SLIP CORRECTOR MEANS AND METHOD FOR MULTISTATION NETWORKS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP81-00120R000100020007-8
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 10
• Document Creation Date: December 20, 2016
• Document Release Date: December 4, 2003
• Sequence Number: 7
• Content Type: CONT

Approved For Release 2007/09/21 : CIA-RDP81-0012OR000100020007-8
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MODULO-2
ADDER
Oct. 31, 1967 R. J. MONAIR
AhA Se S'LIF CORRECTOR MEANS AND METHOD FOR
STABLE
CLOCK
PSEUDO
RANDOM
SEQUENCE
GENERATOR
lot
DATA 14
SAMPLING
AND
DECISION
3,350,644
BALANCED
(81-PHASE)
MODULATOR
X17
INTEGRATE
AND DUMP
CORRELATOR
27 --W
R. F. AMPL.
AND
L F AM PL.
20
13
PSEUDO
RANDOM
SEQUENCE
GENERATOR
STABLE
CLOCK
INVENTOR.
ROBERT J. McNAIR
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OUTPUT
22 BALANCED LOCAL OSCILLATOR
MODULATOR
102
 Approved For Release 2007/09/21 : CIA-RDP81-0012OR000100020007-8
Oct. 31, 1967 R. J. MCNAIR 3,350,644
SLIP CORRECTOR MEANS AND METHOD FOR MULTISTATION NETWORKS
Filed May 20, 1965 3 Sheets-Sheet 2
STATION
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1-1
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(NET CONTROL
1 i
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STATION 4
STATION
3
REFERENCE POINT IN CODE
109 8 7 6 5 4 3 2 I I 2 3 4 5 6 7 8 9 1011 1213141516 BAUDS
0 1 1 1 0 0 1 , 0 1 1 -roll 1 0 0 l 1 1 0 0 0 1 11111' 0 O!06
STATION 1 P R S G
15MI OR bits
allill 1111101010[011 I I 1 0 0 0 I I 1001011 1110 1 I 0 I I 0
STATION 2 PRSG
I2Mi.OR bits
1 1 1 0 1 0 1 0 1 0 [ 1 1 1 1 1 1 0 1 0 1 0 1 1 1 1 1 1 1 0 0 1
STATION 3 P R S G
75Mi.oRbits
0 0 1 1 1.1 0 0 0 1 1 1 0 0 1 1 0 1 1 0 1 1 0 0 1
STATION 4 P R S G
INVENTOR.
ROBERT J. M c NAIR
ATTORNEYS.
/
CENTRAL / /
- _ _ 12 MILES
STATION-""
I R F ENERGY TRAVELS ONE BAUD/MILE FOR CLOCK RATE
i EQUAL TO 186,000 b/s
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 Approved For Release 2007/09/21 : CIA-RDP81-0012OR000100020007-8
Oct. 31, 1967 R. J. MCNAIR 3,350,644
SLIP CORRECTOR MEANS AND METHOD FOR MULTISTATION NETWORKS
Filed May 20, 1965 3 Sheets-Sheet 3
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INVENTOR.
RO
BERT J. McNAIR
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41 :=D-j
 Approved For Release 2007/09/21: CIA-RDP81-0012OR000100020007-8
644
United States Patent Office ...Patented ct 3131350,644
O.
,
2
capabilities thereof, reference is made to the following
description of the appended drawings, in which:
FIG. la is a block diagram, in functional form, of the
transmitting equipment included at each station in a wide-
band network of the type under consideration;
FIG. lb is a block diagram, in functional form, of the
receiving equipment at each such station;
FIG. 2 is a map of a multistation network including
a Control Central station and a plurality of other stations
hereinafter collectively referred to as "outlying stations;"
FIG. 3 is a series of block diagrams showing the time
lags between transmission of a given bit at the Control
3,350,644
SLIP CORRECTOR MEANS AND METHOD
FOR MULTISTATION NETWORKS
Robert John McNair, Cincinnati, Ohio, assignor to Avco
Corporation, Cincinnati, Ohio, a corporation of Dela-
ware
Filed May 20, 1965, Ser. No. 457,345
12 Claims. (Cl. 325-58)
ABSTRACT OF THE DISCLOSURE
In certain two-way communications systems involving
a multiplicity of stations, pseudo-random sequence gener.
ators are used to code mark-space symbols transmitted
by wide band radio frequency waves. The data transmitted
are encoded by multiplexing the information with the out-
put of a pseudo-random sequence generator. One such
generator is provided at each station for both transmission
and reception. Assuming the designation of a particular
station as Control Central, the prior art shows arrange-
ments for so synchronizing the various pseudo-random se-
quence generators so that the sequence provided at any
outlying station which is receiving will lag, in real time,
behind that generated and used for transmission at Con-
trol Central, by an amount proportional to signal-travel
time, i.e., the distance between the stations. However,
assuming such synchronization, systemwise, there is need
to make suitable adjustments to permit a given outlying
station to transmit back to Control Central. Two-way
communication between outlying stations complicates the
Central station and receptions of the same bit at the
several outlying stations; and
FIG. 4 is a schematic diagram, generally in block
form, of a slip corrector device in accordance with the
invention, this slip corrector having what is hereinafter
referred to as a "principal reference output" and a plu-
rality of "selected time advanced outputs" provided for
reasons hereinafter expalined. Each outlying station uses
the combination of a pseudo-random generator and a slip
corrector device in lieu of a simple pseudo-random se-
quence generator.
