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Amsat-UK's Oscar News, 1986 Apr, No.58 p.29



James Miller G3RUH

The root of the "alligator" problem is attributable to the use of a
wideband, flat response linear transponder.  The strongest signal sets the
spacecraft ALC level, which reduces all other signals as well.  This
effect causes the others to increase their uplink power which depresses
the ALC still further....

This kind of linear transponder has the virtue of simplicity of design,
and is fine for phase 3 satellites, conceived and implemented as they are
with essentially late 70's technology.

Probably, phase 4 satellites will embody similar concepts.  Now is the
time to learn from our mistakes.  The problem of one or two over-strong
signals is overcome by splitting the transponder spectrum into small
sub-bands, and selectively controlling the power in each band.  This can
be done with banks of filters, as in ARSENNE.  (It wasn't - jrm 94/Nov).

Yer phase 5 satellite will do it all-digitally via the Discrete Fourier
Transform and cheap silicon.  

Assume a transponder pass-band of 64 kHz.  This will be digitally sampled
at say 128,000 samples/second.  Now let's suppose we want our sub-bands or
spectral cells  1  kHz  wide.  For this, samples are taken in bursts of 1
milli-second duration, making  128  samples  per  burst, 1000
bursts/second.  Now the on-board signal processor applies the fourier
transform to these 128 samples, so converting the signal from the time
domain to the frequency  domain or spectrum.  Each of the resulting  64
spectral cells is checked and adjusted (even to  zero!) so that
inappropriate amplitude variations are evened out.  The 64 cells are then
inversely transformed back to the time domain (1000 times/sec), and pass
on through the transponder as per  normal.  By this means every 1 kHz of
transponder passband is allocated the same amount of power, so every user
gets much the same service.  This simple technology  is  available  NOW.

Doubtless the  Dalligator  or  Digital Alligator will emerge to defeat it!
For further reading see "Poisson, Shannon, and the Radio Amateur",
J.P.Costas, Proc. IRE vol 47, pp 2058-2068, Dec 1959.

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Created: 1994 Nov 13 -- Last modified: 2005 Oct 29