Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`wireless modems, the communication channel 18 is the air through which the transmission
`
`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,
`
`5
`
`and a phase scrambler 66. The DMT transmitter 22 can also include a bit scrambler 74, as
`
`described further below. The remote transmitter 30 of the 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
`
`receivers 34, 263-6 which have similar components to that of the DMT transmitter 22, but
`
`10
`
`perform inverse functions in a reverse order.
`
`The 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 from the bit stream 54. In general, the QAM encoder 42 maps the input serial bit(cid:173)
`
`stream 54 in the time domain into parallel QAM symbols 58 in the frequency domain. In
`
`15
`
`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 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
`
`20
`
`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 of this modulation, carrier signals have phase and amplitude
`
`25
`
`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 comprised of a sequence ofDMT symbols
`
`70. The modulator 46 changes the QAM symbols 58 into DMT symbols 70 through
`
`modulation of the carrier signals. In another embodiment, the modulator 46 uses the inverse
`
`30
`
`discrete Fourier transform (IDFT) to change the QAM symbols 58 into DMT symbols 70. In
`
`6
`
`DISH
`Exhibit 1002 Part 2 Page 285
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`one embodiment, a pilot tone is included in the transmission 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
`
`5
`
`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 characteristics of the carrier signals, the
`
`resulting transmission signal 38 has a substantially minimized peak-to-average (PAR) power
`
`10
`
`ratio. The phase scrambler 66 can be part of or external to the modulator 46. Other
`
`embodiments of the phase scrambler 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 &ffflH'f=t{'l-1-eRkscrambler 66' for use when
`
`15
`
`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
`
`20
`
`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 hyperframe count, and the like, as described
`
`in more detail below. Irrespective of the technique used to produce each value, the same
`
`25
`
`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 channel
`
`18.
`
`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
`
`30
`
`effects the output of the equation. Any equation suitable for computing phase shifts can be
`
`7
`
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`Exhibit 1002 Part 2 Page 286
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`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 transmitter 22 includes a bit
`
`5
`
`scrambler 74, which receives the 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 transmission signal 38 has a Gaussian
`
`probability distribution and, therefore, a substantially minimized PAR.
`
`10
`
`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
`
`the transmission 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
`
`characteristics of the carrier signal, both the DMT transmitter 22 and the remote receiver 34
`
`15 must recognize the value as being associated with the carrier signal. Either the DMT
`
`transmitter 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
`
`transmitter 22 can derive the value from a pseudo-RNG and then transmit the generated value
`
`to the remote receiver 34. In another embodiment, the remote receiver 34 similarly derives
`
`20
`
`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 of random numbers as the 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 increments
`
`25
`
`its symbol counter upon transmitting a DMT symbol; 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.
`
`8
`
`DISH
`Exhibit 1002 Part 2 Page 287
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`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
`
`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
`
`5
`
`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 5 lst DMT carrier signal which is located at the frequency
`
`10
`
`of215.625 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 DMT carrier number) to make the
`
`value-carrier signal association. In other embodiments (as exemplified above with the
`
`15
`
`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
`
`20
`
`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 superframe count reaches 255.
`
`25
`
`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 generates a new random value for each carrier signal.
`
`Other predefined parameters produce values that vary from DMT symbol 70 to DMT
`
`30
`
`symbol 70. For example, when the predefined parameter is the symbol count, the superframe
`
`9
`
`DISH
`Exhibit 1002 Part 2 Page 288
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`count, or hyperframe count, values vary based on the numerical position of the DMT symbol
`
`70 within a sequence of symbols, superframes, or hyperframes. Predefined 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
`
`5
`
`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,
`
`10
`
`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.
`
`15
`
`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 of each 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
`
`20
`
`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 #3, illustrate methods used by the phase scrambler 66 to combine a computed phase
`
`25
`
`shift to the phase characteristic of each carrier signal.
`
`10
`
`DISH
`Exhibit 1002 Part 2 Page 289
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`Phase Shifting Example #1
`
`Phase shifting example #1 (PS #1) corresponds to adjusting the phase characteristic of
`
`the QAM-modulated carrier signal associated with a carrier number N by
`
`Jr Nx-
`3, modulo
`(mod) 2n. In this example, a carrier signal having a carrier number N equal to 50 has a phase
`
`2
`Jr
`50x-
`shift added to the phase characteristic of that carrier signal equal to
`3 (mod 2n) = 3 n.
`
`5
`
`The carrier signal with a carrier number N equal to 51 has a phase shift added to the phase
`
`Jr x-
`characteristic of that carrier signal equal to 51 3 (mod2n)=n. The carrier signal with the
`
`carrier number N equal to 0 has no phase shift added to the phase characteristic of that carrier
`
`signal.
