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A question about OFDM

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(joined December 2015)
Participant

Hi all!

Sorry for my stupid question. But i drew my project OFDM same as OFDM Demodulator.osd file in OptiSystem 13 Samples. I set value and everything same too. But the BER and Constellation results aren’t in the proper way.

Please tell me why, I’m a newbie. My file is in the attachments.

Thank you very much.

Responses (10):

    • #28725
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      NewBie
      Participant

      Hi all!

      Sorry for my stupid question. But i drew my project OFDM same as OFDM Demodulator.osd file in OptiSystem 13 Samples. I set value and everything same too. But the BER and Constellation results aren’t in the proper way.

      Please tell me why, I’m a newbie. My file is in the attachments.

      Thank you very much.

    • #28745
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      alistu
      Participant

      Hi there, This type of question is usually asked when software users want to implement the same system to get the same results, so it is a valid question in my opinion. You need to make two changes: First, change the unit of low-pass filter cutoff and frequency of the quadrature demodulator from MHz to GHz. Second, change PIN photodiode center frequency to 193.1075 (in Downsampling tab).

      Regards

    • #28746
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      NewBie
      Participant

      Thank you, alistu!

      It’s my mistakes. I did it yesterday, like you say. But I still don’t know why we must change the frequency to 193.1075 of the Optical Filter and the PIN photodiode too.

      More question, when i change the QAM modulation from 4 to 16 or 64 then the BER Analyzer isn’t work. What happened, so what should i do to correct?

      Sorry to bother you, i’m not good student! Answer me if you have free time.

      Best regards.

      • #28748
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        alistu
        Participant

        You’re welcome. 193.1 THz is the frequency of the laser, and 7.5GHz is the frequency of the quadrature modulator. So we add these to numbers to obtain the center frequency of the transmitted signal: 193.1THz+0.075THz=193.1075THz. After making the changes and getting results, change symbol rate in layout parameters according to the M-ary scheme and you’ll hopefully have no problem.

    • #28757
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      NewBie
      Participant

      Thank you very much!

      I tried your idea, but the BER Analyzer still doesn’t work. No results there.

      My Bit rate was 10e+009, so I chose Symbol rate was 2.5e+009 ( in 16-QAM). What are my mistakes?

      And one more question please, about Min BER. In my result at 4-QAM, the Min BER is the horizontal line, how can I adjust it ?

      I look forward to hearing from you.

      • #28758
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        alistu
        Participant

        Please attach the OptiSystem .osd file in which you have tried to implement 16-QAM OFDM so that I would be able to check it. The minimum BER that is shown in the way you described in the above comment corresponds to BER analyzer not showing any results due to signal being corrupted, as you have pointed out yourself.

    • #28759
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      NewBie
      Participant

      Sorry about that, it’s here sir. In the attachments.

      • #28769
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        alistu
        Participant

        The system seems to be working fine according to constellation diagram, even though there seems to be symbol rotation (you can use OptiSystem 13 OFDM components to be able to use channel estimation). I suggest you use BER test set instead of BER analyzer to get an empirically calculated value for BER.

    • #28772
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      I made the change that Alistu mention them, and I was able to get BER=0.

      if you would like to change the QAM Modulator to 16, you have to change the following
      – cut off frequency in both Low Pass Cosine Roll Off Filter
      – Frequency of both Quadrature Modulator and Demodulator, also the cut off frequency of the Quadrature Demodulator, and the phase
      – Frequency of the optical fiber and optical filters, also the bandwidth of the optical filter.
      – change the algorithm to Gaussian in the BER analyser, unlock “the eye must be open”, and change eye opening tolerance to 100
      – reduce the # of the subcarriers.

      hopefully after you make the change you got good results

    • #28779
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      NewBie
      Participant

      Thank you very much, alistu and Mohamed. The BER analyzer worked and 16-QAM- OFDM system performance is increased.

    • #28979
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      Dat Tran
      Participant

      I do not see the attachment, if you still keep the file can share with me?

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