Phase shift at ofdm demodulation

[thavamaran]

Hi there guys, I am having some problem at OFDM receiver. Actually I am doing optical ofdm.

My back to back ofdm is perfect. zero erros at no noise and with noise it matches the theoritical bit error rate of 16 qam.

But including my laser model, my laser will be modulated by the ofdm signal, and after i receive it. The constellation point shifted by 45 degree.

I tried to equalize it which is very trivial in OFDM. But after equalizing it, I realize its not linear phase shift, it shifts by 45, then 135 randomly.

I modelled the laser rate equation which is nonlinear differential equation with Euler method numerical method. And my receiver doesnt have any synchronization.

Can anyone suggest me where the problem would be? Need more explanation, please let me know and I will be more than happy to explain it in detail. Thank you.

 

[Communications_Engineer]

Please put up a model of what you are trying to do.

I'd also suggest that you start with BPSK, figure this problem out and go back to 16-QAM

 

[thavamaran]

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.