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C-C Rider Revisited

I just read the C-C Rider Revisited paper in the proceedings of the 22nd
ASMAT-NA Space Symposium. The concept has progressed from a simple "bent-pipe"
transponder to a transponder using DSP on the uplink and downlink. The new
concept is more flexible and can provide major improvements to amateur satellite
capability and accessibility. I have a few comments.

The original proposal required the use of a C-band uplink and downlink for
Doppler shift cancellation. The new proposal can use DSP for Doppler correction
and that removes the single-band constraint. The authors still assume that
C-band uplinks and downlinks are used even though there is a performance penalty
in the satellite receiver. A multiple-band design should be considered. The
uplink can be in a band where the cost of power amplifiers is minimized. Perhaps
multiple uplink bands should be considered, so that users with 435 or 1260 MHz
uplink amplifiers can use them and then migrate to higher frequency bands for
smaller antennas or higher data rates. The downlink can be in a band where
terrestrial interference is minimized.

The new design assumes a 30 Watt ground station transmitter and a 20 dBic gain
antenna. This is only true if TDMA is used on the uplink and each station must
transmit a brief high-power burst during its time slot. Since DSP can accomodate
it, FDMA is a better choice as ground stations can use a 2 Watt power level for
digital voice but transmit continuously. This results in a much simpler and
lower cost ground station. TDMA can still be used on the downlink as it results
in a more efficient power amplifier on the spacecraft and the power spectral
density at the ground station receiver is maximized.

The uplink receiver DSP should accomodate variable bandwidth uplink channels.
Some users may be interested in low bit rate keyboard-to-keyboard communication
or APRS. This could be accomplished with very low power levels or very simple
antennas at the ground station if narrow-bandwidth uplink channels are
implemented. Others may be interested in exchanging images at a high bit rate
using higher power or larger antennas. They could be accomodated with a few
wide-band uplink channels. Since DSP is used in the spacecraft, the width of
uplink channels can be set so that Doppler shift does not cause overlap of
uplink signals and the bandwidth of each virtual receiver can be set to optimize
the SNR for various uplink data rates.

There should be a link between digital and analog voice modes (and possibly
other modes like PSK31) so that cross-mode contacts can be made. The
simultaneous use of V and S band downlinks on AO-13 was very useful as I could
listen on S band where interference (at the time) was less and work the majority
of stations that only had V band receivers. Cross-mode contacts would allow new
users to install lower-cost/lower-profile digital stations and still work all
the existing analog users. This is absolutely necassary to make digital modes
viable in the short term.

I like the new C-C Rider concept and it has a lot of potential for all amateur
satellite users. It seems to me that if DSP is going to be used on all uplinks
and downlinks with a digital switch matrix, there is an extreme amount of
flexibility in configuring the satellite transponders. A mix of analog and
digital modes could be supported on all uplinks simultaneously and two downlinks
(one digital and one analog on separate transmitters) could be supported.


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