CSCO-1001
`Cisco v. TQ Delta
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`US 8,718,158 B2
`Page 2
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`References Cited
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`
`Page 2 of 12
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`

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`U.S. Patent
`
`May 6, 2014
`
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`US 8,718,158 B2
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`Page 3 of 12
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`

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`U.S. Patent
`
`May 6, 2014
`
`Sheet 2 of2
`
`US 8,718,158 B2
`
`STEP 100
`
`GENEPIATE VALUE(S)
`
`STEP 110-1
`TO SYNCHRONIZE
`'
`._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __|
`
`STEP 115
`
`STEP 12°
`
`COMBINE PHASE SHII——r WITH
`PHASE CHARACTERISTIC
`
`STEP 130
`
`COMBINE CARRIER SIGNALS INTO
`A TRANSMISSION SIGNAL
`
`F "T"I=I7c\T\I_S"I\/I-1?" '
`\
`/
`'
`DETECT CLIPPING ‘\
`,
`STEP 140 ~<\/ 0|: TRANSMISSION
`/Q—{E§>| PREDEFINED
`I
`,
`SIGNAL
`L___§.l.(_3'_NAI:___J
`—\ STEP
`I so
`
`STEP 150
`
`THANSMIT TRANSMISSION SIGNAL
`
`Page 4 of 12
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`

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`US 8,718,158 B2
`
`1
`SYSTEM AND METHOD FOR SCRAMBLING
`THE PHASE OF THE CARRIERS IN A
`MULTICARRIER COMMUNICATIONS
`SYSTEM
`
`R 5 LA1 5 D APPLICATION
`
`This application is a Continuation of U.S. patent applica-
`tion Ser. No. 12/783,725, filed May 20, 2010, now U.S. Pat.
`No. 8,090,008, which is a Continuation of U.S. patent appli-
`cation Ser. No. 12/255,713, filed Oct. 22, 2008, now U.S. Pat.
`No. 7,769,104, which is a Continuation ofU.S. patent appli-
`cation Ser. No. 11/863,581 , filed Sep. 28, 2007, nowU.S. Pat.
`No. 7,471,721, which is a Continuation ofU.S. application
`Ser. No. 11/211,535, liled Aug. 26, 2005, now U.S. Pat. No.
`7,292,627, wl1icl1 is a Continuation of U . S. patent application
`Ser. No. 09/710,310, filed Nov. 9, 2000, now U.S. Pat. No.
`6,961,369, which 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 Randomizing The
`Phase Of The Carriers In A Multicarrier Communications
`System To Reduce The Peak To Average Power Ratio OfThe
`Transmitted Signal,” each of which are incorporated herein
`by reference in their entirety.
`FIELD OF TI IE INVENTION
`
`This inve11tion 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 signals.
`BACKGROUND OF THE INVENTION
`
`,,
`
`In a conventional multicarrier communications system,
`transmitters communicate over a communication channel
`using multicarrier modulation or Discrete Multitone Modu-
`lation (DMT). Carrier signals (carriers) or sub—cha1mels
`spaced within a usable frequency band of the cormnunication
`channel are modulated at a symbol (i.e., block) transmission
`rate ofthe 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 characteris-
`tic, or phase. and amplitude of the carrier signals 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 transmission
`signal, which is a linear combination of the multiple carriers,
`to a DMT receiver over the communication charmel.
`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 a11 arbi-
`trary sequence of input data bits comprising the transmitted
`information. Therefore, under the condition that the modu-
`lated data bit stream is random. the DMT transmission signal
`can be approximated as having a Gaussian probability distri-
`bution. A bit scrambler is often used in the DMT transmitter
`to scramble the input data bits before the bits are modulated to
`assure that the transmitted data bits are random and, conse-
`quently, that the modulation of those bits produces a DMT
`transmission signal with a Gaussian probability distribution.
