In article , David H.
writes:


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.

While I can't give you a detailed example of how a portable field radio
(such
as the U.S. military's standard small-unit SINCGARS), there are a host of
already-working-for-a-long-time examples. One is the ordinary modem which
has both a "fast clocking" (almost like coarse acquisition) initial
synchronization that stabilizes the receiver decoder's internal bit rate,
then
an actual sync decoder from that to "line up" everthing.

With repetitive digital signals incoming, an ordinary modem borrows a
technique from magnetic data recording to set the decoding bit rate on the
transition of each pulse. Usually that is done at twice the bit rate to
grab
either transition. That can be a simple PLL with emphasis on the Phase,
but it can also be a simple R-C differentiator thing since the initial
timing
doesn't have to be precise. Once that is locked into place, the "fine
tuning"
for time synchronization can be done in several ways. One way is very
much like the old radar/transponder "coincidence detectors" which run a
pulse pair into a delay line and look for an AND (or coincidence) of the
input with the output, the pair's spacing equal to the delay line delay.
That
old "analogue" method was done in prehistoric times of tubes/valves and
useful in aviation radionavigation (IFF transponders, DME, all of which had
to coexist with unsynchronized pulse pairs from other interrogators).

A more modern way with digital logic is to use a multiple-stage Shift
Register, clocking the S-R with the "coarse" bit rate achieved with the
first data stream arrival. The S-R outputs can be Exclusive-ORed with the
incoming signal and all the Ex-ORs ANDed to select the coincidence which
was known for that particular system. The versatility of that is being able
to
pick any sequence pattern of 1s and 0s desired to get that "time sync" (some
prefer "framing" synchronization as a term) to get in step with the
transmitter
signal. Once that framing lock has been achieved, it is not difficult to
keep a
crystal oscillator phase-locked to the incoming bit rate (NTSC and PAL TV
do that). The framing sync lock pattern could be 4 to 256 bits long,
whatever
is system-desireable and there are no great demands on timing accuracy
for that (fairly easy to work out if thinking in terms of time). With
phase-
locking internal to the receiver, it aligns itself to the transmitter, no
sweat.
There are no great demands on jitter specs to get initial bit rate sync or
even to get framing sync; once the first sync is done, the system can fine
tune itself to stay in timing.

A "framing sync" pattern is slightly wasteful overhead in that it can't
carry
any data during its existance. From there on, there's all kinds of system
variations possible depending on modulation rate, sampling times, time-
multiplexing (if used), and all kinds of other things. Once a Tx-to-Rx sync
has been achieved, the data portion can be tailored to fit...along with the
internal stability necessary to limit the number of framing sync sendings to
maintain a good lock. In the initial acquisition of a signal there's bound
to
be some time wasted for the Rx to get in step with the Tx signal but that
is quite short indeed despite a wide variation in such system architecture.

The ubiquitous "keyless" auto lock is an example of a very secure (through
very long) digital bit pattern. Bandwidth isn't very high despite being UHF
(for keyfob size packaging) but it does the whole works and determines
the correct lock/unlock sequence (along with the convoluted decoding
thing for security) very quick - does it before that one "squeep" of the
horn
or speaker sounds.

Those of us who've worked with aircraft DME (Distance Measuring
Equipment) will have seen the ability of simple pulse-pair "coincidence
detection" work through a mass of "fruit" (pulse pairs from dozens of
other aircraft) to pick out the time-delayed ground station response.
No PLL needed there. The return signal is seen like right away on a
scope, the scope synchronized to the DME interrogator signal. I've
forgotten the ARINC spec but it's like 200+ aircraft can be interrogating
the ground station at the same time (ground station replies with a fixed
50 uSec time delay) and nobody interferes with anyone else.

In a modern digital example, the U.S. military SINCGARS field radios
have an extremely tight spec temperature compensated (like its hard
to believe how tight that is) internal crystal oscillator. The "Plugger"
(old AN/PSN-11 GPS receiver) can attach to it and update the time
to accuracy of the GPSS. Strangely enough, that isn't needed to get
SINCGARS sets to work together as much as the precise time is
needed to do networking, to get all the "hops" in sync (SINCGARS
is a frequency-hopper hopping at about 10+ hops/second in addition
to doing digitized voice and data). [see public data on AN/PRC-119
latest models from ITT Fort Wayne IN website] About a quarter
million of the SINCGARS R/Ts have been produced since first
operational around end of 1989. Extemely secure radio in the field
and many can be working at the same time without interference to
one another.

In conventional NTSC TV receivers, the color subcarrier sample of
only 6 to 8 cycles of 3.58 MHz is sufficient to work the PLL in the
receiver for holding on very tightly to that color burst signal. A crystal
oscillator is used to insure minimal jitter and maximum phase hold
for a single horizontal sweep time (NTSC is most unforgiving of phase
errors). "Lost time" due to sending the color burst is only about 2 1/2
percent of a horizontal sweep duration. While that isn't an SS thing
it shows that overhead time to lock in on a transmitter doesn't take
much time nor does it subtract much from the actual data (video)
transmission time.

To work with SS, either Direct Sequency or Frequency-Hopping,
one has to think in terms of TIME rather than frequency. Once that
can be done you will find lots of such examples in the wider field of
radio-electronics done over the last half century.



retired (from regular hours) electronic engineer person