As indicated, there is shown in FIG. 2 a system of
several communications stations, each capable of trans-
mission and reception. It is important_ to note that at any
one station, both the transmitter and the receiver employ
a con Eon pseudo--random sec{uence generator; that is,
one such generator is provided at each station. Assuming
that station No. I is the Control Central station, one of
problem still more. The invention herein disclosed solves is to synchronize the outlying stations with the Control
the problem by synchronizing a pseudo-random sequence Central station in such manner that, when one of the out-
generator at Control Central with principal reference out- lying stations transmits back to the Control Central sta-
puts of pseudo-random generating means at each outly- 35 Lion, the received sequence at the Control Central station
ing station. Such generating means comprises a pseudo- is in precise step with the pseudo-random sequence gen-
random generator per se and a delay line providing a prin- erator at that Control Central station. Another and more
cipal reference- sequence. The delay line is tapped to pro- rigorous objective will be described following the dis-
vide access to any one of a plurality of time-advanced cussion of FIG. 3.
sequences. When it is desired to transmit back to Control 40 Since all of the outlying stations Nos. 2, 3 and 4 are
Central, a predetermined time-advanced sequence is em- displaced from the Control Central station No. 1, by
ployed. Other predetermined time sequences are used for various distances, the first of these desired objectives is
communication with other outlying stations. accomplished by providing a compensating selected time-
The present invention relates to slip correction in syn-
chronized radio communications networks technology and
is of particular utility in side band communications sys-
tems employing two-way communication among a plu-
rality of stations greater than two in number, specifically
those systems in which pseudo-random sequence genera-
tors are used to code Mark-Space symbols transmitted
via wide-band radio frequency waves.
The transmission of data over networks of the type
mentioned above requires that all stations in the network
have their pseudo-random sequence generating means
properly synchronized, the one to the other, so that each
continuously operates at the proper point in the code se-
quence.
The invention herein disclosed applies particularly to a
radio network wherein the data transmitted by the indi-
videal stations are encoded by multiplexing the informa-
tion with the output of pseudo-random sequence generat-
ing means. The invention maintains time synchronization
of all of the pseudo-random sequence generating means
used by the stations in the communications network.
The principal object of the present invention is to pro-
vide slip corrector means which makes such synchroniza-
tion practical.
For a better understanding of the present invention,
together with other and further objects, advantages and
advanced output ahead of the principal reference output
45 in each of the pseudo-random sequence generating means
at the outlying stations for use when such stations are
operating as a transmitter. The magnitude of the time ad-
vance of such sequence transmission at any outlying sta-
tion is a function of its distance from the Control Central
50 station. For example, at station No. 2 the magnitude of
the time advance of the pseudo-random sequence trans-
mitted to station No. 1 is 30 bits, for reasons discussed
below. When the transmission is from station No. 3 to
the Control Central station No. 1, that magnitude is 24
55 bits. When the transmission is from station No. 4 to the
Control Central station No. 1, the displacement is 15 bits.
The employment of such time advanced sequences per-
mits transmit-receive two-way communication between
Control Central -station No. 1 and any of the outlying
60 stations. Expressing these points more succinctly, the
specific time-advanced pseudo-random sequence used
for transmiting messages from station No. 2 to sta-
tion No. 1 will, on receipt, be in act step with the
sequenc oriainaliv venerated and iced 3-79
ion
63 aCdct~t im he times eadvanceZrsequences as re-
ceived from station Nos. 3 or 4 will likewise be in the
step with the sequence as generated at station No. 1,
notwithstanding the variance in the signal-travel distances
involved. The distance factor is compensated for by se-
70 lecting the magnitude of time advance at each outlying
station, depending on its distance from station No. I.
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The invention provides, at each of the outlying sta-
tions, a modified pseudo-random sequence generating
means which comprises not only a pseudo-random se-
quence generator but also a slip corrector in the form of
r"T*t- r e ay me nto which this sequence gen-
erator works. a ou put o said delay line constitutes
the principal reference output and the plurality of tapped
outputs at various time-advanced points along that delay
line constitute the selected time-advanced outputs from
which the time-advanced sequences are taken, as desired,
when transmitting. Each pulse coming from the pseudo-
random sequence generator proper into the delay line
causes all of the "Mark-Space" pulses in the delay line to
advance one stage, with polarity preserved. That is, the
pulse code stream coming out of the principal reference
output of the delay line at the outlying station will be
the same as the one which originally came from the
pseudo-random sequence generator proper at the outlying
station, but delayed in time by an amount equal to the
number of stages in the delay line multiplied by the clock
rate of said pulses. In other words, receivers at the out-
lying stations adjust their pseudo-random generating
means so that the principal reference output from each
delay line matches the incoming signal wave train from
the central control transmitter at station No. 1. That is,
when receiving at an outlying station, it is the sequence
taken from the reference output that is in step with the se-
quence received from station No. 1. When an outlying
station is transmitting to station No. 1, it is the time
advanced output of the slip corrector device that is
brought into precise synchronization with the sequence
generator at station No. 1.
With the pseudo-random generating means synchron-
ized, it is possible for any outlying station to transmit a
properly coded message back to the Control Central
station. The outlying station does this by tapping off the
pseudo-random code sequence at that stage in the shift
register delay line which represents the time required for
the R.F. energy to traverse both ways along the path
separating the two stations. This is possible since each
successive stage of the shift register prior to the final
represents a discrete advance in the time of occurrence
of a specific mark or space in the sequence.
In the following discussion a certain baud rate is as-
sumed for purposes of illustration and certain distances
between stations are arbitrarily specified, but it will be
understood that the invention is not limited to the pa-
rameters so selected.
The radio frequency energy traveling from one station
to another travels at a speed of approximately 186,000
miles/second or stated another way, 5.37 microseconds
are required for the R.F. signal to travel a statute mile.
