United States Patent
`
`[19]
`
`[11] Patent Number:
`
`6,144,695
`
`Helms et al.
`
`[45] Date of Patent:
`
`Nov. 7, 2000
`
`USUUGI44-695A
`
`[54] METHOD AND APPARATUS FOR
`REDUCING NEAR-END CROSSTALK (NEXT)
`IN DISCRETE MULTI-TONE M0DULAT0R/
`DEMODULATORS
`Inventors: Howard David Helms, Brookside;
`Robert Raymond Miller, 11, Township
`of Morris, Morris County; Richard
`Roben Shjve]y, Convcm station, an of
`NJ.
`
`Assignee: AT&T Corp., New York, N.Y.
`
`APPL N0‘: 08/991176
`
`Dec. 23, 1997
`Filed:
`Int. Cl.” ............................. H04B 1/38; H04B 15/00;
`H04L 5/16
`........................ .. 375/222; 375/285; 375/346;
`370/201; 370/480; 370/503; 379/93.01;
`
`U.S. Cl.
`
`379-/9308; 379/406; 379/410; 379/4179
`455/295; 455/501
`Field of Search ................................... .. 375/222, 285,
`375/346: 3563 370/201» 286» 480> 503>
`514: 515? 379/399’ 406’ 410’ 416’ 417’
`90-01> 9301: 93089 455/3-1’ 5-1’ 501’
`502: 295
`
`.
`References Clted
`U_S_ PATENT DOCUMENTS
`
`54797447 12/1995 Chow 9‘ a1~
`5-‘519’731
`5/1996 Cioffi
`..
`5,521,949
`5/1996 Huang et al.
`5,668,802
`9/1997 Chalmers et al
`5,673,290
`9/1997 Cioffi
`.......... ..
`5,680,394 10/1997 Bingham et al.
`
`~~~~~~~~~~~~~~~~~~~~~~~~~~ -~ 375/260
`375/200
`.. 375/377
`.. 370/276
`375/260
`..................... .. 370/294
`
`........... .. 375/257
`. [375//222
`~ 575/260
`. 375/346
`375/222
`
`3/1999
`5,887,032
`5.970.088 10/1999
`"»009»122
`12/1999
`6,014,412
`1/2000
`............
`6,044,107
`3/2000 Gatherer eta].
`OTHER PUBLICATIONS
`‘
`.
`.
`‘
`’
`.
`American National Standards lnstitite, Standards Docu-
`ment—T1.413—1995; “Network and Customer Installation
`Interfaces—AsyIIIII1etric Digital Subscriber Line (ADSL)
`Metallic Interface”; Approved Aug. 18, 1995, pp.: cover
`page; two inside sheets; i—Vi; 1-115.
`Primary ExaIniner%tephen Chin
`Assistant Exumirter—Michael W. Maddox
`
`ABSTRAC1.
`[57]
`This invention reduces near-end crosstalk between identical
`discrete miilti-tone (DMT-type) modems by introducing the
`following modifications to modems at a central oflice or a
`subscriber premises. The modifications include: a mecha-
`nism for aligning the frames of all modems at the central
`0 ice (CO), a mechanism for aligning the frames transmitted
`by Lhc Subscribcrsv modcms with lhs framcs rcccivcd by
`those modems, and lengthening the DMT frame to include
`a cyclic prefix. The cyclic prefix to a frame has been made
`longer than twice the maximum round-trip delav, that is, the
`delay between the central 0 Tee and the most distant sub-
`scriber to be served. By making adjacent DMT carrier
`frequencies be orthogonal to each other, the aforementioned
`modifications Wlll
`reduce both Near End (Iross-Talk
`(NEXT) and Leakage—Excited Cross—Talk (LEXT). These
`benefits apply to inoderns at both the central ofiice (CO) and
`subscriber ends of the communication path (for example a
`-
`.
