Ulllted States Patent [19]
`Shively et al.
`
`US006144696A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,144,696
`Nov. 7, 2000
`
`[54] SPREAD SPECTRUM BIT ALLOCATION
`ALGORITHM
`
`1/1997 Williams ............................... .. 375/261
`5,598,435
`5,832,387 11/1998 Bae et al.
`455/522
`5,960,003
`9/1999 Fischer et al. ........................ .. 370/468
`
`[75] Inventors: Richard Robert Shively, Convent
`Station; Ranjan V. Sonalkar, North
`Caldwell, both of Ni
`
`[73] Assignee: AT&T Corp., New York, NY.
`
`_
`_
`_
`P '' Wary Exam”_1er—StePhen Chm
`Assistant ExamIner—Mohammad Ghayour
`[57]
`ABSTRACT
`
`[21] Appl. No.: 09/000,842
`_
`Dec‘ 31’ 1997
`Flled:
`[22]
`[51] Int. c1.7 ..................................................... .. H04B 1/38
`[52] us CL
`_ 375/222; 375/225; 370/391;
`370/358
`375/222 219
`[58] Field of Search
`375/225 455/127 509 370/391 358
`’
`’
`’
`’
`References Cited
`
`[56]
`
`US. PATENT DOCUMENTS
`6/1984 Skerlos et al. .......................... .. 358/85
`4,456,925
`4 620 289 10/1986 Chauvel ................................ .. 364/521
`4,725,694
`2/1988 Auer et al. .............................. .. 178/18
`4,916,441
`4/1990 Gombrich ____ __
`_ 340/712
`5,014,267
`5/1991 Tompkins et a1,
`370/62
`5,157,717 10/1992 Hitchcock .... ..
`.. 379/96
`5,335,276
`8/1994 Thompson et al-
`-- 380/21
`5a393a964
`2/1995 Hifm?ton eta1~~
`- 235/381
`5,406,615
`4/1995 Miller, 11 et al. ....................... .. 379/59
`5,488,412
`1/1996 Ma]et1 et al. ........................... .. 348/10
`5,512,935
`4/1996 Majeti et al. ..
`348/9
`5,528,593
`6/1996 English et al. .......................... .. 370/84
`5,534,913
`7/1996 Majeti et al. ............................. .. 348/7
`
`High transmission capacity in a twisted pair signal line,
`where power is limited by a power spectral-density mask
`and an aggregate signal power constraint, is obtained by: (1)
`allocating data to multitone sub-bands according to a lowest
`marginal Power-COSt Per bit Scheme and (2) in an environ
`ment where an aggregate power budget remains after all bits
`have been allocated to all sub-bands with suf?cient margins
`to carry a bit, assigning additional bits to sub-bands with
`otherwise insuf?cient power margins to carry a single bit, by
`frequency-domain-spreading a single bit across several sub
`bands at correspondingly reduced power levels, to permit
`the otherwise unacceptable noise levels to be reduced on
`average by despreading at the receiving end. Another feature
`of the mvennon’ apphcable 1“ an envlronment 1“ Whlch
`multiple interfering Channels are emP1°yed> Provides
`increased Signal throughput by (3) transmitting coherently in
`a number of multitone sub-bands, identical blocks of data,
`with the number of multitone sub-bands being equal to a
`number of interfering channels and multiplying the signal
`carried by corresponding sub-bands in the separate interfer
`ing channels by a different respective vector from an
`orthonormal basis Set SO that near_end cross_talk is e1imi_
`Hated u on des readin at the receivin end
`p
`p
`g
`g
`'
`
`7 Claims, 5 Drawing Sheets
`
`DETERMINE CHANNEL TRANSFORM GHARAGTERISTIG
`521, BY TRANSMITTING AND REcEIvING PSEUDO-NOISE
`SIGNAL AND CALCULATING ATTENUATION AND
`NOISE PowER vs. FREQUENCY
`
`ALLocATE BITS To ALL FREQUENCY BINS SUBJECT
`S22, T0 ANY MINIMUM AND MAXIMUM BITS/BIN LIMITS
`AND SUBJECT ID ANY PowER SPEGTRAL DENSITY
`MASK LIMITS
`
`35¢ GALGULATE THE TOTAL POWER REQUIREMENT
`ovER ALL CHANNELS
`
`S24
`
`IS THE POWER REQUIREMENT GREAIER
`THAN THE AGGREGATE POWER LIMIT
`FOR ALL CHANNELS?
`
`N0
`
`YES
`
`DELETE (DEALLocATE) BITS ON A GREATEST
`525w MARGINAL POWER SAvINGS BASIS UNTIL THE
`AGGREGATE PowER LIMIT Is SAIISEIED
`
`END
`
`S12
`3
`8
`SELEGT NUMBER
`ASSIGN oNE BlT ID
`0F FREQUENCY
`DINS TD SPREAD <— m BINS BEGINNING
`OVER (IN) AND
`AT INDEX
`SET INDEX:1
`
`No
`
`S10
`s
`ToTAL POWER \
`EXCEED RESIDUAL
`POWER?
`YES
`
`/
`
`CALCULATE POWER
`BIN ARRAY (To $53
`"WSW ADDmONAL
`DIT) FOR ALL BINS;
`CHECK PSD MASK
`
`SORT POWER BIN
`ARRAY 1" ASCENDING
`ORDER OF POWER ’54
`AND REcALcULATE
`MAN BINS
`
`S7
`
`"0
`
`YES
`
`END
`
`GALGULATE RESIDUAL ,5
`POWER
`[
`
`CALCULATE ToTAL
`POWER TO IRANSMIT
`oNE DIT IN ADJACENT » 59
`m BINS BEGINNING
`AT INDEX
`L
`
`CSCO-1011
`Cisco v. TQ Delta
`Page 1 of 16
`
`

