United States Patent 1191
`Rice
`
`llllillllllllllllllllllllllIllllIIIIHIIIIIIIHlllllIllllllllilllllllllllll
`USOO5210770A
`[11]
`5,210,770
`Patent Number:
`[45] Date of Patent: May 11, 1993
`
`[54] MULTIPLE-SIGNAL SPREAD-SPECTRUM
`‘TRANSCEIVER
`
`[75] Invenwv Bart F- Rice, Santa Cruz, CaIif-
`[73] Assignee: Lockheed Missiles &‘Space Company,
`Inc" Sunnyvale’ Cahf'
`[21] Appl. No.: 766,372
`22] Filed’
`Se 27' 1991
`[
`'
`p.
`'
`Int. CLS ...................... ..
`11:5. Cl. ............................................... ..
`[58] Field of Search .......................................... ,. 375/1
`[56]
`References Cited
`U S PATENT DOCUMENTS
`'
`'
`4,730,340 3/1988 Frazier, J1‘. ........................... .. 375/1
`4,872,200 10/1989 Jansen .......... ..
`375/1
`4,984,247 1/1991 Kaufman et a1_
`375/1
`5,029,180 7/1991 Cowart ......... ..
`375/1
`5,031,173 7/1991 Short et a1. ........................... .. 375/1
`
`375/1
`5,063,571 11/1991 Vanacraeynest .......... ..
`375/1
`5,084,900 1/1992 Taylor ................ ..
`..... .. 375/1
`5,099,493 3/1992 Zeger et a1. .
`5,111,478 5/1992 McDonald ............................ .. 375/1
`Primal? Examiner_sa1vatore Gangialosi
`Attorney, Agent, or Firm-John J. Morrissey
`
`ABSTRACT
`[57]
`A method and apparatus for communication in a spread
`Spectrum network are
`of bits embody.
`information are assigned to corresponding subsets of
`binary spreading Code sequences and at least one of the
`subsets of binary spreading code sequences contains
`more than one binary spreading code sequence. Se
`lected subsets of the binary spreading code sequences
`are then Simultaneously transmitted from a transmitting
`node to a receiving node.
`
`46 Claims, 10 Drawing Sheets
`
`41/
`
`= SEQUENCE 1 CORRELATOR
`
`,
`
`DEMODULATOR
`ouwur
`
`N
`>
`I seem: CEZCORRELATOR
`I
`,
`.
`I
`
`A
`
`11
`
`TODECODER
`
`swam
`DmcmN
`AND
`LOGIC
`UNIT
`
`> SEQUENCE K connsmon
`
`>
`
`ERIC-1004
`Ericsson v. IV
`Page 1 of 25
`
`

`
`U.S. Patent
`
`May 11, 1993
`
`Sheet 1 of 10
`
`5,210,770
`
`SYMBOLseuscnon UNIT
`MODULATOR
`
`2 E
`
`u
`<2
`2:
`ED
`km
`E
`
`Page 2 of 25
`
`

`
`US. Patent
`
`May 11, 1993
`
`Sheet 2 of 10
`
`5,210,770
`
`w~\ zo:<25“_z_
`
`
`U258
`
`ow \\
`
`E5892 5:5
`Alli 2385
`
`=2:
`
`LAAAAA
`
`—_1AAAAAA
`
`Page 3 of 25
`
`

`
`U.S. Patent
`
`May 11, 1993
`
`Sheet 3 of 10
`
`' 5,210,770
`
`A
`
`cat32005
`29.55%
`
`.22:
`
`NR
`
`ON
`
`a
`
`.6925
`
`L+AAAA
`
`\J*
`0+
`
`/\
`\J
`
`4
`
`ff
`9
`
`_|‘ Al/\ A.
`
`\J
`
`Ml M.
`
`A: Q A \J -
`
`f\
`
`A ‘ll \/ Al]
`
`Al r\\/ 4
`
`p
`
`Page 4 of 25
`
`

`
`US. Patent
`
`May 11, 1993
`
`Sheet 4 of 10
`
`5,210,770
`
`m
`
`:2:
`
`ANlm-FF
`
`2255mm
`.EmEw
`zap-(SE87:
`
`wumacw
`
`N:
`
`Page 5 of 25
`
`

`
`U.S. Patent
`
`May 11, 1993
`
`Sheet 5 of 10
`
`’ 5,210,770
`
`SYMBOLSELECTION UNIT
`
`2 E
`
`u:
`<2
`5:
`‘:0
`Eu:
`E
`
`Page 6 of 25
`
`