`~
`_
`~
`~
`hlgh Capacny twlsled pa“ Wm)‘
`
`26 Claims, 7 Drawing Sheets
`
`2232 SAMPLES
`
`FRAMES TRANSMITTED BY ALL
`C0 MODEMS
`
`2232 SAMPLES
`
`FRAME RECEIVED FROM A SUBSCRIBER'S
`MODEM THAT IS VERY NEAR TO THE CD
`
`2232 SAMPLES
`
`FRAME RECEIVED FROM A SUBSCRIBER'S
`MODEM AT MAXIMUM DISTANCE FROM THE CD
`
`F/3}’_~:.—»3952[11ml___.l________
`
`2043 SAMPLES
` -
`INTERVAL OVER WHICH THE CO'S MODEMS SAMPLE A
`RECEIVED FRAME
` ——
`TIME
`
`Dish
`Exhibit 1011, Page 1
`
`

`
`Nov. 7, 2000
`
`Sheet 1 of 7
`
`6,144,695
`
`1
`FIG.
`(PRIOR ART)
`
`CI
`DISTRIBUTION
`
`150
`
`DIGITAL
`NETWORK
`
`SPLITTER
`
`Dish
`Exhibit 1011, Page 2
`
`

`
`U.S. Patent
`
`Nov. 7, 2000
`
`Sheet 2 of 7
`
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`Dish
`Exhibit 1011, Page 3
`
`

`
`U.S. Patent
`
`v.0N
`
`00027:
`
`Sheet 3 of 7
`
`6,144,695
`
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`Exhibit 1011, Page 4
`
`

`
`U.S. Patent
`
`Nov. 7, 2000
`
`7f044|.6BhS
`
`6,144,695
`
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`Exhibit 1011, Page 5
`
`

`
`U.S. Patent
`
`Nov. 7, 2000
`
`Sheet 5 of 7
`
`6,144,695
`
`FIG. 5
`
`2232 SAMPLES
`
`FRAMES TRANSMITTED BY ALL
`CO MODEMS
`
`2232 SAMPLES
`<j‘
`FRAME RECEIVED FROM A SUBSCRIBER'S
`MODEM THAT IS VERY NEAR TO THE CD
`
`2232 SAMPLES
`
`FRAME RECEIVED FROM A SUBSCRIBER’S
`MODEM AT MAXIMUM DISTANCE FROM THE C0
`
`2043 SAMPLES
`
`L,»-—134—~.
`SAMPLES
`E
`
`E
`
`INTERVAL OVER WHICH THE CO'S MODEMS SAMPLE A
`RECEIVED FRAME
`
`
`TIME
`
`Dish
`Exhibit 1011, Page 6
`
`

`
`U.S. Patent
`
`Nov. 7, 2000
`
`Sheet 6 of 7
`
`6,144,695
`
`FIG. 6
`
`2232 SAMPLES
`
`FRAME RECEIVED FROM THE CENTRAL OFFICE AND FROM
`ANY SUBSCRIBER'S MODEM CLOSER TO THE CENTRAL OFFICE
`
`2232 SAMPLES
`
`LATEST POSSIBLE FRAME TRANSMITTED BY ANOTHER
`SUBSCRIBER'S MODEM FURTHER FROM THE CENTRAL OFFICE
`
`2232 SAMPLES
`
`FRAME RECEIVED AND TRANSMITTED BY
`THE SUBSCRIBER'S MODEM
`
`2048 SAMPLES
`
`INTERVAL OVER WHICH A SUBSCRIBER'S MODEM
`SAMPLES ITS RECEIVED FRAME
`I—:.
`
`TIME
`
`Dish
`Exhibit 1011, Page 7
`
`

`
`Nov. 7, 2000
`
`Sheet 7 of 7
`
`6,144,695
`
`FIG. 7
`
`UPSTREAM
`FREQUENCY BAND
`
`Dish
`Exhibit 1011, Page 8
`
`

`
`6,144,695
`
`1
`METHOD AND APPARATUS FOR
`REDUCING NEAR-END CROSSTALK (NEXT)
`IN DISCRETE MULTI-TONE MODULATORI
`DEMODULATORS
`
`BACKGROUND OF THE INVENTION
`
`l. Technical Field
`
`This invention relates to the field of providing high speed
`digital data services to a digital service subscriber and, more
`particularly, to so—called asymmetric digital subscriber line
`(ADSL) services and to a method of reducing near-end
`crosstalk in discrete multi-tone (DMT) modems located at a
`central office and at the subscriber’s premises for providing
`ADSL services.
