Hello fadii,
Can you please upload your osd file or brief us about other parameters apart from bit rate ??
At the moment i would suggest you to use an optical amplifier with output gain. or you can increase the bandwidth of WDM MUX/Demux.
You can also try increasing the symbol rate. and for that matter you can also increase power of the system. It would be convenient if you upload your osd file.
Regards