`With an appropriate allocation of transmit power levels to
`the carriers or sub-cl1am1els, such a system provides a desir-
`able performance. Further, generating a transmission signal
`witl1 a Gaussian probability distribution is important in order
`to transmit a transmission signal with a low peak-to-average
`ratio (PAR), or peak-to-average power ratio. The PAR of a
`
`2
`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-aver-
`aged value ofthe signal parameter. I11 DMT systems, the PAR
`of the transmitted signal is determined by the probability of
`the random transmission signal reaching a certain peak volt-
`age during the time interval required for a 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 ll:'—7 probability of clipping. The PAR
`of a transmission signal transmitted and received in a DMT
`communication system is an important consideration iii the
`design of the DMT communication system because the PAR
`of a signal affects the communication system’s total power
`constunption 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 constella-
`tion rnaps, 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
`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 sig-
`nals in order to provide a low PAR for the transmission signal.
`SUMMARY OF THE INVENTION
`
`The present invention features a system and method that
`scrambles the phase characteristics of the modulated carrier
`signals iii a transmission signal. In one aspect, a value is
`associated with each carrier signal. A phase shill is computed
`for each carrier signal based on the value associated with that
`carrier signal. The value is detennined 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 to substantially 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 powerratio (PAR). The value is derived from
`a predetermined parameter, such as a random number gen-
`erator, a carrier number, a DMT symbol count, a superframe
`count, and a hyperframe count. In another embodiment, a
`predetermined transmission signal is transmitted wl1er1 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 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 con1pris—
`ing a phase scrambler that computes a phase shift for each
`carrier signal based on a value associated witl1 that carrier
`signal. The phase scrambler also combines the phase shift
`computed for each carrier signal with the phase characteristic
`ofthat carrier signal to substantially scramble the phase char-
`
`Page 5 of 12
`
`

`
`US 8,718,158 B2
`
`3
`acteristic of the carrier signals. In one embodiment, a modu-
`lator, in communication with the phase scrambler, modulates
`bits of an input signal onto the carrier signals having the
`substantially scrambled phase characteristics to produce a
`transmission signal witl1 a reduced PAR.
`DESCRIPTION OF THF, DRAWINGS
`
`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 co1nmunica-
`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 ofa process for
`scrambling the phase characteristics ofthe carrier signals in a
`transmission signal.
`DETAILED DESCRIPTION
`
`FIG. 1 shows a digital subscriber line (DSL) co1nmunica—
`tion system 2 including a discrete multitone (DMT) trans-
`ceiver 1 0 in con11nunication witl1 a remote transceiver 14 over
`a communication chamiel 18 using a transmission signal 38 ,
`having a plurality of carrier signals. The DMT transceiver 10
`includes a DMT transmitter 22 and a DMT receiver 26. The
`remote transceiver 14 includes a transmitter 30 and a receiver
`34. Although described with respect to discrete multitone
`modulation, the principles ofthe invention apply also to other , ,
`types of multicarrier modulation, such as, but not limited to,
`orthogonally multiplexed quadrature amplitude modulation
`(OQAM), discrete wavelet multitone (DWMT) modulation,
`and orthogonal frequency division multiplexing (OFDM).
`The communication channel 18 provides a downstream
`transmission path from the DMT transmitter 22 to the remote
`receiver 34, and an upstream transmission path from the
`remote transmitter 30 to the DMT receiver 26. In one embodi-
`ment, the communication charmel 18 is a pair oftwisted wires
`of a telephone subscriber line. In other embodiments, the
`commrmication channel 18 can be a fiber optic wire, a quad
`cable, consisting oftwo pairs oftwisted wires, or a quad cable
`that is one of a star quad cable, a Dieselhorst—Martin quad
`cable, and the like. In a wireless communication system
`wherein the transceivers 10, 14 are wireless modems,
`the ,
`communication channel 18 is the air through which the trans-
`mission signal 38 travels between the transceivers 10, 14.