For the station separation shown in FIG. 2, the delay
time between transmission and receipt of a message will
be as follows (computed to the nearest tenth microsec-
ond):
Between stations Nos.- Microseconds
1 and 2 ------------------------------- 80.4
1 and 3 -------------------------------- 64.5
1 and 4 ------------------------------- 40.2
2 and 3 ------------------------------- 48.3
2 and 4 ------------------------------- 120.6
3 and 4 ------------------------------- 99.3
Next, assume that the clock rate of the pseudo-random
sequence generator is 186,000 bits per second. This bit or
baud rate gives an on-time for each binary bit of 5.37
microseconds. Hence, as regards the signal radiated from
a transmitter, the leading edge of one of the binary bits
making up to the pseudo-random sequence will have
traveled just a mile when that baud period ends at the
transmitter.
For this signaling rate, the four-station network shown
in FIG. 2 will synchronize as follows when a message is
transmitted by station No. 1, the Control Central. The
principal reference output of the pseudo-random sequence
3,350,644
4
generating rpeans (FIG. 4) of station No. 2 will, by
means of correlation detection or some other equally
effective technique, be brought into synchronization ,vitl;
the data wave train as received from station No. 1. Com-
5 puted on the basis of absolute time, the principal refer-
ence output of the random sequence generating means
(FIG. 4) of station No. 2 will be providing a code se-
quence which is delayed 80.4 microseconds relative to
that of station No. 1 since it takers that long for the R.F.
energy to arrive. Taking into account the lengths of the
R.F. paths, the principal reference output of the sequence
generating means (FIG. 4) of station No. 3 will operate
64.5 microseconds behind the pseudo-random sequence
generator at station No. 1 while the principal reference
15 output of the sequence generating means (FIG. 4) at
station No. 4 will operate 40.2 microseconds behind that
of station No. 1.
FIG. 3 depicts the manner in which the outputs of the
sequence generator of station No. 1 and the principal
20 reference outputs of the pseudo.-random sequence gen-
erating systems of the other stations will synchronize for
the case where station No. 1 provides the transmitted
signal train with which the other stations are to be syn-
chronized. On the basis of absolute time the baud signal
25 trains at stations Nos. 2, 3 and 4 will be out of step with
the basic wave train of station No. 1 by amounts directly
proportional to the respective distances between the trans-
mitter station No. 1 and the outlying receiving stations.
The time lags between the first or basic code sequence
used for modulation at station No. 1 and the primary ref-
erence sequences used for demodulation at the other
other stations are as illustrated in FIG. 3.
The description of FIG. 3 shows the time conditions
which prevail when the principal reference outputs of the
pseudo-random sequence generating means of the out-
lying stations are synchronized onto signals received from
Control Central station No. 1, assuming a baud rate of
186,000 per second. In the top line of the figure there is
shown a code or first pseudo-random sequence as gen-
erated and transmitted by the Control Central station.
This discussion assumes that Control Central has just gen-
erated the random binary sequence "001101." The sec-
ond line shows that 15 bits later in time station No. 2
will receive the code 001101. The third line shows that
12 bits later in time station No. 3 receives the code. The
fourth line shows that 7.5 bits later in time, again with
respect to station No. 1, station No. -4 receives the code.
In each instance the respective time delay is due to travel
time of the R.F. signals. These are the conditions which
prevail when the Control Central, station is transmitting
and the outlying stations are receiving, the principal ref-
erence outputs of the pseudo-random generating means
(FIG. 4) being used for demodulation and being syn-
chronized with and appropriately time displaced with
respect to the output of pseudo-random sequence gen-
erator, used for modulation, at transmitting station No. 1.
The equipment provided according to the invention,
as used in each outlying station, and shown in FIG. 4
consists of pseudo-random sequence generator 10, a shift
register delay line 11, a series of gating circuits a-z and
aa-gg (one gate for each stage of the shift register), and
a suitable switching network (not shown). Operation is
as follows. The pulse output from the pseudo-random se-
quence generator along. line 12 consists of a train of
"mark-space" pulses. This train of pulses comprises the
input to the shift register delay line 11. As the end of
each pulse period is reached, all pulses stored in the reg-
ister are shifted to the right one interval. Thus, for the
example previously used, wherein the bit rate from the
pseudo-random generator is 186,000 bauds/second, shift
pulses occur each 5.37 microseconds. For ease of explana-
tion. assume that the shift register delay line has 33 stages
as shown in FIG. 4. This means that it would not be until
the thirty-fourth pulse interval (one+length of dc],1>'
register) that a specific "mark or space" rule = " ` ''- 1
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3,350,644
5
in the pseudo-random sequence generator 10 will ap- antenna 27. At the outlying station, such as station No. 2,
pear at the principal reference output 13 of the shift reg- the signal is intercepted by a receiving antenna, 18, suit-
ister delay line. Thus, the principal reference output of ably amplified and heterodyned down in frequency by
the shift register delay line is an exact duplicate of the network 19, and processed in an integrator and dump
output of the pseudo-random sequence generator 10 but 5 correlator 20 whose output is applied to a sampling and
delayed in time by an amount equal to the number of decision network 21.
stages in the register times the clock rate of the pulses. The pseudo-random sequence generating equipment at
For the example used this amounts to thirty-three stages the receiver is numbered 102 to indicate its relationship to
times 5.37 microseconds clock rate or 177.22 microsec- station No. 2 and FIG. 4 and its output is coupled, via
onds. Further, an identical time-advanced pseudo-random 10 line 13 and a balanced modulator 22 to the sampling and
sequence is available at any of stages Nos. 1-33 along decision network 21. Again the pseudo-random sequence
the shift register delay line. Progressing from right to left generating means 102* is clock-controlled. Now when sta-
along the delay line shown in FIG. 4, the code sequence- tion No. 2 is functioning as a transmitter, then the time-
available from a particular stage of the register progres- advanced outputs of the equipment illustrated in FIG. 4
sively advances as a function of baud time of occurrence. 15 are utilized, rather than the random sequence generator
Using "and" gates as shown in FIG. 4 specific points in principal reference output.
