[MLB-WIRELESS] History of Spread Spectrum with beginners intro

[vak at alphalink.com.au]

Hi Folks,

Some of you might find this interesting, so I've written the email in two
parts.

1) History of Spread spectrum - Credit goes to M Manh for sending this to
me.

2) A beginner's introduction into Spread Spectrum (which I wrote)


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1) You might like to read the story of how Hedy Lamarr and George Antheil
invented Spread Spectrum Communication at:

http://www.ncafe.com/chris/pat2/

--------------------
2) Beginners intro into spread spectrum.

I thought a few people might be interested to learn how the wireless
equipment we use as part of our network actually works. I've tried to make
this as simple to understand without going into too much detail.

As you may or may not know, the 802.11b standard relies on a technique
called DSSS (Direct Sequence Spread Spectrum) to operate. DSSS is a
modulation technique which encodes our digital data bit stream of 1's and
0's into a  radio signal which can be sent "through the air".

Common radio modulation techniques (such as AM and FM on your car radio)
tend to concentrate all their transmit energy in a single "carrier" (ie. a
very small range of frequencies). On a spectrum analyser (a piece of test
equipment) this "carrier" looks like the cross-section of a very narrow
speed hump.

Spread spectrum on the other hand, takes a large frequency range and fills
it with many "carriers". By occupying a larger frequency range (frequency
bandwidth), it allows us to send more 1's and 0's - and this translates
into greater data bandwidth.

The method used for occupying this increased frequency bandwidth is
dependant on the mathematical algorithm used - hence spread spectrum comes
in several  different flavours - FHSS, DSSS, DS/FH, etc.

FHSS (Frequency Hopping Spread Spectrum)
DSSS (Direct Sequence Spread Spectrum)
DS/FH (Direct sequence, frequency hopping hybrid)

On a spectrum analyser, the signal no longer looks like a narrow speed
hump, but a long rectangular block.

If you have any questions, please don't hesitate to ask. I'll do my best to
answer.

Vaskos
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