital modulated signals? $\endgroup$ Commented Jan 20, 2020 at 14:38
  • $\begingroup$ no, it's part of the technologies. It might pay to either read a logically structured textbook from front to back or wait another lecture – pulse shaping is bound to get introduced now, if you haven't had it yet. $\endgroup$ Commented Jan 20, 2020 at 14:41
  • $\begingroup$ Yeah in general the Hochschule doesn't focus on communications, sadly. And we are in Aschaffenburg. $\endgroup$ Commented Jan 20, 2020 at 14:45

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