[opendtv] Re: From Broadcast Engineering - WRAL tests mobile DTV
- From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 9 Aug 2008 18:15:09 -0400
Tom Barry wrote:
> But I'm still not sure at all of the other side. That is, how much
> FEC overhead is needed to get similar M/H reception quality on
> DVB-T. Is it truly 3 or more times better at mobile or does
> DVB-T also have to use massive extra error correction to get
> quality M/H reception. If so, how much?
>
> That is, how many bits / second can reliable emerge from a
> moving handheld receiver using DVB-T in a 6 mhz channel,
> after subtracting FEC overhead?
The results from tests are a good thing, of course, and Mark will have some
available soon. But this doesn't preclude considering what the underlying
principles are.
And among these is that DVB-T does HM by addressing two components: the FEC
(and GI) of the robust channel AND the constellation. HM uses QPSK, which is
analogous to 2-VSB. However, in 8-VSB, if M/H were to use a different
constellation, the entire multiplex would have to use that same constellation.
Since only the 8-VSB one is used, that kind of limits what the M/H channel has
to be built on.
So you can't *directly* compare the FEC mode used in the robust channel,
between the two systems. But you CAN compare the bit rates with the C/N margin
requirements, which ultimately is all that matters.
For DVB-T, the document that shows what one can expect is ETSI 300 EN 744. It
doesn't give all results for 6 MHz channels, but you can draw the same
comparisons using the 8 MHz results.
What that document shows, for 8 MHz channels, is that in the mode that favors
the HM channel most, you can reduce the wide channel's capacity from 24.13 to
12.06 Mb/s, to retain the same robustness you had in non-HM mode. Doing so, you
will get an HM channel that gives you as much as 6.03 Mb/s of capacity, with a
C/N margin of 6.5 dB of in a gaussian channel (this is with the same 1/32 GI as
the wide channel).
So, I suppose you could conclude that this would be the same overall effect as
8-VSB M/H with a 1/2 rate FEC, right? The 12 Mb/s taken away from the wide
channel becomes 6 Mb/s in the HM channel.
So the next question is, what C/N margin is required in the MPH or A-VSB, when
the 1/2 rate option is used? IIRC, A-VSB gives you 4 dB of C/N for a 1/4 rate
channel and a single receive antenna, and something like 1.9 dB of C/N if a
diversity receive antenna is used with 1/4 rate. But I do not remember, or
maybe the numbers were not given, for the 1/2 rate option.
The other consideration is that when putting COFDM into HM, you are going to
get a fairly big robust channel. Whereas with MPH or A-VSB, it's more like
DVB-H. The robust channel can take up less of the wide channel (and of course
provide less M/H capacity as a consequence). This may be desirable, though, for
stations that want to retain the ability to transmit an HD and SD multiplex
along with M/H.
In short, using a simple 6/8 ratio:
COFDM HM in a 6 MHz channel could provide you with 9 Mb/s of wide channel (16.5
dB of C/N margin required) and 4.5 Mb/s in the H/M channel (6.5 dB of C/N).
Echo tolerance would not be impressive, though, because we are assuming 1/32 GI.
The way MPH was being used, the 8T-VSB wide channel was 14.9 Mb/s (15.2 dB of
C/N) and the narrow channel should be something less than 2.2 Mb/s if 1/2 rate
were used, at an undisclosed C/N margin. Or something less than 1.1 Mb/s (~4 dB
C/N) if 1/4 rate is used. The "something less" is to account for the extra
overhead needed for the training and sync sequences, but that shouldn't be a
big number.
Bert
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