I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US007292627B2
`
`c12) United States Patent
`Tzannes
`
`(IO) Patent No.:
`(45) Date of Patent:
`
`US 7,292,627 B2
`*Nov. 6, 2007
`
`(54) SYSTEM AND METHOD FOR SCRAMBLING
`THE PHASE OF THE CARRIERS IN A
`MULTICARRIER COMMUNICATIONS
`SYSTEM
`
`(75)
`
`Inventor: Marcos C. Tzannes, Orinda, CA (US)
`
`4,985,900 A
`5,748,677 A
`6,256,355 Bl *
`
`1/1991 Rhind et al.
`5/1998 Kumar
`7/2001 Sakoda et al. .............. 375/259
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`(73) Assignee: Aware, inc., Bedford, MA (US)
`
`EP
`
`0 584 534 Al
`
`3/1994
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 268 days.
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`This patent is subject to a terminal dis(cid:173)
`claimer.
`
`(21) Appl. No.: 111211,535
`
`(22)
`
`Filed:
`
`Aug. 26, 2005
`
`(65)
`
`Prior Publication Data
`
`US 2006/0002454 Al
`
`Jan. 5, 2006
`
`Baum!, R. W. et al., "Reducing the Peak-to-Average Power Ratio of
`Multicarrier Modulation By Selected Mapping", Electronic Letters,
`GB, IEE Stevenage, vol. 32, No. 22, Oct. 24, 1996, pp. 2056-2057,
`XP000643915 ISSN: 0013-5194.
`
`(Continued)
`
`Primary Examiner-Mohammed Ghayour
`Assistant Examiner-Lawrence Williams
`(7 4) Attorney, Agent, or Firm-Sheridan Ross P.C.; Jason H.
`Vick
`
`Related U.S. Application Data
`
`(57)
`
`ABSTRACT
`
`(63) Continuation of application No. 09/710,310, filed on
`Nov. 9, 2000, now Pat. No. 6,961,369.
`
`(51)
`
`Int. Cl.
`H04B 1138
`(2006.01)
`H04B 17100
`(2006.01)
`(52) U.S. Cl. ....................................... 375/222; 375/219
`(58) Field of Classification Search ................ 375/220,
`375/222, 219, 226, 260, 327, 362; 370/203,
`370/342, 206
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,955,141 A
`
`5/1976 Lyon et al.
`
`A system and method that scrambles the phase characteristic
`of a carrier signal are described. The scrambling of the phase
`characteristic of each carrier signal includes associating a
`value with each carrier signal and computing a phase shift
`for each carrier signal based on the value associated with
`that carrier signal. The value is determined independently of
`any input bit value carried by that carrier signal. The phase
`shift computed for each carrier signal is combined with the
`phase characteristic of that carrier signal so as to substan(cid:173)
`tially scramble the phase characteristic of the carrier signals.
`Bits of an input signal are modulated onto the carrier signals
`having the substantially scrambled phase characteristic to
`produce a transmission signal with a reduced PAR.
`
`39 Claims, 2 Drawing Sheets
`
`66
`
`Phase
`Scrambler
`
`Modulator
`46
`
`38
`
`70
`
`17/
`54
`
`26
`
`Receiver
`
`Transceiver 1Q
`
`18
`
`34
`
`------o/lt/
`
`Receiver
`
`66'
`
`30 v
`
`Transmitter
`
`Remote
`Transceiver
`11
`
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`

`

`US 7,292,627 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`6,507,585 Bl
`6,590,860 Bl *
`6,704,317 Bl
`2005/0141410 Al*
`200610092902 Al *
`200610140288 Al *
`
`1/2003 Dobson
`7/2003 Sakoda et al. .............. 370/203
`3/2004 Dobson
`............... 370/206
`6/2005 Zhang et al.
`512006 Schmidt ..................... 370/342
`612006 Holden ....................... 375/260
`
`GB
`WO
`WO
`WO
`
`2 330 491 A
`WO 98/32065
`WO 99/22463
`WO 99/29078
`
`4/1999
`7 /1998
`5/1999
`6/1999
`
`OTHER PUBLICATIONS
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0 719 004 A2
`
`6/1996
`
`Annex to Form PCT/ISA/206 for PCT/US00/30958, Mar. 23, 2001.
