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Claims  |
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We claim:
1. A method of producing a code with enhanced acquisition security,
comprising the steps of:
generating a plurality of linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n ;
combining said linear component codes to form a linear first composite code
in accordance with a first composite rule of such a character that said
first composite code does not correlate with said component codes;
nonlinearizing said first composite code to form a nonlinear second
composite code; and
combining said linear component codes, C.sub.1, C.sub.2, . . . C.sub.n-1,
with said nonlinear second composite code to form a nonlinear acquisition
composite code in accordance with a second composite rule of such a
character that at least one of said component codes correlates with said
acquisition composite code.
2. A method of producing a code with enhanced acquisition security,
comprising the steps of:
generating a plurality of linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n ;
combining said linear component codes to form a linear first composite code
in accordance with a first composition rule of such a character that said
first composite code does not correlate with said component codes;
nonlinearizing said first composite code to form a nonlinear second
composite code;
time delaying said component codes, C.sub.1, C.sub.2, . . . C.sub.n-1 ; and
combining said delayed codes, C.sub.1, C.sub.2, . . . C.sub.n-1, with said
nonlinear second composite code to form a nonlinear acquisition composite
code in accordance with a second composition rule of such a character that
at least one of said delayed codes correlates with said acquisition
composite code.
3. A method of producing a code with enhanced acquisition security,
comprising the steps of:
generating a plurality of linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n ;
combining said linear component codes in accordance with a modulo-2
addition rule to form a linear first composite code;
nonlinearizing said first composite code by applying the same to an
encrypter operating in a decrypt mode to form a nonlinear second composite
code;
time delaying said component codes, C.sub.1, C.sub.2, . . . C.sub.n-1 ; and
combining said time delayed codes, C.sub.1, C.sub.2, . . . C.sub.n-1, with
said nonlinear second composite code in accordance with a Boolean majority
voting rule to form a nonlinear acquisition composite code.
4. A system for producing a code with enhanced acquisition security,
comprising:
means for generating a plurality of linear component codes, C.sub.1,
C.sub.2, . . . C.sub.n ;
means for combining said linear component codes to form a linear first
composite code in accordance with a first composition rule of such a
character that said first composite code does not correlate with said
component codes;
means for nonlinearizing said first composite code to form a nonlinear
second composite code; and
means for combining said linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n-1, with said nonlinear second composite code to form a nonlinear
acquisition composite code in accordance with a second composite rule of
such a character that at least one of said component codes correlates with
said acquisition composite code.
5. A system for producing a code with enhanced acquisition security,
comprising:
means for generating a plurality of linear component codes, C.sub.1,
C.sub.2, . . . C.sub.n ;
means for combining said linear component codes to form a linear first
composite code in accordance with a first composition rule of such a
character that said first composite code does not correlate with said
component codes;
means for nonlinearizing said first composite code to form a nonlinear
second composite code;
means for time delaying said component codes, C.sub.1, C.sub.2, . . .
C.sub.n-1 ; and
means for combining said delayed codes, C.sub.1, C.sub.2, . . . C.sub.n-1,
with said nonlinear second composite code to form a nonlinear acquisition
composite code in accordance with a second composition rule of such a
character that at least one of said delayed codes correlates with said
acquisition composite code.
6. The system for producing a code as recited in claim 5, wherein:
said first composition rule is a modulo-2 addition rule;
said nonlinearizing means comprises an encrypter operating in a decrypt
mode to which said first composite code is applied to form said nonlinear
second composite code; and
said second composition rule is a Boolean majority voting rule.
7. A coding method for communication between a transmitter and a receiver
with enhanced security, comprising the steps of:
(I) at the transmitter,
(a) generating a plurality of linear acquisition component codes, C.sub.1,
C.sub.2, . . . C.sub.n,
(b) combining said acquisition component codes to form a linear first
composite code in accordance with a first composition rule of such a
character that said first composite code does not correlate with said
component codes,
(c) nonlinearizing said first composite code to form a nonlinear second
composite code,
(d) combining said linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n-1, with said nonlinear second composite code to form a nonlinear
acquisition composite code in accordance with a second composition rule of
such a character that at least one of said component codes correlates with
said acquisition composite code, and
(e) transmitting said nonlinear acquisition composite code; and
(II) at the receiver,
(a) receiving said nonlinear acquisition composite code,
(b) generating a plurality of linear reference component codes, R.sub.1,
R.sub.2, . . . R.sub.n, that correlate respectively with said linear
acquisition component codes, C.sub.1, C.sub.2, . . . C.sub.n,
(c) combining said reference component codes to form a linear third
composite code in accordance with said first composition rule,
(d) nonlinearizing said third composite code in a manner identical to that
of said (I,c) nonlinearizing step to form a nonlinear fourth composite
code,
(e) correlating said reference component codes, R.sub.1, R.sub.2, . . .