`
`US. Patent
`
`May 11, 1993
`
`Sheet 6 of 10
`
`5,210,770
`
`ON
`
`20552;
`
`358
`
`# .
`
`.5925
`
`20-53mm
`
`:2:
`
`2252:95 @
`wumzcw
`
`AAAAAA
`
`AHBAHA
`
`r\
`\J
`r\
`\d
`
`O
`
`A
`
`O
`NH
`
`A
`
`m;
`
`Page 7 of 25
`
`

`
`U.S. Patent
`
`V.aM
`
`la11
`
`3991
`
`Sheet 7 of 10
`
`5,210,770
`
`.5:N+3.8aEu+..o.E+5.3".:3+#389EmnEm
`
`him-.H.
`
`Elm-H:
`
`
`
`
`
`2&5;+5.802..+=~\E+.3.m8Eu+=<\E+.3.m8:3+~38...Em.I.Ew
`
`Page 8 of 25
`
`

`
`U.S. Patent
`
`May 11, 1993
`
`Sheet 8 of 10
`
`5,210,770
`
`INFORMATION
`
`SOURCE
`
`
`
`SYMBOLSELEc11ONuurrMODULATOR
`
`SEQUENCE
`
`GENERATOR
`
`II
`‘V
`
`II
`II "II
`II "II
`II "II
`' ‘VII
`I "II
`V
`
`Page 9 of 25
`
`

`
`U.S. Patent
`
`191V.aM
`
`3991..
`
`Sheet 9 of 10
`
`5,210,770
`
`
`
`
`
`:2:E88n-zos_S8:_m_>o8._9mwmmmmnwwa
`
`zo=.¢,5maBE§_s:m>_ES=._osma
`
`
`
`.§<Ezm_u5<.Ezo:
`
`
`
`mazmsfim.<~_z2_=uz>m
`
`
`
`:o.r<._:oo_2muzmacmm
`
`cE<Ezmu
`
`
`
`zofimdmEaouzm:2:8.58
`
`
`
`amscazos_3om-8#_zcE>8zmzo=<_2=Ez_
`
`
`
`
`
`Page 10 of 25
`
`

`
`US. Patent
`
`May 11, 1993
`
`Sheet 10 of 10
`
`5,210,770
`
`awesome 8
`
`‘llllllllll 7
`U53
`.22:
`az<
`
`.5925
`( zczbwhmc
`
`\V
`
`
`
`ceswmmoo P wuzwzoww ‘II
`
`
`
`meswcmou u muzwzcwm Ill...
`
`
`
`mo._.<._m_mmou v- muzmzcww llll
`
`I
`
`I
`
`I >
`
`l .EEbo 4 messes-2mm
`
`Page 11 of 25
`
`