`* cited by examiner
`
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`

`

`-....l = N
`N °" N
`
`\C
`'N
`-....l
`rJl
`d
`
`N
`0 .....
`....
`.....
`1J1 =- ('D
`
`('D
`
`z 0
`
`....:i
`0
`0
`N
`O'I
`~
`
`~
`
`~ = ~
`
`~
`~
`~
`•
`00
`
`e •
`
`Fig. 1
`
`14
`
`Transceiver
`
`Remote
`
`18
`
`Transceiver 10
`
`\ / I '~---~
`Transmitter VI
`
`I
`
`-:..
`
`30
`I
`
`/IP-I
`
`I
`
`Scrambler
`
`Phase
`
`66'
`
`I I
`
`Receiver
`
`70
`
`38
`
`46
`
`Modulator
`
`Receiver
`
`j
`
`;//
`26
`
`42
`
`Encoder
`
`/Y
`
`54
`
`34
`
`I
`
`V7f
`44
`
`BAT
`
`i.
`
`76
`
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`

`

`U.S. Patent
`
`Nov. 6, 2007
`
`Sheet 2 of 2
`
`US 7,292,627 B2
`
`STEP 100
`
`GENERATE VALUE(S)
`
`r------------ ------------
`STEP 110-1
`COMMUNICATE VALUE(S}
`:
`l
`l
`TO SYNCHRONIZE
`~-------------------------
`
`STEP 115
`
`COMPUTE PHASE SHIFT
`
`1r
`
`,,
`
`-
`STEP 120
`
`COMBINE PHASE SHIFT WITH
`PHASE CHARACTERISTIC
`
`i..-
`
`-----,
`
`-
`STEP 130
`
`1
`
`COMBINE CARRIER SIGNALS INTO
`A TRANSMISSION SIGNAL
`,.
`
`STEP 160
`
`TRANSMIT TRANSMISSION SIGNAL
`
`FIG. 2
`
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`

`US 7,292,627 B2
`
`1
`SYSTEM AND METHOD FOR SCRAMBLING
`THE PHASE OF THE CARRIERS IN A
`MULTICARRIER COMMUNICATIONS
`SYSTEM
`
`RELATED APPLICATION
`
`This application claims the benefit of the filing date of
`copending U.S. Provisional Application, Ser. No. 60/164,
`134, filed Nov. 9, 1999, entitled "A Method For Random- 10
`izing The Phase Of The Carriers In A Multicarrier Commu(cid:173)
`nications System To Reduce The Peak To Average Power
`Ratio Of The Transmitted Signal," the entirety of which
`provisional application is incorporated by reference herein.
`
`FIELD OF THE INVENTION
`
`This invention relates to communications systems using
`multicarrier modulation. More particularly, the invention
`relates to multicarrier communications systems that lower
`the peak-to-average power ratio (PAR) of transmitted sig(cid:173)
`nals.
`
`BACKGROUND OF THE INVENTION
`
`2
`certain number of symbols. An example of the PAR of a
`transmission signal transmitted from a DMT transmitter is
`14.5 dB, which is equivalent to having a lE-7 probability of
`clipping. The PAR of a transmission signal transmitted and
`received in a DMT communication system is an important
`consideration in the design of the DMT communication
`system because the PAR of a signal affects the communi(cid:173)
`cation system's total power consumption and component
`linearity requirements of the system.
`If the phase of the modulated carriers is not random, then
`the PAR can increase greatly. Examples of cases where the
`phases of the modulated carrier signals are not random are
`when bit scramblers are not used, multiple carrier signals are
`used to modulate the same input data bits, and the constel-
`15 lation maps, which are mappings of input data bits to the
`phase of a carrier signal, used for modulation are not random
`enough (i.e., a zero value for a data bit corresponds to a 90
`degree phase characteristic of the DMT carrier signal and a
`one value for a data bit corresponds to a -90 degree phase
`20 characteristic of the DMT carrier signal). An increased PAR
`can result in a system with high power consumption and/or
`with high probability of clipping the transmission signal.
