Design error

[Dhananjay Patel]

None of the design in electrical digital modulation folder in Optiwave sample is working correctly. Kindly provide with the updated design

[Alessandro Festa]

Hi, what do you mean by “not working correctly”? Which version are you using, 13?

Alessandro

[Ashu verma]

hello, dhananjay patel
As far as electrical digital samples in optiwave optisystem library is concerned, all the systems working correctly. Can you please elaborate what kind of issues you are facing with these samples? As there are few examples on different electrical modulations specifically which of them you found working incorrectly? Also, as alessandro said which version of optisystem you are using? Attach the file OR screenshot to clear what you are asking about malfunctioning of optisystem samples.

[Dhananjay Patel]

I am using Optisuite 13. Let me give you an example of DPSK-pulse generator and decoding. I am not getting the output at the DPSK sequence decoder block. The oscilloscope does not give any output. Besides other design are also not giving the output at the decoding end. The samples which I am using are from optiwave sample 11 folder. I have optisim 13.

Please find attached the screenshot.
Also in OFDM direct detection example, the BER value is always shown 0 irrespective to any change in design such as modulator parameters, fiber length, amplifier gain.

Attachments:

1. 

   error-file.png
   

[Damian Marek]

Thanks for bringing this to our attention! This file does indeed not work as intended. Luckily the fix is quite simple, just change the Layout parameter Symbol rate to Bit rate/3 or 10.125 MHz. It seems the example files used the default setting of 2.5 Gbps, which is incorrect for this DPSK system. I’ll go through the samples and make sure we fix the ones that need fixing.

[Damian Marek]

I can’t reproduce the problem you are having with the Direct Detection OFDM sample found in Advanced modulation systems -> OFDM systems.

The BER I get varies with length, and from simulation to simulation. You can reinstall the Samples from the executable found in the installation folder. However, the errors you found will need to be fixed in the next release for the DPSK samples.

[Dhananjay Patel]

Dear Damian,

The BER does not vary with respect to length. I have kept the length of the fiber at 500kms and 5kms respectively but still I do not find any change in eye diagram or the BER. Please find attached the snapshot.

Kindly send me the design file which you have.

Attachments:

1. 

   error-file1.png
   

2. 

   error-file2.png
   

[Damian Marek]

Are you sure you are using OptiSystem 13, this looks like an older OFDM example. All of the new ones should be using the BER Test Set, because of this issue, which is a very common problem with using the BER Visualizer with an advanced modulation scheme. There have been MANY posts about this from me and others, so please try to search for them. Here is one example:

RoF with QPSK doesn’t work

Regards

[Dhananjay Patel]

Yes, I am using the design from optisystem 13 example folder. I have optisystem 13. Please find attached the two outputs of BER analyser at different distance of 10 and 50kms. for 10kms my output is worse than 50kms.

Attachments:

1. 

   error-file-3.png
   

2. 

   error-file21.png
   

3. 

   error-file3.png
   

[Damian Marek]

Ah there were big changes to the OFDM components starting with 13.0.2

[Dhananjay Patel]

Besides I do not find any BER Test Set in my tool box.

I have one more query. How can i read the points of constellation to calculate the Error Vector Magnitude.

[Damian Marek]

The BER Test Set can be found in the Test Sets->Binary component folder. The error vector magnitude can be found from the constellation visualizer.

error vector magnitude

[Dhananjay Patel]

So how can I get the new design of OFDM. We here have purchased entire Optisuite.

[Damian Marek]

Please contact your local support office:

info@gigasoftindia.in

They will supply the instructions and necessary files to update to the current version 13.0.3

Regards

[Ashu verma]

Hi Dhananjay
You can find BER test set and modified OFDM in optisystem 13.0.2.Even this csn be find in trial version given by optiwave for 30 days.I have used this system in one of my project.Rest for updation of your software version Damian give you better answer.

[Dhananjay Patel]

Thank you all. I have updated my version to 13.0.3.

