Dear Adnan,
you need to take into account the driving voltage for the MZM (bias and gain), the M-pulse generator amplitude and 3-dB coupler (3-dB loss). All of these parameters affect the overall QAM signal power. I am attaching an example showing the effect of gain on the results.
If you need further discussion, please contact me at ahmad.atieh@optiwave.com
Regards,
Ahmad