`

`U.S. Patent
`
`Nov. 7,2000
`
`Sheet 1 0f5
`
`6,144,696
`
`FIG. 1
`
`__
`
`E
`
`__________________ "
`
`A
`
`XMIT
`
`
`
`
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`
`///
`A
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`SIGNAL
`POWER lf'L//
`
`FREQUENCY
`
`7
`
`FIG. 2
`
`31
`
`32
`
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`
`g A 00000000
`
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`
`MODEM I: I:
`
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`
`MODEM :I I:
`
`FIG. 3
`
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`SOURCE
`ENCODER
`
`13
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`CHANNEL
`ENCODER
`
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`V
`
`14
`8
`DIGITAL
`MODULATOR
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`:
`
`CHANNEL 4 TWISTED PAIR 15
`
`>
`
`CHANNEL
`DECODER
`S
`17
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`SOURCE
`DECODER
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`18
`
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`
`DATA
`SINK
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`19
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`
`DIGITAL
`DEMODULATOR
`S
`16
`
`;
`
`V
`
`Page 2 of 16
`
`

`

`U.S. Patent
`
`V.0N
`
`00027:
`
`Sheet 2 of 5
`
`<3
`
`E928
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`Page 3 of 16
`
`

`

`U.S. Patent
`
`Nov. 7,2000
`Sheet 3 0f5
`FIG. 5
`
`6,144,696
`
`DETERMINE CHANNEL TRANSFORM CHARACTERISTIC
`BY TRANSMITTING AND RECEIVING PSEUDO-NOISE
`521w
`SIGNAL AND CALCULATING ATTENUATION AND
`NOISE POWER VS. FREQUENCY
`
`ALLOCATE BITS TO ALL FREQUENCY BINS SUBJECT
`TO ANY MINIMUM AND MAXIMUM BITS/BIN LIMITS
`AND SUBJECT TO ANY POWER SPECTRAL DENSITY
`MASK LIMITS
`
`II
`
`32%,, CALCULATE THE TOTAL POWER REQUIREMENT
`OVER ALL CHANNELS
`
`S24
`
`IT
`IS THE POWER REQUIREMENT GREATER
`THAN THE AGGREGATE POWER LIMIT
`FOR ALL CHANNELS?
`YES
`
`N0
`
`TO FIG. 6
`
`IT
`
`DELETE (DEALLOCATE) BITS ON A GREATEST
`525w MARGINAL POWER SAVINGS BASIS UNTIL THE
`AGGREGATE POWER LIMIT IS SATISFIED
`
`Page 4 of 16
`
`