I am working on OFDM system . I have certain query to make:

1. What is the significance of these global parameters:

Sequence length
Samples per bit
Guard Bits
Symbol rate
Number of samples

2. What is the use of position array in OFDM block.

3. What is the relationship between the number of sub carrier, position array, number of FFT point

4. How to decide the roll of rate of the LP cosine filter?

5. Also in my input and output constellation, I am finding a huge difference, kindly let me know the reason behind it.

PFA the design

Attachments:

1. OFDM-basic1.osd

[alistu]

As for what you have asked about the global parameters, I will give a quick explanation:

– sequence length: the number of data bits to be transmitted.
– samples per bit: the number of samples that the software makes in vector and matrix calculations. for example, if your bitrate is 10e9gbps and you have two samples per bit, the your sampling rate is 20e9.
– number of samples: allow me to exemplify: if you have a sequence of length N and your samples per bit is set to M, then the number of samples equals: M x N .
– symbol rate: I think it refers to the type of data coding you use. If you are using 4-qam, for example, your symbol rate is half the amount of your bit rate.

I hope this helps.

[alistu]

About your question on position array, I was looking for a lucid explanation that I got from Damian a while ago when I asked the same question a while ago and found it in one of the forums. Here is the address:

Optisystem OFDM Block

Take a look at the reply #17621. Feel free to ask any further questions you might have.

[Dhananjay Patel]

Thanks Alistu.

I have tried to simulate just OFDM with QAM sequence generator. But there is a huge difference between the input and output constellation. Can U guide on it.

[Dhananjay Patel]

Dear Alistu..

I tried and I have got the output.

I have one more query.

I intend to generate 32 QAM using QAM sequence generator with user defined bit sequence generator with all possible bit combinations. I am not able to get all 32 constellation points.

Please see the image below and let me know.

[alistu]

Dear Dhananjay,

I am glad that the explanations made some clarifications for you. I cannot see any images in your post. So please attach them and I will try to help. By the way, are you simulating OFDM and QAM using Matlab?

Regards

[Dhananjay Patel]

This is the file attached

Attachments:

1. 

   error-file4.png
   

[alistu]

Dear Dhananjay,

Could you please attach your .osd file if it is possible? The image only shows the problem but gives no clue on the reason, except for someone who has gone through the same problem once.

Thank you.

[Dhananjay Patel]

Dear Alistu
I am planning to do simulation of OFDM and QAM using Matlab. But currently I am trying to simulate the same using Optisystem.

Once, I simulate it on Optiwave, I will try to simulate OFDM and QAM using Matlab with Optiwave. Let me know, if you have any idea of proceeding furthur.

[Dhananjay Patel]

Hello All,

I am designing 4QAM OFDM modulation and demodulation, the system gets hanged when the FFT bin is 2048 and above. I have kept my subcarrier = FFTbin/2.

Here in this design, I am not using any optical component.

Kindly, guide me about the same.

[Damian Marek]

Hello,

I think I know the problem here. The problem is your sequence length is so short that those symbols, which are like patterns of bits, have not shown up due to random chance. If you increase the sequence length to a larger number you should start detecting all the different types of symbols.

[alistu]

Thank you Damian.
And if you, dear Dhananjay, insist on using a short user-defined sequence, you had better take a look at the help section for QAM sequence generator block and include all the bit patterns possible for 16-QAM that are shown there in your sequence. For example, yours probably lacks 1011 according to the patterns table.

[Dhananjay Patel]

Dear Damian and Alitsu,

PFA the design to generate the 32QAM. I am not getting all the symbols at the constellation.

Attachments:

1. 32-QAM.osd

[alistu]

Hi Dhananjay,

Did you get the point Damian made in his statement? In your osd file I have noticed you have not changed your sequence according to what I told you to do. Please go to help section on QAM generator block and see the bit pattern table and make a sequence that contains all those patterns and let me know if your problem is solved.

[Dhananjay Patel]

Dear Alitsu,

This are the symbol points I am giving to the Sequence generator. I have gone through the help section on QAM generator block.

00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11011
11100
11101
11110
11111

It has all the 32 symbols required for 32QAM generation.

[Damian Marek]

I ran your project file and I get the 32 QAM constellation diagram attached, which to my understanding is correct.

Attachments:

1. 

