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I'm beginning to learn about the Constellation Modulator block, and just trying it out by setting up a 4-QAM system, with the constellation points arbitrarily set as : [-1-1j, 1-1j, 1+1j, -1+1j]

And the corresponding symbol map set as : [0, 1, 2, 3]

I tested this block by using a Vector Source with a value of '2', which is the third value of the symbol map. The corresponding constellation point (the third value in the list) --- which is 1+1j -- is indeed plotted.

Although, I also notice that another constellation point (which happens to be the first point in the list) - namely -1-1j is also plotted.

Is there a way to get the simulation to focus only plotting the 1+1j point? (that is - to plot a single point that corresponds to the the '2' value (ie. the 1+1j constellation point).

I've attached a screenshot showing the point that I assumed to see only (marked in green) ----- while the point marked in red seems to be plotted as extra. Any recommendations about what I can do to stop that extra constellation point (at -1-1j) from coming into that constellation plot?

As a result of that extra constellation point coming into the picture ----- I think that the constellation plot also picks up points in-between the two constellation points ----- which comes up as points along a straight line. Basically - picking the trajectory in-between -1-j and 1+1j. I'm just trying to get a plot with 1+1j appearing in the constellation plot only.

Thanks very much in advance for any recommendations/help!

Vector '2'

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UPDATE - I have added waveform plots (obtained from a QT GUI Time Sink block) - which shows some interesting waveforms. These waveforms aren't what what we expect to see, right? Thanks in advance for more comments can recommendations about the Constellation Modulator block.

Vector 0 Vector 1 Vector 2 Vector 3

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    $\begingroup$ Strange, could you try with [2,] instead of 2? $\endgroup$
    – mmmm
    Commented Mar 31, 2021 at 6:10
  • $\begingroup$ Absolutely! Thanks for that suggestion regarding [2,] ....with a comma next to the 2. I just tried it out and just reporting back that I obtained the same result when I put in [2,] instead of [2] ---- which also ended up with the set of dots along that straight line. I also found that if I put in other values (instead of '2') ----- the -1-1j point would also be in those plots. The only exception is if I put '0' ---- which is the only case where I get a single plotted point, because '0' corresponds to -1-1j for this setup. Thanks again! $\endgroup$
    – Kenny
    Commented Mar 31, 2021 at 6:32
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    $\begingroup$ Oh wait, the constellation modulator does pulse shaping. Mystery solved! You're not looking at symbol points, but symbol points convolved with a tx pulse shape! $\endgroup$
    – mmmm
    Commented Mar 31, 2021 at 6:38
  • $\begingroup$ Oh ----- very cool! Thanks for your help 'mmmm'. That helped a LOT!!!!!! Very much appreciated! Although - even with pulse shaping ----- also just wondering why the constellation point keeps swinging to -1-1j? Thanks again! $\endgroup$
    – Kenny
    Commented Mar 31, 2021 at 8:37
  • $\begingroup$ Anybody else get the same results from the Constellation Modulator block and GUI Time Sink combination? $\endgroup$
    – Kenny
    Commented Apr 1, 2021 at 19:04

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