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This question already has an answer here:

i need to know the main difference between QPSK and 4QAM, i have searched over the internet and still i have not found a good explenation.

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marked as duplicate by Dilip Sarwate, MBaz, Matt L., Laurent Duval, Peter K. Dec 28 '16 at 18:56

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There are different types of QPSK: Offset (staggerd) QPSK, $\pi/4$-QPSK, etc.

In its most known form, QPSK is identical to $4$-QAM or $4$-PSK. That is, the same constellation can be referred to by different names. So there is no difference.

This constellation can be seen in the picture bellow (from Wikipedia)

enter image description here

Similarly, $2$-QAM is actually the same as BPSK, as is also pointed out here.

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  • $\begingroup$ you mean that there is no difference between them? $\endgroup$ – user24907 Dec 27 '16 at 16:00
  • $\begingroup$ but when applied both of them for AWGN channel QPSK gives better BER values than 4QAM $\endgroup$ – user24907 Dec 27 '16 at 16:03
  • $\begingroup$ If one is a subset of the other, the can be an optional difference. $\endgroup$ – hotpaw2 Dec 27 '16 at 20:44
  • $\begingroup$ @user24907 With equal parameters, their BER is also exactly the same. $\endgroup$ – msm Dec 27 '16 at 21:51

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