QAM Modulation-Demodulation in Loopback

Hello All,

I am using bladerf in loopback mode with a 20 dB attenuator.
I am trying to implement QAM modulation-demodulation and I have attached
the flowgraph I am using.
The problem is I am not getting the demodulated data.
I have used the same flowgraph for DBPSK,DQPSK, etc.

Any idea why it is not working for QAM ?

Also the QAM demod block has a parameter phase bw. In PSK demod, Costas
Loop is used, but costas loop cannot be used for QAM so how is phase
synchronization done for QAM ?

Thanks for any advice.

Kartik

Just a follow up.
I have noticed that data demodulation is intermittent, like if I execute
the
same flowgraph a number of times sometimes I get a waveform after
demodulation but most of the time received scope is blank.

I really have no explanation for this.
Any information regarding this would be really helpful

Thanks in advance


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On Tue, Dec 3, 2013 at 6:07 AM, kt7 [email protected] wrote:

Just a follow up.
I have noticed that data demodulation is intermittent, like if I execute the
same flowgraph a number of times sometimes I get a waveform after
demodulation but most of the time received scope is blank.

I really have no explanation for this.
Any information regarding this would be really helpful

Thanks in advance

Are you transmitting and receiving from the same BladeRF board? That
could be problematic. Are you connecting them with a cable or using
antennas? It’s quite likely that the amplitude is way off. It could
either be too much or too little for the demodulator. First, make sure
you aren’t sending in data that is too large, which could cause
clipping and other non-linearities in the receiver. Second, make sure
the signal is scaled to about +/-1 before going into the QAM demod
block (by using an AGC block, for instance, or manually scaling the
gain of your low pass filter).

Tom

I am transmitting and receiving from the same bladeRF board using a
cable
and a 20 dB attenuator.

If you see in the flowgraph I multiply by 0.3, that is specifically to
prevent clipping and non-linearity.
I have tried using AGC or gain of low pass filter but no success.

The problem is that same flowgraph is working perfectly for BPSK and
QPSK(most of the times), so I think issues with
amplitude/clipping/non-linearity are ruled out.

However, when I move to 8PSK and QAM, I don’t get demodulated output.

Any suggestions whether there are issues with flowgraph or is it some
issue
with hardware?

On Tue, Dec 3, 2013 at 8:24 PM, Tom R. [email protected] wrote:

Thanks in advance

Tom


Regards

Kartik S.

I had already done simulation for PSK as well as QAM in GNURadio and
everything worked fine, even after using Channel Model block in between.

When using with bladeRF in loopback the issues with 8PSK and QAM are
coming. I can see the constellation in scope sink(if that gives any
insight) but nothing is demodulated correctly. Moreover, once in a while
data is demodulated(after many executions of same flowgraph).

So either there are some issues with Demod blocks or with the hardware
(assuming flowgraph is correct).

Has anyone tried this before and can confirm that this works?
It would be helpful if someone can try this on their end and provide
feedback.

Please correct me if I am wrong or anybody has some better explanation.

Kartik

On Tue, Dec 3, 2013 at 1:44 PM, Kartik S. [email protected]
wrote:

However, when I move to 8PSK and QAM, I don’t get demodulated output.

Any suggestions whether there are issues with flowgraph or is it some issue
with hardware?

It very well could be an issue with hardware. Also I wouldn’t be so
quick to rule out amplitude or clipping if BPSK or QPSK works. They
are pretty resilient modulations.

Try removing hardware from the equation and just using GNURadio as
simulation? You may want a throttle block in there.

Take a look at the time domain and see if you are seeing things nicely
between (-1,1) or if you see a big square and are pegged most of the
time?

Brian

On Wed, Dec 4, 2013 at 5:46 AM, Kartik S. [email protected]
wrote:

Has anyone tried this before and can confirm that this works?
It would be helpful if someone can try this on their end and provide
feedback.

If you put your flowgraph online somewhere that I can grab, I’d give it
a shot.

Maybe push it up to github and send a link to that?

Given that you’ve tried it out with even the channel model between the
blocks, it’s probably a configuration item or hardware misbehaving.

Brian

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