[opendtv] Re: SFN response to Craig's post

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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