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Claims  |
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I claim:
1. A method for communicating information between a transmitting node and a
receiving node of a multi-node communication network, said method
comprising:
a) assigning blocks of bits embodying said information to corresponding
subsets of a set of binary spreading-code sequences, at least one of said
subsets of said set of binary spreading-code sequences containing more
than one of said binary spreading-code sequences; and
b) transmitting simultaneous signals containing selected subsets of said
set of binary spreading-code sequences from said transmitting node to said
receiving node.
2. The method of claim 1 wherein said binary spreading-code sequences are
generated by combining contents of specified stages of a first binary
shift register with contents of specified stages of a second binary shift
register.
3. The method of claim 1 wherein all of said blocks of bits embodying said
information are of equal fixed length.
4. The method of claim 1 wherein one bit of each of said blocks of bits
embodying said information determines a polarity at which the
corresponding subset of said set of binary spreading-code sequences is
transmitted.
5. The method of claim 1 wherein an additional bit of information is
conveyed by transmitting the binary spreading-code sequences in each of
said subsets of said set of binary spreading-code sequences with a
polarity that is related to the polarity of said additional bit of
information.
6. The method of claim 1 wherein each of said selected subsets of said set
of binary spreading-code sequences comprises precisely two of said binary
spreading-code sequences.
7. The method of claim 6 wherein the two binary sequences comprising each
of said selected subsets are transmitted simultaneously by modulating a
first one of said two binary sequences onto a first sinusoidal carrier,
and by modulating a second one of said two binary sequences onto a second
sinusoidal carrier, said first and second sinusoidal carriers being of the
same frequency but out of phase with respect to each other.
8. The method of claim 7 wherein said first and second sinusoidal carriers
are out of phase with respect to each other by 90.degree..
9. The method of claim 1 wherein each of said selected subsets of said set
of binary spreading-code sequences comprises precisely three of said
binary spreading-code sequences.
10. The method of claim 9 wherein the three binary sequences comprising
each of said selected subsets are transmitted simultaneously by modulating
a first one of said three binary sequences onto a first sinusoidal
carrier, by modulating a second one of said three binary sequences onto a
second sinusoidal carrier, and by modulating a third one of said three
binary sequences onto a third sinusoidal carrier, said first, second and
third sinusoidal carriers being of the same frequency but out of phase
with respect to each other.
11. The method of claim 10 wherein said first and second sinusoidal
carriers are out of phase with respect to each other by 60.degree., said
second and third sinusoidal carriers are out of phase with respect to each
other by 60.degree., and said first and third sinusoidal carriers are out
of phase with respect to each other by 120.degree..
12. The method of claim 1 wherein each of said selected subsets of said set
of binary spreading-code sequences comprises precisely four of said binary
spreading-code sequences.
13. The method of claim 12 wherein the four binary sequences comprising
each of said selected subsets are transmitted simultaneously by modulating
a first one of said four binary sequences onto a first sinusoidal carrier,
by modulating a second one of said four binary sequences onto a second
sinusoidal carrier, by modulating a third one of said four binary
sequences onto a third sinusoidal carrier, and by modulating a fourth one
of said four binary sequences onto a fourth sinusoidal carrier, said
first, second, third and fourth sinusoidal carriers being of the same
frequency but out of phase with respect to each other.
14. The method of claim 13 wherein said first and second sinusoidal
carriers are out of phase with respect to each other by 45.degree., said
second and third sinusoidal carriers are out of phase with respect to each
other by 45.degree., said third and fourth sinusoidal carriers are out of
phase with respect to each other by 45.degree., said first and third
sinusoidal carriers are out of phase with respect to each other by
90.degree., said second and fourth sinusoidal carriers are out of phase
with respect to each other by 90.degree., and said first and fourth
sinusoidal carriers are out of phase with respect to each other by
135.degree..
15. The method of claim 1 wherein:
a) each of said subsets of said set of binary sequences received at said
receiving node is correlated with each binary sequence of said set of
binary sequences so as to produce a set of correlation outputs, each
correlation output corresponding to a specified one of said binary
sequences; and
b) said set of correlation outputs is evaluated to identify a particular
one of said subsets of said set of binary sequences as being most likely
to have been transmitted from said transmitting node to said receiving
node.
