[opendtv] Re: DTT tuner design
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 04 Jul 2007 20:51:45 -0400
Dale Kelly wrote:
I misspoke, either modulation system requires increased C/N
to handle multipath.
I this strictly true? I don't think so. And I believe that's why the Chinese
standard adopted a training sequence in the GI (same one they use in their
single carrier mode), rather than using the end of the previous symbol, as
standard COFDM does. And that's also my bet for DVB-T2. (Just speculation.)
The ideal equalizer should not require a higher C/N to twist multipath
distortion back into shape.
Here is an example. I'm assuming here that the numbers provided are C/(N+I),
and that when this value is less than 15 dB, what you're actually seeing is
a (C+I)/N that is close to 15 dB.
(I also believe that eventually we'll be seeing (C+I)/N better than 15 dB,
when the combined trellis and R-S FEC schemes are more cleverly exploited.)
These are C/N numbers that apply to the Samsung Gemini chip.
No multipath 15.2 dB
Brazil A 15.0
Brazil B 17.0
Brazil C 13.5
Brazil D 13.8
Brazil E 19.9
Compare this to the CSA values for 64-QAM and 3/4 FEC (same spectral
efficiency if GI is 1/16).
No multipath is 18 dB, Ricean is 18.6 dB, and their Rayleigh, which is less
strenuous than Brazil E, is 21.7 dB. (Brazil E has 3 dB echoes, vs two.)
So basically, the planning factors used by the CSA do assume a deficit of ~3
dB or more in C/N margin, compared with a well executed equalizer demod. And
that's reflected in the field strength minima. If good demods are used, the
FCC and CSA planning factors should provide very close to the same coverage,
is my thinking.
Bert
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