Ok let me explain what I am doing.....
Im generating OFDM signal, from random data, to 16 QAM mapping and also QPSK mapping, then IFFT of them and then add Cyclic prefix, then the DAC and upconversion to 1 GHz.
Then use this signal to drive the laser, laser has its own rate equation which consists of 2 nonlinear ordinary differential equation, so this equations will modulate the OFDM signal into optical ofdm and i will detect them by square law detection using photodetector.
Then I downconvert the signal to baseband, perform the ADC, then remove cyclic prefix, FFT and demodulate and perform the BER.
But after FFT, when plot the constellation, the phase of the constellation shifted by 45 degree and when I equalize it, I realize its nonlinear phase shift, where the phase shifts by 45 and when you run the program again, it looks like it shifts at 135 and then 45 and so on.
So I really dont have the idea where the problem occur.
If you need any further details, please let me know. Thank you very much. The phase shift is same with QPSK as well.