`

`U.S. Patent
`
`Nov. 7,2000
`Sheet 4 of5
`FIG. 6
`
`6,144,696
`
`$12
`s
`ASSIGN ONE BIT T0
`m BINS BEGINNING
`AI INDEX
`
`$10
`
`TOTAL POWER
`EXCEED RESIDUAL
`POWER?
`YES
`
`=
`CM)
`
`I’
`
`FROM FIG. 5
`
`s2
`3
`SELECT NUMBER
`0F FREQUENCY
`BINS Io SPREAD
`OVER (m) AND
`SET INDEX=T
`
`:
`
`v‘
`CALCULATE POWER
`BIN ARRAY (T0
`TRANSMIT ADDITIONAL J‘ 53
`BIT) FOR ALL BINS;
`CHECK PSD MASK
`
`V
`SORT POWER BIN
`ARRAY IN ASCENDING
`ORDER OF POWER ~/‘S4
`AND RECALCULATE
`MAX BINS
`
`,,
`57
`< NAx BINS >=m? /:N0
`"YES
`
`CALCULATE RESIDUAL ,
`POWER
`
`II
`CALCULATE TOTAL
`POWER TO TRANSMIT
`ONE BIT IN ADJACENT J‘ S9
`m BINS BEGINNING
`AT INDEX
`
`Page 5 of 16
`
`

`

`U.S. Patent
`
`Nov. 7,2000
`
`Sheet 5 of5
`
`6,144,696
`
`FIG. 7
`
`AFTER DE-WEIGHTING
`AND DESPREADING
`
`CODE
`1
`1
`1
`
`1
`
`A
`bed
`CHANNEL1
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`A
`[Fl
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`
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`|?
`cd
`
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`
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`be
`FREQ
`
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`C_0DE_
`1111 H W E III
`CHANNEL 2
`bi
`
`Fl
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`
`1: d
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`
`8 F ,
`lg
`FREQ
`B
`
`1
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`CHANNEL3
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`c
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`\gl lg
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`c
`
`lg
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`3|
`M FREQ
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`CPD}
`1111 Qb|c|
`CHANNEL 4
`
`Elm
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`
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`
`Page 6 of 16
`
`

