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I have confused question regarding the possibility of combination between QAM modulation and PAM in order to improve the spectral efficiency. Is that possible?

As you see in the below Figure, it's the QAM modulation, where the four status of 1+j, 1+j, -1+j and -1-j are existed. where J represents the imaginary part. enter image description here

My question, what's about using the amplitude with that? For example, in case of combination with PAM modulation, we can have eight status instead of four, which are: 1+j, 1+j, -1+j and -1-j at the same time the four added status will be 1/2(1+j), 1/2(1+j), 1/2(-1+j) and 1/2(-1-j), where the amplitude is used to represent additional bits to transmit too.

Is that reasonable and logical to be used ? Is there any articles or books which explain that ?

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It is quite common to combine amplitude and phase modulation. QAM is actually a good example because generally you will have symbols with different phases and different amplitudes. Just in the special case of $4$-QAM (which is actually the same as QPSK), all symbols have the same amplitude. But if you take as an example $16$-QAM then you'll see that the information is encoded in the phase as well as in the amplitude:

enter image description here

[Chris Watts [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons]

Note that in general the exact position of the complex symbols is a compromise between ease of implementation and power efficiency. QAM is especially easy to implement and the penalty compared to more power efficient constellations is relatively small.

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  • $\begingroup$ Yes That's clear, but I think PAM modulation is not similar to Amplitude modulation. for that I'm asking about the possibility to merge PAM with QAM. and the other question, in QAM modulation, real part and imaginary part have the same amplitude during every symbol duration, is that right?? $\endgroup$
    – New_student
    Nov 27, 2018 at 11:08
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    $\begingroup$ @New_student: PAM = pulse amplitude modulation, so it is in fact amplitude modulation. For QAM, real and imaginary parts generally don't have the same magnitude, just look at the figure in my answer: 8 symbols have real and imaginary parts with different magnitudes. $\endgroup$
    – Matt L.
    Nov 27, 2018 at 11:12
  • $\begingroup$ @New_student you have a fundamental misunderstanding of what QAM is. Please consult Wikipedia or your textbook! Matt showed you 16-QAM, and with a single look you could have noticed that no, real and imaginary part aren't the same. $\endgroup$
    – Marcus Müller
    Nov 27, 2018 at 11:38
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    $\begingroup$ @New_student .. Regarding the second question you asked. Take for example 1110 in the above example, you will see the real part and imaginary part have different amplitude. if you work only with QAM, I agree with you that they have same amplitude. BUT, regarding PAM and QAM combination, I advise you to search more about that subject. I've read an article long time ago talking about that idea. it was published in Q3 journal. but be careful you will have some issues in BER. GOOD LUCK $\endgroup$
    – Zeyad_Zeyad
    Nov 27, 2018 at 12:05
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    $\begingroup$ That's why I prefer to say "QPSK" rather than "4-QAM", even though both are correct. $\endgroup$
    – MBaz
    Nov 27, 2018 at 14:01

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