R.sub.n-1, with said nonlinear acquisition composite code by shifting the
phases of said reference component codes, R.sub.1, R.sub.2, . . .
R.sub.n-1, and
(f) correlating said nonlinear fourth composite code with said nonlinear
acquisition composite code by shifting the phase of the remaining
reference component code, R.sub.n.
8. The coding method as recited in claim 7, wherein:
said first composition rule is a modulo-2 addition rule; and
said second composition rule is a Boolean majority voting rule.
9. The coding method as recited in claim 7, wherein:
said (I,c) nonlinearizing step comprises applying said linear first
composite code to an encrypter operating in a decrypt mode to form said
nonlinear second composite code; and
said (II,d) nonlinearizing step comprises applying said linear third
composite code to an encrypter operating in a decrypt mode to form said
nonlinear fourth composite code.
10. The coding method as recited in claim 7, wherein:
said (I,d) combining step comprises time delaying said acquisition
component codes, C.sub.1, C.sub.2, . . . C.sub.n-1, and combining said
delayed codes with said nonlinear second composite code to form said
nonlinear acquisition composite code in accordance with said second
composition rule; and
said (II,e) correlating step comprises time delaying said reference
component codes, R.sub.1, R.sub.2, . . . R.sub.n-1, by the same amount
that said acquisition component codes, C.sub.1, C.sub.2, . . . C.sub.n-1,
are time delayed and correlating said delayed reference component codes
with said nonlinear acquisition composite code.
11. The coding method as recited in claim 7, further comprising the steps
of:
(II) at the receiver,
(g) combining said linear reference component codes, R.sub.1, R.sub.2, . .
. R.sub.n-1, with said nonlinear fourth composite code in accordance with
said first composition rule to form a nonlinear fifth reference composite
code, and
(h) correlating said nonlinear fifth composite code with said nonlinear
acquisition composite code;
(I) at the transmitter,
(f) combining said linear acquisition component codes, C.sub.1, C.sub.2, .
. . C.sub.n-1, with said nonlinear second composite code in accordance
with said first composition rule to form a nonlinear data-carrying
composite code, and
(g) transmitting said nonlinear data-carrying composite code; and
(II) at the receiver,
(i) receiving said nonlinear data-carrying composite code, and
(j) correlating said nonlinear fifth reference composite code with said
nonlinear data-carrying composite code.
12. The coding method as recited in claim 11, wherein:
said first composition rule is a modulo-2 addition rule; and
said second composition rule is a Boolean majority voting rule. |
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Claims |
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Description |
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CROSS REFERENCE TO RELATED APPLICATION
Reference is hereby made to the following co-pending U.S. application
dealing with related subject matter and assigned to the assignee of the
present invention: "A System for the Secure and Rapid Acquisition of
Composite Code Signals" by Earl M. Kartchner et al, U.S. Ser. No. 65,040,
filed Aug. 19, 1970.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to spread spectrum communication
systems and, more particularly, is concerned with a method and system for
nonlinearizing spread spectrum codes for enhanced system security while
still maintaining rapid acquisition thereof.
2. Description of the Prior Art
The present invention constitutes an improvement upon the system described
and illustrated in the above cross-referenced U.S. patent application.
The system of the referenced application utilizes, by way of example, a
plurality of linear PN component codes, C.sub.1, C.sub.2, . . . C.sub.n,
which are all relatively prime with respect to each other, have an
approximately equal number of binary ONES and ZEROES, and, with respect to
linear MAJ and MOD composites thereof, possess the following correlation
properties:
(1) C.sub.1, C.sub.2, . . . and C.sub.n each correlates with MAJ (C.sub.1,
C.sub.2, . . . C.sub.n) fifty percent of the time.
(2) C.sub.1, C.sub.2, . . . and C.sub.n each does not correlate with MOD
(C.sub.1, C.sub.2, . . . C.sub.n).
(3) MAJ (C.sub.1, C.sub.2, . . . C.sub.n) correlates with MOD (C.sub.1,
C.sub.2, . . . C.sub.n) fifty percent of the time.
MAJ (C.sub.1, C.sub.2, . . . C.sub.n) equals a Boolean majority vote of
C.sub.1, C.sub.2, . . . and C.sub.n.