`

`6,144,696
`
`1
`SPREAD SPECTRUM BIT ALLOCATION
`ALGORITHM
`
`TECHNICAL FIELD
`This invention relates to discrete multitone transmission
`(DMT) of data by digital subscriber loop (DSL) modems
`and more speci?cally to the allocation of bits, respectively,
`to the discrete multitones.
`
`2
`poWer spectral graph. The loWer curve, the channel trans
`form characteristic A represents this ?oor, that is, the com
`bined effect of noise and attenuation as a function of
`frequency. A certain margin of poWer is required to meet or
`exceed the minimum threshold of a signal for reliable data
`transmission. In other Words, the poWer of a signal in a given
`sub-band must be sufficiently high to carry a minimal (1-bit)
`QAM tone to obtain a prede?ned bit error rate. The mini
`mum margin, that required to transmit a single bit, is
`represented by curve B. Curve C represents the limits
`imposed by a poWer spectral density mask imposed by an
`external communications standard. The other limit is on the
`aggregate poWer, also de?ned by an external communication
`standard, e.g., ANSI Standard T1.413-1995 limits the total
`poWer for all sub-bands to 100 mWatts. Some coding
`techniques, such as Wei code described in American
`National Standard for Telecommunications—NetWork and
`Customer Installation Interfaces—Asymmetric Digital Sub
`scriber Line Metallic Interface, ANSI T1.413-1995, may
`also require a minimum number of bits in a frequency band
`if the band is to convey any information at all. In other
`Words, if the poWer spectral density mask limit may require
`that less energy be used than the minimum required to
`transmit a single bit.
`Note that the minimum alloWable siZe of the poWer
`margin is, in part, arbitrary since, to an extent, it is de?ned
`in terms of some a priori rules and technical criteria (Which
`are arbitrary to the extent that they establish a dividing line
`betWeen acceptable and unacceptable error rates; Bit Error
`Rate or BER) for the given communication system. Note
`also that the siZe of the margin available for a given
`sub-band corresponds to the dimension of the constellation
`that can be represented in a signal carried in that QAM
`channel. That is, the larger the margin in a band, the greater
`the number of states that can be reliably distinguished in that
`band and the larger the constellation that can be used.
`The above context creates a bit-allocation problem. That
`is, given the constraints, hoW should bits be allocated among
`the available QAM channels to provide the highest possible
`data rates? DSL modems that use DMT modulation concen
`trate the transmitted information in the frequency sub-bands
`that have the minimum attenuation and noise. The optimum
`distribution of transmission poWer is obtained by distribut
`ing the poWer according to the Well-knoWn “Water pouring”
`analogy as described in Robert G. Gallagher, Information
`Theory and Reliable Communication, John Wiley and Sons,
`NeW York, 1968. Such optimal distribution requires a strat
`egy for allocating bits to the sub-bands for the idealiZed
`situation Where the channel sub-bands approach Zero Width
`(AfQO). For discrete bits, the applicable metaphor could be
`described as an ice-cube pouring analogy.
`DSL technology Was conceived to maximiZe the through
`put on tWisted pair copper Wiring With attendant background
`noise, time-variant Far End Cross Talk (FEXT) and Near
`End Cross Talk (NEXT). To determine the transform char
`acteristic of the channel, the modems negotiate during an
`initial channel signal-to-noise ratio (SNR) estimation pro
`cedure. During the procedure, the transmitter sends a knoWn
`pseudo noise (PN) signal. The receiver computes the char
`acteristics of the received signal in the form of a ratio Nk/gk,