`Thus, there remains a need for a system and method that can
`effectively scramble the phase of the modulated carrier
`25 signals in order to provide a low PAR for the transmission
`signal.
`
`In a conventional multicarrier communications system,
`transmitters communicate over a communication channel
`using multicarrier modulation or Discrete Multitone Modu(cid:173)
`lation (DMT). Carrier signals (carriers) or sub-channels
`spaced within a usable frequency band of the communica- 30
`tion channel are modulated at a symbol (i.e., block) trans(cid:173)
`mission rate of the system. An input signal, which includes
`input data bits, is sent to a DMT transmitter, such as a DMT
`modem. The DMT transmitter typically modulates the phase
`characteristic, or phase, and amplitude of the carrier signals 35
`using an Inverse Fast Fourier Transform (IFFT) to generate
`a time domain signal, or transmission signal, that represents
`the input signal. The DMT transmitter transmits the trans(cid:173)
`mission signal, which is a linear combination of the multiple
`carriers, to a DMT receiver over the communication chan- 40
`nel.
`The phase and amplitude of the carrier signals of DMT
`transmission signal can be considered random because the
`phase and amplitude result from the modulation of an
`arbitrary sequence of input data bits comprising the trans(cid:173)
`mitted information. Therefore, under the condition that the
`modulated data bit stream is random, the DMT transmission
`signal can be approximated as having a Gaussian probability
`distribution. A bit scrambler is often used in the DMT
`transmitter to scramble the input data bits before the bits are 50
`modulated to assure that the transmitted data bits are random
`and, consequently, that the modulation of those bits pro(cid:173)
`duces a DMT transmission signal with a Gaussian probabil(cid:173)
`ity distribution.
`With an appropriate allocation of transmit power levels to
`the carriers or sub-channels, such a system provides a
`desirable performance. Further, generating a transmission
`signal with a Gaussian probability distribution is important
`in order to transmit a transmission signal with a low peak(cid:173)
`to-average ratio (PAR), or peak-to-average power ratio. The 60
`PAR of a transmission signal is the ratio of the instantaneous
`peak value (i.e., maximum magnitude) of a signal parameter
`(e.g., voltage, current, phase, frequency, power) to the
`time-averaged value of the signal parameter. In DMT sys(cid:173)
`tems, the PAR of the transmitted signal is determined by the 65
`probability of the random transmission signal reaching a
`certain peak voltage during the time interval required for a
`
`SUMMARY OF THE INVENTION
`
`45
`
`The present invention features a system and method that
`scrambles the phase characteristics of the modulated carrier
`signals in a transmission signal. In one aspect, a value is
`associated with each carrier signal. A phase shift is com-
`puted for each carrier signal based on the value associated
`with that carrier signal. The value is determined indepen(cid:173)
`dently of any input bit value carried by that carrier signal.
`The phase shift computed for each carrier signal is combined
`with the phase characteristic of that carrier signal to sub-
`stantially scramble the phase characteristics of the carrier
`signals.
`In one embodiment, the input bit stream is modulated onto
`the carrier signals having the substantially scrambled phase
`characteristic to produce a transmission signal with a
`reduced peak-to-average power ratio (PAR). The value is
`derived from a predetermined parameter, such as a random
`number generator, a carrier number, a DMT symbol count,
`a superframe count, and a hyperframe count. In another
`embodiment, a predetermined transmission signal is trans-
`mitted when the amplitude of the transmission signal
`exceeds a certain level.
`In another aspect, the invention features a method
`wherein a value is associated with each carrier signal. The
`value is determined independently of any input bit value
`55 carried by that carrier signal. A phase shift for each carrier
`signal is computed based on the value associated with that
`carrier signal. The transmission signal is demodulated using
`the phase shift computed for each carrier signal.