MOD (C.sub.1, C.sub.2, . . . C.sub.n) equals a modulo-2 addition of
C.sub.1, C.sub.2, . . . and C.sub.n.
The chronological sequence of events carried out for achieving rapid code
acquisition by the prior art system may be summarized as follows:
(1) Linear component codes, C.sub.1, C.sub.2, . . . C.sub.n, are generated.
(2) C.sub.1, C.sub.2, . . . C.sub.n are combined in accordance with the
Boolean majority voting rule to form a linear acquisition composite code,
MAJ (C.sub.1, C.sub.2, . . . C.sub.n).
(3) The acquisition composite code is transmitted.
(4) Upon receipt of the acquisition composite code, linear reference
component codes, R.sub.1, R.sub.2, . . . R.sub.n which correlate
respectively with C.sub.1, C.sub.2, . . . C.sub.n, are generated.
(5) First, R.sub.1 is correlated with MAJ (C.sub.1, C.sub.2, . . .
C.sub.n); then R.sub.2 is correlated with MAJ (C.sub.1, C.sub.2, . . .
C.sub.n ; finally, R.sub.n is correlated with MAJ (C.sub.1, C.sub.2, . . .
C.sub.n).
For the transmission and receipt of data, the following chronological
sequence of events is carried out by the prior art system:
(1) Linear reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, are
combined in accordance with the modulo-2 addition rule to form a linear
reference composite code, MOD (R.sub.1, R.sub.2, . . . R.sub.n).
(2) MOD (R.sub.1, R.sub.2, . . . R.sub.n) is then correlated with
acquisition composite code MAJ (C.sub.1, C.sub.2, . . . C.sub.n).
(3) Linear composite codes, C.sub.1, C.sub.2, . . . C.sub.n, are then
combined in accordance with the modulo-2 addition rule to form a linear
data-carrying composite code, MOD (C.sub.1, C.sub.2, . . . C.sub.n).
(4) The data-carrying composite code is then transmitted, instead of the
acquisition composite code.
(5) At the receiver, MOD (R.sub.1, R.sub.2, . . . R.sub.n) now correlates
with MOD (C.sub.1, C.sub.2, . . . C.sub.n).
The above-described sequence of events implies that the total number of
code bits required to be searched for acquisition of the transmitted
composite code is equal to the sum of the individual lengths of the
component codes which form the composite code, rather than the product of
their lengths. Consequently, it is readily appreciated that acquisition
under the prior art system is rapid, thereby leaving little time for an
intelligent jammer to analyze the linear composite code MAJ (C.sub.1,
C.sub.2, . . . C.sub.n), which is transmitted for acquisition, in order to
determine component codes, C.sub.1, C.sub.2, . . . C.sub.n.
Also, the jammer must have knowledge of all of the component codes and
their phase relationship with respect to each other in order to jam MOD
(C.sub.1, C.sub.2, . . . C.sub.n) which is used for data transmission,
since none of the component codes correlate with MOD (C.sub.1, C.sub.2, .
. . C.sub.n).
However, under field conditions where the jammer is capable of intercepting
the transmission of MOD (C.sub.1, C.sub.2, . . . C.sub.n), the latter is
vulnerable to discovery through analysis by the jammer since it is a
linear sequence. By using a computer to perform well known mathmatical
calculations at high speed, the polynomial equation which mathematically
represents the intercepted linear composite code can be determined and a
replica thereof constructed. Therefore, while the overall sequence of
events carried out by the prior art system increases the difficulty of
code analysis by an enemy, it does not preclude such analysis under
certain field conditions in view of the fact that the component codes and
the composites thereof being utilized by the prior art system are all
linear sequences.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, certain
important modifications are made to the prior art system which result in
an improved system being essentially invulnerable to enemy analysis. The
above-summarized correlation properties of the prior art system and its
concomitant rapid acquisition capability are maintained in the improved
system of the present invention, while the likelihood of successful
analysis by an enemy of the codes produced by the improved system is made
infinitesimally small.
The same basic component codes, C.sub.1, C.sub.2, . . . C.sub.n and
R.sub.1, R.sub.2, . . . R.sub.n, as utilized in the prior art system, are
utilized by the improved system during the initial steps in the production
of composite codes and in the later acquisition thereof, respectively.
However, means in the form of an encrypter running in the decrypt mode is
incorporated by the improved system in a manner which insures that the MAJ
and MOD composite codes produced are both nonlinear, have the appearance
of being of infinite length, and, consequently, are essentially
nonanalyzable. Furthermore, the component code, C.sub.n, is not
transmitted as a correlatable component of the MAJ acquisition composite
code; therefore, the sequence of component codes, C.sub.1, C.sub.2, . . .
