I suggest that you take a look at the following thesis done by the same person as the one giving the paper you are trying to reproduce the results of:
High Data Rate Coherent Optical OFDM System for Long-Haul Transmission – by Khaled Alatawi – University of Denver.
At the end of the thesis, the related published papers are there, including the one you have attached here. The thesis demonstrates the steps to simulate the system.