[opendtv] Re: COFDM and equalization

  • From: neilp@xxxxxxxxxxxxxx
  • To: opendtv@xxxxxxxxxxxxx, Henry Baker <hbaker1@xxxxxxxxxxxx>
  • Date: Tue, 20 Apr 2004 17:22:32 +1000 (EST)

DVB-T COFDM needs EQ like any other digital system because of the hostile 
nature of the terrestrial transmission path.  For every symbol transmitted 
using DVB-T in excess of 1 in 12 carriers is a bpsk pilot.  The scattered 
pilots shift 3 carrier positions every symbol to give a regular pilot pattern 
where-by over a 4 symbol timeframe there is an equalisation estimate available 
for 1 in 3 carriers.
  
This combined with the long symbol time ~1 ms (8k) gives quite a reasonable 
estimate of the current channel performance (via interpolation), allowing 
equalisation of all the data carriers being demodulated through the IFFT.
Note: there is a complete channel estimate available every 4 data symbols.

This is one of the major differences between 8-VSB and DVB-T

Neil Pickford
Australia

Quoting Henry Baker <hbaker1@xxxxxxxxxxxx>:

> 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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