I appreciate your help, Dhananjay. As I mentioned, I have used the Optisystem sample file and for the default bit rate, the system is working. It seems that what I refer to as “the problems” are there in your implementations, too. Besides, when I change the bit rate to one GHz in your file, the simulation stops working with an error and the software closes. This is not my concern, though. Thanks again for your help.