   32QAMconstellation.png
   

[Dhananjay Patel]

Also I have kept the sequence length of 16384, and samples per bit = 64. Here in my constellation, I am getting only 16QAM points not 32.

[alistu]

You’re right. Isn’t this the file you have attached? I ran the simulation and got the correct result, so there seems to be no problem. Maybe I made some minor changes in it (like changing the bitrate and so on) but that’s it. here I have attached it again. Please confirm the fact that there is no problem with it.

Regards

Attachments:

1. 32-QAM.doc

[Dhananjay Patel]

Dear Alitsu, I am not able to open the file u have send. Kindly send it over again.

[alistu]

I will send it again, but my guess is that you are using and older version of Optisystem than the version I am using, which is version 13.0.3. In this case, unfortunately you will not be able to open it unless you use the aforementioned version. By the way, as far as I can remember, I made almost no change in your file yet got the result.

Attachments:

1. 32-QAM1.doc

[Dhananjay Patel]

I think you are not sending me the design. The attached file seems to be a word document which I am unable to open.

[alistu]

The file you are referring to as a word document is the Optisystem file. Do not open it with word, try to open it with Optisystem instead. I you did not manage to open it with Optisystem, then you are probably using an older version of Optisystem than the one I am using, which is v13.0.3. I don’t know why it is shown as a .doc file.

[Dhananjay Patel]

Hello Damian,

While I was designing an 4 QAM OFDM system, I am not getting correct results of EVM from the constellation visualizer. I am getting the value of Quality factor and EVM both to be very high. Kindly help. Also find attached the screenshot.

The EVM and Q factor both are 10^99 while my output constellation is almost similar to the input constellation.

[Dhananjay Patel]

Please find file attached

Attachments:

1. 

   error-file22.png
   

[Damian Marek]

If this constellation diagram is at the receiver or after detection where the signals has been regenerated this is not an error. WHen the signals has first been generated or regenerated the constellation should be “perfect”. I can’t see from your screenshots where this visualizer is situated. Generally attaching the project file is much better for troubleshooting.

Regards

[Dhananjay Patel]

Dear Damian,

The constellation which is shown is of output after detection. PFA the project file

Attachments:

1. OFDM-basic-with-4QAM-with-upconversion.osd

[Damian Marek]

As I expected the constellation diagram is being detected after the signal has been regenerated by the M-ary Pulse Generators. The Q factor will be at a maximum because the signal has experienced no noise.

[Dhananjay Patel]

Dear Damian,

Please let me know where are we suppose to detect the constellations. Also Find my other design where I have not up converted the signal to 7.5Ghz but still I am getting Low EVM. Kindly let me know the reason for it.

Attachments:

1. OFDM-basic-with-4QAM.osd

[Dhananjay Patel]

When I upconvert the signal, my EVM becomes very high, but without upconversion my EVM is low. Please justify.

I have send you both the designs.

[Dhananjay Patel]

Dear Damian,

I have designed the entire system with 4QAM OFDM and optical transmission. Please find attached my design. I have following queries which are mentioned below.

1. What is the meaning of a.u (amplitude) shown in the oscilloscope viewer. Is it equal to volts as 1a.u = 1Volt

2. The voltage value at the output of OFDM block is very high (around 60 a.u). I have attached the screenshot of the same. Please check the file named doubt 2.png. With such a high output my optical modulator goes into saturation, hence I have to connect a 35dB attenuator to it.
Please justify.

3. I want to transmit the OFDM signal with optical up conversion with double sideband full carrier. But I am not getting possible results. Please check and let me know if there are other errors in my design. I want to perform analysis using BER, SER and EVM.

I have been struggling to design this system from quite a long time.

[Damian Marek]

Hello,

All of these issues are directly addressed by the new OFDM Modulation and Demodulation, I cannot recommend them enough. You’ll notice that most of the topics on this forum I will probably mention using the new OFDM components and it is for a reason! Now that you have upgraded to 13.0.3 please take a look at the example file “Direct Detection OFDM 4QAM” if for whatever reason after your upgrade the new samples were not installed you can find the installer in the Optiwave Software directory.

