[opendtv] SFNs
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 20 Aug 2004 18:44:18 -0400
The publication of ATSC Standard A/110 brings up the question
of the SFN vs big stick tradeoff again.
NIST provides a useful empirical model of RF propagation in
urban and suburban environments at this site:
http://w3.antd.nist.gov/wctg/manet/calcmodels_dstlr.pdf
Among these is the so-called CCIR, or now ITU-R, model.
Let's hypothesize a 3-small-stick SFN. Each tower is
100' high, and the three are arranged in an equilateral
triangle, 10 miles apart.
Alternatively, we can hypothesize a single big stick, at
the center of this pattern, 500' high.
In all cases, the transmitter ERP is 200 KW. Even for
the one big stick.
How do these two configurations compare?
There are a zillion ways to look at this. I'll acknowledge
that inside the pattern, receivers will do well with the
SFN. But this is a US market, so let's also look outside.
A receiver located 12 miles from the pattern center, with
unity gain antenna 6' off the ground. This would be a
close-in suburb. The station is transmitting on a 600 MHz
center frequency. To make life easier, assume the receiver
is on an azimuth from pattern center on which one of the
small sticks is located.
So this looks like so:
tx tx
^ (^ is the pattern center)
5.8 mi
tx
6 mi
rx
If the receiver is 12 miles from pattern center, it is
about 6 miles from the closest small stick transmitter.
According to the ITU-R empirical model, propagation
loss at the receiver rx is 155.6 dB from the closest
small stick.
Compare this with a 500' big stick at pattern center.
The receiver would be 12 miles from this big stick, but
propagation loss is 150.6 dB -- less than with the
small sticks. Thanks to the greater antenna height.
Now let's look at power levels to get an idea of what
this all means.
Assume the transmitters are 200 KW in either case.
If the big stick is 200 KW ERP, a receiver with unity
gain indoor antenna located 6' up would have to be
capable of accepting a total of ~ 150.6 + 20 dB path
loss (the propagation loss + 20 dB margin for indoor
reception). So being optimistic, receiver sensitivity
of -87.6 dBm would make this possible.
With the small stick approach, even though the
transmitter is closer, receiver sensitivity would have
to be -92.6 dBm, which is possible but not so easy.
Obviously, a little better receiver antenna gain would
ease matters.
Point being, while inside the SFN you might have an
advantage with multiple transmitters, even folks in
the close-in suburbs would have a more difficult time
of it. And this is assuming the single big stick had
no more power than *one* of the small sticks.
To cover large markets, I just don't see that the
small stick SFN approach works well. It just takes
too many sticks, and too much power overall. Not
to mention the difficulty in locating all these
sticks in the community.
But a big stick, with the addition of small repeaters
as necessary, I think works better.
Bert
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