DIRECT DETECTION OFDM

[Idahor Emmanuel]

I am designing a direct detection ofdm on optisystem (file attached below) and am having errors with my ofdm modulator, can someone help please. (file attached beleow)

[alistu]

Hi Idahor,

The maximum number of subcarriers is set to 1024 both in your OFDM modulator and demodulator properties window, while the sequence length determined in the layout parameters is 128. I increased the sequence length to 2048 and “error, calculating component” was not given in the process of simulation.

Regards

[Idahor Emmanuel]

Thank you very much Alitsu for your reply.. I am new to optisystem and am having a bit of the issues with designing this direct detection ofdm. I have designed it on matlab and i want to integrate it to optisystem. But i want the normal direct detection to work before adding my matlab which works perfectly well. i am using subcarriers of 256, sumbol size of 200 and cyclic prefix point of 64 with a QPSK modulation. I am having errors on my OFDM demodulator when i try to simulate it, can you pls help as i have attached the file below.

[alistu]

You’re welcome Idahor. In the attached file, you have chosen the number of subcarriers to be one, while you have set the number of prefix points to 64. Could you please explain what you mean by “symbol size” in your above explanation so that I would be able to make the necessary changes to the implementation?

[Idahor Emmanuel]

Thanks… i wasn’t too sure about the parameters. My matlab implementation is based on 256 subcarriers by 200 symbols used in the ofdm tranceivers. Symbol size is basically the number of ofdm symbols used in its implementation. I dont know how it works on optisystem.. below is my matlab implementation based on subcarriers and sysmbol size. Hopefully this helps because am going to implement this system design with the matlab attached below.

[alistu]

Indeed the information is useful. However, could you please determine the exact position of the subcarriers? In older optiSystem versions, the position was determined by a parameter named “position array” in case you have worked with them. If this factor is determined, I will make the changes and upload the file.

[Idahor Emmanuel]

The number of subcarriers i used is 256 subcarriers and using a QPSK modulation… i don’t know much about the position on optisystem and i haven’t used optisystem before. Is there any more information you would want to know?

[alistu]

Indeed. Please visit the following forum page where I have asked a question regarding position array in OptiSystem and Damian has given a comprehensive answer. this might also provide some information on other parameters of OptiSystem that we have already discussed:

Optisystem OFDM Block

[Idahor Emmanuel]

My position array would be 128…. I hope this helps. Thanks

[Idahor Emmanuel]

Here is the file… Thanks

[alistu]

I have attached the working file where the system works with 256 subcarriers and 512 possible subcarriers. These have all been set by yourself in the file, though. However, the number of training symbols was not set the same in modulator and demodulator and that was the only reason of the error you were facing.

[Idahor Emmanuel]

Thanks… where can i find the attached file you uploaded.

[Idahor Emmanuel]

ohh i got it now… Thanks

[alistu]

You’re welcome. Sorry I forgot to attach it first.

Regards

[Idahor Emmanuel]

Thanks for your help… But how do i know the BER, because i want a BER value… for its not displaying from the BER test. Thanks for your help once again.

[alistu]

You’re welcome Idahor. To see the BER, right click on BER test set and choose component results. There you can choose the display option for any parameters you want to see from the BER test set in the layout (without having to right click on the component again). I believe some items you have already chosen.

[Idahor Emmanuel]

thanks… I have done this and when i try to play around with it by changing values… the BER value doesn’t change and something caught my attention, From the BER Test set, the “sequence length for BER per iteration” = -2048. The value doesnt change and i have no idea what that is… can you please help. I attached the file below. And also from the OFDM demodulator, i am getting an IND value. which again i dont know what it is.. i will be attaching my matlab code into this system as my next but i want to make sure the system works without my matlab code in the first place.. thanks (attached file below)

[alistu]

You’re right. No matter how much the parameters are changed, the result is the same. Even the receiver RF spectrum analyzer doesn’t show any signal for long fiber line and yet the BER is zero. Unfortunately, I couldn’t find the reason. I will let you know if I could. BTW, the number of training symbols is set to 2 in BER test set while it is zero in OFDM components.

[Idahor Emmanuel]

Ohh Yes… I worked around it, and changed some values from the layout, and i was able to get a value.. but i tried getting a BER of 0 now.. its not happening.. the spectrum analyser also changes now. check the attached file. thanks

[alistu]

Yes, the changes are there is the BER value (although the signal is quite corrupt now). But could you please elaborate more on this? What were the specific changes which were made to make the BER test set work? I assume they did not have anything to do with the BER test set component itself from your reply.

[Idahor Emmanuel]

I didnt touch the BER test component… i basically changed the sequence length from 1024 to 32768 and the samples per bit to 4… thats basically all i did.. i changed it from the layout parameters. The issue now is how i would get a BER of 0.

[alistu]

I couldn’t solve the problem even though I tried. However, two changes have to be made for sure: The number of samples per bit should be increased, and all the filters cut-off frequencies should increase with rgard to bit rate. To be more specific, the cut-off frequency of the quadrature demodulator should increase remarkably.

[Idahor Emmanuel]

Thanks Alistu for the effort, what i tried doing is connecting directly without optics, and the time window for its calculation is longer. when you run the simulation, it takes more time for its calculation. And i dont want to use 256 subcarriers out of 512. I want to use just 256 subcarriers out of 256.. is it possible to do that in the OFDM blocks… i have connected directly without optics it takes more time for its calculation. I attached the file below.

[alistu]

The settings for the number of subcarriers is not right in your implemetation. If you want to use all the subcarriers, you should enter 1#256 in the subcarrier location. Right now it is written “256” there, meaning only there is one subcarrier being used. If you correct this, the simulation speed is reduced to what you expect.

[Idahor Emmanuel]

hi, please help with these questions.
In capture1, how can i change the number of subcarriers per port to 256 assuming i want to use just 256 subcarriers out of 256 subcarriers.

In Capture2 how does the bit rate and the sequence length work?

In capture4 i understand that the symbol rate is half of the bit rate, but the other parameters are quite confusing like sequence length.

[alistu]

I already answered the first question in my previous reply.

In the second image, bit rate is calculated as the bit rate multiplied by the ratio of the used subcarriers to all the subcarriers.

In the last image, the sequence length is the number of bits created by PRBS generator or BER test set and is transmitted. The other common name which you might have come across in the papers is “word length”.

Regards

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