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

[Kartik Seth]

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 Rondeau <address@hidden> wrote:

On Tue, Dec 3, 2013 at 6:07 AM, kt7 <address@hidden> 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

--
Regards

Kartik Seth