the pseudo-random code train can be selected and these It will be understood of course that there is only one
points will represent advances in time relative to the ref- random sequence generator at each station and switching
erence output. I.e., selected time-advanced sequences, for networks for encircuiting it as desired are well known to
transmitting, can be gated out of any of the "and" gates 20 those skilled in the invention. While the description of
a-z and aa-gg. FIG. 1 is on the footing that the transmitting equipment
Suppose now that it is desired to set up the four-station as at station No. 1 and the receiving equipment at station
communications network shown in FIG. 2. To establish No. 2, this being done to facilitate the description of oper-
initial synchronization of the network, station No. 1, the ation, it will be understood that it each station there are
Control Central, transmits a synchronizing burst of suffi- 25 both transmitting and receiving equipments functionally
cient length so that the principal reference outputs of the along the lines shown in FIGS. la and 1b, with a common
pseudo-random pulse sequence generating means in the pseudo-random sequence generator.
receiving equipment at stations Nos. 2, 3 and 4 can be With all stations in the FIG. 2 network now syn-
brought into synchronization with that of station No. 1. chronized to the pseudo-random sequence generator of
It is not the direct outputs of the pseudo-random sequence 30 station No. 1, the Control Central, two-day communica-
generators at stations Nos. 2, 3 and 4 which are brought tions can begin. It will be undertaken as follows. Sup-
into time synchronization with the received signal, but, pose station No. 2 transmits a message to station No. 1.
rather, in each instance it is the principal reference output Knowing the range to station No. 1 (15 miles for the ex-
pseudo-random sequence available at the output of the ample used) the selected pseudo-random code sequence
'shift register delay line which is used for demodulation ?5 used for modulation at station No. 2 is taken from the
and made to correlate with the received signal train. Thus, sift register delay line at that stage or gate which repre-
for the conditions previously assumed in the example, the sents a time advance in relation to the primary reference
pseudo-random sequence generating means at stations sequence equal to the round trip delay from station No. 1
Nos. 2, 3 and 4 will be so arranged as to make the prin- to station No. 2 and return. The reason for this is that
cipal reference outputs from their shift register delay lines 40 the primary reference sequence employed for demodula-
appear as shown in FIG. 3 when compared one to the tion at station No. 2 lags behind the sequence generator
other on an absolute time basis. at station No. 1 by signal travel time for a single trip. In
Describing FIG. 4 structurally, it comprises a pseudo- order to make a selected sequence, used for modulation at
random sequence generator proper 10, having a "Mark- station No. 2, "arrive at" station No. I in step with the
Space" pulse output line 12. In FIG. 4 there is also shown 45 sequence generated at station No, 1, the selected sequence
the 33 bit shift register delay line 11 having an input must lead the primary reference sequence by the amount
coupled to line 12 and also having a principal reference of this lag plus the time for a signal trip from station No.
output 13. It is the principal reference sequence or output 2 to station No. 1. For the example used, this round trip
at 13, at each outlying receiver, which is used for demodu- delay is 30 miles times 5.37 microsec./mile. Stage 30 in
lation and synchronized with the pseudo-random sequence 60 the shift register provides the required time advance. This
generator at the transmitter of the Control Central station point in the shift register 11 is available through "and"
No. 1 when station No. 1 is transmitting. In order to pro- gate (dd) (see FIG. 4) and station No. 2 data encoded
vide for the time advanced sequences utilized for modu- through multiplexing with the time-advanced pseudo-ran-
lation, in accordance with the invention, when transmis- dam sequence available at stage 30 in the shift register
sion is back to station No. 1, or, as will be shown, when 65 delay line will be in the proper time phase for immediate
it is between outlying stations, "and" gates designated a-z detection at station No. 1. Similarly, for the example used,
and aa-gg are individually provided at each stage of the station No. 3 would transmit to station No. 1 by tapping
delay line, so that the desired time advance is selected in its shift register delay line at stage 24 (see FIG. 4) which
accordance with range, as has been indicated. can be reached through "and" gate (x), and station No. 4
The particulars of the transmitter and receiver func- eo would transmit to station No. 1 by tapping off stage 15 of
tionally shown in FIGS. la and lb are not important here its shift register delay line reached via "and" gate (o). In
and the functional block diagrams there shown simply all cases then, tapping off the shift register delay line at
indicate a type of system equipment with which the slip that stage of the register which represents the time advance
corrector in accordance with the invention is of utility. A in the code sequence needed to account for the two-way
message or other intelligence to be communicated is furn- 65 travel time of the R.F. energy will properly synchronize
isised by a source of electronic data 14 and applied to the the data being sent back to the Control Central station
input of a modulo-2 adder 15 which also receives an input No. 1 from the outlying stations, i.e., the selected time-
from a pseudo-random sequence generator 101*, which advanced sequences are used.
is timed by a stable clock. The output of the modulo adder In the case of transmission from station No. 2 to sta-
is modulated onto the radio frequency wave output of a 70 tion No. 1 the time advance is equal to twice the radio
radio frequency amplifier 16, via a balanced modulator signal travel time between stations. Using a more general
17 and the composite signal is radiated out of the trans- expression, the time advance is in fact the sum of the sig-
mitter, for example, by station No. 1 transmitting by an nal travel time between stations and the time differential
between the sequences used for demodulation at the trans-
The 'ubscript figures simply indicate location at station
No. 1. station No. 2, etc. T6 ? See Pootaote, column.5.