16. The method of claim 15 wherein said set of correlation outputs is
evaluated according to a selection rule in which a particular one of said
subsets for which a sum of the correlation outputs corresponding to said
binary sequences is largest is identified as being the subset most likely
to have been transmitted from said transmitting node to said receiving
node.
17. The method of claim 15 wherein, when each one of said subsets of said
set of binary sequences comprises the same number of binary sequences as
every other one of said subsets, said set of correlation outputs is
evaluated according to a selection rule in which a particular one of said
subsets for which a ratio of the largest sum of the correlation outputs
corresponding to a first one of said subsets of said binary sequences to
the next-largest sum of the correlation outputs corresponding to a second
one of said subsets of said binary sequences is greater than a
predetermined threshold is identified as being the subset most likely to
have been transmitted from said transmitting node to said receiving node.
18. The method of claim 17 wherein polarity of the subset identified as
being most likely to have been transmitted from said transmitting node to
said receiving node conveys additional information.
19. A communication network comprising a transmitting node and a receiving
node, said transmitting node simultaneously transmitting signals
containing individual spreading-code sequences in a designated first
subset of a set of spreading-code sequences during a specified first time
interval, and simultaneously transmitting signals containing individual
spreading-code sequences in a designated second subset of said set of
spreading-code sequences during a specified second time interval, each of
said designated first and second subsets of said set of spreading-code
sequences being selected from a plurality of subsets of said set of
spreading-code sequences identified with one of said transmitting and
receiving nodes, at least one of said designated first and second subsets
of said set of spreading-code sequences containing more than one
spreading-code sequence.
20. The communication network of claim 19 wherein each of said designated
first and second subsets of said set of spreading-code sequences comprises
more than one spreading-code sequence, said transmitting node comprising
means for modulating the sequences of each of said designated first and
second subsets of said set of spreading-code sequences onto corresponding
carriers of the same frequency but of different phases.
21. The communication network of claim 20 wherein each of said designated
first and second subsets of said set of spreading-code sequences consists
of two sequences, said transmitting node comprising means for modulating
the two sequences of each of said designated first and second subsets of
said set of spreading-code sequences onto two corresponding carriers of
the same frequency.
22. The communication network of claim 21 wherein said two corresponding
carriers are out of phase by 90.degree..
23. The communication network of claim 20 wherein each of said designated
first and second subsets of said set of spreading-code sequences consists
of three sequences, said transmitting node comprising means for modulating
the three sequences of each of said designated first and second subsets of
said set of spreading-code sequences onto three corresponding carriers of
the same frequency.
24. The communication network of claim 23 wherein a first one and a second
one of said three corresponding carriers are out of phase by 60.degree.,
the second one and a third one of said three corresponding carriers are
out of phase by 60.degree., and the first one and the third one of said
three corresponding carriers are out of phase by 120.degree..
25. The communication network of claim 20 wherein each of said designated
first and second subsets of said set of spreading-code sequences consists
of three sequences, said transmitting node comprising means for combining
the three sequences of each of said designated first and second subsets of
said set of spreading-code sequences into two output sequences, and for
modulating the two output sequences onto two corresponding carriers of the
same frequency, said two corresponding carriers being out of phase by
90.degree., the combining of said three sequences of each of said
designated first and second subsets of said set of spreading-code
sequences into said two output sequences and the modulating of said two
output sequences onto said two corresponding carriers being equivalent to
modulating each of said three sequences of each of said designated first
and second subsets of said set of spreading-code sequences onto three
carriers of the same frequency, a first one and a second one of said three
carriers being out of phase by 60.degree., the second one and a third one
of said three carriers being out of phase by 60.degree., and the first one
and the third one of said three carriers being out of phase by
120.degree..
26. The communication network of claim 20 wherein each of said designated
first and second subsets of said set of spreading-code sequences consists
of four sequences, said transmitting node comprising means for modulating
the four sequences of each of said designated first and second subsets of
said set of spreading-code sequences onto four corresponding carriers of
the same frequency.
27. The communication network of claim 26 wherein a first one and a second
one of said four corresponding carriers are out of phase by 45.degree.,
the second one and a third one of said four corresponding carriers are out
of phase by 45.degree., the third one and a fourth one of said four
corresponding carriers are out of phase by 45.degree., the first one and
the third one of said four corresponding carriers are out of phase by
90.degree., the second one and the fourth one of said four corresponding
carriers are out of phase by 90.degree., and the first one and the fourth
one of said four corresponding carriers are out of phase by 135.degree..
