On 18 Mar, 2012, at 10:52 , John Seamons wrote:
> They do talk about using the 11-bit Barker code for autocorrelation. But the 
> sync bits transmitted only match the Barker code if you interpret them a 
> little bit out-of-order.

The part of the paper that talked about the Barker code confused me
somewhat since I couldn't quite figure out how it was relevant.  The
autocorrelation property of the Barker code is only interesting if
the Barker code is the only thing being sent (over and over), but
in this case the concerns are more about spurious correlations with
the variable data, something for which no solution seems to be

It is the case, however, that (non-circular) autocorrelations of
the fixed sequence are relevant at small offsets.  In your data the
fixed sequence seems to be

    -1, 1, -1, 1, 1, -1, 1, 1, 1, 1, -1, -1, -1, -1

which, ignoring the contribution of the variable data (which increases
with increasing offset), gives this basic result for offsets from 0 to
13 seconds:

    14, 1, 2, 1, -6, 1, -2, -3, 0, -3, 0, 1, 0, 1

So there is a quite large autocorrelation at 4 seconds offset.  If I
weight the search pattern by the fixed pulse widths (there are 3 0.2
second pulses and 3 0.8 second pulses in the fixed sequence; I gave
the rest a weight of 0.5) that gets a little better, i.e.

    7.0, 1.4, 0.4, 0.5, -2.1, -0.4, -0.4, -0.9, -0.3, -0.9, 0.0, 0.5, 0.0, 0.2

if I did that correctly, though at the apparent cost of making the
autocorrelation at a 1 second offset a bit worse.

In any case, if this is the pattern they selected I really would have
liked to have seen a discussion of the tradeoffs involved in picking
it, along with the assumptions they made about how it would be
detected.  And I kind of hope I don't have to read about that in
someone's patent since technical descriptions written by lawyers are
really boring.

In any case, I think the paper left out the good parts.

Dennis Ferguson
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