|||||||||||||||
`USOO5353352A
`11
`Patent Number:
`5,353,352
`45
`Date of Patent:
`Oct. 4, 1994
`
`United States Patent 19
`Dent et al.
`
`54)
`
`(75)
`
`MULTIPLE ACCESS CODNG FOR RADIO
`COMMUNICATIONS
`Paul W. Dent, Stehag, Sweden;
`Gregory E. Bottomley, Cary, N.C.
`
`Inventors:
`
`(73)
`
`Assignee:
`
`Ericsson GE Mobile Communications
`Inc., Research Triangle Park, N.C.
`866,865
`Apr. 10, 1992
`
`Appl. No.:
`Filed:
`
`Int. Cl. ............................................... H04K 1/00
`U.S. C. .......................................... 380/37; 37.5/1;
`380/34
`Field of Search ................. 375/1; 380/34, 37, 36,
`380/49, 50
`
`21
`22)
`(51
`(52)
`58)
`
`56)
`
`a
`
`OTHER PUBLICATIONS
`Tzannes, N. S., Communication and Radar Systems,
`N.J.: Prentice-Hall, Inc., 1985, pp. 227-239.
`Stremler, F. G., Introduction to Communication Systems,
`Mass.: Addison-Wesley Publishing Co., 1982, pp.
`406-48.
`"Introduction to Spread-Spectrum Antimultipath
`Techniques and Their Application to Urban Digital
`Radio', G. Turin, Proceedings of the IEEE, vol. 68,
`No. 3, Mar. 1980.
`(List continued on next page.)
`Primary Examiner-Tod R. Swann
`Attorney, Agent, or Firm-Burns, Doane, Swecker &
`Mathis
`ABSTRACT
`57
`Individual information signals encoded with a common
`block error-correction code are assigned a unique
`scrambling mask, or signature sequence, taken from a
`set of scrambling masks having selected correlation
`properties. The set of scrambling masks is selected such
`that the correlation between the modulo-2 sum of two
`masks with any codeword in the block code is a con
`stant magnitude, independent of the mask set and the
`individual masks being compared. In one embodiment,
`when any two masks are summed using modulo-2 arith
`metic, the Walsh transformation of that sum results in a
`maximally flat Walsh spectrum. For cellular radio tele
`phone systems using subtractive CDMA demodulation
`techniques, a two-tier ciphering system ensures security
`at the cellular system level by using a pseudorandomly
`generated code key to select one of the scrambling
`masks common to all of the mobile stations in a particu
`lar cell. Also, privacy at the individual mobile sub
`scriber level is ensured by using a pseudorandomly
`generated ciphering key to encipher individual informa
`tion signals before the scrambling operation.
`
`67 Claims, 9 Drawing Sheets
`
`88
`92
`A/FESF
`Alia
`f D JSElit
`p
`Altai
`
`96
`
`References Cited
`U.S. PATENT DOCUMENTS
`Baxter et al. .
`10/1977
`4,052,565
`Ohnsorge .
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`Gutleber .
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`Gutleber .
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`Gutleber .
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`4,470,138
`Gutleber .
`2/1986
`4,568,915
`Torre et al. .
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`4,644,560
`Gilhousen et al. .
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`4,901,307
`Cripps et al. .
`5/1990
`4,930,140
`Albrieux et al. .
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`4,933,952
`Kaufmann et al. .
`1/1991
`4,984,247
`Abrahamson et al. .
`6/1991
`5,022,049
`Saleh et al. ....................... 380/34 X
`9/1991
`5,048,057
`Dent .
`9/1991
`5,048,059
`Gilhousen et al. .
`10/1991
`5,056,109
`Dent ...................................... 380/49
`10/1991
`5,060,266
`Dent .
`2/1992
`5,091,942
`Gilhousen et al. .
`3/1992
`5,101,501
`Gilhousen et al. .
`4/1992
`5,103,459
`Gilhousen et al. .
`4/1992
`5,109,390
`Bi ............................................ 375/1
`8/1992
`5,136,612
`Dent ........................................ 375/1
`9/1992
`5,151,919
`Falconer et al. ........................ 375/1
`10/1992
`5,159,608
`FOREIGN PATENT DOCUMENTS
`336832 10/1989 European Pat. Off. .
`2172777 9/1986 United Kingdom .
`80
`
`82
`
`86
`Offiti
`All
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
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`
`
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`40),
`Aff a
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`(IPHF
`AEYA,
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`AEP
`Sil/
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`Att
`SEFE
`FAIOP
`
`F-SPA
`witHPIAEA
`
`
`
`till
`Al
`tape?
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 1
`
`