`By way of example, the DMT transmitter 22 shown in FIG.
`1 includes a quadrature amplitude modulation (QAM)
`encoder 42, a modulator 46, a bit allocation table (BAT) 44,
`and a phase scrambler 66. The DMT transmitter 22 can also
`include a bit scrambler 74, as described further below. The
`remote transmitter 30 ofthe remote transceiver 14 comprises
`equivalent components as the DMT transmitter 22. Although
`this embodiment specifies a detailed description of the DMT
`transmitter 22, the inventive concepts apply also to the receiv-
`ers 34, 24 wl1icl1 have similar components to that of the DMT
`transmitter 22, but perfonn inverse frmctions in a reverse
`order.
`Tl1e QAM encoder 42 has a single input for receiving an
`input serial data bit stream 54 and multiple parallel outputs to
`transmit QAM symbols 58 generated by the QAM encoder 42
`
`4
`from the bit stream 54. In general, the QAM encoder 42 maps
`the input serial bit—stream 54in 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) constel-
`lation points 58, or QAM symbols 58, where N represents the
`number of carrier signals generated by the modulator 46. The
`BAT 44 is in communication with the QAM encoder 42 to
`specify the number of bits carried by each carrier signal. The
`QAM symbols 58 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 ol'a plurality of ti1ne—
`domain samples. The modulator 46 modulates each carrier
`signal with a different QAM symbol 58. As a result of this
`modulation, carrier signals have phase and amplitude cl1ar-
`acteristics based on the QAM symbol 58 and therefore based
`011 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 comprised of
`a sequence of DMT symbols 70. The modulator 46 changes
`the QAM symbols 58 into DMT symbols 70 through modu-
`lation of the carrier signals. In another embodiment, the
`, modulator 46 uses the inverse discrete Fourier transform
`(IDFT) to change the QAM symbols 58 into DMT symbols
`70. In one embodiment, a pilot tone is included in the trans-
`mission signal 38 to provide a reference signal for coherent
`demodulation of the carrier signals in the remote receiver 34
`during reception of the transmission signal 38.
`The modulator 46 also includes a phase scrambler 66 that
`combines a phase shift computed for each QAM-modulated
`carrier signal with the phase characteristic of that carrier
`signal. Combining phase shifts with phase characteristics, in
`accordance with the principles of the invention, substantially
`scrambles the phase characteristics of the carrier signals in
`the transmission signal 38. By scrambling the phase charac-
`teristics ol‘ the carrier signals, the resulting transmission sig-
`nal 38 has a substantially minimized peak—to—average (PAR)
`power ratio. The phase scrambler 66 can be part of or external
`to the modulator 46. Other embodiments of the phase scram-
`bler 66 include, but are not limited to, a software program that
`is stored in local memory and is executed on the modulator
`46. a digital signal processor (DSP) capable of performing
`mathematical functions and algorithms, and the like. The
`remote receiver 34 similarly includes a phase descrarnbler 66'
`for use when demodulating carrier signals that have had their
`phase characteristics adjusted by the phase scrambler 66 of
`the DMT transceiver 10.
`To compute a phase shift for each carrier signal, the phase
`scrambler 66 associates one or more values with that carrier
`signal. The phase scrambler 66 determines each value for a
`carrier signal independently of the QAM symbols 58, and,
`therefore, independently of the bit value(s) modulated onto
`the carrier signal. The actual value(s) that the phase scrambler
`66 associates with each carrier signal can be derived from one
`or more predefined parameters, such as a pseudo-random
`number generator (pseudo-RNG), a DMT carrier number, a
`DMT symbol count, a DMT superframe count, a DMT hyper-
`frame count, and the like, as described in more detail below.
`Irrespective of the technique used to produce each value, the
`same technique is used by the DMT transmitter 22 and the
`remote receiver 34 so that the value associated with a given
`carrier signal is known at both ends of the communication
`cha11nel 18.