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miffing and receiving stations when they are synchronized
wtih station No. 1. This general rule will be further dem-
onstrated below.
After initial synchronization of the network a problem
arises if there are more than two stations which transmit
data. For example, assume that stations Nos. 2 and 3 of
the network shown in FIG. 2 wish to carry on two-way
communications subsequent to synchronization as shown
in FIG. 3. When station No. 2 transmitter comes on, its
R.F. energy must travel 9 miles to station No. 3, taking
48.3 ms's. With a clcck rate of 186,000 b./s. this requires
9 baud times.
For the postulated situation, the receiving station No.
3 must theoretically slip the principal reference output of
its pseudo-random sequence generating means back 12
bits to be in synchronism with the principal reference
sequence of the transmitter at the station No. 2, because
the principal reference sequence at station No. 3 was
originally three baud times ahead of the principal refer-
ence sequence at station No. 2 sequence (for receiving).
Note that the theoretical requirement is for slippage at
the receiver of station No. 3 relative to station No. 2.
Assuming such slippage at the receiver of station No. 3,
now suppose that station No. 3 desires to re-transmit to
station No. 2. The principal reference sequence of the
pseudo-random sequence generating means of receiving
station No. 2 must then theoretically slip 6 bits behind
that of station No. 3 in order to synchronize with the
incoming message from station No. 3. Assuming slippage
at the receivers and two-way message transfer between
stations Nos. 2 and 3, both of these stations would soon
be out of readily acquired synchronization range so far
as any of the other stations in the network are concerned.
Now, in accordance with the invention, this problem
is solved by an arrangement (FIG. 4) in which the re-
quired theoretical slippage is accomplished at the trans-
mitters, not the receivers. It is accomplished by the re-
verse of slippage, i.e., by selected time advances at the
transmitters, and it is accomplished while maintaining the
principal reference outputs (i.e., on lines 13 at the out-
lying stations) in synchronism with the pseudo-random
sequence generator at the Control Central station No. 1.
Again assume that station No. 2 wishes to communi-
cate with station No. 3 (see FIG. 2). Selection of the
proper tap (FIG. 4) on the shift register delay line at
station No. 2 depends on knowledge of three things, range
of each station from Control Central and range from each
other. For the example shown in FIG. 2, these ranges are
known to be 15 miles. from station No. 1 to station No. 2,
12 miles from station No. 1 to station No. 3, and 9 miles
from station No. 2 to station No. 3. Computing R.F.
travel time over these ranges and knowing the clock
rate of the pseudo pulses shows that station No. 2 (see
FIGS. 3 and 4) should transmit to station No. 3 using
stage 12 of his shift register delay line which is reached
via "and" gate (I). The time advance at stage 12 is the
sum of 9 units and 3 units. The 9 units represent signal
travel time for one trip. The 3 units represent the. time
differential between the sequences used for demodulation
at the transmitting and receiving stations when they are
synchronized with station No. 1, i.e. 15 minus 12. Similar-
ly, station No. 3 should transmit to station No. 2 using
stage 6 of his shift register delay line which is reached
via "and" gate (f). The time advance at stage 6 is the
sum &f 9 units and minus 3 units. The minus 3 units rep-
resent 12 minus 15. This is a third application of the
general rule.
It was indicated above that when station No. 2 is trans-
mitting to station No. 3, the latter must theoretically slip
its principal reference sequence back by 12 bits but this
slippage is actually accomplished in accordance with the
invention by a time advance of the effective operation of
the pseudo-random sequence generating means at station
No. 2. The sequence selected for transmission being ad-
vanced by 12 bits in time at station No. 2, relative to the
3,350,644
ically slip its sequence generating means 6 bits behind sta-
tion No. 3. This requirement is met, in accordance with
the invention, by a time advance of the effective opera-
tion of the sequence generating means at station No. 3.
The selected sequence being advanced by 6 bits in time
at station No. 3, relative to the principal reference at sta-
tion No. 3, arrives at station No. 2 in step with the prin-
cipal reference pseudo-random sequence generated at sta-
tion No. 2.
8
principal reference sequence at station No. 2, arrives at
station No. 3 in step with the principal reference pseudo-
random sequence generated at station No. 3.
It was also indicated above that when station No. 3
5 then re-transmits to station No. 2? the latter must theoret-
15 In a similar fashion, it may be shown for the sample
network used here that station No. 4 would transmit to
station No. 3 using stage 14 on its shift register, whereas,
station No. 3 would transmit to station No. 4 using stage
23 of its shift register.
t10 Now assume that station No. 2 wishes to communicate
with station No. 4 (see FIG. 2). Since a line connecting
them passes through station No. 1, the Control Central,
the proper tap on their respective shift register delay lines
to use in communicating one with the other, assuming
25 both have been initially synchronized by the Control Cen-
tral, is for each to use the same tap used for transmitting
to station No. 1, that is, station No. 2 uses stage 30. 30
is the sum of 22.5 plus (15 minus 7.5). When the trans-
mission is from station No. 4, it uses stage 15. 15 is the
30 sum of 22.5 plus (7.5 minus 15).
From the foregoing it will be seen that the invention
functions in such manner as to cause:
(1) The principal reference sequence outputs of any
and all pseudo-random sequence generating means at the
35 outlying stations to be synchronized with the pseudo-
random sequence generator at the Control Central sta-
tion, when the outlying stations are receiving, in such
manner as to time-lag behind the first or basic sequence
at station No. 1 as a function of their respective distances
40 from station No. 1.
