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Re: Geosynchronous Satellites



>For nearly all forms of digital modulation you need some degree of linearity
>in the amplification.  I think the only one you can really get away with
>using a truly saturated amplifier would be GMSK which forms the basis for
>GSM.  But I'm not an expert on digital modulations and am unsure how

Any constant-envelope mode, analog or digital, can operate through a
saturating amplifier.  This includes PM, FM, BPSK, QPSK, 8PSK and MSK
but not QAM.

Because they encode data in instantaneous phase transitions,
bandpass-filtered BPSK, QPSK and 8PSK will broaden somewhat as they go
through the saturating amplifier. A suitable passive bandpass filter
on the amplifier output can remove the excess sidebands without
lowering the efficiency of the amplifier. A modified version of QPSK,
variously called offset-keyed QPSK or staggered QPSK, eases the
problem somewhat by changing the phase of only one quadrature channel
at a time.

QAM won't go through a nonlinear amplifier because it uses both
amplitude and phase variations to encode data, and the amplitude
changes are stripped off by a saturating amplifier.

Phil




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