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Digital modes on future satellites



Tony Langdon wrote:

I certainly like the way TDMA would eliminate the alligator problem.  A big
plus for us QRPers - everyone gets a fair go.

Wayne replies:

TDMA would be useful for a QRP station IF you could be heard on the
satellite's control channel in between the alligators.  You probably have a
good chance of that as long as there is no interference (malicious or
accidental) on the control channel.

My biggest concern for a TDMA or CDMA transponder is that a single
interferer can make the entire transponder unusable.  It's possible to make
the control channels frequency-agile, but the complexity goes up for both
the satellite and the ground stations.

My relatively uninformed opinion is that complexity and interference
problems would make TDMA and especially CDMA very risky on a transponder
that is "open to the public".  At the very least, they should only be used
on the microwave bands where there is not a large installed base of
potential interferers.  FDMA with "LEILA alligator control" would be more
immune to single interference sources, and would be much simpler to
implement.

Another thought comes to mind when using CDMA or TDMA for interactive voice
communications.  Does every ground station participating in a conversation
need its own time slot or code?  If so, the ground station must transmit
continuously in order to maintain the assigned time slot or code even when
the operator is listening.  A conversation between two stations would
require two time slots or codes, and a round table discussion would require
several time slots or codes.  In theory, stations could each talk one at a
time sharing the same time slot or code (working half-duplex).  But then you
have the probability that the time slot or code will get reassigned to
somebody else during the handoff.

Back to the present.  I'm psyched about AO-40, and optimistic that the
crashed IHU can be rescued.  Hopefully I can complete my satellite DXCC.
I've been "stuck" at 95 entities confirmed for too long!

Wayne Estes W9AE
Mundelein, IL, USA

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