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VE2HAR > MT63 11.03.05 19:29l 59 Lines 2116 Bytes #-7344 (0) @ WW
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Subj: Re: [MT63] 20 kHz wide Digital Proposal (Was: ARRL MUST GET
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Sent: 050311/1711z @:VE2HAR.#MTL.QC.CAN.NOAM Laval #:41361 $:13736sentto
Karl Larsen wrote:
> Tomi Manninen wrote:
> >Please remember that you *can* trade power for bandwidth. That's what
> >Shannon said over 50 years ago and most hams don't get today.
> >
> God. It just yesterday that I read the Bell Labs paper Shannon
> wrote. Later I taught other EE students about bandwidth and power. Then
> came spread spectrum and now your cell phone uses that plus a wide signal
That is a common misconception. Spread Spectrum has nothing to do with
that paper by Shannon. SS works simply by taking a normal signal, making
it wider (not by adding information to it), transmitting/receiving that,
and then narrowing it back down to the original signal. The normal signal
must obey 'Shannons law', but the SS signal does not--it can be well
below the noise.
The reason the SS signal doesn't have to obey Shannon is that it does not
contain information--Shannons law is about *information*--as the contents
of the spreading signal is knows by the receiver a priori. Well, to be
strict, it contains one bit of information by wether or not it is present.
That part of it will have to obey Shannon, but one bit with all of that
signal power. So, basically, if the transmitter were sending a dead
carrier and you can see that 3db over the Shannon limit, then you should
be able to decoder the output of that transmitter if it is spread.
Cheers,
David n0ymv
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