Dear Vamsi,
Please note that the OFDM Demodulator has two components (I/Q) on each polarization (X/Y). Thus you need to have two QAM Sequence Decoders, one for each polarization.
Please check the example (OFDM Coherent Detection Single Port Dual Polarization 16QAM.osd) at the following location for more information C:UsersUSER NAMEDocumentsOptiSystem 20.0 SamplesAdvanced modulation systemsOFDM systems.
Regards,
Ahmad