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Re: Amateur radio module on commercial Geo Sat?



>I was thinking of FDMA or CDMA on the uplink and TDMA on the downlink. This
>should minimize costs at both ends. The satellite would emulate multiple
>digipeaters with different access methods and modulation on the uplink and
>downlink.

As soon as you use a different modulation/access scheme on the uplink
than on the downlink, the satellite complexity takes a big step
because that usually implies that the satellite has to demodulate and
remodulate everything.  That doesn't mean it can't ever be justified,
but you have to be careful.

>CDMA would require power control on the ground, but that should be less
>costly than a higher power PA and more easily allow portable operation. I
>would think that CDMA would reduce peak power output requirements by a
>factor of 10-100.

You can't say anything about these ratios without additional data. The
peak-to-average power ratio in a TDMA uplink depends directly on what
fraction of the transponder's capacity is being used by that station.
If the station is using half of the transponder time, either because
the station needs a high data rate or the transponder capacity is low,
then the peak-to-average ratio is only 2:1. Your 10-100 numbers
implicitly assume that there are from 10 to 100 users sharing the
transponder at once. Actually, they assume more since you have to
allow for the mutual interference in CDMA. They *would* be correct
for FDMA, which like TDMA is orthogonal (no inter-user interference,
at least in theory.)

>For an FDMA uplink (actually multiple uplinks) the satellite receiver could
>easily handle a 40 dB variation in signal strength. This should eliminate
>most jamming problems. There should be a few physical receivers anyway for

I don't understand what you mean here. If a single transponder is
used, then it must be linear. And if it's linear, the ratio of powers
going out has to equal the ratio going in. And few (if any) linear
transponders have intermodulation products anywhere near 40 dB down.

If you're concerned about jamming, you have a different
problem. Basically, if the signal parameters are known (ie., you're
not using CDMA with cryptographic spreading codes) then all the
multiple access schemes are equally susceptible to optimal jamming.

>An FDMA uplink might be advantageous in that you could create some narrow
>bandwidth channnels for APRS users and other low power portable
>applications.

Low rate support for low power uplinks could still be handled by a
single TDMA transponder by establishing a variety of FEC code rates
and modulation techniques. The low power uplinks would use a low code
rate and BPSK to offset the additional uplink receiver noise
associated with the low power uplink not saturating the
transponder. Higher (peak) power stations would use higher FEC code
rates and/or higher-order modulation methods (QPSK, 8PSK) to more
effectively use the available C/No. The only problem I see here is
that I've been assuming the use of a squelch on the transponder to
save DC power when there's no uplink, and making a squelch that works
reliably for low power uplinks may be hard.

Phil
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