Constellation graph problem

[manpreet3637]

I am implementing Optical Ofdm in optisystem. It show fine constellation graph at reciever with 4 QAM but when i use 16 QAM encoding the constellation graph is not like the graph that i need. I am using 512 subcarriers.

 

[cwjcwjcwj]

What signal constellation do you expect?

You will not get a 16-QAM signal constellation if you check the signal after OFDM modulation.

 

[manpreet3637]

I did not undestand what you said.
I replace 4QAM modulators and demodulators with 16QAM.
The constellation diagram at transmitter side is perfect 16 QAM.
But it is not as expected at 16 QAM demodulator.

 

[cwjcwjcwj]

First of all, it is not called constellation graph. It is called constellation diagram or just signal constellation.

Do you have a channel between tx and rx? Say, AWGN or fading?