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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

On Sun, 09 Jan 2005 18:51:50 -0800, Bill Turner
wrote:

_________________________________________________ __________

Spread Spectrum = Legalized Jamming.

Is it even legal for hams, though??
The authorities seem to feel they have to be able to monitor what we
put out (somewhere in the licence conditions, I believe) and SSC makes
that extremely difficult for them to say the least.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul Burridge wrote:
On Sun, 09 Jan 2005 18:51:50 -0800, Bill Turner
wrote:

_________________________________________________ __________

Spread Spectrum = Legalized Jamming.

Is it even legal for hams, though??
The authorities seem to feel they have to be able to monitor what we
put out (somewhere in the licence conditions, I believe) and SSC makes
that extremely difficult for them to say the least.

It is legal, provided you use the FCC-approved spreading polynomials
and whatnot and hold your mouth right. So long as you do it the FCC
way, they have no trouble monitoring.

--
Mike Andrews, KE5DMQ

Tired old sysadmin

On Tue, 11 Jan 2005 16:10:50 +0000 (UTC), (Mike
Andrews) wrote:

Paul Burridge wrote:
On Sun, 09 Jan 2005 18:51:50 -0800, Bill Turner
wrote:

_________________________________________________ __________

Spread Spectrum = Legalized Jamming.

Is it even legal for hams, though??
The authorities seem to feel they have to be able to monitor what we
put out (somewhere in the licence conditions, I believe) and SSC makes
that extremely difficult for them to say the least.

It is legal, provided you use the FCC-approved spreading polynomials
and whatnot and hold your mouth right. So long as you do it the FCC
way, they have no trouble monitoring.

I see. Well we're not governed by the FCC here in Little Britland, so
I guess it's probably illegal to use it in the UK. Perhaps a G with a
licence to hand can verify...

--

"What is now proved was once only imagin'd." - William Blake, 1793.

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

In article ,
Paul Burridge wrote:

Spread Spectrum = Legalized Jamming.

Is it even legal for hams, though??

Yes, with restrictions. See Part 97 section 311 for the detailed rules.

Briefly:

- SS is allowed, but must not be used for the purpose of obscuring
the meaning of the transmissions.

- SS transmissions must not cause harmful interference to non-SS
stations, and must accept all interference from non-SS stations.

- SS transmissions are limited to 100 watts, and if over 1 watt must
implement automatic transmitter power control which limits power to
the minimum needed to maintain a specified SNR.

- SS stations can be ordered to shut down or restrict their
transmissions, and/or required to maintain a complete record of
information transmitted via SS.

If I recall correctly, the FCC rules used to restrict amateur SS
tranmsissions to using certain specified coding techniques, but this
restriction appears to have been relaxed. I infer that the broader
rule about using publicly-documented modulation techniques is the one
which now applies.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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.