`

`5,353,352
`Page 2
`
`OTHER PUBLICATIONS
`“A Communication Technique for Multipath Chan
`nels', R. Price et al., Proceedings of the IRE, Mar.
`1958, pp. 555-570.
`"Fading Channel Communications", P. Monsen, IEEE
`Communications Magazine, Jan. 1980, pp. 16-25.
`Proakis, J. G., Digital Communications,
`McGraw-Hill 1989, pp. 728-739.
`“Origins of Spread-Spectrum Communications',
`Scholtz, IEEE Transactions on Communications, vol.
`COM-30, No. 5, May 1982, pp. 18-21.
`“Very Low Rate Convolutional Codes for Maximum
`Theoretical Performance of Spread-Spectrum Multi
`ple-Access Channels' A Viterbi, IEEE Journal on
`Selected Areas in Communications, vol. 8, No. 4, May
`1990.
`MacWilliams, F., The Theory of Error-Correcting Codes,
`Part I and II, N.Y.: North-Holland, 1988, pp. 93-124,
`451-465.
`
`“A Class of Low-Rate Nonlinear Binary Codes”, A.
`Kerdock, Information and Control, vol. 20, pp. 182-187
`(1972).
`Tatsuro Masamura, "Spread Spectrum Multiple Access
`System with Intrasystem Interference Cancellation,'
`Transactions on the Institute of Electronics, Information d8.
`Communication Engineers, vol. E 71, No. 3, Mar. 1988,
`pp. 224-231.
`Mahesh K. Varanasi et al., “An Iterative Detector for
`Asynchronous Spread-Spectrum Multiple-Access Sys
`tems,' Proceedings of the IEEE Global Telecommunica
`tions Conference, vol. 1, Nov. 28-Dec. 1, 1988, pp.
`556-560.
`Ryuji Kohno et al., "Adaptive Cancellation of Interfer
`ence in Direct-Sequence Spread-Spectrum Multiple
`Access Systems,” Proceedings of the IEEE/IEICE
`Global Telecommunications Conference, vol. 1, Nov.
`15-18, 1987, pp. 630-634.
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 2
`
`

`

`U.S. Patent
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`Oct. 4, 1994
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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 3
`
`

`

`U.S. Patent
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`Oct. 4, 1994
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`
`

`

`US. Patent
`
`Oct. 4, 1994
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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 5
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 5
`
`

`

`US. Patent
`
`Oct. 4, 1994
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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 6
`
`

`

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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 7
`
`
`

`

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`
`Oct. 4, 1994
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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 8
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`

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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 9
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 9
`
`
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`

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`Oct. 4, 1994
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`
`
`
`

`

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`Oct. 4, 1994
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`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 11
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 11
`
`
`
`
`
`
`
`
`
`
`

