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Sun Aug 8 10:35:36 CEST 2010

LFSR beacon detection

Actually, this might be interesting to use together with the krikit
idea.  Or how to make a variant of GPS using LFSR audio-beacons.

Problem: we have plenty of bandwidth (say 3 - 10 kHz) but not dynamic
range (environment noise, and "noisyness" of the transmitter).  We
want to transmit a low bitrate signal.

Solution: using spread-spectrum techiques we can trade bandwidth for
dynamic range.

A PSK baseband signal after heterodyne filtering by the carrier wave
is a complex phase signal.  It might have some small residual
modulation that is easy to track, so let's ignore it.

The basic idea is to use the LFSR to perform a local prediction for
the I and Q signals separately, keeping the operation itself linear.
(I.e. if more than one bit of information is available, it can be
used.)

We integrate the output of this predictor to yield a slow-varying
information carrier.  A Schmitt trigger on the output of the
integrator should be enough.

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