`The phase scrambler 66 then solves a predetermined equa-
`tion to compute a phase shift for the carrier signal, using the
`
`Page 6 of 12
`
`

`
`US 8,718,158 B2
`
`5
`value(s) a ssociated with that carrier signal as input that effects
`the output ofthe equation. /\ny equation suitable for comput-
`i11g 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-
`mitter 22 includes a bit scrambler 74, which receives the input
`serial bit stream 54 and outputs data bits 76 that are substan-
`tially scrambled. The substantially scrambled bits 76 are then
`passed to the QAM encoder 42. \Vhen the bit scrambler 74 is
`included in the DMT transmitter 22, the operation of the
`phase scrambler 66 fiirther assures that the transmission sig-
`nal 38 has a Gaussian probability distribution 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 a11d combining these carrier signals to produce
`the transmi ssion signal 38. The DMT transmitter 22 generates
`(step 100) a value that is associated with a carrier signal.
`Because the value is being used to alter the phase character-
`istics of the carrier signal, both the DMT transmitter 22 and
`the remote receiver 34 must recognize the value as being
`associated with the carrier signal. Either the DMT transmitter
`22 and the remote receiver 34 independently derive the asso-
`ciated value, or one informs the other of the associated value.
`For example, in one embodiment the DMT transmitter 22 can
`derive the value from a pseudo-RNG and then transmit the
`generated value to the remote receiver 34. I11 another embodi-
`ment, 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-RNG produces the
`same series ofrandom numbers as the receiverpseudo -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 increments
`its symbol counter upon transmitting a DMT symbol; the
`remote receiver 34 upon receipt. 'lhus, when the DMT trans-
`mitter 22 a11d the remote receiver 34 both use the symbol
`count as a value for computing phase shifts, both the DMT
`transmitter 22 a11d remote receiver 34 “know” that the value is
`associated with a particular DMT symbol and with each car-
`rier signal of that DMT symbol.
`Values can also be derived from other types of predefined
`parameters. For example, if the predefined parameter is the
`DMT carrier number, then the value associated with a par-
`ticular carrier signal is the carrier number ofthat signal within
`the DMT symbol. The number of a can*ier 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 spainiing the frequency band-
`width from 0 kIIz to 1104 kIIz. The DMT transmitter 22
`numbers the carrier signals fiom 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.,
`51><4.3125 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 DMT carrier
`number) to make the value-carrier signal association. I11 other
`embodiments (as exemplified above with the transmitter
`pseudo -RNG), the DMT transmitter 22 ca11 transmit the value
`to the remote receiver 34 (or vice versa) over the communi-
`cation channel 18.
`
`,
`
`,,
`
`6
`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 cotmt 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 exemplary
`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-
`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 lrequency of the
`carrier signal. As another example, the pseudo-RNG gener-
`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 numeri-
`cal position of the DMT symbol 70 within a sequence of
`symbols, superframes, or hyperframes. Predefined paran1-
`eters such as the pseudo—RN G, symbol count, superframe
`count, and superframe can also be understood to be para1n—
`eters that vary values over time. Any one or combination of
`the predefined parameters can provide values 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
`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
`with the phase characteristic ofeach QAM-modulated carrier
`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
`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 three
`phase shifting examples, PS //1—PS I/3,
`illustrate methods
`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-
`ing the phase characteristic of the QAM-modulated carrier
`signal associated with a carrier number N by
`
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`US 8,718,158 B2
`
`8
`
`A
`3/
`1
`(9) X %(1TlULl2fI) = ;(Nole that 9 is the 5"’ value in XN.)
`
`modulo (mod) 27:. In this example, a carrier signal having a
`carrier number N equal to 50 l1as a phase shift added to the
`phase characteristic of that carrier signal equal to
`
`The cancier signal with a carrier number N equal to 6 has a
`phase shift added to the phase characteristic of the carrier
`signal equal to
`
`5|": >< §(mod27r) : art.