28. The communication network of claim 20 wherein each of said designated
first and second subsets of said set of spreading-code sequences consists
of four sequences, said transmitting node comprising means for combining
the four sequences of each of said designated first and second subsets of
said set of spreading-code sequences into two output sequences, and for
modulating the two output sequences onto two corresponding carriers of the
same frequency, said two corresponding carriers being out of phase by
90.degree., the combining of said four sequences of each of said
designated first and second subsets of said set of spreading-code
sequences into said two output sequences and the modulating of said two
output sequences onto said two corresponding carriers being equivalent to
modulating each of said four sequences of each of said designated first
and second subsets of said set of spreading-code sequences onto four
carriers of the same frequency, a first one and a second one of said four
carriers being out of phase by 45.degree., the second one and a third one
of said four carriers being out of phase by 45.degree., the third one and
a fourth one of said four carriers being out of phase by 45.degree., the
first one and the third one of said four carriers being out of phase by
90.degree., the second one and the fourth one of said four carriers being
out of phase by 90.degree., and the first one and the fourth one of said
four carriers being out of phase by 135.degree..
29. A method for communicating information between a transmitting node and
a receiving node of a multi-node communication network, said method
comprising:
a) generating a set of binary spreading-code sequences by combining
contents of specified stages of a first binary shift register with
contents of specified stages of a second binary shift register, said set
of binary spreading-code sequences containing more than one binary
spreading-code sequence;
b) assigning blocks of bits embodying said information to corresponding
subsets of said set of binary spreading-code sequences, each of said
subsets of said set of binary spreading-code sequences containing at least
one of said binary spreading-code sequences; and
c) transmitting signals containing selected subsets of said set of binary
spreading-code sequences from said transmitting node to said receiving
node.
30. The method of claim 29 wherein all of said blocks of bits embodying
said information are of equal fixed length.
31. The method of claim 29 wherein one bit of each of said blocks of bits
embodying said information determines a polarity at which the
corresponding subset of said set of binary spreading-code sequences is
transmitted.
32. The method of claim 29 wherein an additional bit of information is
conveyed by transmitting the binary spreading-code sequences in each of
said subsets of said set of binary spreading-code sequences with a
polarity that is related to the polarity of said additional bit of
information.
33. The method of claim 29 wherein each of said selected subsets of said
set of binary sequences comprises precisely one binary sequence.
34. The method of claim 29 wherein each of said selected subsets of said
set of binary sequences comprises precisely two of said binary sequences.
35. The method of claim 34 wherein the two binary sequences comprising each
of said selected subsets are transmitted simultaneously by modulating a
first one of said two binary sequences onto a first sinusoidal carrier,
and by modulating a second one of said two binary sequences onto a second
sinusoidal carrier, said first and second sinusoidal carriers being of the
same frequency but out of phase with respect to each other.
36. The method of claim 35 wherein said first and second sinusoidal
carriers are out of phase with respect to each other by 90.degree..
37. The method of claim 29 wherein each of said selected subsets of said
set of binary sequences comprises precisely three of said binary
sequences.
38. The method of claim 37 wherein the three binary sequences comprising
each of said selected subsets are transmitted simultaneously by modulating
a first one of said three binary sequences onto a first sinusoidal
carrier, by modulating a second one of said three binary sequences onto a
second sinusoidal carrier, and by modulating a third one of said three
binary sequences onto a third sinusoidal carrier, said first, second and
third sinusoidal carriers being of the same frequency but out of phase
with respect to each other.
39. The method of claim 38 wherein said first and second sinusoidal
carriers are out of phase with respect to each other by 60.degree., said
second and third sinusoidal carriers are out of phase with respect to each
other by 60.degree., and said first and third sinusoidal carriers are out
of phase with respect to each other by 120.degree..
40. The method of claim 29 wherein each of said selected subsets of said
set of binary sequences comprises precisely four of said binary sequences.
41. The method of claim 40 wherein the four binary sequences comprising
each of said selected subsets are transmitted simultaneously by modulating
a first one of said four binary sequences onto a first sinusoidal carrier,
by modulating a second one of said four binary sequences onto a second
sinusoidal carrier, by modulating a third one of said four binary
sequences onto a third sinusoidal carrier, and by modulating a fourth one
of said four binary sequences onto a fourth sinusoidal carrier, said
first, second, third and fourth sinusoidal carriers being of the same
frequency but out of phase with respect to each other.