`

`1.
`
`MULTIPLE ACCESS COOING FOR RADIO
`COMMUNICATIONS
`
`The present invention relates to the use of Code Divi
`sion Multiple Access (CDMA) communications tech
`niques in radio telephone communication systems, and
`more particularly, to an enhanced CDMA encoding
`scheme involving scrambling sequences for distinguish
`ing and protecting information signals in a spread spec
`10
`trum environment.
`BACKGROUND
`The cellular telephone industry has made phenome
`nal strides in commercial operations in the United States
`15
`as well as the rest of the world. Growth in major metro
`politan areas has far exceeded expectations and is out
`stripping system capacity. If this trend continues, the
`effects of rapid growth will soon reach even the small
`est markets. Innovative solutions are required to meet
`20
`these increasing capacity needs as well as maintain high
`quality service and avoid rising prices.
`Throughout the world, one important step in cellular
`systems is to change from analog to digital transmission.
`Equally important is the choice of an effective digital
`25
`transmission scheme for implementing the next genera
`tion of cellular technology. Furthermore, it is widely
`believed that the first generation of Personal Communi
`cation Networks (PCNs) employing low cost, pocket
`size, cordless telephones that can be carried comfort
`30
`ably and used to make or receive calls in the home,
`office, street, car, etc. would be provided by cellular
`carriers using the next generation of digital cellular
`system infrastructure and cellular frequencies. The key
`feature demanded of these new systems is increased
`35
`traffic capacity.
`Currently, channel access is achieved using Fre
`quency Division Multiple Access (FDMA) and Time
`Division Multiple Access (TDMA) methods. As illus
`trated in FIG. 1(a), in FDMA, a communication chan
`nel is a single radio frequency band into which a signal's
`transmission power is concentrated. Interference with
`adjacent channels is limited by the use of bandpass fil
`ters that only pass signal energy within the filters' speci
`fied frequency bands. Thus, with each channel being
`45
`assigned a different frequency, system capacity is lim
`ited by the available frequencies as well as by limitations
`imposed by channel reuse.
`In TDMA systems, as shown in FIG. 1(b), a channel
`consists of a time slot in a periodic train of time intervals
`over the same frequency. Each period of time slots is
`called a frame. A given signal's energy is confined to
`one of these time slots. Adjacent channel interference is
`limited by the use of a time gate or other synchroniza
`tion element that only passes signal energy received at
`55
`the propertime. Thus, the problem of interference from
`different relative signal strength levels is reduced.
`Capacity in a TDMA system is increased by com
`pressing the transmission signal into a shorter time slot.
`As a result, the information must be transmitted at a
`correspondingly faster burst rate that increases the
`amount of occupied spectrum proportionally. The fre
`quency bandwidths occupied are thus larger in FIG.
`1(b) than in FIG. 1(a).
`With FDMA or TDMA systems or hybrid
`65
`FDMA/TDMA systems, the goal is to ensure that two
`potentially interfering signals do not occupy the same
`frequency at the same time. In contrast, Code Division
`
`5,353,352
`2
`Multiple Access (CDMA) allows signals to overlap in
`both time and frequency, as illustrated in FIG. 1(c).
`Thus, all CDMA signals share the same frequency spec
`trum. In both the frequency and the time domain, the
`multiple access signals overlap. Various aspects of
`CDMA communications are described in "On the Ca
`pacity of a Cellular CDMA System,' by Gilhousen,
`Jacobs, Viterbi, Weaver and Wheatley, IEEE Trans, on
`Vehicular Technology, May 1991.
`In a typical CDMA system, the informational datas
`tream to be transmitted is impressed upon a much
`higher bit rate datastream generated by a pseudoran
`dom code generator. The informational datastream and
`the high bit rate datastream are typically multiplied
`together. This combination of higher bit rate signal with
`the lower bit rate datastream is called coding or spread
`ing the informational datastream signal. Each informa
`tional datastream or channel is allocated a unique
`spreading code. A plurality of coded information sig
`nals are transmitted on radio frequency carrier waves
`and jointly received as a composite signal at a receiver.
`Each of the coded signals overlaps all of the other
`coded signals, as well as noise-related signals, in both
`frequency and time. By correlating the composite signal
`with one of the unique spreading codes, the correspond
`ing information signal is isolated and decoded.
`There are a number of advantages associated with
`CDMA communication techniques. The capacity limits
`of CDMA-based cellular systems are projected to be up
`to twenty times that of existing analog technology as a
`result of the wideband CDMA system's properties such
`as improved coding gain/modulation density, voice
`activity gating, sectorization and reuse of the same spec
`trum in every cell. CDMA is virtually immune to multi
`path interference, and eliminates fading and static to
`enhance performance in urban areas. CDMA transmis
`sion of voice by a high bit rate encoder ensures superior,
`realistic voice quality. CDMA also provides for vari
`able data rates allowing many different grades of voice
`quality to be offered. The scrambled signal format of
`CDMA completely eliminates cross-talk and makes it
`very difficult and costly to eavesdrop or track calls,
`insuring greater privacy for callers and greater immu
`nity from air time fraud.
`Despite the numerous advantages afforded by
`CDMA systems, the capacity of conventional CDMA
`systems is limited by the decoding process. Because so
`many different user communications overlap in time
`and frequency, the task of correlating the correct infor
`mation signal with the appropriate user is complex. In
`practical implementations of CDMA communications,
`capacity is limited by the signal-to-noise ratio, which is
`essentially a measure of the interference caused by other
`overlapping signals as well as background noise. The
`general problem to be solved, therefore, is how to in
`crease system capacity and still maintain system integ
`rity and a reasonable signal-to-noise ratio. A specific
`aspect of that problem is how to optimize the process of
`distinguishing each coded information signal from all of
`the other information signals and noise-related interfer
`eCe.
`Another issue to be resolved in CDMA systems is
`system security and individual subscriber privacy. Since
`all of the coded subscriber signals overlap, CDMA
`decoding techniques typically require that the specific
`codes used to distinguish each information signal be
`generally known. This public knowledge of the actual
`codes used in a particular cell invites eavesdropping.
`
`SO
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 12
`
`