`Where gk is the channel gain (inverse of the attenuation) in
`frequency band k and Nk is the noise poWer in the band k.
`The literature contains many algorithms for determining the
`poWer distribution across the full frequency bandWidth for
`maximum data throughput. As noted above, the optimum
`approach for non-uniform Gaussian noise channel divided
`such that each band can be considered an additive White
`
`BACKGROUND OF THE INVENTION
`In digital communication systems employing multi
`channel or multi-carrier transmission, the most effective
`allocation of bits to the channels has been discussed in the
`literature. The Well-knoWn solution from information theory,
`analogiZed to pouring Water over a terrain de?ned by the
`noise/attenuation of the channel transform characteristic, has
`been found to insure ef?cient use of signal poWer Within
`limits de?ned by aggregate poWer and poWer spectral den
`sity mask limits. HoWever, the method in some instances
`may not go as far as possible in exploiting available poWer
`imposed by these limits.
`For heuristic purposes, the prior art and the invention are
`discussed in terms of N quadrature amplitude modulation
`(QAM) channels With a uniform symbol rate and a non
`uniform (unique to each channel) QAM constellation. QAM,
`a form of combined amplitude and phase modulation, rep
`resents k-bit sets of data by modulating tWo (orthogonal)
`quadrature carriers, cos ZJ'IZfCI and sin ZJ'IZfCI to generate a
`pulse Whose phase and amplitude convey the encoded k-bits
`of information. The QAM signal tone can be vieWed as a
`phasor in the complex plane, each distinguishable phasor
`representing a unique state of the tone identi?ed With one
`unique value in a range. Thus, if the channel and signal
`poWer are such that 4 separate phasors can be reliably
`distinguished, the scheme alloWs tWo bits to be represented.
`For 3 bits to be represented, 8 phasors must be distinguished
`and so on. The number of different phasors or states that are
`distinguishable in a single tone (pulse), the QAM
`constellation, is limited by the signal to noise ratio of the
`channel and limits imposed by external standards as dis
`cussed beloW.
`In a DMT modem, a transmission frequency band is
`separated into N sub-bands or frequency bins, each corre
`sponding to one QAM channel. In a non-ideal channel each
`sub-band has a different capacity as a result of the variation
`of noise and attenuation With frequency. In addition, external
`standards impose limits on the aggregate poWer of a signal
`(the poWer applied in all sub-band channels) and a cap on the
`poWer as a function of frequency de?ned by a poWer spectral
`density mask.
`The poWer spectral density mask may be dictated by the
`standard used in a particular country implementing the
`standard (such as A.N.S.I. standard T1.413-1995). The mask
`may also be a design constraint intentionally imposed by a
`modem designer for some other reason. For example, a
`designer may intentionally impose a constraint that no more
`than n bits are to be transmitted on transmit channel fre
`quency. Similarly, the designer may impose a constraint that
`a minimum of bits (or no bits) must be transmitted on a
`particular tone or frequency. For example, the poWer limit
`for frequencies or tones betWeen 0 and 200 kilohertZ must be
`less than -—40 dBm/HZ (a poWer level referenced to one
`milliWatt over 1 HZ bandWidth). Above 200 kHZ (to fre
`quencies in the megahertZ of spectrum), the constraint may
`be —34 dBm/HZ.
`Referring to FIG. 1, the attenuation +noise characteristics
`of a medium can be graphically represented by a ?oor in a
`
`10
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`Page 7 of 16
`
`

