[opendtv] Re: Ofcom paves the way for UK HD on DTT
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 4 Apr 2008 16:43:42 -0400
> DVB-T2 depends on keeping an H and a V path separated, to
> attempt to provide twice the bandwidth in the same spectrum.
> Their tests have shown that this is not always possible, in
> different clusters of the coverage area, and that to
> compensate for the problem areas, they have to increase the
> C/N margin in their planning factors.
>
> MPH goes exactly opposite of this. It looks for maximum
> robustness, i.e. lowest possible C/N. Instead of ~24 db C/N
> required for trying to achieve reliable DVB-T2,
To be more exact, instead of ~24 dB required to achieve *44 Mb/s in the
8 MHz channel*, ...
If you need less than twice the bit rate, then you don't need 24 dB of
C/N or more.
Basically, twice the bit rate is the very best DVB-T2 can hope for. Any
time you get significant cross components in the channel matrix H, i.e.
overly strong paths from the H transmitter to the V receiver and vv,
that double bit rate will not be possible.
http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP144.pdf
And you can also change the constellation in DVB-T2, but of course then
you have to compare the C/N margin required with DVB-T2 vs what is
required in DVB-T for that same bit rate.
For example, in an 8 MHz channel, you can achieve 22.12 Mb/s with DVB-T
at 64-QAM, 2/3 FEC and 1/8 GI. And the C/N margin, in a gaussian
channel, is 16.5 dB of C/N.
You can also get that same bit rate with DVB-T2 at 16-QAM, 1/2 FEC, and
1/8 GI. But to achieve that reliably, what C/N margin is required?
That's the tradeoff that has to be made. If the C/N margin required for
DVB-T2 is realistically greater than 16.5 dB of C/N, you're better off
sticking with DVB-T. (By realistically, I mean everywhere in the
coverage area that can be achieved with plain DVB-T.)
Bert
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