`In another aspect, the invention features a system com(cid:173)
`prising a phase scrambler that computes a phase shift for
`each carrier signal based on a value associated with that
`carrier signal. The phase scrambler also combines the phase
`shift computed for each carrier signal with the phase char(cid:173)
`acteristic of that carrier signal to substantially scramble the
`phase characteristic of the carrier signals. In one embodi(cid:173)
`ment, a modulator, in communication with the phase scram-
`bler, modulates bits of an input signal onto the carrier signals
`
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`

`US 7,292,627 B2
`
`3
`having the substantially scrambled phase characteristics to
`produce a transmission signal with a reduced PAR.
`
`DESCRIPTION OF THE DRAWINGS
`
`15
`
`20
`
`DETAILED DESCRIPTION
`
`The invention is pointed out with particularity in the
`appended claims. The advantages of the invention described
`above, as well as further advantages of the invention, may be
`better understood by reference to the following description
`taken in conjunction with the accompanying drawings, in
`which:
`FIG. 1 is a block diagram of an embodiment of a digital
`subscriber line communications system including a DMT
`(discrete multitone modulation) transceiver, in communica(cid:173)
`tion with a remote transceiver, having a phase scrambler for
`substantially scrambling the phase characteristics of carrier
`signals; and
`FIG. 2 is a flow diagram of an embodiment of a process
`for scrambling the phase characteristics of the carrier signals
`in a transmission signal.
`
`4
`encoder 42 maps the input serial bit-stream 54 in the time
`domain into parallel QAM symbols 58 in the frequency
`domain. In particular, the QAM encoder 42 maps the input
`serial data bit stream 54 into N parallel quadrature amplitude
`modulation (QAM) constellation points 58, or QAM sym(cid:173)
`bols 58, where N represents the number of carrier signals
`generated by the modulator 46. The BAT 44 is in commu(cid:173)
`nication with the QAM encoder 42 to specify the number of
`bits carried by each carrier signal. The QAM symbols 58
`10 represent the amplitude and the phase characteristic of each
`carrier signal.
`The modulator 46 provides functionality associated with
`the DMT modulation and transforms the QAM symbols 58
`into DMT symbols 70 each comprised of a plurality of
`time-domain samples. The modulator 46 modulates each
`carrier signal with a different QAM symbol 58. As a result
`ofthis modulation, carrier signals have phase and amplitude
`characteristics based on the QAM symbol 58 and therefore
`based on the input-bit stream 54. In particular, the modulator
`46 uses an inverse fast Fourier transform (IFFT) to change
`the QAM symbols 58 into a transmission signal 38 com(cid:173)
`prised of a sequence ofDMT symbols 70. The modulator 46
`changes the QAM symbols 58 into DMT symbols 70
`FIG. 1 shows a digital subscriber line (DSL) communi(cid:173)
`through modulation of the carrier signals. In another
`cation system 2 including a discrete multitone (DMT) trans- 25
`embodiment, the modulator 46 uses the inverse discrete
`ceiver 10 in communication with a remote transceiver 14
`Fourier transform (IDFT) to change the QAM symbols 58
`over a communication channel 18 using a transmission
`into DMT symbols 70. In one embodiment, a pilot tone is
`signal 38 having a plurality of carrier signals. The DMT
`included in the transmission signal 38 to provide a reference
`transceiver 10 includes a DMT transmitter 22 and a DMT
`receiver 26. The remote transceiver 14 includes a transmitter 30 signal for coherent demodulation of the carrier signals in the
`remote receiver 34 during reception of the transmission
`30 and a receiver 34. Although described with respect to
`signal 38.
`discrete multitone modulation, the principles of the inven(cid:173)
`The modulator 46 also includes a phase scrambler 66 that
`tion apply also to other types of multicarrier modulation,
`combines a phase shift computed for each QAM-modulated
`such as, but not limited to, orthogonally multiplexed quadra(cid:173)
`carrier signal with the phase characteristic of that carrier
`ture amplitude modulation (OQAM), discrete wavelet mu!- 35
`signal. Combining phase shifts with phase characteristics, in
`titone (DWMT) modulation, and orthogonal frequency divi(cid:173)
`accordance with the principles of the invention, substantially
`sion multiplexing (OFDM).
