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Re: PSK, Costas Loops, and simplicity (was: RE: AO16/LO19 Uplink w ANY PC!)



Laura:

While the Costas loop is fine, it is not a decision directed
demodulator.  In software, there is little difference in the
two in terms of computational complexity and a significant
enough difference in performance to warrant the extra
time to understand it and do it.

To do it you jointly estimate the data values, clock phase,
and carrier phase and frequency at the current "sampling
instant".  This sounds complicated but in reality it isn't
since you get almost all of the performance out of some
pretty gross approximations, enough so that it still
outperforms the Costas loop.

To determine the sampling instant, I suggest implementing
an approximate maximum likelihood tracker such as the
one describe in the mid - to - early 1960's (!!) by Umberto
Mengali in an IEEE journal.   The estimate of the clock
error generated is "weighted" by your belief in all of
your parameters (thus the "joint").  Use the data value
generated to estimate carrier phase and frequency offset.  You
could use Costas loop to get close and the gear shift to
decision directed algorithm for tracking/estimation and that
would work.

Proakis book on Digital Communications has a section on
the joint esimation for  QAM (including BPSK, QPSK, etc.).
Remind me to point out the pages after I return from
Phoenix on Monday.

Bob

At 10:43 AM 09/21/1999 -0400, Laura Halliday wrote:
>When I described a Costas Loop PSK decoder as simple, I hope people
>realise that I was taking a system view - while the software is
>indeed non-trivial (though well understood), the hardware is
>straightforward, and it allows a far simpler downlink receiver,
>more than offsetting (IMHO) the complexity of the demodulator.
>Which is all software anyway...
>
>Laura Halliday VA3LDH   "Que les nuages soient notre pied
>Grid: FN03gs                a terre..." - Hospital/Shafte
>----
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