`

`20
`
`35
`
`10
`
`15
`
`3
`SUMMARY
`The encoding of individual information signals is
`simplified by encoding each signal with a common
`block error-correction code, which may be readily
`decoded using a correlation device such as a Fast Walsh
`Transform circuit. Each coded information signal is
`then assigned a unique scrambling mask, or signature
`sequence, taken from a set of scrambling masks having
`certain selected auto- and cross-correlation properties.
`These scrambling masks are ordered based on the signal
`strength of their respectively assigned coded informa
`tion signals. To enhance the decoding process, the high
`est ordered scrambling masks are initially selected in
`sequence to descramble the received composite signal.
`In general terms, the scrambling mask set is selected
`such that the sum of any two scrambling masks, using
`modulo-2 arithmetic, is equally correlated in magnitude
`to all codewords of the common block error-correction
`code. For the case where the block error-correction
`code is a Walsh-Hadamard code, if any two scrambling
`masks are summed using modulo-2 arithmetic, and the
`binary values of the product are represented with +1
`and -1 values, then the Walsh transform of that sum
`results in a maximally flat Walsh spectrum. Sequences
`25
`with such a spectrum are sometimes referred to as
`“bent' sequences.
`In the context of cellular radio telephone systems
`using subtractive CDMA demodulation techniques, the
`present invention incorporates a two-tier ciphering
`30
`system to ensure security at the cellular system level
`and privacy at the individual mobile subscriber level. At
`the system level, a pseudorandomly generated code key
`is used to select one of the scrambling masks common to
`all of the mobile stations in a particular cell. At the
`subscriber level, a pseudorandomly generated ciphering
`key enciphers individual information signals before the
`scrambling operation.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention will now be described in more
`detail with reference to preferred embodiments of the
`invention, given only by way of example, and illustrated
`in the accompanying drawings, in which:
`FIGS. 1(a)-(c) are plots of access channels using
`45
`different multiple access techniques;
`FIG. 2 shows a series of graphs illustrating how
`CDMA signals are generated;
`FIGS. 3 and 4 show a series of graphs for illustratin
`how CDMA signals are decoded;
`50
`FIG. 5 shows a series of graphs illustrating a subtrac
`tive CDMA demodulation technique;
`FIG. 6 is a generalized schematic showing a spread
`spectrum communications system;
`FIG. 7 is a functional block diagram of a system that
`55
`may be used to implement one of the preferred embodi
`ments of the present invention;
`FIG. 8 is a block diagram of another receiver in ac
`cordance with the present invention; and
`FIG. 9 is a functional block diagram of a system that
`may be used to implement another of the preferred
`embodiments of the present invention.
`DETAILED DESCRIPTION
`While the following description is in the context of 65
`cellular communications systems involving portable or
`mobile radio telephones and/or Personal Communica
`tion Networks (PCNs), it will be understood by those
`
`5,353,352
`4
`skilled in the art that the present invention may be ap
`plied to other communications applications. Moreover,
`while the present invention may be used in a subtractive
`CDMA demodulation system, it also may be used in
`applications of other types of spread spectrum commu
`nication systems.
`CDMA demodulation techniques will now be de
`scribed in conjunction with the signal graphs shown in
`FIGS. 2-4 which set forth example waveforms in the
`coding and decoding processes involved in traditional
`CDMA systems. Using the waveform examples from
`FIGS. 2-4, the improved performance of a subtractive
`CDMA demodulation technique is illustrated in FIG. 5.
`Additional descriptions of conventional and subtractive
`CDMA demodulation techniques may be found in co
`pending, commonly assigned U.S. patent application
`Ser. No. 07/628,359 filed on Dec. 17, 1990, which is
`incorporated herein by reference, and in co-pending,
`commonly assigned U.S. patent application Ser. No.
`07/739,446 filed on Aug. 2, 1991, which is also incorpo
`rated herein by reference.
`Two different datastreams, shown in FIG.2 as signal
`graphs (a) and (d), represent digitized information to be
`communicated over two separate communication chan
`nels. Information signal 1 is modulated using a high bit
`rate, digital code that is unique to signal 1 and that is
`shown in signal graph (b). For purposes of this descrip
`tion, the term “bit” refers to a binary digit or symbol of
`the information signal. The term "bit period' refers to
`the time period between the start and the finish of one
`bit of the information signal. The term "chip” refers to
`a binary digit of the high rate code signal. Accordingly,
`the term "chip period' refers to the time period be
`tween the start and the finish of one chip of the code
`signal. Naturally, the bit period is much greater than the
`chip period. The result of this modulation, which is
`essentially the product of the two signal waveforms, is
`shown in the signal graph (c). In Boolean notation, the
`modulation of two binary waveforms is essentially an
`exclusive-OR operation. A similar series of operations is
`carried out for information signal 2 as shown in signal
`graphs (d)-(f). In practice, of course, many more than
`two coded information signals are spread across the
`frequency spectrum available for cellular telephone
`communications.
`Each coded signal is used to modulate a radio fre
`quency (RF) carrier using any one of a number of mod
`ulation techniques, such as Binary Phase Shift Keying
`(BPSK) or Quadrature Phase Shift Keying (QPSK). In
`a cellular telephone system, each modulated carrier is
`transmitted over an air interface. At a radio receiver,
`such as a cellular base station, all of the signals that
`overlap in the allocated frequency bandwidth are re
`ceived together. The individually coded signals are
`added, as represented in the signal graphs (a)-(c) of
`FIG. 3, to form a composite signal waveform (graph
`(c)).
`After demodulation of the received signal to the ap
`propriate baseband frequency, the decoding of the com
`posite signal takes place. Information signal 1 may be
`decoded or despread by multiplying the received com
`posite signal shown in FIG. 3(c) with the unique code
`used originally to modulate signal 1 that is shown in
`signal graph (d). The resulting signal is analyzed to
`decide the polarity (high or low, +1 or -1, “1” or "0")
`of each information bit period of the signal. The details
`of how the receiver's code generator becomes time
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 13
`
`