C.sub.n, which drives the encrypter cannot be determined by analysis of
the MAJ composite code even if component codes, C.sub.1, C.sub.2, . . .
C.sub.n-1, become known by an enemy. Still further, the sequence of
component codes, C.sub.1, C.sub.2, . . . C.sub. n-1, which goes into the
formation of the MAJ and MOD composite codes is time delayed relative to
the sequence thereof which is utilized in the driving of the encrypter in
the decrypt mode. This delay time is nonanalyzable by an enemy.
Accordingly, the present invention broadly relates to a method of producing
a code with enhanced acquisition security, and a system incorporating
means for producing the same, wherein the steps carried out comprise: (a)
generating a plurality of linear component codes, C.sub.1, C.sub.2, . . .
C.sub.n ; (b) combining the linear component codes to form a linear first
composite code in accordance with a first composite rule of such a
character that the first composite code does not correlate with the
component codes; (c) nonlinearizing the first composite code to form a
nonlinear second composite code; and (d) combining the linear component
codes, C.sub.1, C.sub.2, . . . C.sub.n-1, with the nonlinear second
composite code to form a nonlinear acquisition composite code in
accordance with a second composition rule of such a character that at
least one of the component codes correlates with the acquisition composite
code. The first composition rule is a modulo-2 addition rule and the
second composition rule is a Boolean majority voting rule.
More particularly, the nonlinearizing step comprises applying the linear
first composite code to an encrypter operating in a decrypt mode to form
the nonlinear second composite code. Further, the (d) combining step
comprises time delaying component codes, C.sub.1, C.sub.2, . . .
C.sub.n-1, and then combining the delayed codes with the nonlinear second
composite code to form the nonlinear acquisition composite code in
accordance with the Boolean majority voting rule.
The present invention also relates to a coding method for communication
between a transmitter and a receiver with enhanced security. At the
transmitter, the above-outlined (a) through (d) steps are performed for
producing the nonlinear acquisition composite code, followed by (e)
transmitting the acquisition code. At the receiver, the following steps
are performed for acquiring the acquisition code: (a) receiving the
nonlinear acquisition composite code; (b) generating a plurality of linear
reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, that correlate
respectively with the acquisition component codes, C.sub.1, C.sub.2, . . .
C.sub.n ; (c) combining the reference component codes to form a linear
third composite code in accordance with the first composition rule; (d)
nonlinearizing in a manner identical to the same step performed at the
transmitter the third composite code to form a nonlinear fourth composite
code; (e) correlating the reference component codes, R.sub.1, R.sub.2, . .
. R.sub.n-1, with the nonlinear acquisition composite code by shifting the
phases of the component codes; and (f) correlating the nonlinear fourth
composite code with the nonlinear acquisition code by shifting the phase
of the remaining component code, R.sub.n. The (e) correlating step
comprises time delaying the reference components, R.sub.1, R.sub.2, . . .
R.sub.n-1, by the same amount that C.sub.1, C.sub.2, . . . C.sub.n-1 were
time delayed at the transmitter and correlating the time delayed codes
with the nonlinear acquisition composite code by shifting the phases of
the time delayed codes, R.sub.1, R.sub.2, . . . R.sub.n-1.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block schematic diagram of the transmitting portion of the
prior art data communication system.
FIG. 2 is a block schematic diagram of the improvement provided by the
present invention to the transmitting portion of the prior art system of
FIG. 1.
FIG. 3 is a block schematic diagram of the receiving portion of the prior
art data communication system.
FIG. 4 is a block schematic diagram of the improvement provided by the
present invention to the receiving portion of the prior art system of FIG.
3.
DETAILED DESCRIPTION OF THE INVENTION
Introduction
The present invention may be best understood through an explanation of the
improvements it makes to the prior art system. Therefore, in the following
detailed description, the prior art system as illustrated in FIGS. 1 and 3
will first be described, keeping in mind that for a more complete
description thereof attention should be directed to the above-referenced
U.S. patent application. Then, the modifications made to the prior art
system will be described with reference to FIGS. 2 and 4.
Prior Art System
Referring now to FIG. 1, there is illustrated the transmitting portion of
the prior art data communication system, generally designated 10. The
system 10 includes a plurality of acquisition component code generators
12, 14 and 16 for generating a plurality of component codes, C.sub.1,
C.sub.2, . . . C.sub.n, respectively, which may, for example, be binary
state pseudorandom codes. Ideally, the codes should possess the following
properties:
(1) the cyclic lengths of the codes should be relatively prime with respect
to each other;
(2) the codes should possess substantially ideal auto-correlation
functions; and
(3) the codes should be balanced, i.e., each cycle should include
approximately equal numbers of binary ONES and ZEROES.
