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Re: Amateur radio module on commercial Geo Sat?

> 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.)
I think that we need to assume something between 10 and100 simultaneous
users to have digital transponders that can replace analog transponders like
those on AO-13.
> >For an FDMA uplink (actually multiple uplinks) the satellite receiver
> >easily handle a 40 dB variation in signal strength. This should eliminate
> >most jamming problems. There should be a few physical receivers anyway
> 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.
The idea is to have have many lower speed digital uplinks multiplexed onto a
high speed TDM digital downlink so that the transponder behaves like a
digital multiplexer or cross-connect. Multiple PSK or FSK uplinks could
share a common analog front end and then be demodulated individually
similarly to the way AO-16 uplinks work. Today, the demodulation could be
accomplished using DSP rather than multiple FM receiver chips as is done in
AO-16. The difference in the strength of the individual carriers arriving at
the satellite could easily be 40 dB as the analog circuitry is all low-level
and IMD can be minimized without consuming a lot of power. The level of IMD
in A/D converters is -70 to -90 dBc so they can also handle signals at
multiple frequencies within the passband without much IMD.
> 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.
Yes, the total power required to jam the entire uplink is the same, but the
unintentional jamming caused by one high-power station activating the AGC on
a linear transponder and attenuating the signals of all lower power stations
would be avoided.
> >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.
Your transponder sounds like a soft-limiting analog transponder like the
mode-S transponder on AO-13, but it would repeat signals that used digital
modulation and they would be time division multiplexed.

If this is correct, aren't the low power stations are a big disadvantage
because their only get a small percentage of the peak downlink power? This
forces their data rates to be lower than they otherwise could be.

Also, if the transponder is going to operate in a non-saturated mode,
doesn't this mean that the transponder gain must be reduced so that the
wideband noise does not cause limiting? This increases the uplink power
required for all stations.

There is another issue. All forms of PSK have amplitude variations if they
are band limited and the uplink signal with have to be bandlimited to fit
within the transponder passband. When the transponder limits the amplitude
of the repeated signal it will convert the baseband signal from whatever
waveshape it was to a rectangular wave and generate spurious sidebands. It
also means that the shape of the optimal filter at the receiver varies
depending on whether limiting occurs.



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