Hello yasir.
I am attaching a link of an ieee paper which would give you a brief idea about the OSNR margin itself. please find it below.
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5405282
And in case you want to increase OSNR , you may increase the symbol rate in the simulation layout parameters. Just double the symbol rate , keeping the bit rate half of the symbol rate.
Best regards
aabid