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Re: July 2001 President's Letter

>  So actually people are thinking on a simple bent-pipe transponder without 
>any kind of regeneration? Wouldn't  it work better if in some way the data 
>frames would be decoded, checked and corrected for errors and retransmited? 

Yes, there's no question that a regenerative transponder that decodes
the signal and remodulates it would work better. You wouldn't waste
any transponder power repeating noise, nor would the repeated noise
interfere with demodulation at the ground station. That's precisely
the idea behind RUDAK.

Problem is, it is difficult to demodulate and decode energy-efficient
high rate signals with space qualified hardware. I'd rather keep the
transponder itself simple, dumb and reliable, and place all the smarts
on the ground.

One possibility is to use a less energy efficient scheme on the uplink
that spacecraft hardware can readily decode. The decoded data would
then be relayed back in the more energy efficient mode. It turns out
that most of the complexity in these energy efficient modes lies in
the receiving, not the transmitting.

For certain parameters in the link budget, particularly for low uplink
EIRPs, this hybrid combination of "dumb uplink, smart downlink" does
make sense in that the uplink losses due to the less efficient uplink
modulation are less than the repeated noise losses in the bent pipe.

I've created a spreadsheet to analyze combined uplink/downlink link
budgets on hard-limited bent pipe transponders.  I'll post it on my
web page later this afternoon and give a pointer here with some


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