`
`1
`
`MULTIPLE-SIGNAL SPREAD-SPECTRUM
`TRANSCEIVER
`
`20
`
`30
`
`TECHNICAL FIELD
`This invention relates generally to digital communi
`cation systems, and more particularly to a spectrum
`spreading technique for use in multi-node digital com
`munication systems such as digital networks and digital
`radios.
`BACKGROUND OF THE INVENTION
`Spectrum spreading techniques for use in digital com
`munication networks have been described in many
`books and papers. A classic publication in this ?eld is
`Spread Spectrum Communications by M. K. Simon, J. K.
`Omura, R. A. Scholtz and B. K. Levitt, Computer Sci
`ence Press, 11 Taft Court, Rockville, Md. 20850, 1985.
`Particular kinds of spectrum spreading techniques that
`have been implemented in digital communication net
`works in the prior art include “direct-sequence spread
`ing”, "frequency hopping”, “time hopping”, and vari
`ous hybrid methods that involve combinations of the
`aforementioned techniques.
`Multi-node spread-spectrum communication net
`works developed in the prior art were generally charac
`terized as code-division multiple-access (CDMA) net
`works, 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 re
`quires that the simultaneously used spreading codes be
`substantially “mutually orthogona ”, so that a receiver
`with a ?lter 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 communica
`tion network using either a conventional direct
`sequence spectrum spreading technique or a hybrid
`technique involving,—e.g., direct-sequence and fre
`quency-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 l, and each 1 is changed to 0) or left
`unchanged, depending on whether the next bit of infor
`mation 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
`55
`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 trans
`60
`mitting one bit of information (either a 0 or a l) 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
`
`5,210,770
`-
`2
`(MLLRSs), often called “M-sequences”, whose cross
`correlations at all possible off-sets had been computed
`and found to be acceptably low. However, this tech
`nique 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
`10
`Access Performance of Orthogonal Codes: Linear Re
`' ceivers” 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 nar
`row bandwidth are assigned to each node, whereby a
`corresponding multiplicity of information bits can be
`simultaneously transmitted and/0r received by each
`node--thereby providing a correspondingly higher
`data rate. A speci?ed segment of each sequence avail
`able to a node of the network is designated as a “sym
`bol”. 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 orthogo
`nal 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 a1. and
`Pahlavan et al. papers is on network performance, espe
`cially 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 dis
`closes how to derive or generate suitable mutually or
`thogonal spreading codes. However, methods of gener
`ating 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-l3, Octo
`ber 1967, pp. ll9-l21), so-called “Gold codes” were
`proposed for use as spreading codes in multi-node di
`rect-sequence spread-spectrum communication net
`works of the CDMA type. A Gold code is a linear
`recursive sequence that is generated by a product f lfz,
`where f1 and f2 comprise the members of a so-called
`“preferred pair” of primitivepolynomials of the same
`degree 11 over a ?eld GF(2). A primitive polynomial of
`degree n is de?ned as a polynomial that generates a
`maximal-length linear recursive sequence (MLLRS),
`which has a period of (2"- l). The required relationship
`between f1 and f2 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 “sym
`metric codes” and “Kasami codes”. A symmetric'code
`is similar to a Gold code in being generated by a prod
`uct flfz of a pair of primitive polynomials, except that
`for a symmetric code the polynomial f; is the “reverse”
`of primitive polynomial f1, i.e., f2(x)=x"f1(l/x), where
`
`40
`
`45
`
`50
`
`65
`
`Page 12 of 25
`
`