`
`2. Description of the Related Arts
`In the field of cable television, cable modem technology
`is emerging wl1icl1 provides increased bandwidth services to
`the home. Cable television equipment manufacturers are
`promoting the upgrading of cable television distribution
`plant to comprise so-called hybrid optical fiber and coaxial
`cable (hybrid fiber/coax) facilities. It is anticipated in the
`cable field that bandwidths to and from the subscriber can be
`increased so that bi-directional voice and data services may
`be provided in addition to traditional cable television pro-
`gramming. In the related field of telecommunications, there
`exists a considerable amount of embedded distribution plant
`comprising high capacity twisted wire pair cable.
`Historically, each cable pair was specially loaded with
`inductance coils at periodic intervals along the path from a
`serving central office to the subscriber’s premises to improve
`voice telephony performance. The inductance loading coun-
`tered the effects of the high capacity cable and provided a
`flat bandwidth on each twisted cable pair at voice frequen-
`cies. On the other hand, frequencies higher than voice
`bandwidth were intentionally attenuated to such a degree
`that the twisted wire cable pair was unusable for other than
`a voice channel. When the loading is removed, the twisted
`cable pair bandwidth improves and becomes more compa-
`rable to that of coaxial cable.
`
`An emerging technology in the telecommunications arts
`that competes with cable modern technology is so—called
`asymmetric digital subscriber line (ADSL)
`technology.
`Referring to FIG. 1 taken from American National Standards
`Institute standards document Tl.4l3—1995, there is shown a /
`public switched telecommunication network (PSTN) 105
`and a digital network (for example, a frame relay, asynchro-
`nous transfer mode (ATM), Internet or other digital network)
`110 at the left. At the right is the subscriber’s premises. The
`digital network 110 is coupled via a logical interface V to an _
`ADSL transceiver unit (ATU) at the serving central office
`(C). Also at a serving central o ice are located a splitter 120
`for splitting the telecommunications services from the digi-
`tal services, typically based on frequency. For example, a
`voice channel may still be preserved at from 0-4000 Hz. The
`splitter function may be integrated into ATU-C H5 (and at
`the remote subscriber site, into ATU—R 135). Interface U—C
`represents the subscriber loop (twisted pair) interface at the
`central office C and interface U-R represents the subscriber
`loop interface at the remote subscriber terminal end of the
`twisted wire cable pair or other facility 125. Facility 125
`may comprise, for example, a twisted wire pair or a hybrid
`optical fiber/twisted wire pair facility or other wired or
`wireless facility having comparable or greater bandwidth.
`Service module (SM) 150 or 155 at the remote location may
`comprise an intelligent telecommunications terminal, a per-
`sonal computer, a television terminal, an energy manage-
`
`_
`
`2
`ment system, a security system or other service module
`known in the art. Plain old telephone service (POTS) module
`145 represents a traditional
`telecommunications terminal
`such as a facsimile terminal, voice bandwidth modem or
`standard telephone. Facility C1 distribution 140 within the
`subscriber premises may comprise, for example, a bus such
`as a home bus or a star network or other configuration. By
`bus as used herein is intended a communications link that
`may be wired or wireless connecting a plurality of devices
`together. The bus may be arranged so that there is contention
`for access to the bus according to priorities or be provided
`suflicient capacity to alleviate the likelihood of contention.
`Interface T represents the interface between a service mod-
`ule (SM) and/or a bus/star 140 to other service modules
`(SM’s).
`Referring to FIGS. 2A and 2B, there are shown respec-
`tively an ATU—C transmitter whose reference diagram is
`taken from A.N.S.I. Tl.4l3—l995 and an ATU-C receiver
`whose reference diagram is derived therefrom. In FIG. 2A,
`there is shown an expanded functional block diagram of the
`transmitter portion of ATU-C ll5 of FIG. I. A multiplexer/
`sync control unit 200 provides the interface to the digital
`network 110. Various high speed data rate links ASO, AS1,
`AS2 and AS3 at multiples of 1.536 Mbits/sec are provided
`toward digital network 110. In particular, each AS link
`represents an independent downstream simplex
`(unidirectional downstream) bearer of data traffic. Lower
`speed data services are also shown and represented by LSO
`(16 or 64 kbits/sec), LS1 (160 kbits/sec) and LS2 (384 or
`576 kbits/sec). Each LS link may represent a duplex bearer
`(bi-directional) carrying both downstream and upstream
`traffic or, in the alternative, a unidirectional simplex bearer.