`

`5,353,352
`5
`6
`synchronized to the transmitted code are known in the
`lation technique, “noise' includes both hostile and
`friendly signals, and may be defined as any signals other
`art.
`These decisions may be made by taking an average or
`than the signal of interest, i.e., the signal to be decoded.
`majority vote of the chip polarities during each bit
`Expanding the example described above, if a signal-to
`period. Such "hard' decision making processes are
`interference ratio of 10:1 is required and the processing
`acceptable as long as there is no signal ambiguity. For
`gain is 1000:1, conventional CDMA systems have the
`example, during the first bit period in the signal graph
`capacity to allow up to 101 signals of equal energy to
`(f), the average chip value is -- 1.00 which readily indi
`share the same channel. During decoding, 100 of the
`cates a bit polarity +1. Similarly, during the third bit
`101 signals are suppressed to 1/1000th of their original
`period, the average chip value is -0.75, and the bit
`interfering power. The total interference energy is thus
`polarity is also most likely a -1. However, in the sec
`100/1000, or 1/10, as compared to the desired informa
`ond bit period, the average chip value is zero, and the
`tion energy of unity. With the information signal energy
`majority vote or average test fails to provide an accept
`ten times greater than the interference energy, the infor
`able polarity value.
`nation signal may be correlated accurately.
`Together with the required signal-to-interference
`In such ambiguous situations, a "soft' decision mak
`15
`ing process must be used to determine the bit polarity.
`ratio, the processing gain determines the number of
`For example, an analog voltage proportional to the
`allowed overlapping signals in the same channel. That
`received signal after despreading may be integrated
`this is still the conventional view of the capacity limits
`over the number of chip periods corresponding to a
`of CDMA systems may be recognized by reading, for
`single information bit. The sign or polarity of the net
`example, the above-cited paper by Gilhousen et al.
`20
`integration result indicates that the bit value is a +1 or
`In contrast to conventional CDMA, an important
`“1.
`aspect of the subtractive CDMA demodulation tech
`The decoding of signal 2, similar to that of signal 1, is
`nique is the recognition that the suppression of friendly
`illustrated in the signal graphs (a)-(d) of FIG. 4. How
`CDMA signals is not limited by the processing gain of
`ever, after decoding, there are no ambiguous bit polar
`the spread spectrum demodulator as is the case with the
`25
`suppression of military type jamming signals. A large
`ity situations.
`Theoretically, this decoding scheme can be used to
`percentage of the other signals included in a received,
`decode every signal that makes up the composite signal.
`composite signal are not unknown jamming signals or
`Ideally, the contribution of unwanted interfering signals
`environmental noise that cannot be correlated. Instead,
`is minimized when the digital spreading codes are or
`most of the noise, as defined above, is known and is used
`30
`thogonal to the unwanted signals. (Two binary sequen
`to facilitate decoding the signal of interest. The fact that
`ces are orthogonal if they differ in exactly one half of
`the characteristics of most of these noise signals are
`their bit positions.) Unfortunately, only a certain num
`known, including their corresponding spreading codes,
`ber of orthogonal codes exist for a given word length.
`is used in the subtractive CDMA demodulation tech
`nique to improve system capacity and the accuracy of
`Another problem is that orthogonality can be main
`35
