[opendtv] Re: Opinion: Mobile TV's New Free Market Economy
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 22 Mar 2009 19:34:41 -0400
John Shutt wrote:
>> True. Another important factor is min power density, but even
>> there, single carrier schemes do well. Ultimate echo tolerance
>> is certainly still favoring OFDM or CDMA, however, although that
>> comes at a price (spectrum efficiency).
>
> Now you get into the trade-offs between receiving 2.2. bits/Hz
> 50/90 vs. receiving 3.3 bits/Hz 50/50, and that becomes a
> business, not an engineering, decision.
True. Although even there, there are different tradeoffs that different
systems can use, to make their approach work better. It's not all about
modulation. It's about using the right design approach.
So let's take a real-world decision. The French TNT goes for good
spectral efficiency, so they choose the same 2/3 convolutional FEC we
have, they choose 64-QAM which is equivalent to 8-VSB, they use the
minimum GI (1/32), and they design for a healthy power density in areas
where they want coverage. Their criteria for coverage vary from 3 dB
stronger power density than the FCC requires, in the VHF band, to as
much as 5 dB better than FCC in the UHF band.
The 1/32 GI in 8K mode translates to good echo tolerance of +/- 28.8 uS,
so that's certainly within the capabilities of newer 8-VSB receivers.
And the French usually go for very low-powered tall sticks, and closely
spaced translator towers. That design approach is perfectly compatible
with ATSC as well, although of course the infrastructure costs more than
our broadcasters can probably afford.
So I agree with your point that COFDM gives more flexibility. But once
they opt for high spectral efficiency, their techniques to make the
system work well start looking an awful lot like ours would. Less
compatible with SFNs, more dependent on tall sticks for coverage. Of
which they have some 3000 to cover France, btw, so compare that number
with our total of transmitters and translators.
I'm saying that our OTA ease of reception could certainly be improved,
even without changing modulation scheme, if we adopted some of the same
techniques Euro OTA infrastructures use.
Bert
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