Subject: [NEXTEL-1] radio evolution, defcon, and a rant on "Shannon's Limit"

From: [email-masked]
Date: Fri, 06 Aug 2004 13:24:51 -0400

Comments from Frankston, Reed, and Friends
Friday, August 06, 2004

DPR at 8:52 AM [url]:
radio evolution, defcon, and a rant on "Shannon's Limit"
Enjoyed this coverage from Defcon on bluetooth hacking with a smile.
Hackers include some of the most creative people I know.
But it also reminds me (and hopefully in a tangible way reminds you) of
several quite important things about radio receiver evolution:
Receivers evolve to fit markets (even hackers are a market);

"Shannon's limit"* at a point in space is obviously not independent of
receiver/antenna design; useful systems designs are not closed or
isolated - new applications get invented, and sometimes are bad, but
sometimes good ('toothing as a social phenomenon for meeting desirable
partners has grown from a security bug in phones to a feature for users
- it's not too hard to find people who prefer Nokia phones to Motorola
phones, because the latter are so secure they don't allow 'toothing).

Don't get me wrong - I don't think that yard-long mobile microwave yagis
are likely to be a common direction of evolution, at least not in my
probable future prediction. But I wouldn't bet a lot.

* [why the "quotes"?] Note that Shannon himself never claimed he was
describing a physical limit. He was describing a limit to the amount of
information capacity that could be realized via an abstract channel, not
a physical channel, given a prior probability distribution on noise. I
use
the term in quotes above in the sense that engineers commonly misuse it
today, not in the sense of Shannon's actual theory - hence the
quotes, since I try to use the term correctly at all times. The yagi
extracts a huge amount of information from temporal/spatial correlations

within the electromagnetic field, while decorrelating randomly generated
energy in the field. Thus the yagi is an essential part of the system
that must be considered when measuring "noise" not a separable
component. The noise energy and entropy are still there (the temptation
to
think that a bigger antenna works because it collects more signal energy
is not at all correct - it also collects more noise energy, so the ratio

remains constant...). The yagi doesn't make the noise go away.

Spatial correlation of electromagnetic noise is one of the reasons that
it's foolish to speak of "noise temperature at a point in space" as a
limit
on capacity. If there is even such a real thing as noise temperature, it
is too abstract to be useful as a limit on systems capacity. Noise at a
point is not independent of noise nearby - and correlated noise can be
removed (just as the yagi does in its simple way, and the Slepian-Wolf
theorem does in a more subtle way). Capacity has everything to do with
the transport system's architecture, starting with the antennas.

http://www.satn.org/archive/2004_08_01_archive.html#109179751200687308

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