Matlab QPSK exmaple for Adalm Pluto

Question

Alexey Petrenko 2021-12-28

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https://ww2.mathworks.cn/matlabcentral/answers/1618655-matlab-qpsk-exmaple-for-adalm-pluto

回答： Winfried Johannes 2022-3-17

采纳的回答： Alexey Petrenko

Greetings everyone!

I have encountered a problem which I have tried to solve for a long time but eventually failed.

I think that it is due to my lack of knowledge for signal processing.

I have used a single Adalm Pluto module as a Transceiver and two of them as a Receiver and a Transmitter.

I am working within Simulink under Ubuntu.

As for the learning process - I have tried to launch examples for QPSK in Communication Toolbox Simulink.

In both cases I do not see the parcel as it is described in the Receiver QPSK example. If I comment both the Pluto Tr module and Pluto Rx module and than connect QPSK transmitter with QPSK Receiver, I do see a good picture on the Constelation Diagram and I do see all messages as they should be (Hello World 000, Hello World 001 etc). When I am using Pluto modules - I am not getting anything at all, and I see garbage on the constelation diagrams (I do see that something is going on there but it looks bad).

Could anyone help me with this please?

P.S.

I have tried to change Pluto Modules parameters but I did not help.

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Answer 1

Alexey Petrenko 2022-2-8

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Finally I got a result.

Following through Kevin's advices and my own ideas I ended up having all parcels got by the receiver.

The correct way was to lower down the sampling rate withtin pluto TX and RX down to 100k samples.

Although it is totally not the end because as I understand it - I should be able to receive up to 20MHz which is by definition pluto's rf band.

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Kevin 2022-2-8

This is great news. Good to know that finally it is working perfectly for you.

You said that lower sampling works (but not higher sampling rate). This seems to point to bandwidth of your signal. I am guessing you are sending your signal over the air. Try using cable connection and see if this problem goes away. I think it is the wireless channel causing you problem.

Have fun!!!

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Answer 2

Kevin 2021-12-28

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Hi Alexey Petrenko,

It can be tricky debugging hardware problem.

So let's step back and do something really simple.

Can you use the transmitter pluto to send a single tone and use the receiver pluto to receive it? Plot the received signal in time domain to make sure that the signal is not too large or too small (use the tx gain and rx gain to adjust). Also plot the received signal in frequency domain to see if you get a tone.

There are a few other things we can try. But let's do this first and see what we get.

Kevin

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Alexey Petrenko 2022-1-13

Dear Kevin!

Following your advices dedicated to frequency deviation, I have tried to manually change the Rx carrier.

As a result I have seen a little difference in const. diagrams but inspite of this good result I am catching this "bursts" anyway.

I have tried to do something with Tx and Rx Pluto parameters but it did not help at all.

So now it's all about this bursts I am catching and I have no idea of thier nature...

Maybe it has something to do with me using only one Pluto? Maybe I should use two of them - one for Rx and one for Tx?

Regards,

Alex

Kevin 2022-1-13

Hi Alex,

Yes, this can be something related to using just one pluto, especially (I think) changing rx center frequency will also change tx center frequency.

So let try the experiment with two Plutos.

There is also one thing we should be careful. When using one Pluto to transmit, I am assuming that you are using transmitRepeat, then you ideally the signal should not have glitch between the end of the signal and the beginning of the signal. If there is glitch, it may (or may not) cause problem to the receiver recovering the bits. This is something you need to keep in mind. But for now, assuming that this is not a problem.

Kevin

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Answer 3

Kevin 2021-12-28

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I see. OK. Let's go back to your QPSK example.

Do you use a cable to connect transmitter and receiver? If not, can you try this first?

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Alexey Petrenko 2021-12-29

Still I do not get all the messages clearly. I am trying do something with damping factor of symbol and carrier sync and with the normalised loop bandwidth but still the effect I get is questionable... I mean that I am not sure that tuning of those parameters does anything at all.

Alexey Petrenko 2022-2-8

Dear Kevin!

Many thanks for your responses and help.

Seems that I finally got it - I have got 100% results dealing with sample rate. For some reason 400k sampling rate is big in the way that I am having the result I presented to you later. Lowering sample rate to 100k gave me 100% of parcels coming to be correct.

I will look forward to deal with it later anyway.

Thank you very much!

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Answer 4

Alexey Petrenko 2021-12-29

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Greetings Kevin!

I have tried the coax connection earlier but the results on const diagrams were totally the same as it was with antenna connection.

So now I am using antennas.

Also I have tried to manage rx parameters after square root block and was able to get a result :) so now I am getting a part of a parcel or a whole parcel from time to time. I have changed the timing error detection parameter within symbol synchronizer block to zero crossing now I got something like this (see pic).

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Alexey Petrenko 2022-2-22

编辑：Alexey Petrenko 2022-2-22

Greetings Winfried!

I am no guru as you have probably noticed from my other posts, but if I may suggest - try to lower down sample rate as I did and try to manually adjust the carrier within your PlutoRX.

Hope that helps - please feel free to tell of your results.

Regards,

Alex

Winfried Johannes 2022-3-17