[opendtv] Re: Louisiana governor blasts faulty wireless networks
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 14 Sep 2005 15:19:15 -0400
Dan Grimes wrote:
> I would find it hard to believe that a series
> of small transmissions would be less robust than
> one large transmission both physically and
> electronically.
All of this can be calculated, Dan. In practice,
to cover a market the size of most US TV markets,
with small 100 W sticks as Bob Miller mentioned,
would take a huge number of sticks. Assuming you
want continuous coverage throughout the area.
Mount them up high and the number is reduced.
Increase the power and the number is reduced.
In the Berlin example, they use either two or
three (depending on channel) sticks, in a SFN,
with power ranging from 10 KW to 120 KW, and
these are tall. (This may be changing. Their
site explaining this has gone dark.)
The 120 KW example, Ch 44, has a second tall
stick of 50 KW in a SFN. Their coverage goes out
to about 57 miles, with rooftop receiver antennas.
At less than 2 b/s/Hz, which they dialed in to
make the SFN work well. To me, the power and
reach is about the same ballpark as what we need
here. We would want over 60 miles of range often,
and, with 8-VSB, we have to use 3.3 b/s/Hz.
Paris has the big stick on the Eiffel Tower,
which is on the west side of town, and then
three much smaller transmitters to the N, E, and
S, in a SFN.
However, a mere 30 miles away, there's another
large stick, on a different set of frequencies,
because that's too far to be included in the
Paris SFN, and the Paris signal is getting too
weak by then. So unless you go to the trouble of
synchronizing transmitters, you need translators.
> But when looking at spectral efficiency, aren't
> the problems the same, just on a smaller scale?
Exactly. See above the Paris example. One could
in principle design very large SFNs, but that
introduces complexities that no TV system has so
far wanted to tackle. Besides, in the US anyway,
markets are kept separate. Broadcasters want
their individual stations to be unique, unlike
most European networks, where transmitters are
mere translators in a network.
In terms of spectral efficiency, SFNs help because
all your repeaters can be set to the same
frequency. The reality is, no one wants to create
huge SFNs, for technical reasons. So the real
value of the SFN becomes ease of reception within
a confined area. And then often more difficult
reception far away.
And you're still using translators to extend
coverage, which means you have NOT gained any
spectral efficiency.
Bert
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