`

`3
`Gaussian noise channel has been proved to be the “Water
`pouring” algorithm of power distribution. In this case, the
`gk/Nk. Pro?le is compared to a terrain and the available
`aggregate poWer limit to a ?xed supply of Water poured over
`the terrain. The depth of the Water corresponds to the poWer
`spectral density. The Water pouring analogy is inappropriate
`to allocation of poWer in digital channels intended for
`transmission of binary data (bits).
`According to one method of allocating bits (John A. C.
`Bingham, Multicarrier Modulation for Data Transmission:
`An Idea Whose Time Has Come, IEEE Communications
`Magazine, May 1990, pp5—14), frequency sub-bands or bins
`are “?lled” With data bits one bit at a time. Abit is added to
`the bin for Which the marginal poWer cost is the loWest. That
`is, a bit is added to the bin such that transmission in that bin
`is the least expensive, relative to an additional bit in any
`other bin, in terms of poWer needed for the resulting signal
`constellation to be received at a prede?ned BER. The ?lling
`procedure is folloWed until the total PoWer Budget is used
`up. Since poWer can only be allocated in discrete amounts
`corresponding to each bit, the procedure is likened, as
`mentioned, to an ice-cube ?lling procedure rather than a
`Water-?lling procedure.
`
`15
`
`OBJECTS AND SUMMARY OF THE
`INVENTION
`
`25
`
`It is an object of the invention to provide a method for
`transmission in a multitone communication system together
`With an algorithm for allocating bits in the system.
`It is an object of the invention to provide a method for
`transmission in a multitone communication system subject
`to an aggregate signal poWer constraint together With an
`algorithm for allocating bits in the system.
`It is an object of the invention to provide a method for
`transmission in a multitone communication system subject
`to a signal-poWer spectral density mask constraint together
`With an algorithm for allocating bits in the system.
`It is an object of the invention to provide a method for
`transmission in a multitone communication system subject
`to an aggregate signal poWer constraint and a signal-poWer
`spectral density mask constraint together With an algorithm
`for allocating bits in the system.
`It is another object of the invention to provide a method
`for transmitting data over multiple interfering channels.
`It is another object of the invention to provide a method
`for transmitting data over multiple interfering channels and
`a method for reducing interference betWeen the interfering
`channels.
`It is another object of the invention to reduce near end
`cross talk betWeen DSL modems communicating over the
`same cable.
`Brie?y, high transmission capacity in a tWisted pair signal
`line, Where poWer is limited by a poWer spectral-density
`mask and an aggregate signal poWer constraint, is obtained
`by: (1) allocating data to multitone sub-bands according to
`a loWest marginal poWer-cost per bit scheme and (2) in an
`environment Where an aggregate poWer budget remains after
`all bits have been allocated to all sub-bands With suf?cient
`margins to carry at least one bit, assigning additional bits to
`sub-bands With otherWise insufficient poWer margins to
`carry a single bit, by frequency-domain-spreading a single
`bit across several sub-bands at correspondingly reduced
`poWer levels, to permit the otherWise unacceptable noise
`levels to be reduced on average by despreading at the
`receiving end. In an environment in Which multiple inter
`
`35
`
`45
`
`55
`
`65
`
`6,144,696
`
`4
`fering channels are employed, signal throughput is increased
`by (3) forming a number of sub-bands for spreading blocks
`of data that is equal to a number of interfering channels and
`multiplying the signal carried by corresponding sub-bands in
`the separate interfering channels by a different respective
`vector from an orthonormal basis set so that near-end
`cross-talk is eliminated upon despreading at the receiving
`end.
`Note that “spreading” as used in the present application,
`refers to a process applied at a stage Where the signal is
`decomposed into spectral elements, so that it can be applied
`selectively to frequency components, in contrast to conven
`tional spreading found in, for example, Wireless (cellular)
`telephony, Where spreading is applied to the signal time
`series, and affects (spreads) all elements of the spectrum
`equally as a consequence.
`According to the invention bit allocation may be per
`formed to optimiZe throughput Within aggregate poWer and
`poWer spectral density mask limits. Some method, such as
`the approach identi?ed above With the Water pouring
`analogy, may be used for this bit allocation. The process of
`bit allocation Will be limited either by the mask limit or the
`aggregate signal poWer limit. If after ef?cient allocation, the
`total signal poWer is less than the aggregate poWer limit,
`there Will usually be unused sub-bands. These unused sub
`bands Were rejected in the initial bit-allocation process
`because the available poWer margin in them Was insuf?cient
`to transmit a single bit. That is, the channels Were identi?ed
`as unusable because transmitting a single bit Was found to
`exceed the mask limit for the channel. In this case, Where the
`bit allocation process is limited by the mask, the channels
`With loW poWer margins are used to transmit information by
`spreading a single block of data (one or more bits) over
`multiple channels and then despreading them at the receiver.
`The device of spreading and despreading over multiple
`channels also provides a mechanism for reducing near end
`cross talk (NEXT). The context to Which this device applies
`is a packet consisting ofI interfering channels and n1 carriers
`in each channel. For example, the channels could be four
`Wire pairs, in each, some multiple of four carriers are used
`to convey information by spreading a single block over each
`of four carriers to transmit, and then despreading at the
`receiver. At the transmitter, hoWever, the signals in each
`interfering channel are multiplied by one element of an
`I-dimensional orthogonal code (such as a binary code). At
`the receiving end, the signals are multiplied again by the
`respective opposite orthogonal code and then despread. The
`process of despreading not only reduces incoherent noise as
`in the embodiment discussed above, but it also substantially
`eliminates NEXT because the interference generated in all
`the frequency channels, being derived from orthogonal set,
`cancel each other. Thus in a channel of four tWisted pairs of
`Wires, each pair transmits a different block of data but every
`different block is spread over four carriers in a given Wire
`pair. The signal transmitted over each of the four Wire pairs
`is assigned one of four orthogonal codes. Summing each
`block spread over the four frequency channels causes mutual
`cancellation of the four induced cross-talk signals of the four
`Wires that Were multiplied by the four orthogonal codes.
`Discrete Multitone (DMT) modulation serves as a frame
`Work to demonstrate the spreading process. An input data
`stream is segmented into small blocks of bits, and each such
`block is re-expressed as a complex number. For example, a
`constellation of 16 possible discrete complex number values
`can be used to convey 4 bits, since 16 different states are
`required to represent 4 bits. The resultant array of complex
`numbers is inverse-Fourier transformed to synthesiZe a time
`
`Page 8 of 16
`
`