`tained only when the relative time alignment between
`the signal decoding process. Rather than simply decode
`two signals is strictly maintained. In communications
`each information signal from the composite signal, the
`environments where portable radio units are moving
`subtractive CDMA demodulation technique also re
`constantly, such as in cellular systems, precise time
`moves each information signal from the composite sig
`alignment is difficult to achieve. When code orthogo
`nal after it has been decoded. Those signals that remain
`nality cannot be guaranteed, noise-based signals may
`are decoded only from the residual of the composite
`interfere with the actual bit sequences produced by
`signal. Consequently, the already decoded signals do
`different code generators, e.g., the mobile telephones.
`not interfere with the decoding of the remaining signals.
`In comparison with the originally coded signal energies,
`For example, in FIG. 5, if signal 2 has already been
`however, the energy of the noise signals is usually small.
`decoded as shown in the signal graph (a), the coded
`45
`"Processing gain is a parameter of spread spectrum
`form of signal 2 can be reconstructed as shown in the
`systems, and for a direct spreading system it is defined
`signal graphs (b) and (c) (with the start of the first bit
`as the ratio of the spreading or coding bit rate to the
`period of the reconstructed datastream for signal 2
`underlying information bit rate, i.e., the number of chips
`aligned with the start of the fourth chip of the code for
`per information bit or symbol. Thus, the processing gain
`signal 2 as shown in FIG. 2 signal graphs (d) and (e)),
`is essentially the bandwidth spreading ratio, i.e., the
`and subtracted from the composite signal in the signal
`ratio of the bandwidths of the spreading code and infor
`graph (d) (again with the first chip of the reconstructed
`mation signal. The higher the code bit rate, the wider
`coded signal 2 aligned with the fourth chip of the re
`the information is spread and the greater the spreading
`ceived composite signal) to leave coded signal 1 in the
`ratio. For example, a one kilobit per second information
`signal graph (e). This is easily verified by comparing
`55
`signal graph (e) in FIG. 5 with signal graph (c) in FIG.
`rate used to modulate a one megabit per second code
`signal has processing gain of 1000:1. The processing
`2 (truncated by removing the first three and the very
`last chip). Signal 1 is recaptured easily by multiplying
`gain shown in FIG. 2, for example, is 8:1, the ratio of the
`code chip rate to the information datastream bit rate.
`the coded signal 1 with code 1 to reconstruct signal 1.
`Large processing gains reduce the chance of decod
`Note that because the bit periods of datastreams for
`ing noise signals modulated using uncorrelated codes.
`signals 1 and 2 are shifted relative to one another by 2
`For example, processing gain is used in military con
`chips there are only six + 1 chips in the first bit period
`texts to measure the suppression of hostile jamming
`of the recaptured signal 1 shown in FIG. 5 signal graph
`signals. In other environments, such as cellular systems,
`(f). It is significant that while the conventional CDMA
`processing gain helps suppress other, friendly signals
`decoding method was unable to determine whether the
`that are present on the same communications channel
`polarity of the information bit in the second bit period
`of signal 1 was a +1 or a -1 in signal graph (f) of FIG.
`but use codes that are uncorrelated with the desired
`code. In the context of the subtractive CDMA demodu
`3, the decoding method of the subtractive CDMA de
`
`65
`
`O
`
`50
`
`Petitioner Sirius XM Radio Inc. - Ex. 1011, p. 14
`
`

