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I'm developing a Multi-user MIMO communication model in MATLAB where a base station communicates with 2 users. The model uses OFDM (QPSK) modulation. Channel model, precoding and covolutional encoder is also used. Iam sending single stream of data to each user and I'm trying to plot the receive constellation diagram for my analysis. At the receiver side, I have single antenna users. Ideally, I'm supposed to get a rectangle lattice configuration for QPSK, however am getting weird geometrical designs which am not able to understand why. I tried to check at the transmitter side how the constellation would be and it seems to be okay. I would be glad if anyone can help me out to debug this issue.

[Transmit configuration before precoding] Receive constellation for both users 1&2

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  • $\begingroup$ Isn't a transient of your MIMO, if you have no inter symbolic interference and your mimo supports a complex filter this could explain the nice diagram you have. Because it is varying phase and amplitude, trying to go somewhere. This is reasonable only if you wave a filter with length 1 sample. $\endgroup$
    – Bob
    May 13, 2021 at 8:54
  • $\begingroup$ What is the difference between U1,DS1 and U2,DS1 constellation diagram with grid? On U1,DS1 it looks like it is on a transient and the gain is converging (Maybe the AGC or an equalizer...) It looks synchronized (angle ok) but as if the magnitude is increasing or decreasing. No idea for U2,DS2 $\endgroup$
    – user51024
    May 13, 2021 at 11:13

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