`scrambles the phase characteristics of the carrier signals in
`The communication channel 18 provides a downstream
`the transmission signal 38. By scrambling the phase char(cid:173)
`transmission path from the DMT transmitter 22 to the
`acteristics of the carrier signals, the resulting transmission
`remote receiver 34, and an upstream transmission path from 40
`signal 38 has a substantially minimized peak-to-average
`the remote transmitter 30 to the DMT receiver 26. In one
`(PAR) power ratio. The phase scrambler 66 can be part of or
`embodiment, the communication channel 18 is a pair of
`external to the modulator 46. Other embodiments of the
`twisted wires of a telephone subscriber line. In other
`phase scrambler 66 include, but are not limited to, a software
`embodiments, the communication channel 18 can be a fiber
`program that is stored in local memory and is executed on
`optic wire, a quad cable, consisting of two pairs of twisted 45
`the modulator 46, a digital signal processor (DSP) capable
`wires, or a quad cable that is one of a star quad cable, a
`of performing mathematical functions and algorithms, and
`Dieselhorst-Martin quad cable, and the like. In a wireless
`the like. The remote receiver 34 similarly includes a phase
`communication system wherein the transceivers 10, 14 are
`scrambler 66' for use when demodulating carrier signals that
`wireless modems, the communication channel 18 is the air
`have had their phase characteristics adjusted by the phase
`through which the transmission signal 38 travels between 50
`scrambler 66 of the DMT transceiver 10.
`the transceivers 10, 14.
`By way of example, the DMT transmitter 22 shown in
`To compute a phase shift for each carrier signal, the phase
`FIG. 1 includes a quadrature amplitude modulation (QAM)
`scrambler 66 associates one or more values with that carrier
`encoder 42, a modulator 46, a bit allocation table (BAT) 44,
`signal. The phase scrambler 66 determines each value for a
`and a phase scrambler 66 The DMT transmitter 22 can also 55
`carrier signal independently of the QAM symbols 58, and,
`include a bit scrambler 74, as described further below. The
`therefore, independently of the bit value( s) modulated onto
`remote transmitter 30 of the remote transceiver 14 comprises
`the carrier signal. The actual value(s) that the phase scram(cid:173)
`equivalent components as the DMT transmitter 22. Although
`bler 66 associates with each carrier signal can be derived
`this embodiment specifies a detailed description of the DMT
`from one or more predefined parameters, such as a pseudo(cid:173)
`transmitter 22, the inventive concepts apply also to the 60
`random number generator (pseudo-RNG), a DMT carrier
`receivers 34, 36 which have similar components to that of
`number, a DMT symbol count, a DMT superframe count, a
`the DMT transmitter 22, but perform inverse functions in a
`DMT hyperframe count, and the like, as described in more
`reverse order.
`detail below. Irrespective of the technique used to produce
`The QAM encoder 42 has a single input for receiving an
`each value, the same technique is used by the DMT trans(cid:173)
`input serial data bit stream 54 and multiple parallel outputs 65
`mitter 22 and the remote receiver 34 so that the value
`to transmit QAM symbols 58 generated by the QAM
`associated with a given carrier signal is known at both ends
`encoder 42 from the bit stream 54. In general, the QAM
`of the communication channel 18.
`
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`US 7,292,627 B2
`
`5
`The phase scrambler 66 then solves a predetermined
`equation to compute a phase shift for the carrier signal, using
`the value(s) associated with that carrier signal as input that
`effects the output of the equation. Any equation suitable for
`computing phase shifts can be used to compute the phase
`shifts. When the equation is independent of the bit values of
`the input serial bit stream 54, the computed phase shifts are
`also independent of such bit values.
`In one embodiment (shown in phantom), the DMT trans(cid:173)
`mitter 22 includes a bit scrambler 74, which receives the 10
`input serial bit stream 54 and outputs data bits 76 that are
`substantially scrambled. The substantially scrambled bits 76
`are then passed to the QAM encoder 42. When the bit
`scrambler 74 is included in the DMT transmitter 22, the
`operation of the phase scrambler 66 further assures that the 15
`transmission signal 38 has a Gaussian probability distribu(cid:173)
`tion and, therefore, a substantially minimized PAR.