`(‘RC 205 and CRC 210 represent cyclic redundancy
`check in each direction of transmission. Scrambler and
`forward error correction 215, 220 represent scrambling and
`forward error correction, for example, using Reed-Solomon
`error correction coding, in each direction of transmission.
`Interleaver 225 provides a data interleaving function as is
`further described in A.N.S.I. Tl .4l3-l995, incorporated by
`reference as necessary. Tone ordering function 230 provides
`tone selection and control functions as are also described by
`A.N.S.I. Tl.4l 3-1 995. (Constellation encoder (if used) and
`gain scaling functions are represented by block 235. The
`inverse discrete Fourier transform function applied for data
`modulation is represented by block 240. Two data directions
`are shown coupling IDFT 240 and output parallel to serial
`buffer 245 where a cyclic prefix is added to each data frame.
`Finally, a digital
`to analog converter and analog signal
`processing function are represented by block 250 which
`interfaces the subscriber facility 125.
`Referring to FIG. 2B, the ADSL receiver at the central
`office is shown. The horizontal arrows are reversed in
`direction from FIG. 2A. Data demultiplexer 255 interfaces
`the digital network 110. Descrambler 265, 258, deinterleaver
`270, decoder 280, DFT 285, input serial to parallel buffer
`290 and analog to digital converter 295 represent the sig-
`nificant changes in function between ATU-C transmitter and
`receiver.
`
`Referring to FIG. 3, at a subscriber terminal, the trans-
`mitter (ATU—R) is similarly configured as ATU—C but it is
`assumed that channels operate at LSO, LS1 or LS2 toward
`the subscriber’s equipment. Cyclic redundancy checks 305/
`310 are provided for each direction of transmission to/from
`subscriber equipment 375. Scrambler and forward error
`correction circuits 315 and 320, for example, using Reed-
`Solomon error correction coding, are provided for especially
`secure data transmission. An interleaver 325 is provided in
`
`Dish
`Exhibit 1011, Page 9
`
`

`
`6,144,695
`
`3
`one transmit path. Tone ordering circuitry 330 is necessary
`for generating and ordering the discrete multi tones of the
`discret multi—tone (DMT) modem. The constellation encoder
`and gain scaler 335 may or may not provide a form of trellis
`data encoding and gain scaling for controlling the tone
`ordering.
`lDl"l‘ block 340 performs an inverse discrete
`Fourier transform for modulating the digital data. The output
`parallel to serial buffer 345 is provided for providing parallel
`to serial conversion to a digital to analog converter and
`analog signal processing interface 250 which interfaces the
`subscriber loop 125.
`The analog signal framing (FIG. 4) used in ADSL tech-
`nology is obtained by passing quadrature amplitude modu-
`lation (QAM) samples through a D/A converter 250 or 350.
`These samples are arranged in a superframe of 69 frames
`(frames 0-68) totaling approximately 17 milliseconds. Alto-
`gether 512 samples (256 real and 256 imaginary, (P511) are
`taken of the data. An additional 32 samples contain a cyclic
`prefix making a total of 544 samples in each frame. The
`cyclic prefix CP 401 is added, for example, to signal 402 of
`frame 0 (FIG. 4) at the output parallel/serial buffer 245 and
`345 shown in FIGS. 2A and 3 respectively.
`Every 69th frame contains a pse11do-random number
`(PRN) sequence with a nominal length of 544 samples. This
`PRN sequence (the so-called synch symbol) permits recov-
`ery of the frame boundary after interruptions.
`The sub-carrier tones are spaced at 4.3125 kHz according
`to the ANSI Standard Tl.4l3—1995 and at carrier 64 where
`the frequency is 276 kHz, a pilot carrier is inserted. The data
`modulated on that pilot is a constant bit value (for example,
`0,0). Other details of frame construction, data modulation,
`tone ordering and the like may be found in the Standard and
`are not believed to be particularly relevant to the principles
`of the present invention.
`Near—end crosstalk, hereinafter referred to as NEXT, is a
`potentially severe problem for operating multiple Digital
`Subscriber Line (DSL) modems over twisted-pair wires
`between a Central Oflice (CO) and a subscriber’s location.