1) Electrical signals are given arbitrary units and the component that uses the signal assigns its own units for its internal calculation. For example, the FP Cavity Laser would assign a current value to any electrical input.

2) Yes, in the new components you can set the average output power of the electrical signal.

3)The BER Test Set will give you an empirical solution to the BER, whereas the new OFDM Demodulation will display the Constellation diagram and give you the SER and EVM. After the calculation look at the Project Browser and find the graphs section of the OFDM Demodulation.

[Dhananjay Patel]

Here is the design

Attachments:

1. OFDM-basic-with-4QAM-with-upconversion1.osd

[Dhananjay Patel]

Here is the screeenshot of doubt2.png

Attachments:

1. 

   doubt-2.png
   

[Dhananjay Patel]

Dear Damian,

My email id is not updated with the optiwave system. Hence all the mails are bounced back.
I am not able to update my email id.

My official email id is d14ec003@eced.svnit.ac.in

I have also not received the amazon voucher after completing 1000 points. Kindly help

[Damian Marek]

You will be awarded the gift card when Bryan returns to the office next week. Your email has been updated for your account.

[Dhananjay Patel]

I have one query. For SSB generation, the bias voltage of dual electrode MZM needs to be set to VPi/2.

I am designing OFDM system in which I am trying to vary the bias of MZM and checking the EVM at the output. But I am not getting the required result.

According to the theory, if VPi = 4volts, and if I am varying my Vdc (Bias voltage) from 0.2 to 4 volts, then initially with the increase in bias the EVM should drop and reaches its lowest value at Vdc =2 volts. Now when Vdc is increased beyond 2volts, then the EVM should increase. This is due to the MZM non linear distortion.

But I am not getting proper results. I have attached my design below. Please look into it.

Attachments:

1. OFDM-4-QAM_-MI-vs-EVM-design-.osd

[Dhananjay Patel]

One more query.

1. Why there is no option for FFT size in new OFDM block of 13.0.3 version.

2. If I want to change the number of carriers in OFDM such as 512,256,128,64,32,16,8…. Do I need to change the cyclic prefix point accordingly.

[Damian Marek]

1. Now you can explicitly choose which subcarriers you want to use in the simulation please see the Help in the Component Properties for the examples.

2. The cyclic prefix is not dependent on the number of carriers.

[Dhananjay Patel]

Dear Damian,

Kindly go through my design and let me know why am not getting the required results. Please refer reply number 23170.

[Damian Marek]

The first thing I see is that the constellation diagram is slightly off from what our default QAM looks like, rotated by 90 degrees, so that means the EVM will actually be incorrectly calculated. Inspect the Constellation before channel estimation graph to confirm. I tried fixing this by using a phase delay, but it didn’t work as expected! Maybe you have an idea to rotate the constellation diagram.

The other problem here is that it looks like after a sweep the EVM result is losing its value. This is more of a bug than anything and I have encountered it on other components that calculate results dynamically. You could plot the bit error rate as a function of the bias voltage, but depending on the system you would need to use a very large sequence length to catch one bit error.

[Dhananjay Patel]

I tried keeping the sequence length very large as 262144 but still for the distance of 20kms, I am not getting any BER.

The constellation can be rotated by giving a phase angle value to the quadrature demodulator. I gave 15 degrees and got the constellation restored, but the problem still persists. Please help

[alistu]

Hi Dhananjay, The large sequence will increase your accuracy of bit error rate measurement, but it does not improve your system. So if you want to measure BER with accuracy, before increasing sequence length you need to troubleshoot your system and then enhance the length. The accuracy for your sequence length is 3.8e-6 right now.

[Dhananjay Patel]

Dear Damian,

I tried to execute the design “Direct Detection OFDM 4QAM “given in the samples. Here, When we upconvert the frequency to 7.5GHz then the center frequency should be 7.5GHz but as shown the RF spectrum does not gives the correct output. I have attached the RF spectrum output.

Besides, When I try to run the same design from the samples by generating single sideband modulation from MZM (Keeping the angle to 90 degrees), my EVM and BER is too high.

I guess there is some problem with the design in sample folders.