`FIG. 2 shows embodiments of a process used by the DMT
`transmitter 22 for adjusting the phase characteristic of each
`carrier signal and combining these carrier signals to produce 20
`the transmission signal 38. The DMT transmitter 22 gener(cid:173)
`ates (step 100) a value that is associated with a carrier signal.
`Because the value is being used to alter the phase charac(cid:173)
`teristics of the carrier signal, both the DMT transmitter 22
`and the remote receiver 34 must recognize the value as being 25
`associated with the carrier signal. Either the DMT transmit-
`ter 22 and the remote receiver 34 independently derive the
`associated value, or one informs the other of the associated
`value. For example, in one embodiment the DMT transmit-
`ter 22 can derive the value from a pseudo-RNG and then 30
`transmit the generated value to the remote receiver 34. In
`another embodiment, the remote receiver 34 similarly
`derives the value from the same pseudo-RNG and the same
`seed as used by the transmitter (i.e., the transmitter pseudo(cid:173)
`RNG produces the same series of random numbers as the 35
`receiver pseudo-RNG).
`As another example, the DMT transmitter 22 and the
`remote receiver 34 can each maintain a symbol counter for
`counting DMT symbols. The DMT transmitter 22 incre(cid:173)
`ments its symbol counter upon transmitting a DMT symbol; 40
`the remote receiver 34 upon receipt. Thus, when the DMT
`transmitter 22 and the remote receiver 34 both use the
`symbol count as a value for computing phase shifts, both the
`DMT transmitter 22 and remote receiver 34 "know" that the
`value is associated with a particular DMT symbol and with
`each carrier signal of that DMT symbol.
`Values can also be derived from other types of predefined
`parameters. For example, ifthe predefined parameter is the
`DMT carrier number, then the value associated with a
`particular carrier signal is the carrier number of that signal
`within the DMT symbol. The number of a carrier signal
`represents the location of the frequency of the carrier signal
`relative to the frequency of other carrier signals within a
`DMT symbol. For example, in one embodiment the DSL
`communication system 2 provides 256 carrier signals, each
`separated by a frequency of 4.3125 kHz and spanning the
`frequency bandwidth from 0 kHz to 1104 kHz. The DMT
`transmitter 22 numbers the carrier signals from 0 to 255.
`Therefore, "DMT carrier number 50" represents the 51st
`DMT carrier signal which is located at the frequency of
`215.625 kHz (i.e., 51x4.3125 kHz).
`Again, the DMT transmitter 22 and the remote receiver 34
`can know the value that is associated with the carrier signal
`because both the DMT transmitter 22 and the remote
`receiver 34 use the same predefined parameter (here, the 65
`DMT carrier number) to make the value-carrier signal
`association. In other embodiments (as exemplified above
`
`6
`with the transmitter pseudo-RNG), the DMT transmitter 22
`can transmit the value to the remote receiver 34 (or vice
`versa) over the communication channel 18.
`In other embodiments, other predefined parameters can be
`used in conjunction with the symbol count. One example of
`such a predefined parameter is the superframe count that
`increments by one every 69 DMT symbols. One exemplary
`implementation that achieves the superframe counter is to
`perform a modulo 68 operation on the symbol count. As
`another example, the DMT transmitter 22 can maintain a
`hyperframe counter for counting hyperframes. An exem(cid:173)
`plary implementation of the hyperframe count is to perform
`a modulo 255 operation on the superframe count. Thus, the
`hyperframe count increments by one each time the super(cid:173)
`frame count reaches 255.
`Accordingly, it is seen that some predefined parameters
`produce values that vary from carrier signal to carrier signal.
`For example, when the predefined parameter is the DMT
`carrier number, values vary based on the frequency of the
`carrier signal. As another example, the pseudo-RNG gener(cid:173)
`ates a new random value for each carrier signal.
