[opendtv] Re: SFN response to Craig's post
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 25 Apr 2007 23:44:01 -0400
At 8:59 PM -0400 4/25/07, Tom Barry wrote:
Manfredi, Albert E wrote:
Now, I will grant you that ease of reception should be a factor in
making that comparison. However, you won't find the dense and wide area
SFN any easier to receive a long distance away than the big sticks. More
the opposite, thanks to the mutual interference.
I know little about SFN's. Are they supposed to also work from a
long distance away?
Tom
It depends on the architecture of the SFN.
One approach is to use one high powered stick and gap fillers to
serve areas where it is difficult to receive the big stick. In this
case you "may" be able to receive the signal from the big stick from
distances comparable to NTSC/ATSC big sticks today.
A second approach is to place 3-5 medium powered transmitters around
the market being served. Typically a receiver will "see" more than
one of these signals, and in the case of DVB-T the power from these
signals may add together to improve reception. I do not know if the
AVSB signals can add together to improve reception. The ability to
receive at a distance in this configuration is likely to be
comparable to one big stick thanks to the geographic disparity of the
SFN sites. But this will depend in large measure on the topography og
the market and the location of the main transmitters. With this
configuration there may also be gap fillers around the edges of the
market with controlled contours, typically radiating signals back
into the market being served. Thus at greater distances from the
mains you may get a good signal from a lower powered stick at the
edge of the coverage area. The ability to precisely control coverage
using these techniques reduces the market-into-market emissions that
limit the ability to re-use frequencies in adjacent markets. Thus it
may be possible to utilize many more channels in each market greatly
increasing spectral efficiency.
The third approach is to use a forest of trees as Bert calls it. In
essence this is a dense network of low powered stick with many
similarities to cellular telephone networks. The major difference is
that there is no need to increase capacity (the number of towers and
frequencies) in densely populated areas, as the desired result for a
DTV network is a uniform signal level across the market, while
cellular networks must increase density to match the usage demand.
The second approach is the most efficient solution in areas where
markets are in close proximity and you want to maximize channel
reuse. In rural areas where market-into-market interference is not a
major issue, the first approach is the still the most efficient. The
third approach is primarily relevant to bi-directional networks such
as cellular telephony.
MediaFlo is building out a low density mesh of medium powered
transmitters to launch their service, then will fill in the gaps with
low powered sticks as the service grows.
Regards
Craig
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