Coherent OFDM

[FadyParticipant]

Why the BER equals 0 in all coherent OFDM examples in the dvanced modulation Folder !
Thanks

[RavilParticipant]

Are you using the scheme from Optiwave website?

[Jaffar Emad KadumParticipant]

OFDM system can support very long distances over optical link up to 2500 km, try to increase the length of the optical link in these examples and the BER will be increased as well.

Best

+4

[RavilParticipant]

Agree with Jaffar. Try to examine dependence of BER on distance and transmission speed increase.

+1

[RavilParticipant]

Please, let me know about your progress!

[Damian MarekParticipant]

The BER visualizers are in “Measured” operating mode. This means that it directly compares an input signal to the transmitted signal and counts the errors. The sequence lengths are set to 16384 bits, which makes it hard to estimate BER values smaller than 1/16384. You can try increasing the sequence length, which will increase the simulation time or change the lengths of the fibers to introduce more distortion.

[FadyParticipant]

Thanks for your replies. I have tried both increasing the sequence length and increasing the length but without success. Increasing the length will give MIN BER of 0 and reducing the length will give a MIN BER of 1.

[Damian MarekParticipant]

What length are you changing the fiber to? I get the expected min BER of 0 for short lengths.

Attachments:

1. 

   minBERProof.png
   

+1

[FadyParticipant]

Thanks for your reply. I have made length sweep from 50 to 100 km, I got min BER of 0 for lengths from 50 km to around 75km then I got minBER of 1 for longer lengths.

[Damian MarekParticipant]

This is not entirely unexpected as the longer fiber lengths leads to more loss and noise. However, the sharp drop off of the BER from 0 to 1 suggests that at one point the OFDM demodulator is completely wrong at detecting the input. This suggests to me that the delay of the optical signal is large enough (from the length), so that the demodulator is misinterpreting the symbols of the constellation diagram, because of this phase shift. This sensitivity is a problem with the OFDM demodulator and one the OptiSystem team is fixing in the next update coming later this year.

You might be able to work through this by changing the delay compensation parameter in the OFDM Demodulator manually.

Regards

[FadyParticipant]

Thanks for your response but what are typical values should I try for the delay compensation parameter in the OFDM Demodulator?

[RavilParticipant]

I don’t think BER should change as gradually. By the way, Fady, what is the bit rate in a channel of the system? Did you keep it the same for both scenario?

[FadyParticipant]

I kept it as in the example 10Gbps

[RavilParticipant]

For this bit rate both distances should work fine in terms of BER with OFDM from theoretical point of view. Did you try to tune delay compensation parameter?

[FadyParticipant]

I tried several values but without success

[Jaffar Emad KadumParticipant]

Dear Fady

Attach the example file that you you are working with and I will try to modify it

[Jaffar Emad KadumParticipant]

Optical OFDM is very sensitive to all simulation parameters, here I attached the variation of BER with number of loops in my OFDM system which are varied from 30 to 42 loops, each loop consist from 80 km SMF and 16 km DCF with their optical amplifiers.

Attachments:

1. 

   Image.png