`
`10
`
`Phase Shifting Example #2
`
`Phase shifting example #2 (PS #2) corresponds to adjusting the phase characteristic of
`
`the QAM-modulated carrier signal associated with a carrier number N by
`
`Jr
`(N +M)x-
`4, mod
`
`2n, where M is the symbol count. In this example, a carrier signal having a carrier number N
`
`15
`
`equal to 50 on DMT symbol count M equal to 8 has a phase shift added to the phase
`
`Jr
`Jr
`-
`(50+8)x-
`4 (mod 2n)= 2 . The carrier signal with
`characteristic of that carrier signal equal to
`
`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 equal to
`
`Jr
`(50 + 9)x-
`4 (mod
`
`3tr
`2n) = 4 .
`
`20
`
`Phase Shifting Example #3
`
`Phase shifting example #3 (PS #3) corresponds to adjusting the phase characteristic of
`
`the QAM-modulated carrier signal associated with a carrier number N by
`
`Jr
`(XN)x-,mod2tr,
`6
`
`11
`
`DISH
`Exhibit 1002 Part 2 Page 290
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`where XN is an array of N pseudo-random numbers. In this example, a carrier signal having
`
`a carrier number N equal to 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
`
`J[
`
`3Jr
`(9) x-(mod2Jr) = -
`6
`
`2
`
`(Note
`
`that 9 is the 5th value in XN.) The carrier signal with a carrier number N equal to 6 has a
`
`5
`
`phase shift added to the phase characteristic of the carrier signal equal to
`
`5Jr
`J[
`(5) x-(mod2Jr) = -
`6
`6
`
`It is to be understood that additional and/or different phase 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
`
`10
`
`carrier signals to form the transmission signal 38. If the transmission signal is not clipped, as
`
`described below, the DMT transmitter 22 consequently transmits (step 160) the transmission
`
`signal 38 to the remote receiver 34.
`
`15 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
`
`20
`
`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
`
`25
`
`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 transmitter
`
`12
`
`DISH
`Exhibit 1002 Part 2 Page 291
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`22 substitutes (step 150) a predefined transmission 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
`
`5
`
`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 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, the predefined transmission signal 78 is an "all zeros"
`
`10
`
`signal, which is a zero voltage signal produced at the DMT transmitter 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 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 demodulation of
`
`15
`
`the carrier signals in the remote receiver 34 during reception of the predefined transmission
`
`signal 78.
`
`After the remote receiver 34 receives the transmission signal 38, the remote receiver
`
`34 determines if the transmission signal 38 is equivalent to the predefined transmission signal
`
`20
`
`78. In one embodiment, when the remote receiver 34 identifies the predefined transmission
`
`signal 78, the remote receiver 34 ignores (i.e., discards) the predefined transmission signal
`
`78.
`
`Following the transmission of the predefined transmission signal 78, the phase
`
`scrambler 66 shifts (step 120) the phase characteristic of the QAM-modulated carrier signals
`
`25
`
`(based on one of the predefined parameters that varies over time). For example, consider that
`
`a set of QAM symbols 58 produces a DMT symbol 70 comprising a plurality of time domain
`
`samples, and that one of the time domain samples is larger than the maximum allowed digital
`
`value for the DMT symbol 70. Therefore, because the transmission signal 38 would be
`
`clipped when sent to the remote receiver 34, the DMT transmitter 22 sends the predefined
`
`30
`
`transmission signal 78 instead.
`
`13
`
`DISH
`Exhibit 1002 Part 2 Page 292
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`After transmission of the predefined transmission signal 78, the DMT transmitter 22
`
`again attempts to send the same bit values that produced the clipped transmission signal 38 in
`
`a subsequent DMT symbol 70'. Because the generation of phase shifts in this embodiment is
`
`based on values that vary over time, the phase shifts computed for the subsequent DMT
`
`5
`
`symbol 70' are different than those that were previously computed for the DMT symbol 70
`
`with the clipped time domain sample. These different phase shifts are combined to the phase
`
`characteristics of the modulated carrier signals to produce carrier signals of the subsequent
`
`DMT symbol 70' with different phase characteristics than the carrier signals of the DMT
`
`symbol 70 with the clipped time domain sample.
`
`10
`
`DMT communication systems 2 infrequently produce transmission signals 38 that
`clip (e.g., approximately one clip every 10 7 time domain samples 70). However, if the
`
`subsequent DMT symbol 70' includes a time domain sample that clips, then the predefined
`
`transmission signal 78 is again transmitted (step 150) to the remote receiver 34 instead of the
`
`clipped transmission signal 38. The clipping time domain sample may be on the same or on a
`
`15
`
`different carrier signal than the previously clipped DMT symbol 70. The DMT transmitter 22
`
`repeats the transmission of the predefined transmission signal 78 until the DMT transmitter
`
`22 produces a subsequent DMT symbol 70' that is not clipped. When the DMT transmitter 22
`
`produces a DMT symbol 70' that is not clipped, the DTM transmitter 22 transmits (step 160)
`
`the transmission signal 38 to the remote receiver 34. The probability of a DMT symbol 70
`
`20
`
`producing a transmission signal 38 that clips in the time domain depends on the PAR of the
`
`transmission signal 3 8.
