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

[Jens Elsner]

(comments inlined)

On Wed, Apr 05, 2006 at 02:02:23PM -0400, Robert McGwier wrote:
> Yes. Here is what you are missing:
> 
> Let us concentrate (as does your nice animated gif) on one channel in 
> the OFDM.
Yes, what's depicted are all decoded symbols in one OFDM frame.

> 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.
Agreed. But this delay will only be fractional, as I can determine the
exact start by correlation. The oscillators are pretty stable, so I can
exactly time one symbol. The offset will thus be fractional and constant.

I verified this by correlating every OFDM symbol with itself. This 
shows that the cyclic prefix does not move. 

> 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.
I agree. But, as stated above, my time offset is fractional and
constant.

Jens