Hi Alireza,
I have also worked on optical OFDM systems and have simulated some of Dr Matin’s students’ works regarding this subject to reproduce the results. I haven’t seen your simulation yet and if you attach it here, we can help you better. But I can tell you that if you use Optisystem’s DDO-OFDM sample file (in Optisystem samples folder) to make this system, you’ll have no problem reproducing it. Just use a demux, copy the optisystem DDO-OFDM sample to make them as many as the number of users, and then set each user’s lasers’ frequencies to those mentioned in the paper and that’s it!
Cheers!