`
`15
`
`20
`
`5,210,770
`4
`3
`n=deg f1=deg f;. The correlation properties of Gold
`been employed in “star network” con?gurations. In a
`star network, the nodes are normally synchronized with
`codes and symmetric codes are discussed in a paper
`entitled “Crosscorrelation Properties of Pseudorandom
`a master controller, so that each node of the network
`and Related Sequences” by D. V. Sarwate and M. B.
`can use a different offset of the same spreading-code
`Pursley, (Proceedings of the IEEE, Vol. 68, No. 5, May
`sequence. False synchronization is not ordinarily en
`1980, pp. 593-619). Kasami codes differ from Gold
`countered with star networks. In circumstances in
`codes in that for Kasami codes, the polynomials f1 and
`which two or more star networks, each utilizing a dif
`f; are not of the same degree. Kasami codes are also
`ferent spreading-code sequence, operate in close prox
`discussed in the aforementioned paper by M. B. Pursley
`imity to each other, composite codes could be used to
`and D. V. Sarwate. The concept of a “composite code”
`advantage to prevent interference between neighboring
`can be broadened to include sequences obtained from a
`star networks. However, in the prior art, reliance has
`two~register sequence generator, where the sequences
`usually been placed upon the‘distance between the indi
`generated in the two registers can be quite general.
`vidual star networks, and upon signal~attenuating struc
`Predominant among the reasons that have militated
`tures (e.g., walls) separating the individual star net
`against using direct-sequence spreading codes for multi
`works, as well as upon cross-correlation properties that
`node spread-spectrum communication networks of the
`are expected of random uncorrelated spreading-code
`prior art is the so-called “near-far” problem. If the
`sequences, to enable one star network to reject signals
`nodes of a multi-node spread-spectrum communication
`from another star network in its vicinity. Consequently,
`network are widely distributed so that power levels for
`composite codes have generally not been used in star
`different nodes can differ markedly at a given receiver
`networks.
`in the network, then at the given receiver the correla
`In PCNs, the use of composite codes as spreading
`tions of a reference sequence with a sequence that is
`code sequences has not yet received much attention,
`transmitted by a nearby node are apt to be stronger than
`because factors such as size, weight and power consid
`correlations of the reference sequence with a version of
`erations have generally favored simplicity over perfor
`the reference sequence that has been transmitted from a
`25
`mance. Techniques involving satellite-based CDMA
`greater distance. Adverse effects of the “near-far” prob
`cellular radio networks have emerged from develop
`lem can include periodic strong correlations in informa
`ments in wireless LANs, but have generally been con
`tion-bit errors, and false synchronization. To avoid such
`cerned with coding and systems engineering rather than
`adverse effects, frequency hopping has been preferred
`with spreading-code sequence generation.
`in the prior art for multi-node spread-spectrum commu
`To date, direct-sequence spectrum spreading tech
`nication networks-especially for tactical networks
`niques have been used primarily in applications requir
`where the nodes are widely distributed. Until recently,
`ing high multipath immunity, good time resolution,
`most of the research funding and efforts in connection
`robustness, privacy and low probability of detection,
`with multi-node spread-spectrum communication net
`and for which in-band interference and the “near/far"
`works have been directed toward tactical networks,
`problem are manageable. Such applications have in
`thereby virtually precluding signi?cant research on
`cluded satellite communications, star networks in of?ce
`direct-sequence spread-spectrum communication net
`environments, mobile radio, and positioning and navi
`works.
`gation applications. The use of composite codes (e.g.,
`Hybrid frequency-hopped and direct-sequence
`Gold codes or symmetric codes) for spectrum spread
`spread-spectrum communication networks have been
`ing in such applications has not heretofore been deemed
`proposed for tactical applications. However, the fre
`appropriate, because composite codes would require
`quency diversity provided by “hopping” of the carrier
`signi?cantly greater hardware complexity to impre
`readily enables rejection of unintended signals, thereby
`ment than MLLRSs without seeming to provide suffi
`making the choice of a particular spreading-code se
`cient compensating advantages over MLLRSs in terms
`quence relatively unimportant. Consequently, there has
`of processing gain, the number of nodes that can be
`been substantially no research in the prior art on the use
`accommodated, the rate of data transmission, or robust’
`of Gold codes and other composite codes for hybrid
`frequency-hopped and direct-sequence spread-spec
`ness.
`trum communication networks.
`Direct-sequence spread-spectrum communication
`networks have received recent attention in connection
`with the development ofv wireless local area networks
`(LANs), personal communications networks (PCNs),
`and cellular telephone networks utilizing communica
`tions 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 appli
`cations, 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
`65
`Spectrum Vol. 27, No. 6, August 1990, pp. 40-45. For
`indoor spread-spectrum communication networks (e.g.,
`wireless LANs), spectrum spreading has commonly
`
`SUMMARY OF THE INVENTION
`It is a general object of the present invention to pro
`vide 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 fam
`ily 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
`speci?ed period of time (i.e., a so-called “symbol inter
`val”) to convey multiple bits of information, if the se
`lected sequence or sequences of the set are modulated
`and transmitted simultaneously.
`
`30
`
`35
`
`40
`
`45
`
`55
`
`Page 13 of 25
`
`