`NEXT occurs when the transmissions from one or more
`modems, particularly those at the central oflice, capacitively
`couple into each other’s twisted—Wire pairs and impair the
`ability of those modems to receive transmissions from the
`other end of the twisted-wire pairs. Moreover, with severe
`NEXT, a subscriber modem cannot receive transmissions
`from its transmitting central office modem. This problem is /
`most severe when a modem operates in f11ll-duplex mode in
`which transmission is simultaneously bi—directional at all
`frequencies.
`Cables that serve subscribers and terminate at a central
`office can comprise thousands of twisted wire pairs that are _
`bundled together in a limited cross—sectional (typically
`circular) area. Electrical signals traveling on the twisted
`pairs can easily electrically couple into physically proximate
`twisted pairs, consequently, near end crosstalk has a detri-
`mental effect on bit error rate. Since the concept behind
`ADSI. technology is to optimize bandwidth use, the phe-
`nomenon of near end cross—talk limits any one of three
`factors:
`the distance a subscriber can be from the central
`offiee, the digital data rate of service and the bit error rate of
`any digital data service. As the number of subscribers
`increase to ADSL technology, the likelihood will increase
`that an ADSI. subscriber will be served by a twisted wire
`cable pair proximate to that of another subscriber and that
`frequencies from one twisted wire cable pair will adversely
`impact the signal
`to noise ratio of digital signals on an
`adjacent or proximate cable pair. Consequently, there is a
`need in the art to alleviate the effects of NEXT.
`
`I
`
`4
`One way to reduce NEXT might be to assign cable pairs
`to subscribers in such a Way that subscribers to ADSL
`services are not in the same bundles as other subscribers. Of
`course, at some point
`in time, as subscribers to ADSL
`increase, so does the likelihood that cable pair assignment in
`such a manner cannot be accomplished. Thus, there is a need
`in the art for reducing NEXT in ADSL services.
`
`SUMMARY OF THE INVENTION
`
`The present invention offers the possibility of substan-
`tially improving one or more of the following parameters:
`distance of the subscriber from the central office, data rate,
`and error rate. For example, the distance of the subscriber
`from the central office may be increased while the data rate
`and error rate are held constant. According to the principles
`of the present invention, the framing suggested by ANSI
`T1.413-1995 is modified and frame alignment is recom-
`mended for all frames transmitted by the central oflice. Also,
`all frames transmitted by subscriber modems should be
`aligned to coincide with received frames. An echo canceler
`may be applied to cancel near end echo as will be further
`discussed herein.
`Interspersing transmit frequencies
`(individual frequencies or bands) between central office and
`subscriber modems may further reduce the effects of near
`end crosstalk.
`
`In particular, in accordance with the present invention, the
`length of the cyclic prefix prepended to each frame of a
`DMT frame should be increased to a value that is at least the
`length of the sum of 1) the maximum round trip delay from
`a central office to a subscriber and 2) the delay required to
`prevent intersymbol interference or ISI (according to the
`ANSI standard, 32 samples or 14.5 microseconds to prevent
`ISI). For example, if the sampling rate is maintained at
`approximately 2.2 megaHertz and if the maximum round-
`trip delay is approximately 80 microseconds for an 18,000
`foot twisted-wire pair loop, one might increase the cyclic
`prefix to about 184 samples from the recommended 32
`samples or the equivalent of 81.3 microseconds of signal
`duration or even higher depending on concerns about ISI.
`Also, it is advantageous to increase the length of the frame
`proportionately to the increase in the cyclic prefix. For
`example, if the cyclic prefix is lengthened from 14.5 micro-
`seconds to 81.3 microseconds,
`then the frame should be
`lengthened from 250 microseconds to approximately 1 mil-
`lisecond. If the same sampling rate is maintained (at around
`2.2 megaHertz), then other parameters will change accord-
`ingly. If the frame length is increased to 1 millisecond, the
`number of sample points of the IDFT should increase from
`512 to,
`for example, 2048,
`the carrier spacing should
`decrease, for example, from 4.1325 kHz to 1.089998 kHz
`and the DMT’s sampling rate should increase slightly from
`2208 kHz to 2263 kHz. These numbers are representative
`only, and other practitioners of the art may deviate depend-
`ing on the application.
