Demodulation

[AnupamaE]

Hello everyone,

I am trying to do EVM measurement for  OFDM signal (modulated using 16QAM) using VST PXIe-5645R.

But I am getting EVM as 0 and a wrong constellation.

I am not able to solve the problem. Can anyone please help me? I have attached the image of the demodulation part of the code with this message.

Thanks,

Anupama

[AnupamaE]

Hello everyone,

Please check the link below to get the complete project.

https://www.dropbox.com/s/g7tlkfdfte8ajix/MIMO_base.zip?dl=0

Thanks,

Anupama

[shawn.t]

Hi Anupama,

I wasn't able to open your project completely. It looks like it's missing the "RX OFDM - USRP.llb" files. Try creating a source distribution from your project (Right-click on Build Specification->New->Source Distribution). This should bring in all the VIs that you're using.

Looking at the VST Streaming (Host).vi, I don't see any way that you're synchronizing the generation and acquisition engines. I don't have a lot of experience with the streaming project for the VST, but maybe you can set the start trigger for the input stream to "Output Stream Start Trigger"?

Is there a reason that you're not using RFSA/G for your project? The project you posted on an earlier post used RFSA/G. Since you're not changing the waveform as you generate, do you need the streaming capability?

[AnupamaE]

Hello,

I am sharing the link to the project here. I hope it works.

https://www.dropbox.com/s/g7tlkfdfte8ajix/MIMO_base.zip?dl=0

I used the VST streaming project example from Labview to start with the project and replaced the input signal using OFDM Signal. Since there are FPGAs configuration I dont think I can use RFSA/G. I am not sure though.

[shawn.t · Accepted solution]

Thanks, Anupama. The new link works.

Your EVM measurement happens only on data passed out of your demod loop. I noticed that you have no exit condition on that loop except for an error. In that case, the data passed out will be empty (the only error could be a timeout from the dequeue element or the MT map symbols call). I'm not sure how the loop could exit at all because there was no timeout set on the dequeue element primitive.

Second, remember from your last post that you have to set the samples per symbol to an even number.

With the samples per symbol being two, the start trigger for the input stream being 'Output Stream Start Trigger", and the read block size being 16000, I was able to get data. Oh, I also wired the demod loop to exit the first time the data array size is greater than 0; otherwise, the EVM measurement is never run. Do you want the EVM measurement within your demod loop? Note that the data did not look very good. I'm not exactly sure why, but could it be an issue with your demod calculations? Is there a reason that you're not using the MT demod VI?

With all of this said, what are you trying to do with this project? If you're trying to measure the EVM of a QAM signal, have you tried running the "MT RFSG Generate QAM" and "MT niRFSA QAM Demod" example VIs at the same time? Are you trying to do some additional calculations?

[AnupamaE]

Thanks Shawn,

I did like what you said and now I am getting the EVM measurement and it is not a great value. I was confused why am I getting wrong constellation. I did the same in the previous project and I was getting correct constellation. But in this when I tried the same I get wrong constellation after demodulating.

EVM calculation is just one part of my project and I have done some other calculations on the same project.  I have to use this streaming project for all this calculations and further modifications.

Can you just have a look on my project and tell me why my constellation is wrong?

Thank you very much for your time.

[AnupamaE]

Hello Shawn,

I got the solution. I changed the IQ data rate also in the project and now I am getting the demodulated constellation perfectly. And I am not sure about the values of EVM but atleast those are working. I just have to modify the project now. 

Thank you very much.