[opendtv] Re: COFDM and equalization
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 20 Apr 2004 18:29:17 -0400
Henry Baker wrote:
> 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?
The experts have already covered your questions, but I'd like
to offer a couple of other thoughts.
1. The convolutional FEC causes each byte of data to be
transmitted with some redundancy over multiple COFDM
subcarriers. Which implies to me that in the presence of deep
and narrow nulls, convolutional FEC alone, over COFDM, should
be much more effective at compensating for such nulls than
FEC used in single carrier schemes like 8-VSB.
2. But deep and narrow nulls are often very localized.
Which implies that diversity antennas ought to work well
with 8-VSB, to correct for deep and narrow nulls. You
only need to move enough from the really deep null to
give the fancy equalizer half a chance.
3. The individual thousands of carriers in COFDM provide
an excellent view of the channel response at any given
time, allowing for quick and dirty equalization. But the
absence of these thousands of carriers has its own appeal,
which would be less power required in transmission and a
nice, flat, even spectrum (i.e. low peak to average
ratio).
4. Availability of the guard interval (GI) in COFDM is a
great way of avoiding the problem of inter-symbol
interference (ISI) without resorting to fancy equalizers.
But the price is reduction in spectral efficiency. The
GI is basically dead air, after all.
5. So single carrier schemes demand more of the receiver
but can also extract more from the RF channel. My
thinking is that as time goes by, with Moore's Law, the
receiver cost will become less of a liability for 8-VSB
(or any other single-carrier option in these modulation
wars). Of course, the single carrier receiver will in
principle always cost more, all else being equal, but
the question is "will the delta matter"?
6. Finally, I think the breakthrough that made OFDM
possible was that guard *bands* around each sub-carrier
were not required. To me, that's the really cool idea
in all of this. The ability to transmit orthogonal QAM
symbols.
BTW, I'm very excited to see in Mark's memo about the
huge and symmetric echo tolerance of "5th gen" Zenith
8-VSB receivers. That's what you need for practical
implementation of gap fillers.
Bert
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