`

`O
`
`5,353,352
`7
`8
`modulation technique effectively resolves that ambigu
`24-bit orthogonal codewords. Decoding an orthogonal
`ity simply by removing signal 2 from the composite
`codeword involves correlation with all members of the
`signal.
`set of N=2M codewords. The binary index of the code
`A general CDMA system will now be described in
`word giving the highest correlation yields the desired
`conjunction with FIG. 6. An information source such as
`information. For example, if a correlation of sixteen
`speech is converted from analog format to digital for
`16-bit codewords numbered 0-15 produces the highest
`mat in a conventional source coder 20. The digital bit
`correlation on the tenth 16-bit codeword, the underly
`stream generated by the transmitter source coder 20
`ing information signal is the 4-bit binary codeword 1010
`may be further processed in a transmitter error correc
`(which is the integer 10 in decimal notation, hence, the
`tion coder 22 that adds redundancy which increases the
`index of 10). Such a code is also termed a 16,4 orthog
`bandwidth orbitrate of the transmission. In response to
`onal block code and has a spreading ratio R= 16/4=4.
`a spreading code selection signal from a suitable control
`By inverting all of the bits of the codewords, one fur
`mechanism such as a programmable microprocessor
`ther bit of information may be conveyed per codeword.
`(not shown), a particular spreading code is generated by
`This type of coding is known as bi-orthogonal block
`coding.
`a transmit spreading code generator 24, which as de
`15
`scribed above may be a pseudorandom number genera
`A significant feature of such coding is that simulta
`tor. The selected spreading code is summed in a modu
`neous correlation with all the orthogonal block code
`lo-2 adder 26 with the coded information signal from
`words in a set may be performed efficiently by means of
`the error correction coder 22. It will be appreciated that
`a Fast Walsh