(2) The selected time advanced outputs of each of
the pseudo-random sequence generators at the outlying
stations, when transmitting to station No. 1, to be in step
as received with the pseudo-random sequence generator
45 at Control Central station No. 1, the time advances being
a function of double the respective distances between the
outlying stations and Control Central station No. 1.
(3) The selected time advanced output of any of the
pseudo-random sequence generating means at any trans-
50 mitting outlying station to be displaced in time in such
manner that the pseudo sequence received at another out-
lying station from such transmitting station is in step with
the principal pseudo-random sequence of the outlying sta-
tion which is receiving, the displacement in that event
55 being a function of the distance between the transmitting
and receiving stations plus (if the transmitting station is
farther from station No. 1 than the receiving station) or
minus (if the transmitting station is closer to station No.
1 than the receiving station) the lag already introduced
80 between the primary reference sequences at transmitting
and receiving stations by reason of the synchronization
with the Control Central station.
Therefore it will be seen that what the invention does
at any outlying station is to advance the transmitted se-
65 quence by an amount which is functionally related to two
variables:
(1) The length of signal travel between the two sta-
tions involved, and
(2) The algebraic difference 'between their relative
70 distances with respect to the Control Central station.
With the network operating in this way all data come
to its addressee in proper phase so that continuous time
synchronization of all pseudo-random sequence generat-
ing means is maintained. The only criterion is that 1111:
75 range from one station to the other is known 10 an
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3,350,644
10
9
curacy equatable to the clock rate of the pseudo-random
sequence generator. For the example discussed the range
accuracy requirement is 0.5 mile but this is not considered
restrictive.
In the operational environment, range from one site
to another can be measured with this same code slip
corrector. Ranging is achieved as follows. One of the
stations acts as master, sending out a synchronizing burst,
utilizing the pseudo-random sequence present at the prin-
cipal reference output of his code slip corrector.
The second station synchronizes its reference output
pseudo-random sequence with the received wave train. He
then transmits a like synchronizing burst back to the
master station using an encoding sequence which is time
locked to the pseudo-random sequence present at the
principal reference output of his shift register delay line.
The operator of the master station without changing his
sequence generating means has only to sample the various
taps on his shift register delay line until he finds a time-
advanced sequence such that correlation is accomplished.
The specific tap giving correlation provides immediate
information as to the range between stations. Transmis-
sion of this information back over a radio link allows
the second station to properly set its transmitting tap.
Other ranging means may be used. For example, if the
station sites are fixed, surveyed range data are probably
available. If the network stations are mobile or com-
prised of both fixed and mobile transceivers, it may be
that data on present position are available from an in-
ertial platform or other navigational aid.
The illustrative system here described to better explain
the operation of the code slip corrector, is not intended
to be restrictive. The code slip corrector is useful for
maintaining time synchronization of all of the pseudo-
random code generating means used in a radio network
having any one of a wide variety of configurations. The
clock rate of the generator and the maximum range ex-
pected between stations will determine the parameters
of the shift register delay line and its associated gating
circuits for a specific implementation. Further, data com-
puters used for processing the station data may be pro-
grammed to automatically select, update, and switch in
the proper tap on the shift register delay line whenever
a particular station of the network is being addressed.
Finally, it should be mentioned that no one station has
f to be the master station. Rather, for a random access
network, any station can initiate a message and temporar-
ily be the Control Central.
It will be understood that the two stable clocks illus-
trated in FIG. 1, for example, are locked in by synchro-
nizing data provided by means known to the art and not
shown herein. In the absence of lock-in, the clock at each
receiver runs faster than the clock at the master trans-
mitter at Control Central. This disclosure postulates syn-
chronization of the clocks and provides novel means for
maintain system synchronization with respect to the
generation of the pseudo-random sequences.
While the above discussion postulates that there is a
simple pseudo-random sequence generator at the Control
Central station No. 1, which generator has a principal
reference output only, it is within the purview of the in-
vention to provide at station No. 1 a pseudo-random se-
quence generator and a shift register slip corrector in ac-
coy,dance with the invention, thus permitting synchronized
reception at station No. 1 from one of the outlying sta-
tions, should such outlying station be selected as the
master station.
This application contains subject matter disclosed but
not claimed herein. This subject matter is claimed in
United States patent application, Ser. No. 656,750 filed
July 28, 1967 in the name of Robert J. 1MicNair, entitled
"Slip Corrector Means and Method for Synchronization
of Pseudo-Random Generating Means in Multistation
Networks" and assigned to Avco Corporation, the as-
signee of the present application and invention.
While .there has been shown and described what is at
present considered to be a preferred embodiment of the
invention, it will be understood by those skilled in the
art that various changes and modifications may be made
therein without departing from the scope of the invention
as described in the appended claims.
Having fully described the invention, I claim:
1. The method of maintaining synchronism between
pseudo-random sequences at first and second transmitter-
receiver stations of a wide band radio network which
comprises the following steps:
generating a first pseudo-random sequence at the first
station;
generating a second pseudo-random sequence at the
15 second station;
delaying the second pseudo-:random sequence to pro-
vide a principal reference sequence, which is de-
layed by a predetermined amount with reference
to the second pseudo-random sequence, and several
selectable sequences which are delayed by inter-
mediate amounts with reference to the second
pseudo-random sequence and time-advanced relative
to the principal reference sequence;
transmitting signals modulated by said first pseudo-
random sequence from the first station to the second
station;
receiving said signals at the second station and demod-
sequence;
synchronizing the principal reference sequence with the
first pseudo-random sequence in such a manner that
the principal reference sequence lags in time behind
the first pseudo-random sequence by an amount
tions;
transmitting signals modulated by a selected one of the
time-advanced sequences from the second station to
the first station; and
receiving said signals at the first station and demodu-
lating them by said first pseudo-random sequency;
the selected time-advanced sequence being time ad-
vanced relative to the principal reference sequence
so as to be synchronized at the first station with the
first sequence there generated, the magnitude of the
time advance being proportioned to double said
distance, and equal to the amount of said lag plus
the time of signal travel between said stations.
