more on FCC votes to let low-power devices use empty TV channels after digital switchover

[David Farber <dave () farber net>]

Begin forwarded message:

From: Brett Glass <brett () lariat net>
Date: October 13, 2006 5:57:28 PM EDT
To: dave () farber net, ip () v2 listbox com
Subject: Re: [IP] FCC votes to let low-power devices use empty TV  
channels after digital switchover


> fyi re fcc and unlicensed spectrum re digital broadcasting etc- big
> news! comments?
>
>
> <http://www.openspectrum.info/news-oct06-014.html>

Dave:

For IP, if you'd like.

The FCC's press releases and statements on this decision can be found at

http://www.fcc.gov/

Unfortunately, some of the Commissioners are leaning toward a Part 15- 
like regime for these frequencies -- the same one used for Wi-Fi and  
the massively congested 900 MHz band. Alas, such a regime would  
vastly decrease the usefulness of the spectrum.

Here's why. Firstly, Because these lower frequencies propagate  
readily through the external walls of buildings, the signal from  
every bit of consumer gear that used these frequencies -- be it a  
cordless phone, a baby monitor, a remote control, or what have you --  
would propagate far outside the area where it was needed, raising  
noise levels on the band. This would create a cacophony similar to  
what is already being experienced on the 900 MHz band, which is now  
dubbed the "band of last resort" for wireless broadband. (See http:// 
www.brettglass.com/Laramie900.jpg for a spectrum measurement showing  
the noise on the 900 MHz band in our small town of 27,000 souls.)

Secondly, there isn't all that much spectrum there. Even if we  
include TV channels 2 through 20 -- the largest swatch that might be  
allocated in some areas -- we're talking one to two hundred  
megahertz, in several discontiguous chunks. Devices which operated on  
these bands would not be faster than the latest Wi-Fi gear, simply  
because there is less spectrum and more noise than in the existing  
unlicensed bands.

Thirdly, as the frequency of a signal gets lower, it becomes less and  
less practical  to build good directional antennas for it. For  
example, a highly focused 21 dBi panel antenna for 5.8 GHz takes up  
less than a square foot. (Note to non-engineers: A "21 dBi" antenna  
is one which focuses its signal in one direction about 128 times more  
intensely than an "isotropic radiator," which sends it in every  
direction, would.) But an equally focused antenna for the 900 MHz  
band would have to be a dish or panel several feet in diameter; the  
larger wavelength simply precludes it from being smaller. So, on the  
relevant TV frequencies, which are much lower than 900 MHz (most are  
in the 4th row of the chart at http://www.ntia.doc.gov/osmhome/ 
allochrt.pdf), trying to "aim at" a particular transmitter and away  
from others which interfere will be difficult.

Finally, there has been no claim -- before the FCC or anywhere else  
-- that the amount of spectrum currently available for indoor  
consumer devices is inadequate. The big spectrum crunch -- the one  
which is producing the comments to which the Commissioners alluded in  
their comments -- is in spectrum for outdoor wireless broadband  
networks, which need to reach across long distances and penetrate  
thick vegetation to provide service to end users.

For these reasons, the best and most productive use of unused TV  
spectrum would be under a regime similar to the "lightly licensed"  
one proposed for the 3.65 GHz band. Under this regime, the band is  
reserved for point-to-point outdoor wireless broadband applications,  
and users are required to cooperate so as not to interfere with one  
another's signals or waste the spectrum. Licenses should not be  
difficult or expensive to obtain, but should be required to allow  
enforcement of the requirement to cooperate and to allow users who  
violate that requirement to be identified.

If new spectrum is needed for indoor uses (though there is currently  
no indication of such a need; the current 2.4 and 5.8 GHz bands seem  
more than adequate), it would be best if it were at higher  
frequencies, most of which are less likely to penetrate to the  
outside. This would permit spectrum reuse within adjacent buildings.  
For example, reclamation of the 23 GHz LMDS "B" band, which has  
proven impractical for outdoor applications because the signals are  
so badly attenuated by rain and obstacles, might well be better used  
indoors. It could provide very high speed communications within a  
room or building with antennas small enough to fit completely within  
the devices.

In short, as new spectrum becomes available, the FCC policy that  
would best serve the public would be this: Allocate frequencies below  
1 GHz, which are sorely needed to provide a "wireless first mile," to  
point-to-multipoint outdoor uses such as the provision of wireless  
broadband, specifically because they penetrate foliage and can  
penetrate some distance into buildings. Require coordination among  
users of these lower frequencies, so that this valuable spectrum does  
not fall prey to tragedies of the commons and is not wasted or abused.

On the other hand, when new frequencies above 5 GHz become available,  
allocate them to unlicensed indoor uses (which will allow maximum  
reuse because the signals will not be able to escape into the  
outdoors as easily) and to "point-to-point" outdoor uses (where the  
shorter wavelength makes it practical to create very well focused  
antennas that allow outdoor reuse). Require that parties who use  
these frequencies for outdoor point-to-point links use very well  
focused, high gain antennas with a minimum gain of 30 dBi -- 1000  
times more focused than an antenna which spewed signals in every  
direction would be. At these frequencies, such a well focused antenna  
would take the form of a dish less than two feet in diameter or a  
panel less than 18 inches on a side, and so would be easy to deploy.

We have here a situation in which the laws of physics can guide  
public policy so as to make the most productive possible use of  
spectrum. Let's not make the mistake of assuming that what worked on  
one band will necessarily work well on another with very different  
properties. If the unused TV spectrum is allocated and regulated  
properly, we have a real opportunity to maximize its utility to the  
public -- specifically in the deployment of wireless broadband --  
rather than repeating the disaster we've already seen on nearby  
frequencies, where consumer electronics ultimately destroyed the  
utility of the band.

--Brett Glass, LARIAT.NET


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