(12)
`
`United States Patent
`Wu
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,219,378 B1
`*Apr. 17, 2001
`
`US006219378B1
`
`(54) DIGITAL SUBSCRIBER LINE MODEM
`INITIALIZATION
`
`TX US
`S W P1
`:
`t
`I
`75
`Ong u’
`ano’
`(
`)
`nven or
`(
`)
`(73) AssigneeZ Texas Instruments Incorporated,
`Dallas’ TX (Us)
`
`(,F) Notice
`'
`
`This patent issued on a continued pros-
`ecution application ?led under 37 CFR
`1.53(d), and is subject to the tWenty year
`patent term provisions of 35 U.S.C.
`154(a)(2).
`
`Subject to any disclaimer, the term of this
`patent iS eXtended 0r adjusted under 35
`U-S-C- 154(k)) by 0 days-
`
`(21) Appl. No.: 08/995,256
`_
`(22) Flled:
`
`Dec‘ 19’ 1997
`Related U-S- Application Data
`(60) Provisional application No. 60/059,182, ?led on Sep. 17,
`1997
`
`(51) Int. Cl.7 ............................ .. H03H 7/30; H03H 7/40;
`H03K 5/159
`
`(52) US. Cl. ......................... .. 375/231; 375/222; 370/484
`_
`
`
`Fleld of Search ................................... .. 375/260; 370/484, 210
`
`(56)
`
`References Cited
`
`US PATENT DOCUMENTS
`5,285,474 * 2/1994 Chow etal. ....................... .. 375/231
`5,910,970 * 6/1999 Lu ............... ..
`375/377
`6,028,891 * 2/2000 Ribner etal. ...................... .. 375/222
`
`FOREIGN PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`“A Discrete Multitone Transceiver System for HDSL Appli
`cations,” Chow, et al., IEEE Journal on Selected Areas In
`Communications, vol. 9, No. 6, Aug. 1991, pp. 895—908.
`“Multicarrier Modulation for Data Transmission: An Idea
`Whose Time Has Come,” John A. C. Bingham, IEEE
`Communications Magazine, May 1990, pp. 5—14.
`
`*
`
`-
`-t d b
`C1 6
`y exammer
`
`Primary Examiner—Chi Pham
`Assistant Examiner—Phuong Phu
`(74) Attorney, Agent, or Firm—J. Dennis Moore; Wade
`James Brady, III; Frederick J. Telecky, Jr.
`
`(57)
`
`ABSTRACT
`
`A method of initializing the operation of a remote modem
`(10) and central of?ce modem (20k) for asymmetric sub
`icrrllier lirile‘ilngdem comrélumcations ptver a ivilsifld plair Wire
`am 1 y (
`)_’
`_1Scre e mu 1_ 9H6 _ec no Ogy’ 1S
`disclosed. The m1t1al1Zat1on process begms With the remote
`
`mofiem (10) lssumg an mmahzanon request’ In response to
`WhlCh the central of?ce modem (20k) issues an acknowl
`d
`t. Th
`t
`d
`10 '
`1 d
`1
`-
`t
`6 gm“
`e “m0 6 m0 6?“ (
`) me ‘1 65 a °.W 6.05
`analog ?lter (21) for separatmg upstream commumcation
`
`eChoes from downstream data~ - - -
`
`
`analog ?lter
`mcreases the duration of impulse response over the channel,
`and thus requires time-domain equalization (TEQ) process
`(31) performed by the remote modem (10) to ?lter relatively
`long circular pre?xes, so that intersymbol interference is
`eliminated. Transceiver training processes (62R, 62C) are
`Carried out, through the generation of a Pseudo-random
`sequence of frames by the central office modem (20k), the
`receipt of Which is used by the remote modem (10) to
`establish its TEQ coef?cients and taps in an optimal manner.
`
`0 806 852 A2 11/1997 (EP) .
`
`18 Claims, 5 Drawing Sheets
`
`REMOTE MODEM 10
`
`CENTRAL OFFICE
`MODEM 20)(
`
`_____
`
`__1N_JT_R_EO_ ‘_
`INIT_A§K__ ACTIVATION AND fsoc
`50R\ ACTIVATION AND
`ACKNOWLEDGEMENT ::"ACK
`ACKNOWLEDGEMENT
`_-——>
`1
`1
`62R\ TRANSCEIVER
`TRANSCEIVER I520
`TRAINING
`TRAINING
`1
`1
`CHANNEL
`CHANNEL
`ANALYSIS
`ANALYSIS
`1
`1
`
`64R/
`
`______
`
`\64C
`
`GGR/
`
`EXCHANGE
`
`~—————> EXCHANGE
`
`\66C
`
`1
`
`I
`
`TO COMMUNICATION OF DATA
`
`

