Generated noise from BladeRF

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samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Generated noise from BladeRF

Post by samv » Fri Nov 10, 2017 6:49 am

Hello everybody,
I have built a QPSK transmitter which send random data. this modulated signal is sent to the osmocom block which interfaces with the bladeRF. When I read the signal on a spectrum analyzer, i get noise around my signal that I don't want. What is noise come from? And how do I remove it?
Here is my osmocom setup

Tx freqency: 1.6GHz
sample rate : 4MHz
Bandwidth filter: 1.5MHz

Links below are screenshots from the spectrum analyzer and the GRC flowgraph
https://ibb.co/nCYOqw
https://ibb.co/dAmWSb
https://ibb.co/dPR9DG
https://ibb.co/j5U70w

Thank you in advance for your help :)

ykhaled
Posts: 7
Joined: Sat Jun 11, 2016 8:38 pm

Re: Generated noise from BladeRF

Post by ykhaled » Fri Nov 10, 2017 5:20 pm

Why don you post the spectrum analyzer plot?
did you calibrate your I/Q?

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Mon Nov 13, 2017 12:53 am

I have posted my spectrum analyser result ( last three link in my fist post). I repost here:
free mode: https://ibb.co/dAmWSb
average: https://ibb.co/dPR9DG
Max holde: https://ibb.co/j5U70w

I made the DC offset correction as Bladerf recommended. I haven't done the I/Q balance calibration. I will try to do it.
Thank you.

[UPDATE] I made the I/Q calibration and DC offset calibration following the guide on bladerf wiki. I have still the same probleme after sending my data. Here is the result:
https://ibb.co/kRi0oG

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rtucker
Posts: 16
Joined: Sun Jan 25, 2015 10:38 am

Re: Generated noise from BladeRF

Post by rtucker » Mon Nov 13, 2017 11:23 am

Greetings!

So I was indeed able to reproduce this issue...

https://photos.app.goo.gl/Y9R5JZVZOjsimQUJ3

Looking at the generated signal, something that caught my eye was the constellation:

https://photos.app.goo.gl/hf3HOSLEgiu0vGsz1

That didn't look quite right, so I looked at the time domain:

https://photos.app.goo.gl/OBMdpXZi3ynnphPB3

There's a lot of really fast switching going on there, like more than I'd expect for 8 samples per symbol at a 4 MHz sample rate. Each symbol should last for about 2 us, but some of these transitions are closer to 1 us.

Long story short, I think the constellation object expects the random source to be in the range [0,3], and it's overflowing and behaving poorly. I set the random source range to [0,4) and life is good...

Flowgraph: https://photos.app.goo.gl/90SRelf6X902fbvK2
Constellation: https://photos.app.goo.gl/kHbMfZJ9xCw6D7zr2
Time domain display: https://photos.app.goo.gl/ADwhBVUIJshc2xS12
Spectrum: https://photos.app.goo.gl/Ng1lyuUqlx2dxVmC2

Let me know if this cleans things up for you. Thanks! -rt
Rey Tucker (she/her)
Systems Engineer, Nuand LLC
Rochester, NY, USA

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Tue Nov 14, 2017 3:08 am

Thank you very much for your answer! It works now!
There is still some point that I don't understand:

1: the spectrum image shows a "house" with a peak in the middle of the signal.
Here is a capture of what I expected:
Image ->https://ibb.co/djEEHb

Here is a capture of what I received:
Image ->https://ibb.co/hF4EHb

How can I avoid this "house effect"? Or is it a normal form?
I did the DC offset calibration by the way.

2. If I want to send data, should I use another block to generate the QPSK or should I unpack all my bit before going the QPSK constellation?

3. My reception does not synchronize every time when I start my flowgraph. I have the stop it and start it again and sometimes it works, sometimes not. this picture shows an unsynchronized reception:
Image ->https://ibb.co/cCoPH

I post the result of my flowgraph , mainly based on the gnuradio qpsk tutorial:
Image ->https://ibb.co/mFKWxb

I have lots of questions sorry, but I would like to understand what my qpsk mod/demod does not work correctly

Thank you in advance ;)

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rtucker
Posts: 16
Joined: Sun Jan 25, 2015 10:38 am

Re: Generated noise from BladeRF

Post by rtucker » Wed Nov 15, 2017 2:22 pm

OK, so I looked through the source code ( :shock: ) and re-implemented the Constellation Modulator using individual blocks and... it worked a lot better.

First: the range on Random Source should indeed be [0,256), as it was in your original version. Each 8-bit byte gets chunked into four 2-bit symbols, so the weird "house" look was because 3 out of every 4 symbols was 00. This makes repeating patterns, which are Bad.

Secondly, the actual problem with the excessively wide signal is because the RRC filter in the Constellation Modulator is producing I/Q values outside of the range (-1,1). When the samples are being converted from complex to int16_t for transmission, an overflow occurs and everything goes bad.

Here it is without scaling the output from the modulator:

https://photos.app.goo.gl/I0MBeFRkVep4IkD02
https://photos.app.goo.gl/sLaQU3dNoVfEUmYt1

Note the numerous points falling outside of (-1,1)

And here it is, multiplying by 0.707 (arbitrary choice of 1/sqrt(2)):

https://photos.app.goo.gl/mx3Z218dkDuBbTS83
https://photos.app.goo.gl/os8aacLSsjvhE74I2

Everything's within (-1,1) and life is good!

Here's my flowgraph for the above (only thing changed was the multiplication constant):

https://photos.app.goo.gl/rIP14a49HatKzOD53

I have reason to believe your receiver will have a better time with this!

Thank you for following up! I need more practice with signal processing, for sure :)
Rey Tucker (she/her)
Systems Engineer, Nuand LLC
Rochester, NY, USA

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Tue Nov 21, 2017 3:25 am

Thank you for your income, after doing this correction, my emission looks great :) thanks again.

Now I am dealing with the reception part. I noticed some phenomenon that I do not understand:
- I Have some lost connection sometimes. My emission fall down for a mili sec and start again. Having this problem, I cannot execute a correct BER test. I run the bladeRF on USB 3.0 but I don't where this problem is coming from.

- The synchronisation needs a lot a of gain even though I use a cable instead of the antennas.

If someone has an idea, it would be amazinh :)

thank you

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Fri Nov 24, 2017 12:48 am

Ok I find the problem with the sync, it was the calibration. So my reception looks good when no cut out appears.

[RESOLVED -> check update]I still have this problem with the cut out or interruption of the signal. It is like the USB cannot support my data rate... Is it even possible?
I am working on a virtual machine which support USB 3.0 as well as my PC has USB 3.0 port onboard.
my sample rate is 4M on osmocom block and the freq is 1.6GHz.

Has anyone noticed a similar trouble?

Thank you
--------------------------------------------------
[UPDATE]
Ok, I've noticed that when I mouve my mouse, I generate the cut off. Is it that normal?
I will try not on a virtual machine if anything change.
TBC


[UPDATE 2]
lost of connection is probably generated by the virtual machine. On a pure ubuntu pc, I have not cut off at all.

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Fri Nov 24, 2017 6:15 am

I have a strange reaction I do not understand:
My qpsk sync works fine when I send random data
https://ibb.co/kJeCYR[img
Image

But when I send PRBS data, my sync does not work...
https://ibb.co/dSAanm
Image

Am I missing something here?

samv
Posts: 7
Joined: Fri Nov 10, 2017 6:32 am

Re: Generated noise from BladeRF

Post by samv » Tue Dec 05, 2017 6:07 am

Anyone any ideas? I still haven't understood this problem?

Thank you :)

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