How to Add PIN Noise to the Sampled Signal using Matlab?

[alistu]

Hi all,

In the new version of Optisystem, the PIN noise is created in a separate vector from the received (main) signal. So what should I do to add the noise from a Coherent Detection block to the signal using Matlab? Both in-phase and quadrature main signals exiting the block have real values, while the noise is complex in both cases.

Damian kindly recommended that I add the magnitude of the noise to the signal in both outputs, like this:
in-phase overall signal = in-phase signal + abs(in-phase complex noise);
quadrature overall signal = quadrature signal + abs(quadrature complex noise);

However, I tried it and the result was the constellation diagram shown in image1 attached. Just adding the magnitude means that all symbols in the diagram are shifted towards the first quadrant and the symmetry is corrupted. So I have doubts about the validity of this method (although what is shown in the image is even different from this, but let’s skip that for now).

With Damian out of the office for a while, I could use your suggestions more than ever. How do you think I should add the noise to the sampled signal?

Thank you all for any help.

Attachments:

1. 

   image1.jpg
   

[alistu]

I have to make a correction to what I have posted above. The output signal from the coherent detection block is not still processed in electrical domain, so the amount of noise magnitude is not added to the symbols, as opposed to what I said about the shift being made and so on. However just to make sure Damian’s suggestion is right (which it most probably is), I would appreciate to know the distribution of the noise added to the signal in theory. I have attached the distribution of the magnitude of the PIN noise that I have plotted using Matlab. Thanks!

Attachments:

1. 

   Photodetector-Noise-Distribution.jpg
   

[Ravil]

Hi Alistu,

I agree with you that addition of just a magnitude (as any deterministic value) will just shift the signal (as you can see from your picture). As you mentioned in your second comment, any noise DOES defined by its distribution function. However, as you know, there are different sources of noise in p-i-n photodiode for different noises: thermal, short and dark. All these sources have different nature that defines their distribution function.

[alistu]

Hi Ravil,

Actually I have explained in my second comment that adding the magnitude to the signal exiting the coherent detection block does not move the symbols, because it is added to a signal that has not been processed and therefore does not represent the symbols yet (although it contains the information that the symbols are supposed to be extracted from, after exiting the coherent detection block and then passing through the demodulation block(which is in my case an OFDM demodulation block)). I was clearly wrong about it in my first post.

[Ravil]

Can you, please, specify what type/types of noises are you modeling at the Coherent Detection block using Matlab?

[alistu]

The noise that I have included in my design actually come from all the sources that you have mentioned, i.e. thermal noise, shot noise and dark noise. You can open the Optisystem OFDM samples and see what is included. Now I would like to know if the magnitude of the noise that I am adding to the signal is a known distribution. Thanks!

[Damian Marek]

If i remember correctly I think I also suggested to enable the convert noise bins in the PIN detector. There should be an option there. If not you could always place a Convert Noise Bins component from the Signal Processing Library. OptiSystem can take care of that for you.

[alistu]

You had not suggested that before but thank you very much for suggesting it now. However, I have problems implementing it. When I connect it to the PIN detectors the way shown in the image attached, I receive no signal at the output of the Coherent detection block (as you can see in the screenshot attached). Please help.

Attachments:

1. 

   ConvertNoise.jpg
   

2. 

   NoSignal.jpg
   

[Damian Marek]

Ok my mistake the component you want to use is in the same folder but it is called Convert to Sampled Signals. In the component parameters you can change the signal type to convert from to Noise bins instead of parameterized.

[alistu]

Thank you Damian. I used Convert to Sampled Signals components the way shown in imag1 attached and I changed signal type to noise bins for each component, but the result did not change for my OFDM system and as you can see in imag2 attached, the output signal from coherent detection block which enters OFDM demodulator still contains noise bins.

Attachments:

1. 

   imag1.jpg
   

2. 

   imag2.jpg
   

[Damian Marek]

From your screenshot is shows the components are set to convert “Parameterized” signals, if you change this to “Noise bins” it should work.

[alistu]

It was so when I took the screenshots but I changed them afterwards yet did not take new screenshots.

[Damian Marek]

What about placing the convert to sampled signals component directly before your Matlab component so that more noise bins are not contributed. If this is still not working please attach the project file.

[alistu]

I could not place “Convert to Sampled Signals” component before the electrical OFDM demodulator, as the input and output ports of the component are defined “optical”. I have sent the files to your mail. Thanks for your help.

[Damian Marek]

So as you might be aware many of the components add noise to the signal through noise bins, EDFAs for example. The PIN adds noise directly to the sampled signal and not as noise bins. However the Electrical Amplifier does add noise as a separate signal. On the Noise tab of these amplifiers there is an option to add noise to signal, if you check the box the separate noise signal will disappear and the total noise will be added to the sampled signal.

Also just use the BER Test Set, instead of the BER Analyzer it can be found in the Test Sets folder.

[alistu]

Thank you very much.
May I ask why you suggest using BER Test Set instead of BER Analyzer? You have suggested it on more than one occasion to me. Doesn’t BER Analyzer have more options than BER Test Set?

[alistu]

Here I have attached an image which shows the output elements of each of the four Convert to Noise Bin blocks. As you can see, the “Sampled” structure is empty. Instead, the information is probably in the Noise structure. But as it can be seen in the other attached image, there are some unknown parameters with strange vector lengths in the Noise structure. Also, the time vector is lost. I have no idea what to do.

[Damian Marek]

Here is an explanation:

RoF with QPSK doesn’t work

Furthermore, now with the new OFDM components and all of the overhead that goes along with it, prefix points, pilot tones, training symbols etc. the BER needs to take these in to account, to give the true BER.

Regards

[alistu]

I am using the BER Test Set as you have suggested Damian, but it doesn’t show the bit-error-rate. In fact, the light green lines with “BER” and “Bit Errors” are not shown. How would I be able to read the BER? Should I do anything to make those lines appear? I couldn’t find anything relevant in the configurations. Thank you.

[Damian Marek]

Sure, just right-click and go to Component Results. You can check the Disp box for the BER and/or Bit errors to make it appear.

[Taiwo Ambali Abiola]

Quite educative. Thanks to Alistu for bringing the topic and Damian for the quick response.

[alistu]

You’re welcome Taiwo Ambali Abiola for your compliment. I welcome any opinion you have on the subject that might be helpful to someone. I would also really appreciate it if you or all the other forum members posted comments regarding the topic in here and share their experiences with us.

Cheers!

[Author]

Posts