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64-QAM OFDM constellation left-shifted

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(joined February 2020)
Participant

Dear all,

I am working on a project with M-QAM OFDM signals and RoF.
My current problem is with 64QAM signal. I get the constellation, which looks like very good, but it is not ok when I compute the EVM by MatLab. The EVM is higher than 30%!
To compute the EVM on MatLab I use to rescale the constellation. For 4, 16QAM signals work super great. The EVM is always as I expected. However, I cannot know what happen with 64QAM. The constellation, after signal processing on MatLab, looks like a left-shifted. I know that the rotation appears with fiber… but there is no fiber! I have also checked the signal at baseband and I found a peak in the center of data band… I also do not know why it is there… because when I use 4 or 16QAM the baseband data is great.

Optisystem constellationMatLab constellation64QAM baseband signal

I know that I have to move OFDM OS12 to new OFDM component, but I need some results now and I do not have lot of time to change everything. I will do it very soon.

Could anyone help me?

Best regards,

Luis

Responses (1):

    • #67126
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      Luis Vallejo
      Participant

      Dear all,

      I am working on a project with M-QAM OFDM signals and RoF.
      My current problem is with 64QAM signal. I get the constellation, which looks like very good, but it is not ok when I compute the EVM by MatLab. The EVM is higher than 30%!
      To compute the EVM on MatLab I use to rescale the constellation. For 4, 16QAM signals work super great. The EVM is always as I expected. However, I cannot know what happen with 64QAM. The constellation, after signal processing on MatLab, looks like a left-shifted. I know that the rotation appears with fiber… but there is no fiber! I have also checked the signal at baseband and I found a peak in the center of data band… I also do not know why it is there… because when I use 4 or 16QAM the baseband data is great.

      Optisystem constellationMatLab constellation64QAM baseband signal

      I know that I have to move OFDM OS12 to new OFDM component, but I need some results now and I do not have lot of time to change everything. I will do it very soon.

      Could anyone help me?

      Best regards,

      Luis

    • #67151
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      Ahmad Atieh
      Moderator

      Hello Luis,
      I’ve replied to your enquiry through email. There were few issues related to the settings of the Quadrature Modulator gain and the LP Cosine Roll Filters after OFDM cut off frequency.
      Regards,
      Ahmad

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