The cyclic code lengths are said to be relatively prime with respect to
each other if there exists no integer other than unity that divides more
than one of the respective lengths of the codes, that is, they have no
common factors. An autocorrelation function is said to be ideal when it
possesses a correlation peak of substantial magnitude and has no
significant side lobes. The acquisition code generators 12, 14 and 16 may
be of the conventional shift register type that provides pseudorandom
codes of the linear maximal length variety.
The system 10 also includes a clock source 18 which provides the timing
signals for the acquisition component code generators 12, 14 and 16 such
that all of the codes are provided at the same clock rate and have the
same bit intervals in time. It is understood that the relative bit phases
of the codes provided by the generators 12, 14 and 16 may be adjusted by
means that, for the sake of clarity, are not shown.
The pseudorandom binary codes, C.sub.1, C.sub.2, . . . C.sub.n, provided by
the component code generators 12, 14 and 16 are applied as inputs to a MAJ
combiner 20 of the system 10. The MAJ combiner 20 combines the bits of the
component codes in accordance with a conventional Boolean majority logic
voting rule thereby providing a linear composite code hereinafter referred
to as the MAJ composite code for convenience. The output of the MAJ
combiner 20 is a binary ZERO during a bit interval if, in the bit
interval, half or more than half of the component codes are binary ZERO.
The output of the MAJ combiner 20 is binary ONE during the bit interval if
more than half of the component codes are binary ONE. The MAJ composite
code may be utilized as an acquisition composite code in the system 10. It
is understood that the MAJ combiner 20 may comprise conventional binary
majority voting circuits of a type well known in the digital electronics
art.
As well as being provided as inputs to the MAJ combiner 20, the
pseudorandom binary acquisition component codes, C.sub.1, C.sub.2, . . .
C.sub.n, are also applied as inputs to a MOD combiner 22 of the system 10.
The MOD combiner 22 combines the bits of the component codes in accordance
with a conventional modulo-2 addition rule thereby providing a linear
composite code hereinafter referred to as the MOD composite code for
convenience. The MOD combiner output is ZERO when the number of binary
ONES being summed in a bit interval is even, and the MOD combiner output
is ONE when the number of binary ONES being summed in a bit interval is
odd. It is understood that the MOD combiner 22 may comprise a conventional
binary arithmetic circuit of a type well known in the digital electronics
art. The MOD composite code may be utilized as a verification composite
code or as a data carrier composite code in the system 10.
The MAJ and MOD composite codes from the combiners 20, 22, respectively,
are applied as inputs to a mode control 24 of the system 10. The mode
control 24 provides as its output either the MAJ composite code or the MOD
composite code selected in accordance with the mode in which the system 10
is operating. The selected composite code from the mode control 24 is
applied as an input to a modulator 26 of the system 10. An input 28 to the
modulator 26 provides a data signal which may modulate the selected
composite code in any convenient manner.
The selected composite code passing through the modulator 26 is applied as
an input to a transmitter 30 of the system 10. The transmitter 30 may
provide the selected code for transmission in a conventional manner. The
output provided by the transmitter 30 is transmitted for receipt by the
receiving portion of the system illustrated in FIG. 3.
Turning to FIG. 3, the output transmitted by transmitter 30 of FIG. 1 is
received by a conventional receiver 32 of the system 10. The receiver 32
processes the received signal in a conventional manner and its output is
applied as a common input to a number of correlators 34 of the system 10.
Other inputs to the correlators 34 are applied from a mode control 36 in a
manner to be explained.
The receiving portion of the prior art system 10 also includes a plurality
of reference component code generators 38, 40 and 42 which may be
identical to acquisition component code generators 12, 14 and 16 of FIG.
1. The reference generators 38, 40 and 42 generate a plurality of linear
reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, respectively,
which may, for example, be binary state pseudorandom codes and are
preferably identical respectively to linear acquisition component codes,
C.sub.1, C.sub.2, . . . C.sub.n, for optimum performance of the system 10.
However, it is to be understood that the reference component codes need
only correlate with the acquisition component codes and not be identical
therewith. While only three acquisition component code generators and
three reference component code generators are illustrated, it should be
understood that the pluralities thereof may include greater than three
generators and that an acquisition generator and a reference generator for
respectively providing acquisition component code C.sub.n-1 and reference
component code R.sub.n-1 are assumed to be present in the system 10
without illustration of the same.