42. The method of claim 41 wherein said first and second sinusoidal
carriers are out of phase with respect to each other by 45.degree., said
second and third sinusoidal carriers are out of phase with respect to each
other by 45.degree., said third and fourth sinusoidal carriers are out of
phase with respect to each other by 45.degree., said first and third
sinusoidal carriers are out of phase with respect to each other by
90.degree., said second and fourth sinusoidal carriers are out of phase
with respect to each other by 90.degree., and said first and fourth
sinusoidal carriers are out of phase with respect to each other by
135.degree..
43. The method of claim 29 wherein, when at least one subset of said set of
binary spreading-code sequences comprises more than one sequence:
a) each of said subsets of said set of binary sequences received at said
receiving node is correlated with each binary sequence of said set of
binary sequences so as to produce a set of correlation outputs, each
correlation output corresponding to a specified one of said binary
sequences; and
b) said set of correlation outputs is evaluated to identify a particular
one of said subsets of said set of binary sequences as being most likely
to have been transmitted from said transmitting node to said receiving
node.
44. The method of claim 43 wherein, when each one of said subsets of said
set of binary sequences comprises the same number of binary sequences as
every other one of said subsets, said set of correlation outputs is
evaluated according to a selection rule in which a particular one of said
subsets for which a sum of the correlation outputs corresponding to said
binary sequences is largest is identified as being the subset most likely
to have been transmitted from said transmitting node to said receiving
node.
45. The method of claim 43 wherein said set of correlation outputs is
evaluated according to a selection rule in which a particular one of said
subsets for which a ratio of the largest sum of the correlation outputs
corresponding to a first one of said subsets of said binary sequences to
the next-largest sum of the correlation outputs corresponding to a second
one of said subsets of said binary sequences is greater than a
predetermined threshold is identified as being the subset most likely to
have been transmitted from said transmitting node to said receiving node.
46. The method of claim 45 wherein polarity of the subset identified as
being most likely to have been transmitted from said transmitting node to
said receiving node conveys additional information. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates generally to digital communication systems, and more
particularly to a spectrum spreading technique for use in multi-node
digital communication systems such as digital networks and digital radios.
BACKGROUND OF THE INVENTION
Spectrum spreading techniques for use in digital communication networks
have been described in many books and papers. A classic publication in
this field is Spread Spectrum Communications by M. K. Simon, J. K. Omura,
R. A. Scholtz and B. K. Levitt, Computer Science Press, 11 Taft Court,
Rockville, Md. 20850, 1985. Particular kinds of spectrum spreading
techniques that have been implemented in digital communication networks in
the prior art include "direct-sequence spreading", "frequency hopping",
"time hopping", and various hybrid methods that involve combinations of
the aforementioned techniques.
Multi-node spread-spectrum communication networks developed in the prior
art were generally characterized as code-division multiple-access (CDMA)
networks, which utilized "code-division multiplexing" (i.e., a technique
in which signals generated by different spreading-code sequences
simultaneously occupy the same frequency band). Code-division multiplexing
requires that the simultaneously used spreading codes be substantially
"mutually orthogonal", so that a receiver with a filter matched to one of
the spreading codes rejects signals that have been spread by any of the
other spreading codes.
In a typical multi-node spread-spectrum communication network using either
a conventional direct-sequence spectrum spreading technique or a hybrid
technique involving,--e.g., direct-sequence and frequency-hopped spectrum
spreading, only a single spreading code is employed. At regular intervals,
the polarity of the spreading code is either inverted (i.e., each 0 is
changed to 1, and each 1 is changed to 0) or left unchanged, depending on
whether the next bit of information to be transmitted is a 1 or a 0. The
resulting signal is an "information-bearing" sequence, which ordinarily
would be transmitted using some type of phase-shift keyed (PSK)
modulation--usually, binary phase-shift keyed (BPSK) modulation or
quaternary phase-shift keyed (QPSK) modulation.