[Damian Marek]

The sample rate is large enough to represent the upconverted signal so it wraps around the frequency domain. If you increase the Samples per bit to 4 you’ll see that it is centered at 7.5 GHz.

[Damian Marek]

Using your attached project from an earlier reply I do not get any bit errors over 20 km of propagation even for very long lengths which means the BER is quite low. I have disabled the Dispersion compensation from the OFDM Demodulation because it depends on fiber length and the Equalizer actually does a pretty good job without it.

[Dhananjay Patel]

Here is the RF spectrum

Attachments:

1. 

   error-file.png
   

[Dhananjay Patel]

Dear Damian,

How can I introduce OSNR block in the above mentioned system (Optical OFDM). Should I connect the OSNR block after the fiber or before it.

[Damian Marek]

You mean “Set OSNR”? Since it only adds noise bins I don’t think it matters if you put it before or after. Only matters if there is a wavelength dependent loss in the fiber.

[Dhananjay Patel]

Dear Damian,

I tried to connect OSNR block before and after fiber and varied the OSNR value from 2 to 30dB. But in all the cases my EVM is not affected. Kindly help.

[Dhananjay Patel]

The value of the EVM shown remains the same for all the values of OSNR.

[Dhananjay Patel]

Dear All,

I have found one problem in the software. I am trying to build optical OFDM system. When I try to keep the value of order of the PRBS generator to be 30 or when I keep my sequence length equal to 2^30, The system takes lot of time to eualuate and in the end it hangs (shows Not responding).

I need this use sample length to calculate a BER value of 10^-9 and below. How can I proceed?

[alistu]

Dear Dhananjay,

As far as much experience can tell, it is typical fort the simulation to take more time with the sequence having been increased that much, so it cannot necessarily be considered a bug. However, I suggest that you reduce the number of samples per pit if it is possible so as to make less samples all in all that way. You can also improve your hardware (as I did to overcome these problems), or install 64-bit windows if you are currently using 32-bit version and if it is compatible with your software.

Regards

[Dhananjay Patel]

Dear Alitsu,

I have a 64 bit version and it is compatible with software. If I reduce the number of samples per bit, the RF spectrum shifts from its center position. Refer reply number 23354.

So in this case I have to keep my samples per bit equal to 4. This takes a lot of time to execute and finally the software goes in to not responding state. Tell me if you have some other option.

I will try to reduce the size of my block diagram and will try to calculate BER. Hope this works…

[alistu]

Unfortunately I cannot think of any other suggestions, except for promotion of your RAM if it is possible for you. The more you promote it, the more you see the effect. Either doing this or reducing the size of block as you mentioned, I hope your problem will be over.

[Dhananjay Patel]

I will try to reduce the blocks and check if I can get the output.

[Dhananjay Patel]

Hello All,

can anyone please tell me, how supression of higher order sidebands at the output of MZM, helps in overcoming the non linearity of MZM. I want a mathematical proof for the same.

[alistu]

Hi Dhananjay,

Can you please first introduce some reference that has claimed the aforementioned statement about overcoming MZM non-linearity by suppressing higher order sidebands? I will try to find the proof if it was possible then (since you have asked for a “mathematical” proof).

Regards

[Dhananjay Patel]

Dear Alitsu,

I am trying to understand how the higher order sidebands will affect the transmission through the fiber. I am not able to find any mathematical proof stating that the higher order sidebands will lead to more dispersion and other losses. So I cant attach any document regarding the same.

It would be great if you can through some light on it….

[Dhananjay Patel]

I am working on non linearity of modulators so am trying to understand the effect of higher order sidebands generated by MZM on the fiber transmission.

[alistu]

Hi Dhananjay,

The reason I asked for a paper or something in which what you are trying to prove is stated is just to make sure if it is a fact before trying to prove it. I will try to find mathematical proof and let you know about it. Meanwhile, I suggest that you go through basic communication books to find solution to these types of problems.

Regards

[Dhananjay Patel]

Thanks Alitsu,

I would also ike to know wht is the relationship between the second & third order side bands and Second and third order intermodulation products.

Mathematical connection between them will be fine

[Author]

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