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CO-OFDM Architecture

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(joined September 2014)
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In CO-OFDM transmitter, a 90degree phase shift is used (as indicated in the file attached which shows a block diagram of the system). Why has not this phase shift block been used in Optisystem sample file “Coherent Detection Optical OFDM system.osd”?

Responses (16):

    • #14745
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      alistu
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      In CO-OFDM transmitter, a 90degree phase shift is used (as indicated in the file attached which shows a block diagram of the system). Why has not this phase shift block been used in Optisystem sample file “Coherent Detection Optical OFDM system.osd”?

    • #14746
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      alistu
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      And here is the block diagram of the CO-OFDM system simulated by Optisystem which does not contain the phase shifter.

      Attachments:
      • #14815
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        Damian Marek
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        Couldn’t see your attached pic of the block diagram. Can you try reattaching as a jpg or png?

        I may be able to guess at the answer though. Those X couplers used in the OptiSystem design actually cause a 90 degree phase shift when the input is coupled to the opposite output. The bottom branch of the transmitter will have a 90 degree phase shift compared to the upper branch.

    • #14767
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      Ravil
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      Hi alistu, a phase shift is required block in OFDM to create orth. components. It is included in “Coherent detection” block. In my opinion, it should be placed after MZM.

    • #14768
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      Ravil
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      Did you try to ‘run’ that scheme, by the way?

    • #14965
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      alistu
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      Here is the block diagram of the system which was not attached.

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    • #14968
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      alistu
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      Yes, I have run it and it works.
      There has been used a phase shifter in the coherent detection block, which is according to the theory. But none in the transmitter…

      • #14997
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        Damian Marek
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        Ok there is actually a 90 degree phase shift in the OptiSystem project layout, but it is incorporated into the X coupler. In the simulation (and also in physical X couplers) the signal that is coupled to the adjacent waveguide experiences a 90 degree phase shift. If you look at the transmitter and receiver you will see X couplers that incorporate this phase shift.

        +2
    • #15107
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      alistu
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      That’s the point I hadn’t noticed. Thanks.

    • #15115
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      Ravil
      Participant

      Thank you for the explanation Damain! Do you know the purpose of including Electrical Gain in one of the MZM arms at the transmitter?

      • #15135
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        Damian Marek
        Participant

        The electrical gain is there to facilitate an efficient dual driven modulator. Each branch does not have to modulate the entire Vpi, only half of it.

        In a phase shift keying scheme then both branches would have the same amplitude.

    • #15116
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      Ravil
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      Did anybody test this scheme for its max transmission capacity? (Just wandering…)

      • #15136
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        Damian Marek
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        By max transmission capacity, do you mean max bit rate to maintain a certain BER?

    • #16814
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      alistu
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      If by “capacity” you mean Bitrate-Distance product, I have tested it and to my surprise I could only achieve 10Gbps for 360km, which is much less than that of any other paper I’ve studied about CO-OFDM since 2007 (for example, in Alatawi’s 2013 master’s thesis at University of Denver, a system with 50Gbps for 6000km was achieved using Optisystem v.12 without exploiting any types of equalizers and so on). So I would also like to know if anyone has tested the system in this regard. Thanks in advance.

    • #17734
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      alistu
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      How is receiver sensitivity considered in Optisystem coherent optical OFDM example discussed above? I believe there should be such a parameter in PIN photodetectors, but there isn’t. And sensitivity is not something that can be neglected. Can anyone help please?

      • #17743
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        Damian Marek
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        The receiver sensitivity is not explicitly shown or set within the PIN Photodiode, as it depends on the users definition (which Q factor is chosen as the threshold). However, the noise and bandwidth which can be set within the Photodiode changes the sensitivity of the photodiode and a nice little example of this is found at this link:

        http://psut.edu.jo/sites/yazan/classes/Spring2010/MicrowaveLab/FExp4.pdf

        Let me know if this helps!

        +1
    • #17753
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      alistu
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      Thanks a lot! I got what I needed to know.

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