[opendtv] Re: DVB-T2 news
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 3 Mar 2011 18:14:53 -0600
Olivier Houot wrote:
> I saw a new issue of DVB scene magazine :
>
http://www.dvb.org/news_events/dvbscene_magazine/DVB-SCENE37.pdf
They did themselves a disservice, it looks like. Something didn't look right,
so I did a quick check:
The Shannon limit at 15 dB C/N over an "8 MHz" (actually 7.6 MHz) channel is
only 38.2 Mb/s. The graph on p. 10 shows the orange line way up at 43 Mb/s or
so. Similarly, for 20 dB of C/N, the Shannon limit is 50.6 Mb/s, and they show
about 57 Mb/s.
And even more, the Shannon limit for an actual 8 MHz channel, which does not
apply here because that's not reality, is still only 40.2 Mb/s at 15 dB C/N, or
53.3 Mb/s at 20 dB of C/N. So in fact, DVB-T and T2 are quite a bit closer to
the limit than they show there, assuming their other curves are correct. It
looks like they used a GI of 1/4 for the DVB-T curves, so they look worse than
they otherwise might.
> They say Finland is deploying the largest SFN. It is not specified if it
> is nation-wide, though.
>
> They are doing it on VHF III (174-230 Mhz) for improved propagation.
> They have tweaked the parameters to reach 37 Mbps in a 7Mhz channel.
> Transmitters use 1.2 to 3.4 KW ERP.
> 80% (population) coverage is tentatively expected in mid-2011.
First of all, let me say how happy I am we have had free HDTV available to us
since 1998, while Finland got their first HD broadcasts one year ago and HDTV
is only available by subscription, since January. That's just a parenthetical
comment. I feel so blessed!
They're using DVB-T2 for the SFNs, two 7 MHz VHF channels, 32K mode, 19/256 GI,
only a 4/5 FEC, and low power transmitters (1.2 to 3.4 KW ERP).
This shows how good DVB-T2's 32K mode is for SFNs. You only waste 7.4 percent
channel capacity, and you get a guard interval that is about equal to a 1/4 GI
in DVB-T 8K mode. So, that makes it about 256 usec in a 7 MHz channel.
"The transmitters have been positioned in existing mobile phone and local radio
towers allowing the network to be flexibly modified as coverage requirements
change in relation to, for instance, the expansion in population of cities," it
says.
In order to make this work reliably, assuming synchronized symbol
transmissions, the towers can't be more than 48 miles apart. But at those super
low power levels, they can't be that far apart anyway. So it should be a good
deal.
Bert
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