WORTH READING ur comments appreciated: Beyond White Spaces

[David Farber <dave () farber net>]

Begin forwarded message:

From: "David P. Reed" <dpreed () reed com>
Date: October 27, 2008 9:57:47 AM EDT
To: dave () farber net
Cc: ip <ip () v2 listbox com>, Brett Glass <nnsquad () brettglass com>
Subject: Re: [IP] Re:   Your comments appreciated: Beyond White Spaces

I don't want to argue with Brett on the broader issues of policy.  (I  
have my own concerns about white spaces approaches being proposed, but  
they relate to the FCC's approach to the whole matter, rather than the  
uncontroversial facts of electromagnetic wave physics).

However, it's worth noting that his comments about "wall penetration"  
are extremely misleading.  Not that the other side conveys the physics  
correctly, either.  Here's the real scoop:

A) Low frequency signals like TV have long wavelengths.  *At the same  
radiated energy*, low frequency signals do not propagate well through  
walls that contain conductors or *close to  the ground*.  The "fresnel  
zone" is a direct linear function of the wavelength, and when a  
conductor is in the fresnel zone, signals are severely attenuated.  A  
smaller fresnel zone is a big win, and you get that by going with  
higher frequencies.

This is confusing our intuition that broadcast TV "goes far" -- TV  
signals propagate into buildings because they are radiated at 10^6  
Watts, but most microwave signals (2.4 and 5 GHz) are radiated at  
10^-2 Watts.   That's a power ratio of 100,000,000 (27 dB)!  It's hard  
to attenuate such a signal - like attenuating the acoustic shock wave  
of a nuclear blast compared to the output of a loud boom box. At the  
same radiated power as WiFi, TV signal propagation would literally  
suck in many cases because of these issues.  The other confusing fact  
is that TV antennae are *super high*, thus dealing with the curve of  
the earth and intervening terrain and trees.   Put appropriate  
antennae that high up and angled down, and 2.4 and 5 GHz signals will  
travel VERY far.

B) In addition, the absorption of building materials (wood, concrete,  
glass) of RF is not particular different between TV signals and 5  
GHz.   This is known physics, and I can provide references.

C) The "noise" present in the environment is dramatically lower above  
1 GHz - essentially zero.  Below 1 GHz, there is lots of data to show  
that nature and electrical equipment creates a noise level that is  
many dB higher than it is above.

D) The shorter the wavelength, the more rapidly a receiver's acquired  
signal varies in space due to multipath effects (confusingly called  
"fading," because in fact multipath is equally as likely to amplify as  
to attenuate).  This is why multi-antenna (MIMO) systems work in  
microwave systems, and also why adaptive SDR works, but these  
approaches work less well at sub GHz frequencies, because they *win*  
more with more rapid spatial variability.

E) Finally, constructing antennas of the same aperture (angular) and  
efficiency requires the lower frequency antennas to be much longer  
(larger) - dimension grows proportional to size.

In summary: When the debaters invoke *utter ignorance* of propagation  
physics, don't trust their policy pronouncements.  This is true for  
both sides of the debate in this case, I'm sad to say.

David Farber wrote:
> Begin forwarded message:
>
> From: Brett Glass <brett () lariat net>
> Date: October 26, 2008 4:56:39 PM EDT
> To: dave () farber net, "ip" <ip () v2 listbox com>
> Subject: Re: [IP] Your comments appreciated: Beyond White Spaces
>
> Dave, and everyone:
>
> Making the TV white spaces into Part 15-style unlicensed bands is a  
> HORRIBLE idea. Why? Well, firstly, the spectrum isn't needed for  
> that purpose. With 500 MHz of unlicensed spectrum at 5 GHz, plus 50  
> MHz at 2.4 GHz, PLUS the 20 MHz of spectrum at 900 MHz, do we really  
> need more spectrum for cordless phones and baby monitors? No. What  
> we need the spectrum for is to deliver wireless broadband, on  
> frequencies which are NOT polluted by an untold number of consumer  
> devices.
>
> Secondly, the laws of physics come into play here, in a big way.  
> Signals transmitted on the former TV frequencies can penetrate walls  
> and go for miles, interfering with all the other devices in town  
> operating in the same band. Is this what we want for consumer  
> devices? Of course not! If anything, we should allocate higher  
> unlicensed frequencies -- which have a HARDER time penetrating walls  
> -- so that each user's home is his or her "spectrum castle." If any  
> more spectrum is needed for indoor wireless (though it's not clear  
> that it is), how about liberalizing the rules for the 24 GHz  
> unlicensed "low millimeter wave" band so that it can be used in the  
> home? This would make far more sense than turning consumer devices  
> loose on the TV bands. The "white spaces," in turn, could be  
> reserved for services that deliver wireless broadband right through  
> the walls of these "castles," so as to connect them to the Internet  
> without drilling holes or mounting outdoor antennas.
>
> Thirdly, since the radios used on these frequencies have to be  
> "cognitive" anyway (they have to make sure not to step on TV  
> channels), they might as well use their intelligence to make better  
> use of the airwaves. As I explain in my talk at
>
> http://www.brettglass.com/CR/
>
> Shannon's Law dictates that the way to get the greatest utility out  
> of spectrum is not to slice it up into little chunks and license it  
> exclusively, but rather to share large swaths of spectrum,  
> intelligently, using cognitive radio technology. In fact, if we do  
> not do the latter, wireless will never be useful to deliver "ultra- 
> broadband" -- that is, speeds of 100 Mbps and up -- to multiple  
> users from a single transmitter. The laws of physics and information  
> theory indisputably dictate this; again, see the slides at the link  
> above.
>
> Fourthly, using this spectrum to deliver service TO the home rather  
> than for unlicensed devices WITHIN the home would help ensure that  
> this use would not interfere with television broadcasters, because  
> providers' antennas -- which would be located outdoors on towers and  
> buildings and would be very sensitive -- would be much better  
> situated to detect any broadcast signals with which they might  
> interfere.
>
> So, what we really need is NONEXCLUSIVE licensing of the TV white  
> spaces -- similar to the licensing which has been established for  
> the 3.65 GHz band, but with a mandatory spectrum etiquette that  
> allows the spectrum to be shared among multiple broadband providers  
> and their customers. This would maximize the use of the spectrum and  
> also consumer choice. It would this be far better than Tom Hazlett's  
> concept of total exclusive licensing (which would just allow the big  
> cell phone providers to warehouse yet more spectrum) or the New  
> America plan (which would turn the spectrum into a useless "junk  
> band." A simple understanding of physics, radio technology, and  
> consumer needs leads inevitably to this as the best possible use of  
> the TV "white spaces."
>
> --Brett Glass
>
>
>
>
>
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