OFDM-4QAM-SSB- WITH OS12 AND OS13

[Gokul Boro .]

Hi all,
Please let me know why does this difference in the two blocks.

I am attaching both the files here.

See the output after the Quadrature Modulator in RF spectrum analyzer. Simulation using OS12 has RF OFDM spectrum centered at 7.5 GHz. While that using OS13 has OFDM spectrum not centred at 7.5, instead the spectrum has a bandwidth of only 5GHz. How can a signal of bandwidth 5GHz after being up-converted with a frequency of 7.5 GHz have a bandwidth of 5GHz?

Please help me to understand it.

Thank you.

[Gokul Boro .]

I am attaching the other design here, because I couldnot attach it in the topic.

Attachments:

1. Direct-Detection-Optical-OFDM-System.osd

[Duy Le Nguyen]

Hi Gokul!
Sorry, but can i ask one question? Why in the DD-OOFDM sample we can choose the frequency of Quadrature Modulator is 7.5 GHz and the cut off frequency of low-pass filter is 0.62*bit rate.

Thanks and regards.

[Gokul Boro .]

Hi Duy Le Nguyen,
I am explaining it based on some papers I went through. In the attached files, the theoretical bandwidth of the OFDM signal is 5 GHz. In order to avoid the Signal-Signal beat interference (SSBI) in the photodiode at the receiver, we have to generate a gap of 5GHz(equal to the bandwidth of the OFDM signal itself) between the Optical OFDM Signal and the main optical carrier. Since we have to generate a gap of 5GHz, we need to center the electrical OFDM signal at 7.5 GHz (calculated as 1.5*bandwidth of OFDM) so that the useful bandwidth of OFDM signal does not get affected by the SSBI noise. Now, about LPF what I can say is that 0.61*bitrate=3.1GHz. If you see at the spectrum analyzer, you can see that the useful BW is within 3.1GHz. So to shape the pulse we use these filters as pulse shaping filters. This is what I have understood. If you think that there is some problem with my explanations (because I am still very weak in communication engg.) you can point it out. Your effort will be appreciated.

Now if I am clear, can you please help me with my problems stated above?
Thank you

[Rajguru M. Mohan]

Hi,
As OS12 doesn’t have training and pilot symbol ports.
And Pilot sequences are the unmodulated data we are transmitting along
with the data.Pilots are used for syncronization and Channel estimation purposes.
Channel estimation is the main thing we need to concentrate for improving the channel capacity .
More the number of pilots more efficeintly we can estimate the channel and increase the capacity .
But as pilots increases overhead increases. so the selection of pilots is such that
tradeoff should be considered between pilots symbols and spectral efficiency.
As we know OFDM Modulation (OS12) component modulates a digital signal into multiple orthogonal sub-carriers.
Orthogonal Frequency Division Multiplexing [1] is a multi-carrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream.

Due to presence of training and pilot symbol ports in OS13 causes difference in results to that with OS12.

Thanks

[Gokul Boro .]

Hi Rajguru,
Thanks for replying.
I am aware of those pilots and training symbols. I once somewhere saw that the new OFDM component is completely re-written. Moreover in the simulation files I have attached, these things doesnot seem to be problem because as seen in the OS12 component the BER rate for transmission distance of 100KM the BER was 0, same is the case with OS13 block. But what difference I saw was in the RF spectrum analyzer outputs at both the designs. Do you have any answer for that?

[Rajguru M. Mohan]

Hi,
the new block in Version 13 seems to be much more complex than OS12.
I would suggest you please go through the following links:

Extra point in constellation point in OOFDM

OFDM in Latest Version of OptiSystem

Thanks,

[Gokul Boro .]

Thank you Rajguru, but the links you have attached are the ones that were started by myself. If you have any newer linkk, please share it.

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