`It is a further principle of the present invention to align
`frames transmitted from the central office toward the sub-
`scriber. All frames transmitted by all central office modems
`are aligned and synchronized to begin at the same time as
`they are transmitted toward subscribers. That is, all frames
`transmitted by all central olfice modems to all subscribers
`(especially served on the same cable of twisted wire pairs)
`start and end at the same times. One way to accomplish the
`alignment is to provide a common shared timing signal to all
`(10 modems from a common source. For example, the signal
`may comprise a train of narrow , for example, 1 microsecond
`pulses spaced by the period of a frame (for example, one
`
`Dish
`Exhibit 1011, Page 10
`
`

`
`6,144,695
`
`5
`millisecond). Each central office modern then would align its
`frames that are transmitted toward respective subscribers
`over different twisted wire cable pairs with this pulse train.
`The pulse train from the common source may be accom-
`panied by a higher frequency signal (for example, a sinusoic
`or a pulse train at the frequency specified in Section 6.9 of
`ANSI T1.413-1995) to which each modern could be syn-
`chronized. Alternatively, the pilot tone of 276 kHz specifiee
`at Section 6.9.1.2 might by applied as the shared resource.
`Either alternative will save the costs of providing an oscil-
`lator within each central oflice modern.
`Now, it is a further principle of the present invention tha
`all frames transmitted by the subscriber’s modem toward the
`central o ice modem be aligned with the frames that i
`receives from the central oflice.
`Its transmitted frames
`should coincide with frames it receives from the centra
`
`5
`
`oflice as may be seen from the FIGS. 6a) and 6c).
`With the frames aligned as described and per FIG. 6 ane
`with the lengthened cyclic prefix (for example, 184 samples
`long), each central oflice modem samples its received frame
`wholly within the frame 1) received from its subscriber’s
`modem and 2) transmitted by all other subscriber’s modems
`as in FIG. 5 which shows that each central o ice modem
`samples its received frame wholly within the frames trans-
`mitted by all other subscribers’ modems.
`Referring again to FIG. 5, there is shown the frames that
`a central ofliice modem would receive if the principles of the
`present invention were followed for two extremes of round-
`trip delay. When the subscriber is almost adjacent to the
`central oflice, the leading edge of the subscriber frame is
`received 184 samples before the leading edge of the central
`office sampling. The leading edge of the distant subscriber’s
`frame coincides with the leading edge of the CO’s sampling.
`For these and any delays between these extremes, the CO
`modem receiver takes its samples entirely within both the
`frame received from all subscribers’ modems and the frames
`transmitted by all modems at the central office. Thus, a
`frame transmitted at one carrier frequency is guaranteed to
`not interfere with a frame received at another carrier fre-
`quency. Taking samples from inside the frame preserves the
`orthogonality of the sinusoids in the discrete Fourier trans-
`form (DFT), since the modem’s DFT modulator operates on
`an integral number of periods of each carrier frequency.
`NEXT is reduced if not elir11ir1ated via this orthogonality
`requirement.
`Finally, it is a further principle of the present invention
`that
`the central office and subscriber modems’
`transmit
`frequencies may be interspersed. For example, one or more
`bands of frequencies toward the subscriber from the central
`oflice may be interspersed with a band of frequencies in the _
`return direction from the subscriber n1oder11 to the central
`office. At the extreme, all odd multipled carriers may be used
`in the downstream path from the central office and even
`multipled carriers be used in the upstream path (or vice
`versa). That is, for example, odd multiples of 1.089998 kl-I7
`may be used in the downstream and even multiples in the
`upstream path exclusive of any band reserved for POTS
`telephony. This even-odc distribution of upstream and
`downstream frequencies 0 ers the advantage of degrading
`nearly symmetrically as distance from the central office
`and/or NEXT interference increase.
`Referring again to FIG. 6,
`it may be seen that similar
`benefits are obtained at a subscriber modem. A subscriber’s
`modem will sample only inside the frames transmitted by
`itself and by other subscriber modems of the same type.
`Again, sampling inside the frames preserves the orthogo-
`nality of sinusoids at adjacent frequencies.
`
`6
`Further advantages and features of the present invention
`will be understood fror11 studying the accompanying draw-
`ings and from reading the following detailed description
`thereof.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a system reference model showing the primary
`components of an asymmetric digital subscriber line metal-
`lic interface between a subscriber and a central office and
`between the central oflice and a digital or public switched
`telecommunications network.
