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

[Ben Reynwar]

Wouldn't one normally use differential encoding to avoid this problem?
Maybe I'm missing something important.

Ben

On Thu, May 19, 2011 at 12:21 PM, Robert McGwier <address@hidden> wrote:
> 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
>> > _______________________________________________
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>>
>> --
>> View this message in context:
>> http://old.nabble.com/QAM-demod-Error-in-GRC-tp23722153p31655276.html
>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>
>>
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>
>
> --
> Bob McGwier
> address@hidden
> Amateur Radio Station: N4HY
> Engineer, Mathematician (Ph.D Brown University)
>
>
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