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64 QAM

(joined April 2014)
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Hello All
I am working on radio over fiber. I am sending 1GHz 64 QAM Rf signal for the length of 1000 km. At receiving side the constellation diagram is attached, What is significance or meaning of this type constellation diagram.

Responses (23):

    • #12128
      Ajay Vyas
      Participant

      Hello All
      I am working on radio over fiber. I am sending 1GHz 64 QAM Rf signal for the length of 1000 km. At receiving side the constellation diagram is attached, What is significance or meaning of this type constellation diagram.

    • #12135
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      Damian Marek
      Participant

      Hi,

      It looks like a PSK type modulation scheme. Does the transmission side have the same circular type constellation diagram?

      Regards

    • #12142
      Ajay Vyas
      Participant

      no sir, in tranmitter side we have 64 QAM cansetellation daigram

      Attachments:
      • #12153
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        Damian Marek
        Participant

        Could you please attach your project file? The constellation looks too ordered to be a result of a noise source.

    • #12164
      Ajay Vyas
      Participant

      Thanks
      Please find the attached project file

      +1
      • #12172
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        Damian Marek
        Participant

        I tried out your project and there was one or two problems I saw. First of all I didn’t get the ordered circular signal at the receiver side. I got a very noisy QAM signal with an almost random distribution. The problem here is the implementation of the Laser source. If you are going to use the measured component then it cannot be run in digital mode, because the input signal is not as simple as a digital signal. You will also need to provide an electrical bias to the input signal and attenuation so that Laser is operating above threshold and below saturation (Max power). Try playing around with the Laser source and look at the optical signal until you can get a cleaner output.

        Hope this helps!

    • #12192
      Ajay Vyas
      Participant

      Thanks
      I make changes as per ypur guideline, but still not got the cleaner out put, I am trying again. When i increase the length of fiber 1000km then I got Circular constellation diagram for 64 QA.

      • #12206
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        Damian Marek
        Participant

        You might also want to try using a CW Laser and a Mach Zehnder modulator, so that the input signal is less distorted by chirp (and saturation/threshold effects). Once the transmission link is working you can change to directly modulated laser source.

    • #12256
      Ajay Vyas
      Participant

      Thanks for you suggestion. I was change the CW inplace of DML. There is improvement at receving side & I again used DML with your suugested parameters there was also some improvment in Magnitute. But still need for some improvment please let me know for further action

    • #12292
      Ajay Vyas
      Participant

      Hello Sir,
      Please suggest me modification to get 64QAm recevied signal , I got received signal as attached

      • #12302
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        Damian Marek
        Participant

        Could you please attach your most up-to-date project file with the CW Laser?

    • #12323
      Ajay Vyas
      Participant

      Please find the update project file

      • #12341
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        Damian Marek
        Participant

        Ok so there are few changes you should make:

        1) The Mach-Zehnder Modulator automatically normalizes the electrically input, so the gain and bias components are not really doing anything. The same goes for the gain on the output. However, you can keep them there if you want.

        2) The real problem is the Low Pass Filter in the Quadrature Demodulator, you should play around with this value to get better results. I was getting pretty good constellations for a cutoff frequency of 50-100 MHz.

        You might also want to create your own demodulator circuit so you can see view the spectrum before the filter. Double click the Quadrature Demodulator and look at the help file to see how it is built.

        Hope this helps!

        +1
    • #12372
      Ajay Vyas
      Participant

      Thanks for your valuble suggestions. I got very good result for CW in case of 64 QAM & for Direct Modulated Laser in case of 4 QAM, but for Direct Modulated Laser in case of 64 QAM. I am attached all results. Please let me know how i can get good result for 64 QAM

      • #12575
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        Damian Marek
        Participant

        The problem with using a directly modulated laser source for QAM, especially 64 QAM, is that they inherently introduce a lot of phase noise and also a chirp across each pulse. For QAM, where phase coherence is very important to the signal integrity, I don’t see a very simple solution to this problem. Now there might be a way to compensate for the chirp over each pulse, but this is not my area of expertise.

        I did read an article recently on mitigating chirp in directly modulated lasers:

        A. Karar, M. Yañez, Y. Jiang, J. Cartledge, J. Harley, and K. Roberts, “Electronic dispersion pre-compensation for 10.709-Gb/s using a look-up table and a directly modulated laser,” Opt. Express 19, B81-B89 (2011).

        But again even this paper was for NRZ, where phase information plays a lesser role. However, if you figure out a way for reliably transmitting QAM with a directly modulated laser source, I’m sure you will have a bright future in academia!

        Good luck!

    • #12946
      Ajay Vyas
      Participant

      Thanks for your suggestions & guidance . I made changes with the combination of bias voltage, phase & magnitude> I got some improve result for 16 QAM using direct modulated laser for radio over fiber link & 64 QAM for the same. I attached those result. Please let me know if you have any reference or recommdation for optimization

      • #27709
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        karthikeyan
        Participant

        hi ajay……. i m doing the research about same field, i need to share something with u…. pls help and send your mail id

    • #13049
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      Selvendran S
      Participant

      try to implement the CML concept, it may helpful to u to controlled the phase related noise … but i don’t know exactly

    • #13073
      Ajay Vyas
      Participant

      Do you mean by continuous modulated Laser? i am alredy try with CML but I have to work with DML… Thanks

    • #13074
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      Selvendran S
      Participant

      No, Chirp-Managed Directly Modulated Laser (CML) is a special kind source to mitigates the dispersion in a fiber for long reach PON network.
      CML is nothing but combutination of (DML Laser + Passive Optical Filter)…
      As per Reply #12575 the problem is raised from the phase noise or chrip of DML laser mean, CML can be a better alternate to rectify your problem.

      herewith i have attached some journal papers regarding Chirp-Managed Directly Modulated Laser for your reference… all the best for your research..

      Attachments:
      +2
    • #14157
      Ajay Vyas
      Participant

      I try with CML but still pphase noise is raise please let me know

      +1
    • #41099
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      Chandana S
      Participant

      Hello,

      I am implementing CO-OFDM MIMO system using spatial CW laser. I am getting a constellation diagram as below. Can any one help me fix the issue?

      Attachments:
      +1

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