After reading the whole topic on Radio over fiber system, I concluded that I have the same problem with my system. Although my system is a bit more simple than the above mentioned system was but I am not getting the desired results (BER=10^-12). currently the BER is 0.
system specifications: bit rate= 240 Mbps ; symbol rate= 120 Msps; bits per symbol= 2 bits;
frequency of first DPSK modulator is 0.7 GHz; frequency of second DPSK modulator is 1.5 GHz.
I am attaching the snapshot and the system file below.