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Re: TDMA - Reading the mail





Phil Karn wrote:

> >One question about a TDMA or CDMA type satellite signal:  How would one go
> >about "reading the mail"  or joining in a round table QSO?  The beauty of
> >analog is we all share the same spectrum/time space.  That is also the worst
> >part about it as well.
>
> The same way you do it now. Because the satellite downlink is
> inherently a broadcast medium, there's nothing to keep any interested
> station from demodulating the same transmission. In fact, with TDMA
> (but not FDMA) it's trivial to demdulate everything at once. But you
> probably wouldn't want to actually *listen* to 20 QSOs at the same
> time. :-)

In fact, you CAN "listen" to 20 QSOs at one time --  your hand-held GPS CDMA
receiver typically does 10 now. It's all a question of the number of xDMA
channels you have in the box.

Let me use GPS to illustrate CDMA: each of the GPS satellites (more than 24, less
than 32, and currently I think 27) are all on the same frequency (1575.42 MHz).
Each has an orthogonal spreading code (a unique 1023 bit long sequence of bits)
transmitted at a rate of 1.023 Mb/sec (i.e. a new "frame" is sent every msec).

Each GPS satellite (inefficiently, but that's another story) takes 20 such frames
to send you 50 bit/sec data (which contains high accuracy navigation data for the
satellite -- keps, information on satellite health, clock data, etc). And all GPS
satellites transmit continuously,  all at the same time!

In that 50 bit/sec message (1500 bits long, and hence repeating every 30
seconds)  is also contained some crude keps for all the other satellites.
Therefore, after a couple of minutes, your GPS receiver knows which other
satellites are in view and approximately what doppler shift should be applied to
find them. Your receiver then tunes in some other satellites and when it has
detected (at least) 4, it can tell you the grid square where you are located.
[If the receiver wakes up totally dumb (including being at a QTH more than ~1000
km from where it thinks it is, or with a really bogus internal clock), it simply
starts searching all the 32 CDMA "channels" and a wide range of doppler offsets
until it finds signals to becomes smart enough to do a more intelligent search].

The typical handheld GPS receiver today has 12 channels (i.e. listens to 12
"qso's"), works on the dash of your car with 4 AA cells, and costs about $100 at
K-Mart. And the 1575 MHz GPS frequency is roughly  midway between the L & S-band
frequencies that Phil has been suggesting.

I think what Phil is suggesting is that we could use some of these same basic
schemes, add a transmitter, and have an amateur "qso box" terminal and matching
satellite system that would work jus as as well as GPS.

No, the hardware -- either the satellites or the user ground terminals -- do not
exist today. But AMSAT has never let that stop us. The only thing  we can't do is
to violate the fundamental laws of physics!

73 de Tom, W3IWI



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