`

`6,144,696
`
`5
`series, Y(t), that represents a sum of multiple distinct sinu
`soids. (A complex conjugate array of complex numbers is
`used as an input to the Inverse Fast Fourier Transform
`process to assure a real resultant time series.)
`Each of the complex numbers used to encode data there
`fore plays the role of a complex spectral coefficient. That is,
`each de?nes the amplitude and phase of one of the orthogo
`nal sinusoids included in the transmitted Waveform. The
`number of discrete points in the constellation for each of the
`bands is a consequence of the measured attenuation and
`noise level in that frequency band, based on a bit-allocation
`process that need not be described here.
`In both of the above schemes, the signal poWer in each
`frequency carrier is reduced in proportion to the number of
`carriers used. Also, in both schemes, the information relating
`to the number of bits per block, the frequency channels over
`Which blocks are to be spread, etc. must be shared betWeen
`the transmitter and the receiver. Regarding the latter scheme,
`the transmitter and receiver must also share the orthogonal
`codes to be used for each tWisted pair, though these can be
`established on a permanent basis.
`According an embodiment, the invention provides a trans
`mitting modem that receives digital data from a data source
`and modulates separate carriers to represent the digital data.
`The modulated signal is applied to a channel connected to a
`receiving modem. The channel is subject to a poWer spectral
`density mask. The transmitting modem includes ?rst,
`second, and third signal modulators, each With an input. The
`modem also has a signal combiner With a combined output
`connected to the channel and a serial-to-parallel converter
`connected to the data source and to each of the ?rst, second,
`and third signal modulator inputs. The connection is such
`that the digital data from the data source is converted to
`multiple parallel streams applied respectively to the ?rst,
`second, and third signal modulators. Each of the ?rst,
`second, and third signal modulators has a respective output
`connected to the signal combiner such that a sum of output
`signals of the ?rst, second, and third signal modulators is
`applied to the channel. A transfer characteristic of the
`channel is such that a ?rst minimum poWer required to
`represent a speci?ed minimum number of bits by modulat
`ing in a ?rst frequency sub-band falls beloW the poWer
`spectral density mask and such a that a second minimum
`poWer required to represent a second speci?ed minimum
`number of bits by modulating in each of second and third
`frequency sub-bands exceeds the poWer spectral density
`mask. The serial-to-parallel converter is programmed to feed
`a ?rst bit of the digital data to the ?rst signal modulator to
`represent the ?rst bit by modulating in the ?rst frequency
`sub-band at a ?rst poWer level at least as high as the ?rst
`minimum poWer. The serial-to-parallel converter is also
`programmed to feed a second bit of the digital data to the
`second and third modulators to represent the second bit by
`coherently modulating in both the second and the third
`frequency sub-bands at a second poWer level beloW the ?rst
`poWer level, Whereby resulting signals applied in the second
`and third frequency sub-bands may be combined by the
`receiving modem to retrieve the second bit. The ?rst and
`second minimum number of bits are both equal to one in the
`absence of some other speci?ed constraint.
`According another embodiment, the invention provides a
`frequency division multiplexor transmitting data from a data
`source over a channel. The multiplexor has a signal modu
`lator With an input and ?rst, second, and third outputs, each
`output transmitting data in a respective one of ?rst, second,
`and third frequency bands. A channel response detector
`connected to the channel detects a transfer characteristic of
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`the channel, the transfer characteristic including a noise
`poWer level and an attenuation of the channel. A controller
`connected to the signal modulator controls an allocation of
`?rst and second blocks of data from the data source for
`transmission in the ?rst, second, and third frequency bands.
`The controller is programmed to transmit the ?rst block of
`data in the ?rst frequency band and transmit the second
`block redundantly in each of the second and third frequency
`bands at a ?rst poWer level When the channel transfer
`characteristic is such that a poWer level required to transmit
`the second block, at a speci?ed bit error rate, in the second
`frequency band alone is a ?rst poWer level. HoWever, When
`the channel transfer characteristic is such that a poWer level
`required to transmit the second block, at the speci?ed bit
`error rate, in the second frequency band alone at a second
`poWer level, the second poWer level being higher than the
`?rst poWer level, the controller transmits the second block in
`the second frequency band alone.
`According still another embodiment, the invention pro
`vides a modem With a frequency-division modulator and a
`controller. The modulator transmits input data in separate
`frequency channels. The controller has a memory that stores
`a poWer spectral density (PSD) mask specifying the maxi
`mum poWer levels permitted for each of the frequency
`channels. The controller’s memory also stores an aggregate
