Re: [Discuss-gnuradio] OFDM / DAB demodulation

[Jens Elsner]

Hi Clark,

thanks for trying out my code. The problem is that what you are seeing
is *already* DQPSK. 

The pilot is accounted for, it is the first symbol in every frame.

Demodulating might make sense, but I guess the BER is still way too high
to compensate it with channel coding. There has to be some kind of
reasonable explanation for the phenomenon we are seeing...

For those of you working with real OFDM data: What could be the cause of a 
"phase jitter", even after differential demodulation?

Jens

On Sun, Apr 02, 2006 at 10:01:29AM -0400, Clark Pope wrote:
> Tried your matlab code. Looks like fine QPSK symbols to me. The absolute 
> phase reference is not zero but that should not affect DQPSK demodulation 
> since the last symbol is the phase reference for the current symbol.
> 
> Maybe you need to adjust phase based on pilot bins?
> 
> I would proceed to demodulating the bins to see if the data you recover 
> makes sense. There must be some regular structure to the data stream that 
> you can identify.
> 
> -Clark
> 
> >From: Jens Elsner <address@hidden>
> >To: address@hidden
> >Subject: Re: [Discuss-gnuradio] OFDM / DAB demodulation
> >Date: Sat, 1 Apr 2006 18:30:33 +0200
> >
> >You are absolutely right - the matlab code is attached.
> >You can get my sample data (local radio station, 3 MBytes) from
> >http://www.1c3.de/dab_ml.dat.bz2.
> >
> >Jens
> >
> >On Sat, Apr 01, 2006 at 10:27:57AM -0500, Robert McGwier wrote:
> >> Why don't you allow us to provide you with help based on the reality as
> >> opposed to conjecture.  Post your matlab scripts.  This "remote viewing"
> >> exercise is inefficient.
> >>
> >>
> >> Bob
> >>
> >>
> >>
> >> Jens Elsner wrote:
> >> >I resampled all the data to do a proof-of-concept. The frames are only 
> >a
> >> >small part
> >> >of the "chunk" variable. What I am getting is a fairly nice pi/4 DPSK.
> >> >Only that
> >> >in between OFDM Symbols (there are 76 in a frame) there is some sort of
> >> >random
> >> >phase jitter. And I am clueless what it could be. Frequency offset is
> >> >compensated,
> >> >the channel doesnt vary that fast.
> >> >
> >> >Jens
> >> >
> >> >
> >>
> >>
> >> --
> >> AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
> >> NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
> >> Laziness is the number one inspiration for ingenuity.  Guilty as 
> >charged!
> >>
> 
> 
> ><< dabtest_ml.m >>
> 
> 
> ><< prs.m >>
> 
> 
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> 
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