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Re: Geosynchronous Satellites

> QAM is great when SNR is high, the channel is reasonably linear and
> bandwidth is limited. But it's a poor choice on a satellite channel
> when the SNR is low, the channel is quite possibly nonlinear, and
> bandwidth is more plentiful. That's why you see QAM used on telephone
> lines and in cable modems, but not on satellites, where BPSK and QPSK
> are usually the way to go.

A year or so ago as part of a "work" project, I was looking at hardware
intended to be used to implement a fixed-wireless network using the
MMDS spectrum.  At the time, a bunch of the hardware was essentially
the same stuff used to implement Internet service on CATV plants.  These
typically used QAM (16- and 64-QAM if I recall on vendor in particular).
They then had to explain all of the nifty new adaptive equalization they
had to bolt-on to the modem design because, well, you don't get
very much multipath on a CATV plant, but free-space MMDS was a different
matter completely :-)

The clever vendors figured this out before they shipped their products..

Interestingly, they'd typically use QPSK in the upstream direction
because of the training time required for the equalizer used in
the QAM modem wouldn't be nearly fast enough for the TDMA uplink..

As an aside, the reason you see all this cheap "analog TV" MMDS hardware
is because most of the operators were having their lunch eaten by
both the cable operators with greater penetration on the one side,
and the DBS carriers on the other.  Much of that spectrum is going
to be re-used for data applications, so perhaps there will be another
generation of interesting surplus to be had in a few years.


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