Hello Rajveer,
I went through your project, you could simulate 16-QAM and obtain a proper constellation. However, you need to set properly the following:
1. the global parameter symbol rate must be 2.5Gsymbol/s due to using 4bits/symbol
2. The OFDM Demodulator reference bit rate should be Bit rate/4
3. The phase of the Quadrature Demodulator should be set at 185 deg
The constellation should looks like the attached file.
I recommend you using new OFDM components instead of the OFDM OS12 version. It is more flexible and has more capabilities.
Regards,
Ahmad