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Plot Noisy MIL-STD-188 QAM Constellation

Apply MIL-STD-188 QAM to random data symbols, pass the modulated signal through an AWGN channel, and then plot the signal constellation.

The cm_ex_mil188_qam_mod model is configured for a modulation order 16. To get desired results all blocks in the model must align their configuration to the same modulation order. The InitFcn callback defines the workspace variable, M = 16, and this variable is used to set:

The Constellation Diagram block has the Reference constellation parameter set to Custom and the Custom value parameter set to align with the modulator block's modulation setting. To see the values, open Reference Constellation in the Plot tab on the Constellation Diagram window.

Run the model with the EbN0 of the AWGN channel block set to 20 dB, and then view the constellation diagram.

Change the EbN0 of the AWGN channel block to 10 dB. Run the model and observe the increase in noise shown in the plotted constellation diagram.

You can try running the model with a different modulation order. To adjust the modulation order you must align:

  • The Modulation order setting in the MIL-188 QAM Modulator Baseband block. To set the Modulation order for the MIL-188 QAM Modulator Baseband block select a value from the dropdown list in the block mask.

  • The value of the workspace variable M.

  • The Reference constellation in the Constellation Diagram block.

If these parameters are not aligned with each other you will notice that the reference constellation and input signal do not produce the desired constellation.