`poWer limit specifying a total permitted poWer for all of the
`signals in all of the channels. The controller is programmed
`to measure and store in the memory the channel transfer
`characteristic of a communications channel through Which
`the input data is to be transmitted. The controller is also
`programmed to transmit respective unique portions of the
`input data in of the frequency channels based on the stored
`aggregate poWer limit, the PSD mask, When the measured
`transfer characteristic is a ?rst transfer characteristic. The
`controller is programmed to transmit a same portion of the
`data in at least tWo of the frequency channels responsively
`to the stored aggregate poWer limit, the PSD mask, When the
`measured transfer characteristic is a different transfer char
`acteristic.
`According still another embodiment, the invention pro
`vides a method for use in a data modulator for allocating bits
`to data channel frequencies. The method includes the fol
`loWing steps. (1) storing mask poWer data representing a
`respective maximum poWer level for each of the data
`channel frequencies; (2) storing aggregate poWer data rep
`resenting a total amount of signal poWer to be applied in all
`of the channel frequencies; (3) allocating bits on a per
`frequency basis, such that bits are successively allocated
`until the respective maximum poWer level is at least sub
`stantially reached for each of the channel frequencies and
`such that each of the bits is allocated to a single respective
`one of the channel frequencies; and (4) When the aggregate
`poWer level is not substantially reached in the step of
`allocating, further allocating bits to multiples of the channel
`frequencies for transmission at reduced poWer rates per
`channel frequency, to permit further bits to be allocated,
`until one of the aggregate poWer limit is substantially
`reached and the respective maximum poWer level is reached
`for each of the data channel frequencies.
`According still another embodiment, the invention pro
`vides an apparatus that allocates bits for data transmission
`via a multiple discrete frequencies. The apparatus has tone
`ordering circuitry, gain scaling circuitry and an inverse
`discrete Fourier transform modulator. The circuitry in com
`bination allocates initial bits to frequencies on a per fre
`quency basis, such that the initial bits are successively
`allocated until a maximum poWer level for each frequency
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`is at least substantially reached, each of the initial bits being
`unique to a given frequency. The circuitry also calculates a
`stored total power level for the initial bits allocated to a
`plurality of transmit frequencies, and if the stored total
`poWer level is not exceeded, allocate further bits to frequen
`cies for Which no initial bits are allocated, such that each of
`the further bits is redundantly allocated to more than one of
`the frequencies.
`According another embodiment, the invention provides a
`frequency-division multiplex (FDM) transmission system
`for a channel having multiple subchannels, each of the
`subchannels being susceptible to cross-talk interference
`from another of the subchannels. The system comprises a
`transmitting modem With a programmable FDM modulator
`connected to modulate ?rst and second frequency carriers,
`representing an input data stream, in each of ?rst and second
`subchannels of the channel. Also, the system includes a
`receiving modem connected to the channel and a modulator
`programmed to modulate the ?rst and second frequency
`carriers coherently to represent a ?rst subportion of the data
`stream in the ?rst and second frequency bands to form ?rst
`and second signals in the ?rst subchannel. The modulator is
`programmed to modulate the ?rst and second frequency
`carriers coherently to represent a second subportion of the
`data stream in the ?rst and second frequency bands to form
`third and fourth signals in the second subchannel. The
`receiving modem has a demodulator con?gured to combine
`coherently the ?rst and second signals. The modulator is also
`programmed to form the third and fourth signals such that
`When the demodulator combines coherently the ?rst and
`second signals, cross-talk interference in the ?rst
`subchannel, caused by concurrent transmission of the third
`and fourth signals in the second channel, is diminished in a
`combined signal resulting therefrom.
`According to still another embodiment, the invention
`provides a method for reducing near end cross talk. The
`method performs the folloWing steps. (1) forming ?rst and
`second signals in respective ?rst and second tones redun
`dantly representing ?rst data to form a ?rst multi-tone signal
`such that the ?rst and second signals are Weighted by a ?rst
`vector of an orthogonal set of codes; (2) applying the ?rst
`multi-tone signal to a ?rst interfering channel; (3) forming
`third and fourth signals in the respective ?rst and second
`tones redundantly representing second data to form a second
`multi-tone signal such that the third and fourth signals are
`Weighted by a second vector of an orthogonal set of codes;
`(4) applying the second multi-tone signal to a second
`interfering channel; and (5) combining the ?rst and second
`multitone signals such that a distortion in the ?rst data
`caused by near end cross talk in ?rst interfering channel is
`diminished.
`According to still another embodiment, the invention
`provides a frequency-di