Hi Alistu,
thanks for your reply, I think this parameter is increasing the signal some how inside the OFDM demodulator, because I notice when I increase the gain then amplitude of the constellation increase, and when I decrease it then the amplitude is decrease it also.
moreover, if I increase any gain at any amplifier (electrical or optical ) before the OFDM demodulator or the power transmitter then I have to decrease the gain at the OFDM demodulator to get better results.