A publication entitled Spread Spectrum Techniques Handbook, Second Edition,
March 1979, which was prepared for the National Security Agency by Radian
Corporation of Austin, Tex. describes a number of spread-spectrum
techniques that had been proposed in the prior art. Of particular interest
is a direct-sequence technique described on page 2-21 et seq. of the
Spread Spectrum Techniques Handbook, which involved transmitting one bit
of information (either a 0 or a 1) by switching between two independent
signals that are generated by different spreading codes. Ideally, the
spreading codes of the two independent signals should be "almost
orthogonal" with respect to each other, so that cross-correlation between
the two sequences is very small. In practice, in such early
spread-spectrum communication systems, the two independent signals were
maximal-length linear recursive sequences (MLLRSs), often called
"M-sequences", whose cross-correlations at all possible off-sets had been
computed and found to be acceptably low. However, this technique of
switching between two independent signals did not achieve widespread
acceptance, mainly because it required approximately twice the electronic
circuitry of a polarity-inversion technique without providing any better
performance.
Two recent papers, viz., "Spread-Spectrum Multiple-Access Performance of
Orthogonal Codes: Linear Receivers" by P. K. Enge and D. V. Sarwate, (IEEE
Transactions on Communications, Vol. COM-35, No. 12, December 1987, pp.
1309-1319), and "Spread-Spectrum Multiple-Access Performance of Orthogonal
Codes for Indoor Radio Communications" by K. Pahlavan and M. Chase, (IEEE
Transactions on Communications, Vol. 38, No. 5, May 1990, pp. 574-577),
discuss multi-node spread-spectrum communication networks in which
multiple orthogonal sequences within a relatively narrow bandwidth are
assigned to each node, whereby a corresponding multiplicity of information
bits can be simultaneously transmitted and/or received by each
node--thereby providing a correspondingly higher data rate. A specified
segment of each sequence available to a node of the network is designated
as a "symbol". In the case of a repetitive sequence, a symbol could be a
complete period of the sequence. The time interval during which a node
transmits or receives such a symbol is called a "symbol interval". In a
multi-node spread-spectrum network employing multiple orthogonal
sequences, all the nodes can simultaneously transmit and/or receive
information-bearing symbols derived from some or all of the sequence
available to the nodes.
The emphasis in the aforementioned Enge et al. and Pahlavan et al. papers
is on network performance, especially in certain kinds of signal
environments. Neither paper recommends or suggests using any particular
set of mutually orthogonal spreading codes for generating multiple
orthogonal sequences; and neither paper discloses how to derive or
generate suitable mutually orthogonal spreading codes. However, methods of
generating families of sequences that are pairwise "almost orthogonal" by
using two-register sequence generators have been known for some time.
In a paper entitled "Optimal Binary Sequences for Spread-Spectrum
Multiplexing" by R. Gold, (IEEE Transactions on Information Theory, Vol.
IT-13, October 1967, pp. 119-121), so-called "Gold codes" were proposed
for use as spreading codes in multi-node direct-sequence spread-spectrum
communication networks of the CDMA type. A Gold code is a linear recursive
sequence that is generated by a product f.sub.1 f.sub.2, where f.sub.1 and
f.sub.2 comprise the members of a so-called "preferred pair" of primitive
polynomials of the same degree n over a field GF(2). A primitive
polynomial of degree n is defined as a polynomial that generates a
maximal-length linear recursive sequence (MLLRS), which has a period of
(2.sup.n -1). The required relationship between f.sub.1 and f.sub.2 that
makes them a preferred pair is described in the aforementioned paper by R.
Gold.
A Gold code is a particular kind of "composite code". Other kinds of
composite codes include "symmetric codes" and "Kasami codes". A symmetric
code is similar to a Gold code in being generated by a product f.sub.1
f.sub.2 of a pair of primitive polynomials, except that for a symmetric
code the polynomial f.sub.2 is the "reverse" of primitive polynomial
f.sub.1, i.e., f.sub.2 (x)=x.sup.n f.sub.1 (1/x), where n=deg f.sub.1 =deg
f.sub.2. The correlation properties of Gold codes and symmetric codes are
discussed in a paper entitled "Crosscorrelation Properties of Pseudorandom
and Related Sequences" by D. V. Sarwate and M. B. Pursley, (Proceedings of
the IEEE, Vol. 68, No. 5, May 1980, pp. 593-619). Kasami codes differ from
Gold codes in that for Kasami codes, the polynomials f.sub.1 and f.sub.2
are not of the same degree. Kasami codes are also discussed in the
aforementioned paper by M. B. Pursley and D. V. Sarwate. The concept of a
"composite code" can be broadened to include sequences obtained from a
two-register sequence generator, where the sequences generated in the two
registers can be quite general.