`
`FIG. 2A is a functional block diagram of an ADSL
`transmitter unit located at a central ofiice and FIG. 2B is a
`functional block diagram of an ADSL receiver unit located
`at a central oflice.
`
`FIG. 3 is a functional block diagram of an ADSL trans-
`mitter unit located at a remote subscriber premises location.
`FIG. 4 is a diagram of a proposed frame/superframe
`structure for ar1 ADSL modem and the cor11por1er1ts thereof
`such as frame length, cyclic prefix and synch symbol that
`may be modified according to the present
`invention to
`prevent near end crosstalk.
`FIG. 5 shows the result of frame lengthening and align-
`r11er1t principles of the present
`invention as applied at a
`central office modem.
`
`FIG. 6 shows the result of the frame lengthening and
`alignment principles of the present invention as applied at a
`subscriber modern.
`
`FIG. 7 shows an example of a frequency interspersing
`step of the present
`invention as applied to splitting the
`frequency bands of upstream and downstream transmission.
`FIG. 8 shows an example of the frequency interspersing
`step of the present invention as applied to utilizing every
`other carrier frequency for upstream or downstream trans-
`mission.
`
`_
`
`Dl:'l'AlLl:'D Dl:'SCRll"'l‘l()N OF THE
`INVENTION
`
`The basis for central oflice or subscriber modern design
`according to the present invention is the Discrete Multitone
`(DMT) modem standardised inANSI Standard T1.413-1995
`for an Asymmetric Digital Subscriber Line (ADSL). The
`terms used herein conform to their usage in the T1.413—l995
`standard document. Frame and superframe are defined by
`Sect. 6.2.1.1; pilot frequency is defined in Sect. 6.9.1.2; the
`inverse discrete Fourier Transform (IDFT) is defined by
`Sect. 6.9.2,
`the synch symbol is shown ir1 FIG. 3 and is
`defined by Sect. 6.9.3; and cyclic prefix is defined by Sect.
`6.10. This invention makes the following changes to the
`Standard:
`
`Aligning the Frames Transmitted by the Central Office (CO)
`Modems
`The frames transmitted by all central office CO) modems
`(for example, that portion of ATU-C 115 of FIG. 1 and FIG.
`2 facing the subscriber loop 125) according to the present
`invention are aligned at the central Office; that is, the frames
`transmitted by all CO modems start and end at the same
`times. To achieve this alignment, the CO modems may share
`a common timing signal. This signal may consist of a train
`of narrow (for example, 1 ysec.) pulses spaced by the period
`of the frame. Each CO modem would align its frames with
`this pulse trair1. This pulse train could be accompanied by a
`higher-frequency signal (for example, a sinusoid or a pulse
`train) to which each modem may be synchronized. For
`example, a pulse train at the sampling frequency specified in
`
`Dish
`Exhibit 1011, Page 11
`
`

`
`6,144,695
`
`7
`Section 6.9.3 for the synch symbol of the ANSI standard
`might be shared among the CO modems covered by this
`invention. Alternatively, a sinusoid at the pilot frequency
`specified in Section 6.9.1.2 of the ANSI standard might be
`shared. Either alternative would save costs by removing the
`need for each CO modem to contain its own oscillator or
`other means for producing a synchronization signal.
`Aligning the Transmitted and Received Frames at the Sub-
`scribers’ Modems
`
`The subscribers’ modem (for example, ATU—R 135 of
`FIG. 1) disclosed in the present invention is designed to
`align the frames that it transmits (per ATU—R transmitter of
`FIG. 3) with the frames that it receives, so that its transmit-
`ted frame coincides with its received frame. This is illus-
`
`trated by the 2”’ arrow from the bottom c) in FIG. 6. The
`reason for this alignment wfll be explained subsequently
`herein.
`Lengthening the Cyclic Prefix and the Frame
`the central
`To implement the changes in alignment at
`office and at the subscriber, the lengths of both the DMT
`frame and the cyclic prefix contained within the frame are
`increased from the values specified in the ANSI standard
`according to the principles of the present invention. The
`cyclic prefix should be made at least as long as the sum of
`1) the maximum round—trip delay from the central oflice to
`a subscriber that is the farthest away from the central oflice
`(e.g., approximately 80 ysec. for an 18,000 foot twisted-wire
`cable pair) and 2) the delay required to prevent intersymbol
`interference (ISI). This delay, for example, is approximately
`14.5 microseconds assuming a 32 sample cyclic prefix.
