In article ,
David H. wrote:
Thanks for the info, Caveat. Looks like I may have had run across one
of your links
in all my searching

There are many question I have regarding SS, but one that's bothering me in
particular. Regarding the PN spreading sequence, these sequences
obviously have to be
aligned perfectly in both transmitter and receiver. Naturally they could
be kept in
sync if both circuits were initialized at the same time.

However, 3 things: 1) The circuits will not be initialized at the same
time in 99% of
most cases, as in the use of, say, a portable field radio. 2) If they were
synchronized at the same time, well, no clock or oscillator is perfect. It would
eventually drift. 3) As I understand it, there is no initial "handshake"
signal at
the beginning of transmission with the receiver to initialize/syncronize the PN
sequences on both ends.

So in short, how do the PN sequences became and remain synchronized
through time?
Thanks.

The ones I'm familiar with - the Barker codes used in 802.11 DSS -
have the interesting characteristic that their auto-correlation is
extremely specific. That is: if you take two copies of the Barker
sequence, put 'em above one another, multiply them together (treating
the two states as 1 and -1 rather than 1 and 0) and then sum up the
products, you get a very high positive value (11, in the case of an
11-bit Barker code). If you invert one of the two before multiplying,
then you end up with a very large negative value (e.g. -11).

However, if you shift one of the sequences one or more bit positions
to either side (in a circular-shift fashion), and do the multiply-
and-add thing again, the final sum is very close to zero. In other
words, the correlation between a Barker code, and a time-shifted
version of that code, is very small.

So - in order to transmit using DSS, you take the incoming bits,
multiply (XOR) them with a Barker sequence of chips running at (e.g.)
11x the bit rate, and transmit. To receive, you take the incoming
(11x rate) pattern, and perform the "multiply and add" process against
the original Barker chip pattern at the full 11x rate. If nothing's
being transmitted, and the input signal is simply noise, the
correlation between the noise and the Barker pattern will be very low,
and the sums coming out of the adder will be close to zero.

During a transmission, the sums from the correleator will also be close
to 0 when the bits don't line up. Every 11th chip-time, though, when
a complete bit's worth of chips from the receiver has entered the
correlator, the sum will jump up to a high absolute value (+11 or -11
in the example case, if none of the chips were corrupted by noise).
This sudden jump to a high absolute value tells you what the original
data bit was (1 or 0) and can also create a synchronization pulse
which you can use to discipline your receive oscillator.

The above description is crude, inexact, and may contain errors, but
perhaps it gives the flavor of the method.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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