are these values of Q-factor normal !!!

[mai fouad]

in this system , i use QPSK modulation and receive in-phase and q-phase from matlab and it passes through that system as shown in the figure then i receive again the in-phase and q-phase back to the matlab and before calculating the Q-factor i estimate the phase shift between transmitted signal and received one and multiply the received signal by the conjugate of this phase then calculating the Q-factor but the value of Q-factor at very low power levels is high that is almost no effect of changing input power so what i can do !!!

Attachments:

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   qpsk-tx-rx.png
   

[Ravil]

Dear Mai Fouad, what is your input sequence bit rate and sensitivity of your PIN-diode? You may obtain this graph due to low bit rate (and therefore high dispersion tolerance) and high sensitivity of PIN-diode.

[Abdallah Ismail]

Use an ideal fiber at first then Try to increase Noise figure of EDFA to 6dB and check the effect.
I think that the non-linearity appears at the powers near 0dBm because the length is 1100km and it may degrades the SNR and hence Q factor.

[Ravil]

I agree with Abdallah about possible non-linear effects and I’d suggest you to do the experiment with different non-linear coef. of NL optical fiber for the same transm. power and see the difference with OSNR and Q factor

[Author]

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