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Re: [Phys-l] WIFI



Thanks John, one further question (below) if you don't mind.

Message: 20
Date: Sat, 07 Apr 2012 13:55:10 -0700
From: John Denker <jsd@av8n.com<mailto:jsd@av8n.com>>
Subject: Re: [Phys-l] WIFI
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu<mailto:phys-l@carnot.physics.buffalo.edu>>
Message-ID: <4F80A9AE.3010205@av8n.com<4F80A9AE.3010205@av8n.com">mailto:4F80A9AE.3010205@av8n.com>>
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On 04/07/2012 12:57 PM, Forinash III, Kyle wrote:
Does anyone know if digital signals (e.g. WIFI) are sent by frequency
modulation or amplitude modulation or something else? I read in
Wikipedia something about the phase difference between two or more
carrier frequencies encoding the ones and zeros but that sounds to me
like basically FM. I know the transmission frequencies are much
higher than AM or FM which makes sense if you want to send lots of
information. I need some simple (simplistic) explanation to follow a
simple explanation of AM and FM for my intro class.

A good starting point is here:
http://en.wikipedia.org/wiki/Constellation_diagram

This is super-relevant to physics, because it motivates people
to think in terms of _phase space_.

As I understand it, basic 802.11b WIFI is just PSK (phase shift
keying) i.e. phase modulation, as shown by the first diagram in
the article cited above.

Ok I think I've got this much. A signal wave is phase shifted from a reference wave. The phase shift is plotted on a 2D graph (constellation diagram) where discrete points on the graph are coded to binary numbers. There are several (basically arbitrary) choices of constellation diagrams.

One nagging question is how the phase difference is detected. Adding two identical waves with different phases creates a new wave with a different amplitude but the frequency and wavelength are the same. So the phase is read from the amplitude of signal plus reference? This doesn't fit with your comment about this being closer to frequency modulation so I am confused.

kyle


On the other hand ... fancier things such as 802.11a and 802.11g
sometimes use PSK but sometimes use 16-QAM or 64-QAM. The latter
use fancier constellations of codewords, as shown by the second
diagram in the article cited above.

The "A" in QAM stands for amplitude, but a glance at the diagram
makes it obvious that both the amplitude and phase are being
modulated.

The idea that the codewords exist in phase space and can be
distinguished based on how far apart they are in phase space
is quite profound. It is relevant to the way we calculate
the entropy of quantum states, relevant to the uncertainty
principle, relevant to the second law of thermodynamics,
relevant to Liouville's theorem and associated notions of
optical brightness, etc. etc. etc.

==

Note that so-called frequency modulation is indistinguishable
from phase modulation (up to irrelevant constants).

Also: WIFI modulation schemes are listed here:
http://www.tutorialspoint.com/wimax/wimax_wifi_comparison.htm


------------------------------


---------------------------------------------------------
"Violence is the last refuge of the
incompetent."
Issac Asimov

kyle forinash 812-941-2039
kforinas@ius.edu<mailto:kforinas@ius.edu>
http://homepages.ius.edu/kforinas/Ebook/Site/Blog/Blog.html

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