[opendtv] Re: Let the games begin
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 9 Nov 2006 12:04:09 -0500
Tom Barry wrote:
> That is, at a reasonable, typical, and equivalent (very bursty,
> non-random) error rate, how many bits per second can be delivered
> in how much channel bandwidth by the various methods?
They are comparable, Tom. You can check for yourself, for example by
comparing the A-VSB numbers in
http://www.iptv.org/dtv/2006/media_pres.cfm?ses=5&id=7&Track=T
(the presentation Craig pointed out yesterday) with the COFDM numbers in
http://www.ttv.com.tw/TVaas/file/En300744.V1.4.1.pdf
You should do this, really, otherwise you seem unable to be convinced.
The short version is this:
The most robust for of COFDM is QPSK, non-HM, which provides 3.7 to 4.5
Mb/s in a 6 MHz channel. (The range is caused by varying the GI.) This
is 0.617 b/s/Hz to 0.75 b/s/Hz.
The C/N margin needed for this varies from 3.1 to 5.4 dB (single
antenna).
The most robust form of A-VSB is 1/4 rate, which provides 4.85 Mb/s in a
6 MHz channel (actually 5.38 MHz channel). This is 0.808 b/s/Hz.
The C/N margin needed for this is 4.5 dB C/N (single antenna) or 1.7 dB
C/N (diversity antenna).
Summary: in the most robust mode, COFDM provides slightly less spectral
efficiency at potentially slightly higher robustness.
The Shannon limit for 4.85 Mb/s in a 5.38 MHz channel is -0.615 dB of
SNR. Which means that with diversity antenna, A-VSB is about 2 dB away
from theoretical best. Which means, you won't get a whole lot better. I
have no COFDM numbers to compare, but as I said, you won't get a whole
lot better no matter what.
Furthermore, if A-VSB is used in an HM-sort of situation, i.e. some
A-VSB and some regular 8-VSB, then the A-VSB part of the signal is just
as robust as it was in the non-HM case. This is not quite true in COFDM,
though. The difference can be small in some modes, but in general, the
robust channel becomes somewhat less robust when 16-QAM or 64-QAM is
transmitted along with QPSK.
So, please check the numbers out. They are comparable.
Bert
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