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Re: July 2001 President's Letter



>Would it add considerable difficulty to use O-QPSK instead. This way class C
>uplink amplifiers could be used, or at least we wouldn't have to worry as
>much about saturating existing class A amplifiers. I don't think many hams
>realize how close to saturation they operate some of their equipment. With a
>PSK uplink, this could be a problem.

This is exactly right, and I'll probably use O-QPSK if I do use
QPSK. (It was late last night when I wrote my message). I have also
considered MSK.  Unlike QPSK and O-QPSK, MSK is truly constant
envelope and will not broaden at all when it is hard limited. Problem
is, MSK is considerably more complex to demodulate coherently at high
speed than QPSK.

I don't yet know whether I have sufficient cycles in a late-model
Pentium to demodulate either QPSK or MSK at the required symbol rate,
so it may be necessary to use a hardware demodulator. This could be an
off-the-shelf ASIC, such as one designed for direct satellite
broadcasting, or one implemented in a gate array. If we use a
commercial ASIC, that may also dictate our choice of mode.

I need to work out the ground received power spectral densities of the
sidelobe levels for each of the schemes (MSK, QPSK, offset QPSK) after
hard limiting and with various degrees of high-level filtering after
the satellite HPA. If, for example, the sidelobe power spectral
densities for hard-limited QPSK as seen at the ground are already low
compared with the sky background temperature, then there's no point in
trying to lower them even further with filtering or a "better behaved"
modulation scheme. 

I also have to consider antenna sidelobes from the uplink antennas if
their HPAs are also to run in a nonlinear mode.

Phil


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