[sdiy] Re: Information Content of Signals

[John L Marshall]

I was just reflecting back.

I the early days of cellular Motorola introduced NAMPS with three 10kHz
channels fit into the space one 30kHz AMPS channel. The expectation was that
a substantially greater signal would be required for the same S+N/N. The
result was that the 10kHz receive filters were so good that the noise floor
went down by approximately one third. Only a slightly stronger signal was
required.

Take care,
John
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Pacific Northwest Synthesizer Meeting
August 9, 2003
www.sound-photo.com
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----- Original Message ----- 
From: "Magnus Danielson" <cfmd at swipnet.se>
To: <john.l.marshall at gte.net>
Cc: <grichter at asapnet.net>; <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, May 18, 2003 6:23 PM
Subject: Re: [sdiy] Re: Information Content of Signals


> From: "John L Marshall" <john.l.marshall at gte.net>
> Subject: Re: [sdiy] Re: Information Content of Signals
> Date: Sun, 18 May 2003 17:37:38 -0700
>
> > But of all of the signals only one is of interest at a time. All of the
> > other signals are noise to desired signal.
> >
> > This is inpart how CDMA works. The desired signal is actually below the
> > noise. All of the other signals are noise contributors. But, we know how
to
> > look for the desired signal.
>
> Indeed. By the lovely tool of correlation impressive gains can be had.
> For instance, the C/A code of L1 GPS signals, is a 1023 chip long Gold
code.
> This have a 30 dB gain in relation to others and the selected codes have
been
> chosen for best suppression of worst case sine correlation.
>
> It is through tools like correlation you may go from a wide bandwidth to a
much
> more narrower while keeping the main signal intact and unwanted signal
becomes
> suppressed. This shift between wanted and unwanted signal is indeed a gain
in
> signal to noise.
>
> TDM = Time Division Multiplexing
> FDM = Frequency Division Multiplexing
> CDM = Code Division Multiplexing
> WDM = Wavelength Division Multiplexing
>
> TDM, FDM and CDM is actually not as different as people is beleived to
make
> them. They are just three special-cases of systems having various forms of
> ortogonal vectors. TDM is simple since then each have their own
"timeslot".
> FDM is simple since then each have their own "frequency". CDM is a little
> harder yeat since then each have their own "code". They all have the
property
> that they have N signals where neither of them contain any part of the
other,
> i.e. they are linearly independent, and this also makes what happends on
one of
> them doesn't affect the other. Another way is to say that they do not
> correlate, which is just the same. Sines of different frequency does not
> correlate, and therefore is usefull as such vectors. Different sequences
of
> digits can be shown to have the same relative property, and thus the code
they
> have can be shown to have the same property. One such binary sequence is
when
> the n-th sequence have the n-th bit (chip) set to one and all the others
set to
> zero, where n is from 0 to N-1, neither of these can be expressed as the
linear
> sum of any other, so they do not correlate, so TDM is also such a system.
>
> It's not all that different in theory when you toss it the right way.
>
> WDM is just a poor choice of name since it really is FDM, the frequency
doesn't
> change, but wavelength might when you changes medium.
>
> Cheers,
> MAgnus
>