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