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If you look for amplitude, frequency, phase modulation, you typically get some picture saying you have a wave and you modulate the corresponding parameter (amplitude, frequency, phase). On the other side, if you look at how digital communication is happening, people say that you have a series of delta impulses (or rectangular signals) representing the data, put that into some transmit-filter that is doing pulse shaping, multiple that by a carrier frequency and emit the signal by the antenna (they call it "sending pulses"). Before it is shifted to the carrier frequency, you have that sinc-function signals that somehow overlay with each other.

I can imagine that in an analogous amplitude modulation case it is enough to multiple the low-frequency baseband signal with a carrier frequency, and get it back on the other side with a similar technique. However, if we go now to a digital signal, can I then do the same with delta impulses that are fed into a square root raised cosine filter that is then multiplied with the carrier frequency? Is that amplitude modulation? However, I have no idea what that should look like for phase shift keying or even frequency. How does a transmit filter chain look, if I have digital signals? It is confusing that some people talk about sending pulses and others about sending waves.

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  • $\begingroup$ no, we don't call it "sending pulses"; that would be physically impossible. We call it modulating a carrier with the output of the pulse-shaping filters. So, I don't really think your premise is correct! $\endgroup$
    – Marcus Müller
    Mar 4, 2022 at 13:43
  • $\begingroup$ your second paragraph can be answered by "exactly the same as in analog modulation, the math is the same, just the signal we deal with at baseband is differently interpreted". $\endgroup$
    – Marcus Müller
    Mar 4, 2022 at 13:45
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    $\begingroup$ You really are just asking for the first N chapters of any modern digital communications textbook. And, honestly, that might be a bit broad for a question here. $\endgroup$
    – Marcus Müller
    Mar 4, 2022 at 13:45
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    $\begingroup$ I think you're on the right track but you're missing some fundamental concepts, such as up and down conversion and the FT's modulation property. Much of what you're missing is covered in textbooks -- I recommend those by Lathi and Haykin, and by Lapidoth on the digital side. $\endgroup$
    – MBaz
    Mar 4, 2022 at 13:46
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    $\begingroup$ a baseband signal includes DC. DC can't radiate, physically. As MBaz said, you're on the right track, but there's a few misunderstandings, and answers would get very long. You're definitely not quite right on what baseband is here, and "modulation" has different meanings in different context, and you're mixing these up. Hit the books again, and read deeper this time - we can't put it any shorter or better than a good book, so really no sense in anyone of us just copying out the first chapters of our favourite textbook, which might confuse you, having started with a different book. $\endgroup$
    – Marcus Müller
    Mar 4, 2022 at 15:03

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