The reference component code generators 38, 40 and 42 are driven by a clock
source 44 of the system 10. The clock source 44 provides clock signals to
the reference component code generators 38, 40 and 42 so that the
component codes are provided at the same clock rate with respect to each
other and have identical bit intervals. The clock source 44, in addition,
includes circuits to continuously and simultaneously adjust the phases of
the clock signals provided to the generators 38, 40 and 42 so that the
component codes generated thereby may together be stepped in time. The
clock source 44 may, additionally, include circuits to delete, one at a
time, clock pulses from any one of the clock signals going to the
component code generators so that the component code provided by the
generator associated therewith may be stepped one bit interval at a time
past the received signal for reasons to be discussed later. The clock
source 44 receives a signal from the mode control 36 to control the
described functions in a manner to be explained later. The circuits of the
clock source 44 may be of any conventional design of a type well known to
those skilled in the art.
The reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, provided by
the respective generators 38, 40 and 42 are combined in MAJ and MOD
combiners 46 and 48 in a manner identical to that described with respect
to FIG. 1, thus providing linear MAJ and MOD reference composite codes
respectively.
The individual reference component codes, R.sub.1, R.sub.2, . . . R.sub.n,
as well as the MAJ and MOD composite codes are provided as inputs to the
mode control 36. The mode control 36 selects, in a conventional manner,
one or more of the codes for application to the correlators 34, for
reasons to be explained later. The mode control 36, in addition, selects
the particular ones of the correlators 34 to be utilized in accordance
with the operational mode of the system 10.
The outputs of the correlators 34 are applied as inputs to respective
threshold detectors 50 of the system 10. The threshold detectors 50 are
conventional circuits that provide signals, respectively, whenever the
respectively applied correlation signals exceed respective predetermined
thresholds. The outputs of the threshold detector 50 are applied as inputs
to the mode control 36 for reasons to be discussed later.
The output signal from one of the correlators 34, selected by the mode
control 36, is provided as an input to a code tracking circuit 52 of the
system 10. The code tracking circuit 52 is utilized to track the peak of
the correlation function provided by the selected correlator. The circuit
52 may comprise a conventional phase locked loop which may conveniently be
instrumented as a dither modulator/demodulator of a conventional type. The
output of the code tracking circuit 52 is applied as an input to the clock
source 44 to adjust the phases of the clock signals for the purpose of
tracking the correlation peaks during the various operational modes of the
system 10 to be described hereinafter.
The output of the same or another selected one of the correlators 34 is
also applied as an input to a data demodulator 53 of the system 10. The
data demodulator 53 is a conventional circuit that demodulates the data
modulated code carrier signal, thereby providing a signal representative
of the data impressed on the code carrier by the modulator 26 of FIG. 1.
In the following description of the operation of the data communication
system 10 illustrated in FIGS. 1 and 3, the acquisition mode of the system
will first be discussed. The acquisition component code generators 12, 14
and 16 are set to generate respective predetermined component codes,
C.sub.1, C.sub.2, . . . C.sub.n, and the relative phases thereof are
adjusted in accordance with a predetermined pattern. The mode control 24
is adjusted to select the MAJ composite code for transmission by the
transmitter 30 as the acquisition composite code.
The receiver 32 of FIG. 3 then receives the transmitted acquisition
composite code. The reference component code generators 38, 40 and 42 are
adjusted to generate reference component codes, R.sub.1, R.sub.2, . . .
R.sub.n, in the same sequence pattern as that of the acquisition component
codes, C.sub.1, C.sub.2, . . . C.sub.n.
The mode control 36 selects one of the reference component codes, for
example R.sub.1, for application to a selected one of the correlators 34
for correlation matching with the received signal, MAJ composite code. The
mode control 36 controls the clock source 44 so that the phase of the
reference component code R.sub.1 is continuously swept past the received
signal, MAJ (C.sub.1, C.sub.2, . . . C.sub.n) composite code, until the
selected one of the correlators 34 indicates a correlation peak by means
of the associated one of the threshold detectors 50. The mode control 36,
in response to the detected correlation peaks, stops the time sweep of the
reference component code R.sub.1. The output of the selected one of the
correlators 34 is utilized, via the code tracking circuit 52, to maintain
the proper adjustment of the clock source 44 so that the correlation peak
of the reference component code R.sub.1 with respect to the incoming
signal is tracked. Thus, the code tracking circuit 52 locks onto the peak
of this correlation function adjusting the clock source 44 to maintain the
reference component code R.sub.1 aligned with the incoming signal. Since
the remaining reference component codes are generated in bit synchronism
with the reference component code R.sub.1, the remaining codes are now in
bit synchronism with the incoming signal, MAJ composite code, although not
necessarily aligned at their respective correlation peaks. The reference
component code R.sub.1 has now been acquired with respect to the received
or incoming signal.