`Since the ISI prevention delay is much smaller than the
`maximum round trip delay, the ISI prevention delay may be
`considered to be immaterial; nevertheless ISI is of concern
`and should be considered. Although the cyclic prefix could
`be increased without lengthening the frame, it is advanta-
`geous to increase the frame length proportionately to main-
`tain the fraction of time during which data is sent. Other
`modifications become proportionately useful, for example,
`decreasing the spacing between carrier frequencies of either
`the central office or subscriber DMT modem. The modified
`cyclic prefix is preferably inserted via the output parallel to
`serial buffers 245 and 345 of the CO and subscriber
`modems.
`Interspersing the CO and Subscribers’ Modems’ Transmit
`Frequencies
`Optionally, the frequencies used by the central ollice and
`subscriber modems can be interspersed—either individually
`or in groups—to virtually eliminate near end crosstalk
`(NEXT), provided that the frames are aligned as described
`above. Referring to FIG. 7, a band of frequencies may be _
`reserved for downstream transmission and a complimentary
`band of all remaining frequencies may be used for upstream
`transmission. Referring to FIG. 8, alternatively, all of the
`central office modems might transmit on the even values of
`the frequency index, while all of the subscribers’ modems
`might transmit on the odd frequency-indiccs (or vice versa)
`where the central oflice transmit on even is shown in FIG.
`8(b) and the subscriber trasnmit on odd is shown in FIG.
`8(a). If the carrier frequency separation is 1.0898 kilohertz
`then odd multiples plus a reserved band x may be used, for
`example, for upstream transmission and even multiples for
`downstream, where f is the frequency separation and x the
`reserved band. Although either of these interspersing
`approaches would halve the number of carrier frequencies
`available to each direction of transmission, it would virtually
`prevent NEXT between all modems utilizing this invention
`without requiring the use of band-separating filters or echo
`
`8
`cancellers. Besides saving costs, interspersing could utilize
`bandwidth symmetrically and consistently. For example,
`each direction of transmission could use a constant fraction
`
`(such as 50%) of the available bandwidth.
`Example of Selecting the Parameters of a Modem According
`to the Present Invention
`
`If the sampling rate (2.2 megaHertz) is maintained
`approximately the same as in the ANSI standard, the ANSI
`standard’s parameters may be modified as follows: 1)
`Increase the DMT frame length from 250 usec. to 1 msec.;
`2) Increase the number of points in the DMT’s Discrete
`Fourier Transform (DFT) from 512 to 2048; 3) Increase the
`number of samples in the DMT’s cyclic prefix from 32 to
`184; 4) Decrease the DMT’s carrier-frequency separation
`from 4.13125 kHz.
`to 1.0898 kl-Iz.; and 5) Increase the
`DMT’s sampling rate from 2208 kHz.
`to 2263 kHz. For
`simplicity, the example shown in FIGS. 5 and 6 do not show
`the additional 32 samples for preventing ISI in cyclic prefix
`(or an increase to 216 samples). Also with the increase in
`sampling rate, instead of the 69th frame being dedicated to
`a synch symbol, the 73rd frame may contain frame synchro-
`nization information.
`With a sampling rate of 2263 KHz., a cyclic prefix of 184
`samples is equivalent to a delay of 81.3 Msec., which exceeds
`the longest anticipated round—trip delay (80 ,usec) over the
`twisted-wire pair. It does not account for intersymbol inter-
`ference (ISI) prevention delay and, to do so, the cyclic prefix
`should be 216 samples. Also, the above-suggested 1 msec.
`frame-length (2 msec. for the transmitter/receiver pair) is not
`likely to be harmful for telephony applications because it is
`much less than the 10 msec. delay at which quality impair-
`ment becomes noticeable in a telephone call. An additional
`small delay may be introduced by the convolutional codec
`and/or interleaver shown in FIGS. 2A and 3.
`_ Benefit of Making the Cyclic Prefix Longer than the Maxi-
`mum Round—tri