On Mon, 10 Jan 2005 11:16:05 -0500, 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.

Dave

*snip*

FYI on CDMA cell phone systems the base station transmits an
"unspread" pilot carrier for the mobile to lock onto. Once the phone
finds a pilot it starts looking for the sync/paging channel, which is,
IIRC, the "101010..." Walsh Code. Can't remenber now but I think one
of the "rake reciever" channels stays glued to this to keep the
timing.

The base stations obtain thier PN offset timing from GPS synced clocks
at each site, all system timing is based on the "even" second output
from the clocks.

Not sure about the current military gear but it used to take a few
seconds after pushing the PTT before a little beep sounded letting you
know it was OK to talk, but they wern't running at anything like the
spreading rates used today, probably a lot easier to find & sync to
the transmission pattern.

H.