Better System Performance Without Optical Amplifiers?

[alistu]

Hi all,

I have been trying to simulate some sort of advance modulation system by OptiSystem and the longest reach I managed to achieve before the constellation diagram becomes completely corrupted was about 500kms, while the transmission line consisted of 3 spans with a SSMF, a DCF and an optical amplifier in each. When I omitted the optical amplifiers, the reach increased significantly up to about 2500kms, but the signal (as shown in the image1 attached) got really weak while the diagram was still clear. So the reason for this signal corruption is the 4dB noise figure of amplifiers. Now I would like to know if we can really do such a thing in practice to improve the constellation diagram at the cost of having a drastic decrease in the received signal power. In other words, can such a weak signal be really detected in practice?
BTW, it’s a 4-QAM constellation diagram and pay no attention to the 4 extra constellation points, which is probably due to some Optisystem bug.

Thank you in advance.

[Alessandro Festa]

Dear Alitsu, the behaviour you describe seems weird, but in order to understand it it is better if you post the simulation file…how much power are the amplifiers launching into SSMF?

[alistu]

Hi Alessandro,

Thank you for trying to help me. If you simulate an OFDM system from optisystem sample files (or probably if you simulate ANY system), the same thing would happen. The amount of amplification is the same as the amount of accumulated attenuation in each span. However, I believe the behavior can be well explained by noticing the fact that the Optical amplifier has a noise figure of 4dB. So when you omit it, the signal becomes less noisy but weaker.

[Alessandro Festa]

Hi Alitsu,

I understand your point, but amplifiers, even if they add noise, are necessary to go below receiver sensitivity. Probably in these particular cases receiver sensitivity is very good (or shot/thermal noise are not taken into account?).

[alistu]

Dear Alessandro,

Both shot noise and thermal noise have been taken into account. Here I have attached the noise characteristics of the PIN photodetectors of the simulated system. Unfortunately, I can’t post my simulation file as it’s a part of a project.

[Alessandro Festa]

Ok I understand thanks!

[Ashu verma]

sEE THE RPLY BELOW.wRONGLY pOSTED IT TWICE

[Ashu verma]

Hello Alisthu
I have also designed long reach OFDM system from one of the optisystem samples examples,But it worked good for me upto around 4500 Km.The only thing i did is, mitigate the effects of non linearities by using matlab.I havent faced this type of problem with my system.also i have used the
the symmetric dispersion compensation with 25Km SMF followed by EDFA (Gain 5dB Nf 4dB) then DCF 10Km again EDFA and 25 KM smf.And my constellation diagrams were noisy but not showed another 4-QAM signals as you have mentioned above.

[alistu]

Hi Sam Sung,

Firstly, may I ask what type of nonlinearity mitigation you used? You have probably used the samples from Optisystem 13 updates. the OFDM blocks in the new update makes use of pilot signals and I believe that’s why you got a better reach than me.
From your comment I understand you haven’t had my experience. I just want to know if I can trust the results obtained. Thank you.

[Abhishek Shrama]

Hello Alistu
you can use optical phase conjugation in your system. I think samsung also talking about this compensation.

[alistu]

Hello Abhishek,
Now that we are talking about optical phase conjugation, could you kindly tell me why this method is useful in mitigating the nonlinearity effects? Or perhaps give a reference on this? However my main question in this topic concerns validity of the result that I have obtained. Please refer to my question and let me know about your opinion. Thank you.

[Ashu verma]

Hello Alisthu
Yes abhishek is rigth.OPC would be the best way to compensate the effects of nonlinearities in ofdm system.I have used optisystem 11 samples to make my long haul OFDM system.If you are using optisystem 13 then you will get more good results.Please attach file to see what the main problem is in your system.

[alistu]

Hello Sam Sung,
You’re right and I think the better results obtained from optisystem 13 are due to the fact that it makes use of pilot signals in the OFDM block. About what you mentioned earlier about your constellation diagram and how it was different from mine, I think the reason is that I am using a Matlab program for the OFDM block, and not the Optisystem block. And although my program is probably correct, in every Optisystem version it gives a different constellation diagram! Si I think it’s a bug related to Optisystem and its co-simulation with Matlab.

[Damian Marek]

Alistu,

I have worked a lot with the OptiSystem 13 OFDM components and also the Matlab component and I have not seen this bug before. If I understand correctly you have implemented your own OFDM Modulation in Matlab and OptiSystem Constellation visualizer is giving the strange sub-QAM constellation as well. I would need your project file to determine whether this is indeed a bug, but my feeling is that it is not in fact a bug.

Regards

[alistu]

Dear Damian,

I cannot upload the files here, but I will send them to your mail. I have implemented my own OFDM modulation and demodulation blocks and writing an OFDM program is not that complicated, so I don’t think the problem stems from my Matlab OFDM program. Besides, as I demonstrated to you before in my project files and screenshots attached via mail, the same OFDM program results in different constellation diagrams in Optisystem 12 and Optisystem 13. In fact, I’m still waiting for your reply to that mail, verifying that the problem was actually because of some bug in Optisystem 12. Here I have attached that constellation diagram what happened to it in Optisystem 12 using the same program as I have used here.

Thank you and appreciate Optiwave Team’s help and support.

[Ravil]

Hi Alistu,

I have a quick question (just for my understanding) to you: did you really achieve 2500 km reach without using any signal amplification? What was the power level of your received signal in this case?

[Alessandro Festa]

That’s also what I am curious about…2500km with the best available fiber means at least 420dB! This should bring any signal below the noise level in Optisystem…

[alistu]

Dear Ravil and Alessandro,

Very good question! I think the attached figure from the optical spectrum analyzer right after the line would give you your answer. Strange for me too, but as I showed you, I haven’t set receiver noises to zero.

Regards

[Alessandro Festa]

As I mentioned above, the signal is below the OptiSystem noise threshold which is -100dBm…I am wondering how it can be received!

[alistu]

Yeah, me too 🙂

[Damian Marek]

Found the problem here if anyone is interested. The signal is still indeed quite low, but if you change the scale to W you will be able to view it. The m-file that alistu sent me was not incorporating the thermal and shot noise from the Photodetectors which is attached to the main sampled signal as a separate entity in the Matlab data structure, essentially making a perfect photodetector.

[Alessandro Festa]

Thanks Damian, this clarifies all and is what I supposed in my previous post…with noise taken into account there is no way the link can be made!

[Ravil]

Thank you very much for your clarification, Damian! It makes a perfect sense know: alistu has the model of noiseless receiver, – the thermal noise, to be considered in matlab file, should bring the received power level around -90dB (if I am not mistaken), and we should add the the shot noise to this result afterwords.

[alistu]

Hi all,

As Damian mentioned, I have implemented OFDM transmitters and receivers using Matlab components in my simulation, but I haven’t noticed the fact that photodetector noise is given in separate vector than the signal and that’s the reason for the aforementioned problem. Thanks for trying to help me here and you were right! It was not possible to find the problem without taking a look at the simulation files.

Regards

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