`Other predefined parameters produce values that vary
`from DMT symbol 70 to DMT symbol 70. For example,
`when the predefined parameter is the symbol count, the
`superframe count, or hyperframe count, values vary based
`on the numerical position of the DMT symbol 70 within a
`sequence of symbols, superframes, or hyperframes. Pre-
`defined parameters such as the pseudo-RNG, symbol count,
`superframe count, and superframe can also be understood to
`be parameters that vary values over time. Any one or
`combination of the predefined parameters can provide val(cid:173)
`ues for input to the equation that computes a phase shift for
`a given carrier signal.
`In one embodiment, the phase scrambling is used to avoid
`clipping of the transmission signal 38 on a DMT symbol 70
`by DMT symbol 70 basis. In this embodiment, the DMT
`transmitter 22 uses a value based on a predefined parameter
`that varies over time, such as the symbol count, to compute
`the phase shift. It is to be understood that other types of
`predefined parameters that vary the values associated with
`carrier signals can be used to practice the principles of the
`invention. As described above, the transceivers 10, 14 may
`45 communicate (step 110) the values to synchronize their use
`in modulating and demodulating the carrier signals.
`The DMT transmitter 22 then computes (step 115) the
`phase shift that is used to adjust the phase characteristic of
`each carrier signal. The amount of the phase shift combined
`50 with the phase characteristic of each QAM-modulated car(cid:173)
`rier signal depends upon the equation used and the one or
`more values associated with that carrier signal.
`The DMT transmitter 22 then combines (step 120) the
`phase shift computed for each carrier signal with the phase
`55 characteristic of that carrier signal. By scrambling the phase
`characteristics of the carrier signals, the phase scrambler 66
`reduces (with respect to unscrambled phase characteristics)
`the combined PAR of the plurality of carrier signals and,
`consequently, the transmission signal 38. The following
`60 three phase shifting examples, PS #1-PS #3, illustrate meth(cid:173)
`ods used by the phase scrambler 66 to combine a computed
`phase shift to the phase characteristic of each carrier signal.
`
`Phase Shifting Example #1
`Phase shifting example #1 (PS #1) corresponds to adjust(cid:173)
`ing the phase characteristic of the QAM-modulated carrier
`signal associated with a carrier number N by
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`7
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`US 7,292,627 B2
`
`8
`5 and XN equal to [3, 8, 1, 4, 9, 5, ... ] has a phase shift
`added to the phase characteristic of the carrier signal that is
`equal to
`
`modulo (mod) 2it. In this example, a carrier signal having a
`carrier number N equal to 50 has a phase shift added to the
`phase characteristic of that carrier signal equal to
`
`n
`
`(9)x 6 (mod 27') = 2 -
`
`3n
`
`7r
`
`2
`50 x 3 (mod 27') = 3n.
`
`10 (Note that 9 is the 5th value in XN.) The carrier signal with
`a carrier number N equal to 6 has a phase shift added to the
`phase characteristic of the carrier signal equal to
`
`The carrier signal with a carrier number N equal to 51 has
`a phase shift added to the phase characteristic of that carrier
`signal equal to
`
`15
`
`n
`
`(5) x (; (mod2n) = 6 .
`
`Sn
`
`7r
`
`51 x 3 (mod2n) = n.
`
`The carrier signal with a carrier number N equal to 0 has no
`phase shift added to the phase characteristic of that carrier
`signal.
`
`It is to be understood that additional and/or different phase
`20 shifting techniques can be used by the phase scrambler 66,
`and that PS #1, #2, and #3 are merely illustrative examples
`of the principles of the invention. The DMT transmitter 22
`then combines (step 130) the carrier signals to form the
`transmission signal 38. If the transmission signal is not
`25 clipped, as described below, the DMT transmitter 22 con(cid:173)
`sequently transmits (step 160) the transmission signal 38 to
`the remote receiver 34.
`
`Phase Shifting Example #2
`Phase shifting example #2 (PS #2) corresponds to adjust(cid:173)
`ing the phase characteristic of the QAM-modulated carrier 30
`signal associated with a carrier number N by
`
`Clipping of Transmission Signals
`A transmission signal 38 that has high peak values of
`voltage (i.e., a high PAR) can induce non-linear distortion in
`the DMT transmitter 22 and the communication channel 18.
