Re: [Discuss-gnuradio] OFDM demodulation problem / example video

[Robert McGwier]

Yes. Here is what you are missing:

Let us concentrate (as does your nice animated gif) on one channel in 
the OFDM.

Let us suppose you have a variable delay into the signal after its 
onset.  This will happen with probability one because your clock and the 
transmitter clock will not be the same except in your computer simulation.

If you took the FFT beginning (say) 39 samples into the symbol at  t,  
and then   52 samples after the beginning time for symbol at  t+1,   
this will be an additional rotation due to the frequency offset of this 
channel from zero.  Notice this means that every channel will have a 
different rotation which will be a multiple of the frequency offset from 
correct and the difference in time after symbol onset you go into the 
symbol to take the FFT.

This is definitely a nontrivial exercise to get right.

Bob



Jens Elsner wrote:
> > Thank you very much for the reference.   This was a very nice link to 
> > Mostofi's work.  The following paper is also pertinent to Jen's thinking 
> > on the guard interval offset being irrelevant.  It is not of course:
> >
> > *Y. Mostofi*, D. Cox and A. Bahai, "Effect of Timing Synchronization 
> > Errors on Pilot-aided Channel Estimation in OFDM: Analysis and 
> > Solution," /Proceedings of 5^th IEEE International Symposium on Wireless 
> > Personal Multimedia Communications (WPMC),/ Honolulu, Hawaii, Oct. 2002, 
> > pp. 1309-1313.
> >
> > http://www.cds.caltech.edu/~yasi/papers/WPMC02.pdf
> >     
> Bob,
>
> I am still convinced that using differential demodulation removes the
> phase shift (see reply to Prateek). Or is there something I am not
> seeing so far?
>
> Jens
>
>
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>   


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