`
`For example, the following phase shifting example, PST #4, illustrates the method
`
`used by the phase scrambler 66 to combine a different phase shift to the phase characteristic
`
`of each carrier signal to avoid the clipping of the transmission signal 38.
`
`14
`
`DISH
`Exhibit 1002 Part 2 Page 293
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`Phase Shifting Example #4
`
`Phase shifting example #4 (PS #4) corresponds to adjusting the phase characteristic of
`
`J[
`-x(M +N)
`, mod 2n, where M is
`the carrier signal associated with a carrier number N by 3
`
`the DMT symbol count. In this example, ifthe DMT symbol 70 clips when the DMT symbol
`
`5
`
`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, 70' are the same.
`
`10
`
`If this different set of time domain samples (and consequently 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
`
`15 without a time domain sample 70 that is clipped. In one embodiment, the transmitter 22 stops
`
`attempting to produce a non-clipped DMT symbol 70' for the particular set of QAM symbols
`
`58 after generating a predetermined number of clipped DMT symbols 70'. At that moment,
`
`the transmitter 22 can transmit the most recently produced clipped DMT symbol 70' or the
`
`predetermined transmission signal 78.
`
`20
`
`The PAR of the DSL communication system 2 is reduced because the predefined
`
`transmission signal 78 is sent instead of the transmission signal 38 when the DMT symbol 70
`
`clips. For example, a DMT communication system 2 that normally has a clipping probability
`
`of 10-7 for the time domain transmission signal 38 can therefore operate with a 10-5
`
`probability of clipping and a lower PAR equal to 12.8 dB (as compared to 14.5 dB). When
`25 I operating at a 10-5 probability of clipping, assuming a DMT symbol 70 has 512 time-domain
`samples 70, the DMT transmitter 22 experiences one clipped DMT symbol 70 out of every
`
`105
`
`512, or 195 DMT symbols 70. This results in the predefined (non-data carrying)
`
`transmission signal 78 being transmitted, on average, once every 195 DMT symbols.
`
`15
`
`DISH
`Exhibit 1002 Part 2 Page 294
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`Although increasing the probability of clipping to 10-5 results in approximately a 0.5%
`
`(1/195) decrease in throughput, the PAR of the transmission signal 38 is reduced by 1.7 dB,
`
`which reduces transmitter complexity in the form of power consumption and component
`
`linearity.
`
`5
`
`While the invention has been shown and described with reference to specific
`
`preferred embodiments, it should be understood by those skilled in the art that various
`
`changes in form and detail may be made therein without departing from the spirit and scope
`
`of the invention as defined by the following claims. For example, although the specification
`
`uses DSL to describe the invention, it is to be understood that various form of DSL can be
`
`10
`
`used, e.g., ADSL, VDSL, SDSL, HDSL, HDSL2, or SHDSL. It is also to be understood that
`
`the principles of the invention apply to various types of applications transported over DSL
`
`systems (e.g., telecommuting, video conferencing, high speed Internet access, video-on
`
`demand).
`
`16
`
`DISH
`Exhibit 1002 Part 2 Page 295
`
`

`
`Marked-Up Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`Abstract
`
`A system and method that demodulates-~e·l'ffff:tJ:i-les 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 substantially 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.
`
`17
`
`DISH
`Exhibit 1002 Part 2 Page 296
`
`

`
`Clean Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
`
`System and Method for Descrambling the Phase of Carriers in a Multicarrier
`Communications System
`
`5
`
`Related Application
`
`This application is a Continuation of U.S. Application No. 13/439,605, filed April 4,
`
`2012, which is a Continuation of U.S. Application No. 13/284,549, filed October 28, 2011,
`
`now U.S. Patent No. 8,218,610, which is a continuation of 11/860,080, filed September 24,
`
`2007, now U.S. Patent No. 8,073,041, which is a divisional of U.S. Application No.
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`10
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`11/211,535, filed August 26, 2005, now U.S. Patent No. 7,292,627, which is a continuation
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`of U.S. Application No. 09/710,310, filed on November 9, 2000, now U.S. Patent No.
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`6,961,369, which claims the benefit of the filing date of copending U.S. Provisional
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`Application, Serial No. 60/164,134, filed November 9, 1999, entitled "A Method For
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`Randomizing The Phase Of The Carriers In A Multicarrier Communications System To
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`15
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`Reduce The Peak To Average Power Ratio Of The Transmitted Signal," each which are
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`incorporated by reference herein in their entirety.