The mode control 36 now selects a second one of the correlators 34 and
applies a second reference component code, for example R.sub.2, thereto
for correlation with the received signal. Reference component code R.sub.2
is stepped one bit at a time until the selected one of the correlators 34
manifests a correlation peak as detected by the associated one of the
threshold detectors 50. The mode control 36 then stops the stepping of
code R.sub.2. The remaining reference component codes are aligned with the
received signal, MAJ composite code, in a similar manner and acquisition
of MAJ composite code is achieved. It should be understood that the
component codes may be selected serially as described above, or in
parallel which means all codes are selected and correlated with the MAJ
acquisition composite code simultaneously.
In order to verify the successful acquisition of the MAJ acquisition
composite code transmitted by the transmitter 30, the mode control 36 of
the receiving portion of the system 10 applies the output of the MOD
combiner 48 to a selected one of the correlators 34 for correlation of MOD
(R.sub.1, R.sub.2, . . . R.sub.n) reference composite code with the
incoming MAJ (C.sub.1, C.sub.2, . . . C.sub.n) acquisition composite code.
Only if all of the reference component codes, R.sub.1, R.sub.2, . . .
R.sub.n, are in proper alignment with the respective acquisition component
codes, C.sub.1, C.sub.2, . . . C.sub.n, of the transmitted MAJ (C.sub.1,
C.sub.2, . . . C.sub.n) composite code, will the selected MOD (R.sub.1,
R.sub.2, . . . R.sub.n) reference composite code correlator provide a
correlation peak.
When successful acquisition has been verified, the mode control 24 of the
transmitting portion of the system 10 may be adjusted to select the MOD
(C.sub.1, C.sub.2, . . . C.sub.n) composite code for transmission by the
transmitter 30. This composite code may function as a carrier for the data
to be impressed thereon via data input 28 to the modulator 26.
Since the receiving portion of the system 10 is now in proper alignment
with the transmitting portion, the output of the last selected one of the
correlators 34 may now be utilized to track the incoming signal. The data
carried by the transmitted composite code, MOD (C.sub.1, C.sub.2, . . .
C.sub.n), may be removed from the carrier by the data demodulator 53 for
utilization in apparatus not shown.
MODIFICATIONS OF THE PRESENT INVENTION
Referring now to FIG. 2, there are shown the modifications introduced by
the present invention to the transmitting portion of the prior art data
communication system 10. The modifications are incorporated between the
component code generators 12, 14, 15, 16 and the MAJ and MOD combiners 20,
22 of the prior art system 10 and result in an improved technique for
acquisition composite code generation wherein the security of the code is
enhanced over that of the prior art system.
The modifications to the transmitting portion of the prior art system take
the form of another MOD combiner 54, a nonlinearizer 56, and delays 58,
60, 62, which are all conventional components. The same basic composite
codes, C.sub.1, C.sub.2, . . . C.sub.n, as utilized in the prior art
system 10, are generated by component code generators 12, 14, 15, 16 in
the initial step of the method of acquisition composite code generation
employed by the improved system. However, the component codes are applied
as inputs to the MOD combiner 54 which is connected to the nonlinearizer
56. The MOD combiner 54 combines the bits of the component codes in
accordance with the modulo-2 addition rule to produce a linear composite
code being substantially identical to the MOD composite code produced by
the MOD combiner 22 of the prior art system 10 of FIG. 1.
The linear MOD composite code output of the MOD combiner 54 is applied as
an input to the nonlinearizer 56. The nonlinearizer 56 may take any
suitable form, such as an encrypter being set for operation in a decrypt
mode. The nonlinearizer 56 is driven by the linear composite code, MOD
(C.sub.1, C.sub.2, . . . C.sub.n) and produces a nonlinear composite code
Z. The nonlinear composite code Z is applied to both the MAJ and MOD
combiners 20, 22 of the prior art system.
