Re: [Discuss-gnuradio] QAM demod Error in GRC

[Robert McGwier]

Vlad

It is apparent to me that you did not understand Josh's explanation.  Possibly he was not forceful enough. ;-).

This is not an error.  It is the mathematical consequence of doing QAM with rotational symmetries.  YOU MUST provide your OWN synchronization to remove the phase ambiguity.  It is not a bug or an error. It is a feature.  It is the mathematical nature of the beast.

Bob

On Thu, May 19, 2011 at 7:55 AM, Vlad Stoianovici <address@hidden> wrote:

Dear list and Josh,
I'm still getting this error when using the QAM Demod block (qamX_demod is
not connected internally).
Are there any new developments concerning these blocks?

Vlad.


Josh Blum-2 wrote:
>
> Well, when you receive a qam signal, and lock to it, you still have some
> unknown phase offset. Lets suppose qam8 has two possible phases: 0 and
> 180 degrees. Since you don't know which phase is correct, you can look
> for the packet header in the received signal at 0 degrees and
> simultaneously at 180 degrees. Whichever phase offset yields a valid
> packet is the winner. Of course, this all assumes that you are framing
> your data with some kind of packet headers.
>
> Anyone else have some qam demodulation ideas to share?
>
> -josh
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>

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Bob McGwier

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Engineer, Mathematician (Ph.D Brown University)