Hello Raini,
You may get the constellation centered if you don’t have proper compensation and recovery of the symbols. You may need to used DSP component.
I recommend you going through the many examples on advanced modulation in OptiSystem Example Library located at the following link for more information about proper design of the system.
C:\Users\xxxxxxx\Documents\OptiSystem 16.0 Samples\Advanced modulation systems\
Regards,
Ahmad