Predominant among the reasons that have militated against using
direct-sequence spreading codes for multi-node spread-spectrum
communication networks of the prior art is the so-called "near-far"
problem. If the nodes of a multi-node spread-spectrum communication
network are widely distributed so that power levels for different nodes
can differ markedly at a given receiver in the network, then at the given
receiver the correlations of a reference sequence with a sequence that is
transmitted by a nearby node are apt to be stronger than correlations of
the reference sequence with a version of the reference sequence that has
been transmitted from a greater distance. Adverse effects of the
"near-far" problem can include periodic strong correlations in
information-bit errors, and false synchronization. To avoid such adverse
effects, frequency hopping has been preferred in the prior art for
multi-node spread-spectrum communication networks--especially for tactical
networks where the nodes are widely distributed. Until recently, most of
the research funding and efforts in connection with multi-node
spread-spectrum communication networks have been directed toward tactical
networks, thereby virtually precluding significant research on
direct-sequence spread-spectrum communication networks.
Hybrid frequency-hopped and direct-sequence spread-spectrum communication
networks have been proposed for tactical applications. However, the
frequency diversity provided by "hopping" of the carrier readily enables
rejection of unintended signals, thereby making the choice of a particular
spreading-code sequence relatively unimportant. Consequently, there has
been substantially no research in the prior art on the use of Gold codes
and other composite codes for hybrid frequency-hopped and direct-sequence
spread-spectrum communication networks.
Direct-sequence spread-spectrum communication networks have received recent
attention in connection with the development of wireless local area
networks (LANs), personal communications networks (PCNs), and cellular
telephone networks utilizing communications satellites. The "near-far"
problem is ordinarily not an issue for LANs and PCNs, because the nodes in
such networks are generally distributed at distances that are not very far
from each other. For cellular telephones, the "near-far" problem is not an
issue in satellite applications, because all transmitters in the "spot
beam" from a satellite are roughly at the same distance from the
satellite.
Several wireless LANs are described in an article entitled "Spread Spectrum
Goes Commercial" by D. L. Schilling, R. L. Pickholtz and L. B. Milstein,
IEEE Spectrum Vol. 27, No. 6, August 1990, pp. 40-45. For indoor
spread-spectrum communication networks (e.g., wireless LANs), spectrum
spreading has commonly been employed in "star network" configurations. In
a star network, the nodes are normally synchronized with a master
controller, so that each node of the network can use a different offset of
the same spreading-code sequence. False synchronization is not ordinarily
encountered with star networks. In circumstances in which two or more star
networks, each utilizing a different spreading-code sequence, operate in
close proximity to each other, composite codes could be used to advantage
to prevent interference between neighboring star networks. However, in the
prior art, reliance has usually been placed upon the distance between the
individual star networks, and upon signal-attenuating structures (e.g.,
walls) separating the individual star networks, as well as upon
cross-correlation properties that are expected of random uncorrelated
spreading-code sequences, to enable one star network to reject signals
from another star network in its vicinity. Consequently, composite codes
have generally not been used in star networks.
In PCNs, the use of composite codes as spreading-code sequences has not yet
received much attention, because factors such as size, weight and power
considerations have generally favored simplicity over performance.
Techniques involving satellite-based CDMA cellular radio networks have
emerged from developments in wireless LANs, but have generally been
concerned with coding and systems engineering rather than with
spreading-code sequence generation.
To date, direct-sequence spectrum spreading techniques have been used
primarily in applications requiring high multipath immunity, good time
resolution, robustness, privacy and low probability of detection, and for
which in-band interference and the "near/far" problem are manageable. Such
applications have included satellite communications, star networks in
office environments, mobile radio, and positioning and navigation
applications. The use of composite codes (e.g., Gold codes or symmetric
codes) for spectrum spreading in such applications has not heretofore been
deemed appropriate, because composite codes would require significantly
greater hardware complexity to imprement than MLLRSs without seeming to
provide sufficient compensating advantages over MLLRSs in terms of
processing gain, the number of nodes that can be accommodated, the rate of
data transmission, or robustness.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a
spread-spectrum technique for use in a multi-node digital communication
network, whereby a unique set of spreading-code sequences is assigned to
each node of the network for transmitting digital signals.
