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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