Can you explain what kind of signal is this or what can you characterize when you see QPSK constellation like this? to get a better signal it needs to be near 1 and be a round circular constellation. What can be represented by the elliptical kind of constellation in this QPSK?

many thanks, SamQPSK_constellation

  • 1
    $\begingroup$ This looks perfect? What's wrong about it? You seem to have basically all noise variance in the radial component of your received symbols, which is the best place for it to go? $\endgroup$ Feb 6, 2021 at 0:42
  • $\begingroup$ I especially don't see anything elliptical about it? The average magnitude in each of the clusters seems to be equal, and they're 90° from each other? $\endgroup$ Feb 6, 2021 at 0:52
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    $\begingroup$ The constellation is circular. If you need the center of the constellation points nearer to $(\pm1.0, \pm1.0)$ then just adjust the gain. If you need better EVM in the radial direction, you may need to adjust your pulse matched filter(s) in some manner to fix the overshoot or ISI. $\endgroup$
    – Andy Walls
    Feb 6, 2021 at 1:56
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    $\begingroup$ ...or in addition to what Andy already mentioned, timing offset could possibly also introduce the amplitude variation shown? $\endgroup$ Feb 6, 2021 at 2:32
  • $\begingroup$ ... and to add yet another idea, the magnitude of the received signal might be slowly varying due to a pheomena at the hardware level, which may be due to a residual sinusoid modulating the amplitude, or an issue in the gain of an automatic gain control loop, to name a few. $\endgroup$
    – cjferes
    Feb 6, 2021 at 6:33


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