[opendtv] Re: C/N comparison analysis
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 14 Jul 2007 23:11:01 -0400
Albert Manfredi wrote:
> Anyway, this is a good read, and what it explains is independent of
> real-world implementations. Meaning, what he explained in 1999 is still
> true today.
Agreed. The explanations of 8vsb today appear equally independent of
real world implementations. ;-)
- Tom
If anyone is interested in the whys and wherefores for the C/N margin
differences between 8T-VSB and COFDM, this 1999 article by Dr. Yiyan Wu,
of the CRC, is an excellent explanation.
http://www.atsc.org/news_information/papers/1999_vsb_cofdm_comparison/ICCE99paper.PDF
In short, the difference is caused by (1) forward error correction
implementation differences and (2) use of a decision feedback equalizer
in 8-VSB, rather than using active pilots as DVB-T does. The article
gives 1.5 dB advantage caused by the better trellis code used in 8-VSB,
slightly better R-S code, and the inner block interleaver, and a 2 dB
advantage given by the DFE vs use of active pilots. That's where the 3.5
dB figure came from.
The article also measured the differences, even as Eb/No.
The article discusses multipath immunity and the faster rolloff of
COFDM, which allows it to take up about 5.7 MHz in the 6 MHz channel,
vs. 5.38 MHz. (The spectra I've seen of 2K COFDM suggest that it is
about the same as 8-VSB in this regard.) So it explains the tradeoffs in
spectral efficiency as that steeper rolloff is countered by the need to
introduce a GI.
Anyway, this is a good read, and what it explains is independent of
real-world implementations. Meaning, what he explained in 1999 is still
true today. It also explains why the Chinese did what they did when they
recently created their standard.
Both systems would definitely benefit from a more clever FEC
implementation, prefereably using the existing transmission standards,
obviously, to make the introduction simple.
Bert
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Tom Barry trbarry@xxxxxxxxxxx
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