[opendtv] Re: Seeing Ghosts on a Single Frequency Network

Craig Birkmaier wrote:

> Assuming that Allen is correct, that multicarrier SFNs are not the best
> solution, one must ask what kind of spectral efficiency (i.e. reuse)
> can be achieved when multiple frequencies (channels) are used to
> deliver the same content?

My summary of Al Limberg's musing might be, "anything an SFN can do, a MFN can 
do better," with the sole exception of spectrum savings. SFNs give me this 
uncomfortable feeling of a seriously compromised and kludgey solution, whole 
ONLY benefit is spectrum savings, which comes at a significant cost. And more, 
to save on spectrum realistically, you need regional broadcasting and you need 
32K mode, in practice.

The much-touted European SFNs do not really save on spectrum. It's not even 
clear whether they help reception. Compared with a MFN to cover those same 
cities, e.g. main tower plus low-powered British-style relays, I think the case 
for SFNs is difficult to make.

> I can see that one could checkerboard the transmission sites to deal
> with on-channel interference from adjacent markets, but it seems that
> to be really useful the power levels and antenna heights for each
> "cell" would need to be reduced, or we are in the same boat as today
> with large dead zones (white spaces) between used channels.

The Brits have done this already, in practice. If they are careful in 
synchronizing their many low-powered relays, and if receivers are designed to 
know which frequencies are associated with any given multiplex in that area, 
then the receivers can behave similar to cell phones, when they do their 
"handover" from one tower to the next. Or even better, if two frequencies are 
available in a location, use both simultaneously to improve the SNR. I think 
all of this is plenty doable, and it would actually work a lot better than any 
SFN. You wouldn't get the destructive interference problems, that you cannot 
avoid in SFNs, even with COFDM. (See that article on the Ibiquity trials in 
Baltimore, for example. They even show the COFDM spectrum being distorted by 
interference.)

White spaces continue to exist. The only difference being, if you scale way 
down the checkerboard, you also scale way down the area covered by each white 
space. But the fraction of white space area won't change. As long as you need 
market-sized coverage, I don't see any spectrum savings achieved by reducing 
the scale at lower than market area. You gain on ease of reception, not on 
spectrum use. If you want that smaller than market area coverage, like for 
hyper-local, then sure, you would be able to do that economically.

> I'm not seeing the benefits of the approach Allen is suggesting...

The main benefit is, IMO, the MFN works without the big kludgey compromises, 
i.e. without creating unavoidable interference zones. It works with single or 
multiple carrier schemes. And it should be doable with fewer towers, since you 
don't need those extra towers to overpower the interference zones.

Qualcomm needed to provide coverage over a very large area with just Ch 55. 
They had no choice but to use an SFN. But look at the complexity of their 
network, and look at the lack of coverage area, compared to what they could 
have done with a checkerboard of frequencies.

Bert
 
 
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