`
`5
`It is likewise a particular object of the present inven
`tion to provide logic circuit designs for hardware imple
`mentation of methods for generating a family of spread
`ing-code sequences for assignment to the nodes of a
`multi-node digital communication network.
`It is a further object of the present invention to pro
`vide 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 trans
`mitted on each carrier of the set.
`It is another object of the present invention to pro
`vide 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 appli
`cation) 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 spread
`ing-code sequences for use in a multi-node digital com
`. munication network.
`It is a further object of the present invention to pro
`vide a technique whereby direct-sequence spectrum
`spreading, or a hybrid combination of direct-sequence
`and frequency-hopped spectrum spreading, can be uti
`lized in conjunction with code diversity or “code hop- _
`ping” in a spread-spectrum digital communication net
`work 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 communi
`cate with the given node.
`
`5,210,770
`6
`available to a given node of the network for modulating
`two sinusoidal carriers, which are of the same fre
`quency but which differ in phase by 90°.
`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 sub
`sets and modulated onto orthogonal carriers.
`FIG. 7 is a schematic representation of a procedure
`according to the present invention whereby three se
`quences are selected from the set of sequences that are
`available to a given node of the network, and are com
`bined so as to be capable in effect of modulating three
`sinusoidal carriers of the same frequency but with rela
`tive phases of 0°, 60° and 120°.
`FIG. 8 is a schematic representation of a procedure
`according to the present invention whereby four se
`quences are selected from the set of sequences that are
`available to a given node of the network, and are com
`bined so as to be capable in effect of modulating four
`sinusoidal carriers of the same frequency but with rela
`tive phases of 0°, 45°, 90° and 135".
`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 present invention.
`FIG. 12 is a block diagram of a correlation unit of the
`receiver of FIG. 11, which correlates each in-coming
`spreading-code sequence detected by the receiver with
`all the spreading-code sequences that are available to
`the node.
`
`10
`
`25
`
`30
`
`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 pres
`ent invention.
`FIG. 2 is schematic illustration of an alternative em
`bodiment 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., “EX
`CLUSIVE OR”) and feedback functions.
`FIG. 3 is a schematic illustration of another alterna
`tive embodiment of a spreading-code sequence genera
`tor 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 alter
`native embodiment of a spreadingcode sequence gener
`ator for use in the apparatus of FIG. 1, which allows
`information to be transmitted by switching in register
`contents (called “f1lls”) 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 se
`quences are selected from the set of sequences that are
`
`45
`
`50
`
`60
`
`65
`
`BEST MODE OF CARRYING OUT THE
`INVENTION
`In accordance with the present invention, a family of
`“almost orthogonal” binary sequences is generated to
`provide disjoint sets of spreading-code sequences that
`can be assigned to corresponding nodes of a multi-node
`digital communication network. Each node of the net
`work is allotted multiple spreading-code sequences,
`which are selected from the total number of available
`sequences provided by the family of “almost orthogo
`nal” binary sequences. The spreading-code sequences
`assigned to the various nodes of the network are all
`modulo-2 sums (i.e., “EXCLUSIVE OR” outputs) of
`the contents (also called the “?lls”) of successive stages
`in two so-called “shift registers”.
`The binary sequences from which the disjoint sets of
`spreading’code sequences are selected for assignment to
`the nodes of the network are said to be “almost orthog
`onal” because the selected binary sequences all have
`low auto-correlation values (except for offset 0), and all
`have low cross-correlation values relative to each
`other, where the auto-correlations and the cross-corre
`lations are performed over a specified number of bits
`that de?nes a so-called “symbol interval”. For algebra
`ically generated periodic linear recursive sequences that
`are selected for their favorable auto-correlation and
`cross-correlation properties, the optimum symbol inter
`val for a given sequence coincides with the period of
`
`Page 14 of 25
`
`