2. The method of maintaining synchronism between
pseudo-random sequences at second and third trans-
mitter-receiver stations of a wide band radio network in-
cluding a first master transmitter-receiver station at which
a first pseudo-random sequence is generated, which com-
prises the following steps:
generating a second pseudo-random sequence at the
55 second station;
generating a third pseudo-random sequence at the third
station;
delaying the second and third pseudo-random se-
quences to provide a second-station principal refer-
ence sequence and a third-station principal refer-
ence sequence, and further to provide secondary
time-advanced sequences 'which are delayed by in-
termediate amounts with reference to the second
pseudo-random sequence and time advanced with
reference to the second-station principal reference
sequence;
synchronizing the second and third stations with the
first, there being time lags between the first pseudo-
random sequence and the second-station principal
reference sequence and between the first pseudo-
random sequence and the third-station principal
reference sequence, the magnitudes of the time lags
being dependent on the respective distances of the
second and third stations, respectively, from the first
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selecting at the second station that one of the secondary
time-advanced sequences which is time advanced
relative to the second-station principal. reference se-
quence by an amount equal to the algebraic sum of
travel time between the second and third stations
and the differential between said time lags, the differ-
ential being equal to the time lag characterizing the
second station minus the time lag characterizing the
third station,
transmitting from the second station to the third sta-
tion signals modulated by the selected secondary
time-advanced sequence, and
receiving said signals at the third station and demodu-
lating them by utilizing the third-station principal
reference sequence.
3. In a wide band radio transmission system, the com-
bination of:
a first transmitter-receiver station having a first pseudo-
random sequence generator;
a second transmitter-receiver station having a second
pseudo-random sequence generator;
a tapped delay line coupled to the second pseudo-
random sequence generator for providing a principal
reference sequence, delayed by a predetermined
amount relative to the second pseudo-random se-
quence, and a plurality of selectable time-advanced
sequences which are delayed by intermediate
amounts with respect to the second pseudo-random
sequence and time-advanced with reference to the
principal reference sequence;
the distance between the stations introducing a time lag
between the first pseudo-random sequence and the
principal reference sequence when the second station
is synchronized to the first;
means modulated by a selected time-advanced sequence
for transmitting signals from the second station to
the first station;
and means demodulated by the first pseudo-random
sequence for receiving signals at the first station;
the selected time-advanced sequence being in advance
of the principal reference sequence by an amount
equal to twice said time lag, which amount is equal
to said lag plus the time of signal travel between
said stations.
4. In a wide band radio data transmission system in
which coded mark-space symbols are transmitted over
wide band radio frequency waves, the combination in ac-
cordance with claim 3 in which the delay line for provid-
ing the principal reference sequence and the time-ad-
vanced sequences is of the clocked shift register delay
type, and comprises a plurality of stages with time-ad-
vanced outputs, said line further having a principal refer-
ence output and an input to which the output of the sec-
ond pseudo-random sequence generator is applied, there-
by providing selectable time-advanced sequences at said
stages.
5. The combination in accordance with claim 4 in
which the means for selecting the desired time advance
comprises a plurality of gate circuits individually coupled
to said stages.
6. In a wide band radio transmission system of the type
which includes a control central transmitter-receiving
station having a first pseudo-random sequence generator,
the combination of:
a second transmitter-receiver station having a second-
station pseudo-random sequence generator,
a third transmitter-receiver station having a third-sta-
tion pseudo-random sequence generator,
a tapped delay line coupled to the second-station
pseudo-random sequence generator for providing a
principal reference sequence, delayed by a prede-
termined amount relative to the second-station
pseudo random sequence, and a plurality of selecta-
ble time-advanced sequence, which are delayed by
intermediate amounts with respect to the second-
3,350,644
station 'pseudo-random sequence and time advanced
with reference to said principal reference sequence,
another tapped delay line coupled to the third-station
pseudo-random sequence generator for providing
5 another principal reference sequence, delayed by a
predetermined amount relative to the third-station
pseudo-randontsequence,
the principal reference sequences for the second and
third stations being synchronized with the first
pseudo-randontsequence and lagging therebehind by
time lags equal to signal-travel time over the respec-
tive distances from the second and third stations to
the control central station,
means modulated by a selected time advanced sequence
15 for transmitting signals from the second station to
the third, and
means demodulated by the third-station principal ref-
erence sequence for receiving signals at the third
station,
20 the selected time advanced sequence being in advance
of the principal reference sequence at the second
station by an amount equal to the algebraic sum of
the travel time between the second and third stations
and the differential between said time lags, said dif-
ferential being equal to the time lag characterizing
the second station means and the time lag charac-
terizing the third station.
7. The method of maintaining synchronism between
code sequences at first and second transmitter-receiver
30 stations of a wide band radio network which comprises
the following steps:
generating a first code sequence at the first station;
generating a second code sequence at the second sta-
tion;
delaying the second code sequence to provide a princi-
pal reference sequence,. which is delayed by a pre-
determined amount with reference to the second code
sequence, and several selectable sequences which are
delayed by intermediate amounts with reference to
the second code sequence and time-advanced rela-
tive to the principal reference sequence;
transmitting signals modulated by said first code se-
quence from the first station to the second station;
receiving said signals at the second station and demodu-
lating them by utilizing said principal reference se-
quence;
first code sequence in such a manner that the prin-
cipal reference sequence lags in time behind the first
code sequence by an amount which is dependent on
the distance between the stations;
transmitting signals modulated by a selected one of
the time-advanced sequences from the second station
to the first station; and
receiving said signals at the first station and demodu-
lating them by said first code sequence;
the selected time-advanced sequence being time ad-
vanced relative to the principal reference sequence
so as to be synchronized at the first station with the
first sequence there generated, the magnitude of the
time advance being proportioned to double said
distance and equal the amount of said lag plus the
time of signal travel between said stations.