`
`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
`
`47
`
`51>< §(modl'z) = 71.
`
`The carrier signal with the carrier number N equal to 0 has no
`phase shift added to the phase characteristic of that carrier
`signal.
`
`Phase Shifting Example #2
`
`Phase shifting example #2 (PS #2) corresponds to adjust-
`ing the phase characteristic of the Q/\M—modulated carrier
`signal associated witl1 a carrier number N by
`
`7:
`/.
`N M)
`+
`,><4
`
`I
`
`n1od 275, where M is the symbol cou11t. In this example, a ——
`carrier signal having a carrier number N equal to 50 o11 DMT
`symbol count M equal to 8 has a phase shift added to the phase
`characteristic of that carrier signal equal to
`
`(5«:::~ + 3) x §(mod2:r) :
`
`7.’
`3.
`
`The carrier signal with the san1e carrier number N equal to 50
`oi1 the next DMT symbol count M equal to 9 has a phase shift
`added to the phase characteristic ofthat carrier signal equal to
`
`_,
`,
`-1110
`‘
`‘
`4
`1fi0+9\><7r(
`
`ctlrr)
`
`:1.
`4
`3”
`
`Phase Shifting Example #3
`
`Phase shifting example //3 (PS //3) corresponds to adjust-
`ing the phase characteristic of the QAM-modulated carrier
`signal associated with a carrier number N by
`
`(Xoxg.
`
`mod 27:, where XN is an array ofN pseudo-random numbers.
`In this example, a carrier signal having a carrier number N
`equal to 5 andXNequal to [3, 8, 1,4, 9, 5, .
`.
`. J l1as aphase shift
`added to the phase characteristic of the carrier signal that is
`equal to
`
`.
`(5)>< g[mod2.'I) =
`
`5.
`
`It is to be understood that additional and/or different phase
`shifting teclmiques ca11 be used by the phase scrambler 66,
`and that PS #1, #2, and #3 are merely illustrative examples of
`the principles oftl1e invention. The DMT transmitter 22 then
`combines (step 130) the carrier signals to fon11 the transmis-
`sion signal 38. If the transmission signal is not clipped, as
`described below, the DMT transmitter 22 consequently trans-
`mits (step 160) the transmission signal 38 to the remote
`receiver 34.
`Clipping of Transmission Signals
`A transmission signal 38 that has high peak values of
`voltage (ie, a high PAR) can induce non—linear distortion in
`the DMT transmitter 22 and the communication channel 18.
`One form of this no11—linear distortion of the transmission
`signal 38 that may occur is the limitation of the amplitude of
`the transmission signal 38 (ie, 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 accu-
`rately 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 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 transmit-
`ter 22 substitutes (step 150) a predefined transmission signal
`78 for the clipped transmission signal 38.
`The predefined transmission signal 78 has the same dura-
`tion as a DMT symbol 70 (eg, 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 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 embodi-
`ment, the predefined transmission signal 78 is a known
`pseudo -random sequence pattern that is easily detected by the
`remote receiver 34. In another embodiment, the predefined
`transmission signal 78 is an “all zeros” signal, which is a zero
`voltage signal produced at the DMT transmitter 22 output
`(ie, zero volts modulated on all the carrier signals). In addi-
`tion to easy detection by the remote receiver 34, the zero
`voltage signal reduces the power consumption 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 signal for coherent demodu-
`lation of the carrier signals in the remote receiver 34 during
`reception of the predefined transmission signal 78.
`After the remote receiver 34 receives the transmission sig-
`nal 38, the remote receiver 34 detennines if the transmission
`signal 38 is equivalent to the predefined transmission signal
`78. In one embodiment, when the remote receiver 34 identi-
`
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`10
`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, 70'
`are the san1e.
`If this different set of time domain samples (and conse-
`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 dillerent set of time domain
`samples 70 (and consequently t