Hello Ramya,
Currently we don’t support ACO-OFDM or DCO-OFDM. Users write their own Matlab code and use the Matlab component in OptiSystem to achieve that.
Regarding demodulation of the signal, make sure that you use the proper parameters for the Quadrature Demodulator especially its low-pass filter . Please refer to the examples in OptiSystem Library located at the following link for more information.
C:\Users\USER NAME\Documents\OptiSystem 16.1 Samples\Advanced modulation systems\OFDM systems
Regards,
Ahmad