8. The method of maintaining synchronism between
code sequences at second and third transmitter-receiver
stations of a wide band radio network including a first
master transmitter-receiver station at which a first code
sequence is generated, which comprises the following
steps:
generating a second code sequence at the second sta-
tion;
generating a third code sequence at the third station;
delaying the second and third code sequences to pro-
vide a second-station principal reference sequence
and a third-station principal reference sequence, :,n.l
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further to provide secondary time-advanced sequen-
ces which are delayed by intermediate amounts with
reference to the second pseudo-random sequence,
and time-advanced with reference to the second-sta-
tion principal reference sequence;
synchronizing the second and third stations with the
first, there being time lags between the first code
sequence and the second-station principal reference
sequence and between the first code sequence and the
third-station principal reference sequence, the magni-
tudes of the time lags being dependent on the re-
spective distances of the second and third stations,
respectively, from the first station;
selecting at the second station that one of the secondary
time-advanced sequences which is time advanced rela-
tive to the second-station principal reference sequence
by an amount equal to the algebraic sum of travel
time between the second and third stations and the
differential between said time lags, the differential
being equal to the time lag characterizing the second
station minus the time lag characterizing the third
station;
transmitting from the second station to the third sta-
tion signals modulated by the selected secondary
time-advanced sequence; and
receiving said signals at the third station and demodu-
lating them by utilizing the third-station principal
reference sequence.
9. In a wide band transmission system, the combina-
tion of:
a first transmitter-receiver station having a first code
sequence generator;
a second transmitter-receiver station having a second
code sequence generator;
a tapped delay line coupled to the second code sequence
generator for providing a principal reference se-
quence, delayed by a predetermined amount rela-
tive to the second pseudo-random sequence, and a
plurality of selectable time-advanced sequences which
3,350,644
are delayed by intermediate amounts with respect 40
to the second code sequence and time-advanced with
reference to the principal reference sequence;
the distance between the stations introducing a time
lag between the first code sequence and the principal
reference, sequence when the second station is syn-
chronized to the first;
means modulated by a selected time-advanced sequence
for transmitting signals from the second station to
the first station;
and means demodulated by the first code sequence for
receiving signals at the first station;
the selected time-advanced sequence being in advance
of the principal reference sequence by an amount
equal to twice said time lag, which amount is equal
to said lag plus the time of signal travel between said 55
stations. / y
10. In a wide band radio data transmission system in
which coded mark-space symbols are transmitted over
wide band radio frequency waves, the combination in
accordance with claim 9 in which the delay line for pro-
viding the principal reference sequence and the time-ad-
vanced sequences is of the clocked shift register delay
type, and comprises a plurality of stages with time-ad-
vanced outputs, said line further having a principal ref-
14
erence output and an input to which the output of the
second -code sequence generator is applied, thereby pro-
viding selectable time-advanced sequences at said stages.
11. The combination in accordance with claim to in
which the means for selecting the desired time advance
comprises a plurality of gate circuits individually coupled
to said stages.
12. In a wide band radio transmission system of the
type which includes a controL central transmitter-receiv-
ing station having a first code sequence generator, the
combination of:
a second transmitter-receiver station having a second-
station code sequence generator,
a third transmitter-receiver station having a third-sta-
tion code sequence generator,
a tapped delay line coupled to the second-station code
sequence generator for providing a principal refer-
ence sequence, delayed by a predetermined amount
relative to the second-station code sequence, and a
plurality of selectable time-advanced sequences which
are delayed by intermediate amounts with respect
to the second-station code sequence and time-ad-
vanced with reference to said principal reference
sequence;
another tapped delay line coupled to the third-station
code sequence generator for providing another prin-
cipal reference sequence, delayed by a predetermined
amount relative to the third-station code sequence,
the principal reference sequences for the second and
third stations being synchronized with the first code
sequence and lagging therebehind by time lags equal
to signal-travel time over the respective distances
from the second and third stations to the control
central station,
means modulated by a selected time advanced sequence
for transmitting signals from the second station to
the third, and
means demodulated by the third-station principal ref-
ence sequence for receiving signals at the third sta-
tion,
the selected time advanced sequence being in advance
of the principal reference sequence at the second sta-
tion by an amount equal to the algebraic sum of the
travel time between the second and third stations and
the differential between said time lags, said differen-
tial being equal to the time lag characterizing the
second station minus the time lag characterizing the
third station.
2,401,406
2,471,416
2,982,853
3,128,465
3,167,770
3,244,808
3,262,111
3,308,431
References Cited
UNITED STATES PATENTS
6/1946 Bedford et al. ------ 325-32 X
5/1949 Deloraine et al. __-___ 343-179
5/1961 Price et al. -------- 325-65 X
4/1964 Brilliant ------------ 343-225
1/1965 Doherty et al. ____ 178-69.5 X
5/1966 Roberts ---------- 325-42 X
7/1966 Grahin ------------- 343-7.5
3/1967 Hopner et al. ------- 340-147
ROBERT L. GRIFFIN, Primary Examiner.
JOHN W. CALDWELL, Examiner.
B. V. SAFOUREK, Assistant Examiner.
3~ ) (Ql ` l ~~ T-Zr~'d iti~ w7g/
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