`
`5,210,770
`8
`7
`where the particular codes and polarities that are se
`the sequence. For sequences generated by a non-linear
`random number generator, and for linear recursive
`lected in a particular case depend upon the information
`sequences of very long period, the symbol interval for a
`to be conveyed. Since information is conveyed in
`given sequence can be chosen arbitrarily-in which
`blocks, Reed-Solomon coding (or any other suitable
`case the auto-correlation and cross-correlation proper
`coding scheme) can optionally be used to provide for
`ties of the sequences cannot be guaranteed, but have the
`ward error control.
`usual statistics for correlations of random sequences.
`Symbol decision methods (i.e., methods that can be
`An example of a set of binary spreading-code sequen
`used by a receiver to determine the most likely transmit
`ces that could be used in a multi-node digital communi
`ted sequence or sequences) can vary for different em
`cation network according to the present invention
`bodiments of the present invention. In each embodi
`would be a set of Gold code sequences, each of which
`ment, the receiver identi?es those particular incoming
`is generated by the product M; of a “preferred pair” (f 1,
`sequences having the strongest correlation values, and
`f2) of primitive polynomials of the same degree n over
`determines their polarities. The decision logic algorithm
`the ?eld GF(2), i.e., the algebraic ?eld of two elements
`for each embodiment determines the most likely trans
`0 and l. A primitive polynomial over GF(2) is a polyno
`mitted sequence or sequences from the correlation val
`rnial that generates a maximal-length linear recursive
`ues.
`sequence (MLLRS). If the degree of the primitive poly
`If Gold code sequences, or “symmetric” sequences,
`nomials f1 and f; is n, the period of the Gold code se
`or Kasami code sequences are used as the spreading
`quences generated by the product flfz is (2"— 1).
`code sequences, mathematically guaranteed cross-cor
`Another example of a set _of binary spreading-code
`relation properties of those sequences over an entire
`sequences that is suitable for use in a multi-node digital
`period can be exploited by taking the symbol interval to
`communication network would be a set of so-called
`be equal to the period of the spreading-code sequences.
`“symmetric” sequences, each of which is generated by
`According to one method for ensuring that modulation
`the product f 1f;, where f1 and f; are primitive polynomi
`is “balanced” (i.e., that equal numbers of 0’s and l’s are
`als, and where f; is the “reverse” of f1, i.e.,
`transmitted during each symbol interval), the symbol
`interval is taken to be equal to twice the period of the
`spreading-code sequences, and the spreading-code se
`quences are transmitted so that a complete sequence is
`transmitted-during the ?rst half of a symbol interval
`and so that the reciprocal of that sequence is transmit
`ted—-during the second half of the symbol interval. This
`method produces a factor-of-two decrease in the sym
`bol rate for a given “chip rate” (i.e., the rate at which
`individual bits of the spreading-code sequences are
`transmitted).
`Acquisition and maintenance of synchronization for
`spread-spectrum signals have been widely discussed in
`published literature. In each embodiment of the present
`invention, synchronization of each incoming sequence
`with the spreading-code sequences that have been as
`signed to a given node is acquired by conventional
`means. Synchronization is maintained, and the possibil
`ity of false synchronization is minimized, by using a
`two-register sequence generator to generate candidate
`spreading-code sequences that are to be correlated with
`each incoming sequence. If synchronization of an in
`coming sequence with the sequences assigned to the
`given node is lost, that incoming sequence does not
`correlate strongly with any of the candidate spreading
`code sequences. However, if synchronization is main
`tained, the incoming sequences that are most likely to be
`signals transmitted by other nodes of the network are
`determined. A stream of information bits is then assem
`bled from the incoming sequences identi?ed as likely to
`be information-bearing signals. If forward error correc
`tion has been used, the information bit stream is de
`coded to determine the information originating at the
`transmitting node of the network.
`A speci?ed number K of available spreading-code
`sequences is assigned to each node of a network accord
`ing to the present invention. The number of information
`bits that can be conveyed per symbol varies directly
`with the value of the number K. If the total number of
`spreading-code sequences available to the network is N,
`then the maximum number of nodes that can be accom
`modated by the network is N/K. Thus, there is a trade
`off between the number of information bits that can be
`
`where n=deg f1=deg f1.
`Yet another example of a set of binary spreading-code
`sequences that could be employed in a multi-node digi
`tal communication network according to the present
`invention would be a set of Kasami code sequences,
`each of which is generated by a product flfz, where f1
`and f; are primitive polynomials such that the degree of
`one of the polynomials divides the degree of the other.
`The auto-correlation properties of composite-code
`sequences (e.g., Gold code sequences, symmetric code
`sequences and Kasarni code sequences), and the cross
`correlation properties of families of such composite
`code sequences over an entire period, are described in
`the aforementioned article by M. B. Pursely et al.
`wherein such sequences are shown to be “almost or
`thogona .”
`Alternatively, a set of random spreading-code se
`quences could also be used in practicing the present
`invention. While composite-code sequences are espe
`cially useful and convenient for particular embodiments
`of a multi-node digital communication network accord
`ing to the present invention, it is not necessary to limit
`the invention in principle to the use'of any particular
`kinds of spreading-code sequences. The salient charac
`teristic of a network according to the present invention
`is a two‘register sequence generator, which enables
`multiple spreading-code sequences to be obtained by
`combining the outputs of selected stages of each of the
`two registers.
`Various embodiments of a multi-node digital commu
`nication network according to the present invention are
`described hereinafter. In each of these embodiments, a
`family of binary spreading-code sequences can be gen
`erated using Gold code sequences, or “symmetric"
`sequences, or Kasami code sequences, or any other
`suitable sequence generation scheme. From the family
`of binary spreading-code sequences so generated, a
`unique set of multiple spreading-code sequences is as
`signed to each node of the network. Speci?ed codes, or
`their reciprocals (i.e., codes of opposite polarity), are
`selected periodically for transmission by each node,
`
`55
`
`65
`
`20
`
`25
`
`45
`
`Page 15 of 25
`
`

`
`5,210,770
`10
`9
`conveyed per symbol and the maximum number of
`tion of the discussion to accommodate a situation in
`which the registers have different “lengths” (i.e., differ
`nodes that can be accommodated by the network.
`In embodiments of the present invention in which
`ent numbers of stages) is straightforward.
`composite codes are employed, the individual spread
`A spread-spectrum digital communication system
`ing-code sequences assigned to a given node of the
`according to the present invention can be constructed
`network may be speci?ed by feedback taps associated
`for the most part from commercially available compo
`with the polynomials f1 and f2, and by the initial “?lls”
`nents. Specially designed components are required only
`(i.e., contents) of shift registers corresponding to the
`for the spreading-code sequence generator and associ
`polynomials f1 and f2. Various methods can be used to
`ated parallel sequence correlators. In FIG. 1, a spread
`specif