[opendtv] Re: China comes up with own DTV standard
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 28 Jul 2006 18:16:17 -0400
Steve Wilson wrote:
> FYI...
> http://www.eetasia.com/ART_8800423182_590626_82328af9200606.HTM[1]
> Posted : 28 Jun 2006
I think we've seen this before, though not with the level of description
provided in this article.
The idea of filling up the COFDM GI with the same training sequence as
the one used in the VSB single-carrier scheme is interesting, but makes
me wonder why do this. If the GI is taken up with a training sequence,
and the training sequence is required to implement equalizers to
eliminate intersymbol interference, then it seems to me some of the
advantages of COFDM have been eliminated. Possibly, the long symbol
duration and the inherent insensitivity to narrow and very deep nulls
are still advantages of a multicarrier scheme. The C/N margin advantage
claimed, about 3 dB, is in fact what you'd expect to see. The two
schemes have just about merged.
From the 30,000 ft level, you can either take up time to send an
adequate number of training sequences in a single-carrier scheme or in a
multicarrier scheme. Saying you're sending these training sequences in
the GI of the multicarrier scheme becomes mostly a matter of symantics.
It's just a time slot dedicated to training.
As to whether it's better to create a standard for low-power devices
that uses narrow bandwidth (e.g. DMB-T) vs time division, I'd say that's
also a toss-up. DVB-H goes for the latter. It seems to me that's
primarily a way to make a transmitter that operates in the TV spectrum
also useful for transmitting signals to hand-helds. Other than that, I
don't see any advantages to the technique over COFDM (or over VSB, for
that matter).
(Time division as used in wide CDMA channels is another matter, I think.
In that case, the advantage is a soft ceiling on max number of
simultaneous channels offered, as opposed to the hard limit created by
these other techniques.)
In short, these look like different ways of doing things. I'm not sure
they are any better.
Bert
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