[opendtv] COFDM and equalization

I have a very dim understanding of COFDM.

I understand that it involves splitting up the
bandwidth into thousands of independent narrow
channels, and that FFT's can be used to efficiently
do a lot of the processing.

The question is the following: if the channel is dispersive,
meaning that the delays are frequency-dependent, then a
COFDM whose channels are narrow enough shouldn't care at all,
since it is unlikely that a single narrow channel would be
wide enough to disperse the frequencies within the channel.

Many equalizers are trying to recover the proper shape of
a signal, but if the signal is very narrow-band, the signal
is always a good approximation to a sine wave, no matter
what happens to it in the channel.

So why would a COFDM ever need equalization?

(I realize that multipath could involve polarization and
cancellation, but this merely kills the capacity of one
or more of the individual narrow-band channels, which should
be taken care of by FEC, not equalization.)

 
 
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