QAM constellation is not what it should be

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Hello.

I was given a Simulink block diagram to simulate QAM modulation and note down the BER for further comparison with M-PSK BER.
In Simulink, when I enter the QAM modulator and set the parameters, there is a button that allows me to see the constellation diagram. This is only a theoretical approach, so, not many technical details were taught to us.

I have attached the file from Simulink so that anyone can help me. The problem is that the constellation diagram looks like the one from M-PSK in a square shape when it should be in a circular shape! How do I set this correctly?

Note:
There are environment variables that were set in MatLab and that Simulink uses as some blocks parameters in the block diagram.
So, for instance, if I want 16-QAM, I have to set M=16 in MatLab window! There is another variable which is SNR but for the constellation purpose, I believe there is no need to change this one. If the simulation doesn't run, set SNR = 20.
 

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fourtytwo

Joined May 2, 2017
31
I think this is simply done for clarity, makes it easier to find a point from its binary value. I have only ever seen square constellations on digital equipment, if I think back to the 70's when some modems output the constellation as an XY for a scope they may have been round. Sorry I don't have simulink.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Ok, I found another modules for what I need, or at least with these modules I get the correct constellation diagrams shape. But the BER is much much lower than with the ones I was using before.

Before I was using "General QAM modulator" and "General QAM demodulator". Now I'm using "Rectangular QAM Modulator" and "Rectangular QAM demodulator". Hope this is correct for what the teacher is expecting in terms of BER!
 
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