`One form of this non-linear distortion of the transmission
`signal 38 that may occur is the limitation of the amplitude of
`the transmission signal 38 (i.e., clipping). For example, a
`particular DMT symbol 70 clips in the time domain when
`one or more time domain samples in that DMT symbol 70
`are larger than the maximum allowed digital value for the
`DMT symbols 70. In multicarrier communication systems
`when clipping occurs, the transmission signal 38 does not
`accurately represent the input serial data bit signal 54.
`In one embodiment, the DSL communication system 2
`avoids the clipping of the transmission signal 38 on a DMT
`symbol 70 by DMT symbol 70 basis. The DMT transmitter
`45 22 detects (step 140) the clipping of the transmission signal
`38. If a particular DMT symbol 70 clips in the time domain
`to produce a clipped transmission signal 38, the DMT
`transmitter 22 substitutes (step 150) a predefined transmis-
`sion signal 78 for the clipped transmission signal 38.
`The predefined transmission signal 78 has the same
`duration as a DMT symbol 70 (e.g., 250 ms) in order to
`maintain symbol timing between the DMT transmitter 22
`and the remote receiver 34. The predefined transmission
`signal 78 is not based on (i.e., independent of) the modulated
`55 input data bit stream 54; it is a bit value pattern that is
`recognized by the remote receiver 34 as a substituted signal.
`In one embodiment, the predefined transmission signal 78 is
`a known pseudo-random sequence pattern that is easily
`detected by the remote receiver 34. In another embodiment,
`60 the predefined transmission signal 78 is an "all zeros" signal,
`which is a zero voltage signal produced at the DMT trans(cid:173)
`mitter 22 output (i.e., zero volts modulated on all the carrier
`signals). In addition to easy detection by the remote receiver
`34, the zero voltage signal reduces the power consumption
`65 of the DMT transmitter 22 when delivered by the DMT
`transmitter 22. Further, a pilot tone is included in the
`predefined transmission signal 78 to provide a reference
`
`7r
`
`(N+M)x 4,
`
`mod 2it, where M is the symbol count. In this example, a
`carrier signal having a carrier number N equal to 50 on DMT
`symbol count M equal to 8 has a phase shift added to the
`phase characteristic of that carrier signal equal to
`
`7r
`
`7r
`
`(50 + 8) x 4 (mod2n) = ;;:·
`
`35
`
`40
`
`The carrier signal with the same carrier number N equal to
`50 on the next DMT symbol count M equal to 9 has a phase
`shift added to the phase characteristic of that carrier signal 50
`equal to
`
`n
`
`(50 + 9) x 4 (mod 2n) = 4 .
`
`3n
`
`Phase shifting example #3 (PS #3) corresponds to adjust(cid:173)
`ing the phase characteristic of the QAM-modulated carrier
`signal associated with a carrier number N by
`
`where XN is an array of N pseudo-random numbers. In this
`example, a carrier signal having a carrier number N equal to
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`US 7,292,627 B2
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`10
`
`7r 3" x(M +N),
`
`mod 2it, where Mis the DMT symbol count In this example,
`ifthe DMTsymbol 70 clips when the DMT symbol count M
`equals 5, the predefined transmission signal 78 is transmitted
`instead of the current clipped transmission signal 38. On the
`following DMT symbol period, the DMT count M equals 6,
`thereby causing a different set of time domain samples to be
`generated for the subsequent DMT symbol 70', although the
`QAM symbols 58 used to produce both DMT symbols 70,
`15 70' are the same.
`If this different set of time domain samples (and conse(cid:173)
`quently the transmission signal 38) is not clipped, the DMT
`transmitter 22 sends the transmission signal 38. If one of the
`time domain samples in the different set of time domain
`samples 70 (and consequently the transmission signal 38) is
`clipped, then the DMT transmitter 22 sends the predefined
`transmission signal 78 again. The process continues until a
`DMT symbol 70 is produced without a time domain sample
`25 70 that is clipped. In one embodiment, the transmitter 22
`stops attempting to produce a non-lipped DMT symbol 70'
`for the particular set ofQAM sym