It is a particular object of the present invention to provide a method for
generating a family of nearly orthogonal spreading-code sequences, and for
assigning a unique set of spreading-code sequences from the family of
sequences so generated to each node of a multi-node digital communication
network.
It is also a particular object of the present invention to provide methods
for selecting a set of one or more spreading-code sequences that can be
used during a specified period of time (i.e., a so-called "symbol
interval") to convey multiple bits of information, if the selected
sequence or sequences of the set are modulated and transmitted
simultaneously.
It is likewise a particular object of the present invention to provide
logic circuit designs for hardware implementation of methods for
generating a family of spreading-code sequences for assignment to the
nodes of a multi-node digital communication network.
It is a further object of the present invention to provide methods for
simultaneously modulating a set of carriers of the same frequency but of
different phases in order to enable multiple bits of information to be
transmitted on each carrier of the set.
It is another object of the present invention to provide a spread-spectrum
technique for use in a multi-node digital communication network, which can
readily incorporate standard error-control coding (whose parameters are
matched to the particular application) into the transmission and reception
of digital signals propagated by the network.
It is also an object of the present invention to provide a technique
whereby conventional equipment designed for generating arbitrary
spreading-code sequences can be adapted to the task of generating a family
of spreading-code sequences for use in a multi-node digital communication
network.
It is a further object of the present invention to provide a technique
whereby direct-sequence spectrum spreading, or a hybrid combination of
direct-sequence and frequency-hopped spectrum spreading, can be utilized
in conjunction with code diversity or "code hopping" in a spread-spectrum
digital communication network designed to have a low probability of
intercept (LPI).
It is also an object of the present invention to provide symbol detection
methods, which enable a receiver at any given node in a multi-node
spread-spectrum digital communication network to determine the most likely
spreading-code sequence or sequences transmitted by another node of the
network attempting to communicate with the given node.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic illustration of an apparatus for generating a family
of nearly orthogonal spreading-code sequences of the composite code type,
and for selecting unique sets of the sequences so generated for assignment
to corresponding nodes of a multi-node digital communication network
according to the present invention.
FIG. 2 is schematic illustration of an alternative embodiment of a
spreading-code sequence generator for use in the apparatus of FIG. 1,
which allows register taps to be arbitrarily selected for summation (i.e.,
"EXCLUSIVE OR") and feedback functions.
FIG. 3 is a schematic illustration of another alternative embodiment of a
spreading-code sequence generator for use in the apparatus of FIG. 1,
wherein one of the modulo-2 adders (i.e., "EXCLUSIVE OR" circuits) shown
in FIG. 1 is omitted, which enables a maximal-length linear recursive
sequence (MLLRS) to be used as one of the possible spreading-code
sequences.
FIG. 4 is a schematic illustration of yet another alternative embodiment of
a spreading-code sequence generator for use in the apparatus of FIG. 1,
which allows information to be transmitted by switching in register
contents (called "fills") obtained from lock-up tables at the beginning of
each symbol interval.
FIG. 5 is a schematic representation of a procedure according to the
present invention whereby two sequences are selected from the set of
sequences that are available to a given node of the network for modulating
two sinusoidal carriers, which are of the same frequency but which differ
in phase by 90.degree..
FIG. 6 is a schematic representation of a procedure according to the
present invention whereby the set of spreading-code sequences available to
a given node of the network is partitioned into two subsets, and whereby
sequences are selected from each of the subsets and modulated onto
orthogonal carriers.
FIG. 7 is a schematic representation of a procedure according to the
present invention whereby three sequences are selected from the set of
sequences that are available to a given node of the network, and are
combined so as to be capable in effect of modulating three sinusoidal
carriers of the same frequency but with relative phases of 0.degree.,
60.degree. and 120.degree..
FIG. 8 is a schematic representation of a procedure according to the
present invention whereby four sequences are selected from the set of
sequences that are available to a given node of the network, and are
combined so as to be capable in effect of modulating four sinusoidal
carriers of the same frequency but with relative phases of 0.degree.,
45.degree., 90.degree. and 135.degree..
FIG. 9 is a schematic representation of a procedure according to the
present invention whereby externally generated spreading-code sequences
serve as inputs to two shift registers for generating unique
spreading-code sequences.
FIG. 10 is a block diagram of a transmitter for use by a node of a
multi-node digital communication network according to the present
invention.
FIG. 11 is a block diagram of a receiver for use by a node of a multi-node
digital communication network according to the | | |
