Re: [Discuss-gnuradio] QAM Modulation-Demodulation in Loopback

[Brian Padalino]

On Tue, Dec 3, 2013 at 1:44 PM, Kartik Seth <address@hidden> wrote:
> 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?

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