Direct Detection OFDM 4 QAM

[Fady]

Could you please let me know what is the role of the Gaussian optical filter which is placed before the photodetector in the example (Direct Detection OFDM 4 QAM.osd) of the advanced modulation systems samples folder? Why its center freq. is 193.1025? Thank you

[Duy Le Nguyen]

Hi Fady.
The center freq. of RF Quadrature modulator is 2.5GHz and freq. of CW laser is 193.1 THz, so we have center freq. of filter is 193.1025THz.
Regards.

[Fady]

Hi Duy,

The center freq. of RF Quadrature modulator is 7.5GHz not 2.5GHz. Where did you get the 2.5GHz from?

[Duy Le Nguyen]

Hi Fady.
You can see it in the RF sectrum analyzer after the Quadrature Modulator. But I don’t know the reason, I’m beginer.
And you can go through the follow link. Gokul has the same question.

OFDM-4QAM-SSB- WITH OS12 AND OS13

If you know the answer, please feedback to me.
Thanks and regards.

[Fady]

Thanks Duy!

[Rajguru M. Mohan]

Hi,
Gaussian filters have a property that their support in the time domain, is equal to their support in the frequency domain.
This comes about from the Gaussian being its own Fourier Transform.
Here gaussian filter is used as low pass filter to remove high frequency component and band limit the signals.
When we use gauusian filter in receivers we would have obtain the Gaussian optical receiver’s eigenmodes exactly in frequency domain. Their eigenfunctions are Hermite functions multiplied by a common phase factor. Using these eigenmodes, we investigate the probability distribution functions of the electrical filter’s output before the decision circuit. We derive a bit-error rate (BER) expression in a closed form using the lowest-order eigenmode. It is valid when the optical filter bandwidth is comparable to the modulation bandwidth as in conventional high capacity optical wavelength-division multiplexing transmission systems. The BER expression can be applied also to non-Gaussian optical receivers.

thanks

[Hamza Ali Abbas Khan]

Hi Fady.
I would agree to little part of Mr Rajguru M Mohan that generally Gaussian filter is used as low pass filter to remove high frequency component and band limit the signals. In your Case i am not sure about if it does perform the same task but you can say that. And as far as the center frequency thing is concerned i cannot disagree with Duy Le Nguyen. Anyway thank you for bringing up a good query.
Thanks

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