Component codes, C.sub.1, C.sub.2, . . . C.sub.n-1, are also applied
individually to MAJ and MOD combiners 20, 22. The identification of the
code C.sub.n-1 is for the purpose of indicating that all of the component
codes, except for a single one of them, C.sub.n, are applied to the MAJ
and MOD combiners 20, 22 in the improved system, in contrast to the prior
art system 10 of FIG. 1 wherein all of the component codes were applied
thereto. Preferably, the codes, C.sub.1, C.sub.2, . . . C.sub.n-1, when
applied to combiners 20, 22 are time delayed relative to these same codes
incorporated by the nonlinear composite code Z. Such time delay for codes,
C.sub.1, C.sub.2, . . . C.sub.n-1, may be brought about by delays 58, 60,
62. The purpose for the time delay is so that the portions of the
component codes, C.sub.1, C.sub.2, . . . C.sub.n-1, incorporated in the
nonlinear composite code Z will not be canceled out by the MAJ and MOD
combining of the code Z with these same component codes.
The MAJ combiner 20 combines the bits of the linear time delayed component
codes, C.sub.1, C.sub.2, . . . C.sub.n-1, and the nonlinear composite
code, Z, in accordance with the conventional Boolean majority voting rule
thereby providing a nonlinear composite code, MAJ (C.sub.1, C.sub.2, . . .
C.sub.n-1, Z) which may be utilized as the acquisition composite code in
the improved system.
The MOD combiner 22 combines the bits of the linear time delayed component
codes, C.sub.1, C.sub.2, . . . C.sub.n-1, and the nonlinear composite
code, Z, in accordance with the conventional modulo-2 addition rule
thereby providing a nonlinear composite code, MOD (C.sub.1, C.sub.2, . . .
C.sub.n-1, Z) which may be utilized as the data-carrying composite code in
the improved system.
The nonlinear acquisition and data-carrying composite codes are applied as
inputs to the mode control 24 of the prior art system and selected for
transmission in the same manner as described above in the prior art
system.
Turning now to FIG. 4, there is shown the modifications introduced by the
present invention to the receiving portion of the prior art system 10. The
modifications are incorporated between the reference component code
generators 38, 40, 41, 42 and the MAJ and MOD combiners 46, 48 of the
prior art system 10.
The modifications to the receiving portion of the prior art system, being
identical to those made to the transmitting portion, take the form of a
MOD combiner 64, a nonlinearizer 66, and delays 68, 70, 72. The same basic
reference composite codes, R.sub.1, R.sub.2, . . . R.sub.n, as utilized in
the prior art system of FIG. 3 are generated by reference component code
generators 38, 40, 41, 42 and in the same manner as described hereinbefore
with respect to the prior art system. The reference component codes are
applied as inputs to the MOD combiner 64 which is connected to the
nonlinearizer 66. The MOD combiner 64 combines the bits of the reference
component codes in accordance with the modulo-2 addition rule to produce a
linear reference composite code being substantially identical to the MOD
reference composite code produced by the MOD combiner 48 of the prior art
system 10 of FIG. 3.
The linear MOD reference composite code output of the MOD combiner 64 is
applied as an input to the nonlinearizer 66. The nonlinearizer 66, being
identical to that of the improved transmitting portion of the system, is
driven by the linear reference composite code, MOD (R.sub.1, R.sub.2, . .
. R.sub.n), to produce a nonlinear reference composite code N, which is
identical to code Z of the transmitting portion.
Component codes, R.sub.1, R.sub.2, . . . R.sub.n-1, or, in other words, all
of the reference component codes with the exception of a single one,
R.sub.n, are also applied individually to the mode control 36, the MAJ
combiner 46 and the MOD combiner 48. The nonlinear reference composite
code N is also applied to the mode control 36, the MAJ combiner 46 and the
MOD combiner 48. Preferably, the component codes, R.sub.1, R.sub.2, . . .
R.sub.n-1, when applied to control 36 and combiners 46, 48 are time
delayed by the same amount and for the same purpose that acquisition
component codes, C.sub.1, C.sub.2, . . . C.sub.n-1, were time delayed in
the transmitting portion of the improved system. Such time delay for
codes, R.sub.1, R.sub.2, . . . R.sub.n-1, may be brought about by delays
68, 70, 72.
The linear reference component codes, R.sub.1, R.sub.2, . . . R.sub.n-1,
and the nonlinear reference composite code N are combined in MAJ and MOD
combiners 46 and 48 in a manner identical to that described with respect
to FIG. 2, thus providing nonlinear composite codes, MAJ (R.sub.1,
R.sub.2, . . . R.sub.n-1, N) and MOD (R.sub.1, R.sub.2, . . . R.sub.n-1,
N), respectively. These latter MAJ and MOD codes are provided as inputs to
the mode control 36, along with delayed component codes, R.sub.1, R.sub.2,
. . . R.sub.n-1, and nonlinear reference composite code N.
The mode control 36 selects as it | | |