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`Field of the Invention
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`This invention relates to communications systems using multicarrier modulation.
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`More particularly, the invention relates to multicarrier communications systems that lower
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`20
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`the peak-to-average power ratio (PAR) of transmitted signals.
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`Background of the Invention
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`In a conventional multicarrier communications system, transmitters communicate
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`over a communication channel using multicarrier modulation or Discrete Multitone
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`Modulation (DMT). Carrier signals (carriers) or sub-channels spaced within a usable
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`25
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`frequency band of the communication channel are modulated at a symbol (i.e., block)
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`transmission rate of the system. An input signal, which includes input data bits, is sent to a
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`DMT transmitter, such as a DMT modem. The DMT transmitter typically modulates the
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`phase characteristic, or phase, and amplitude of the carrier signals using an Inverse Fast
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`Fourier Transform (IFFT) to generate a time domain signal, or transmission signal, that
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`30
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`represents the input signal. The DMT transmitter transmits the transmission signal, which is a
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`Exhibit 1002 Part 2 Page 297
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`Clean Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
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`linear combination of the multiple carriers, to a DMT receiver over the communication
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`channel.
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`The phase and amplitude of the carrier signals ofDMT transmission signal can be
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`considered random because the phase and amplitude result from the modulation of an
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`arbitrary sequence of input data bits comprising the transmitted information. Therefore,
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`under the condition that the modulated data bit stream is random, the DMT transmission
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`signal can be approximated as having a Gaussian probability distribution. A bit scrambler is
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`often used in the DMT transmitter to scramble the input data bits before the bits are
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`modulated to assure that the transmitted data bits are random and, consequently, that the
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`10 modulation of those bits produces a DMT transmission signal with a Gaussian probability
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`distribution.
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`With an appropriate allocation of transmit power levels to the carriers or sub(cid:173)
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`channels, such a system provides a desirable performance. Further, generating a transmission
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`signal with a Gaussian probability distribution is important in order to transmit a
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`15
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`transmission signal with a low peak-to-average ratio (PAR), or peak-to-average power ratio.
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`The PAR of a transmission signal is the ratio of the instantaneous peak value (i.e., maximum
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`magnitude) of a signal parameter (e.g., voltage, current, phase, frequency, power) to the time(cid:173)
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`averaged value of the signal parameter. In DMT systems, the PAR of the transmitted signal is
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`determined by the probability of the random transmission signal reaching a certain peak
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`20
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`voltage during the time interval required for a certain number of symbols. An example of the
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`PAR of a transmission signal transmitted from a DMT transmitter is 14.5 dB, which is
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`equivalent to having a IE-7 probability of clipping. The PAR of a transmission signal
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`transmitted and received in a DMT communication system is an important consideration in
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`the design of the DMT communication system because the PAR of a signal affects the
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`25
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`communication system's total power consumption and component linearity requirements of
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`the system.
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`If the phase of the modulated carriers is not random, then the PAR can increase
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`greatly. Examples of cases where the phases of the modulated carrier signals are not random
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`are when bit scramblers are not used, multiple carrier signals are used to modulate the same
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`30
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`input data bits, and the constellation maps, which are mappings of input data bits to the phase
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`2
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`Exhibit 1002 Part 2 Page 298
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`Clean Substitute Specification
`Attorney Docket No. 6936-47-CON-DIV-CON-3
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`of a carrier signal, used for modulation are not random enough (i.e., a zero value for a data
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`bit corresponds to a 90 degree phase characteristic of the DMT carrier signal and a one value
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`for a data bit corresponds to a -90 degree phase characteristic of the DMT carrier signal). An
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`increased PAR can result in a system with high power consumption and/or with high
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`5
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`probability of clipping the transmission signal. Thus, there remains a need for a system and
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`method that can effectively scramble the phase of the modulated carrier signals in order to
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`provide a low PAR for the transmission signal.
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`Summary of the Invention
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`The present invention features a system and method that scrambles the phase
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`10
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`characteristics of the modulated carrier signals in a transmission signal. In one aspect, a value
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`is associated with each carrier signal. A phase shift is computed for each carrier signal based
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`on the value associated with that carrier signal. The value is determined independently of any
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`input bit value carried by that carrier signal. The phase shift computed for each carrier signal
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`is combined with the phase characteristic of that carrier signal to substantially scramble the
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`15
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`phase characteristics of the carrier signals.
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`In one embodiment, the input bit stream is modulated onto the carrier signals having
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`the substantially scrambled phase characteristic to produce a transmission signal with a
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`reduced peak-to-average power ratio (PAR). The value is derived from a predetermined
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`parameter, such as a random numbe