Hi guys, I am currently working with my OFDM system. My back-to-back system works in perfect condition. My OFDM was done with 16-QAM, 72 subcarrier carries data and 56 carrier zero (zero padding) total 128 subcarries, then IFFT and DAC, then upconverting it to 1 GHz. Perfect back-to-back system. But when I include a laser model in my program to make optical OFDM, there is a phase shift at the constellation point after FFT at the receiver, the laser model is a nonlinear ODE model. Can anyone please advice what I should! I am really lost, I even try to compensate the phase shift which is about 45 degree, but at every loop the phase shift varies, like 45, and then 135, it just jumps with a difference of 90 degrees. Please advice me on this. If you dont understand my explanation, please let me know, I will explain in more detail.