`
`U.S. Patent
`
`Apr. 17, 2001
`
`Sheet 1 of 5
`
`US 6,219,378 B1
`
`FIG.
`
`7
`
`130 kHz
`
`640 kHz
`
`FIG. 2
`
`FIG. 3
`
`M/'
`
`L_______J
`
`FIG. 5
`
`
`
`
`
`

`
`U.S. Patent
`
`Apr. 17, 2001
`
`Sheet 2 of 5
`
`US 6,219,378 B1
`
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`
`U.S. Patent
`
`Apr. 17, 2001
`
`Sheet 3 0f 5
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`_
`
`US 6,219,378 B1
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`
`U.S. Patent
`
`Apr. 17, 2001
`
`Sheet 4 0f 5
`
`US 6,219,378 B1
`
`r-----____------—---_--_-------__q
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`A
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`FIG. 7
`
`REMOTE MODEM 10
`
`CENTRAL OFFICE
`MODEM 20‘(
`
`AC A
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`C C C C O 2 4 6 6 6 6 6
`
`TO COMMUNICATION OF DATA
`FIG. 8
`
`

`
`U.S. Patent
`
`Apr. 17, 2001
`
`Sheet 5 0f 5
`
`US 6,219,378 B1
`
`To
`
`REMDTE MODEM 10
`
`CENTRAL OFFICE mm 201(
`
`' 60R
`
`wAIT FOR
`MDSL PILOT
`
`____PI1gT_____
`
`“
`
`IssuE MDSL
`PILOT SIGNAL
`\
`68C
`
`BZRI
`
`__T_Eg-_c_TRN§__
`\74R 740/
`
`70c
`/
`__R_s_D _RE_vER_D__ MEASURE REC'D POWER
`ISSUE PSD
`AND DERIvE PSD
`REVERB sIcNA1
`II
`II
`__T_Eg-_R_TRN§__ IssuE TEQ TRAlNlNG
`ADJUST AGC AND
`\72R m/ SIGNAL AT DERIvED PSD
`TRAIN TEQ
`II
`{I
`ADJUST AGC
`ISSUE TEO
`AND TRAIN TEQ
`TRAINING SIGNAL
`II
`II
`-—F—E9_—C—T3N9—> ISSUE FEQ TRNG SIGNAL
`ISSUE FEQ TRNG
`SIGNAL AND TRAIN FEQ ._F_EQ-BJEN_G__
`AND TRAIN FEO
`1
`\76R 76(:/
`1
`ISSUE AND REcEIvE -———S1N—C———> ISSUE AND RECEIVE
`SYNC FRAMES
`____SIN_C____
`SYNC FRAMES
`/
`1
`780/
`1
`78R
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`I FIG. 9
`To CHANNEL ANA1YsIs 64R, 64C
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`

`
`US 6,219,378 B1
`
`1
`DIGITAL SUBSCRIBER LINE MODEM
`INITIALIZATION
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims the bene?t, under 35 U.S.C.
`§119(e)(1), of US. Provisional Application Ser. No. 60/059,
`182, ?led Sep. 17, 1997, and incorporated herein by this
`reference.
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH OR DEVELOPMENT
`Not applicable.
`
`BACKGROUND OF THE INVENTION
`
`This invention is in the ?eld of high-speed modem
`telecommunications, and is more speci?cally directed to the
`initialiZation of communications betWeen high-speed
`modems.
`The high-speed exchange of digital information betWeen
`remotely located computers is noW a pervasive part of
`modem computing in many contexts, including business,
`educational, and personal computer uses. It is contemplated
`that current and future applications of high speed data
`communications Will continue the demand for systems and
`services in this ?eld. For example, video on demand (VOD)
`is one area Which has for some time driven the advancement
`of technology in the area of digital information exchanges.
`More recently, the rapid increase in use and popularity of the
`Global Internet (hereinafter, the “Internet”) has further moti
`vated research and preliminary development of systems
`directed to advanced communication of information
`betWeen remotely located computers, particularly in effect
`ing higher bit-rates using existing infrastructure.
`One type of technology arising from the above and
`continuing to evolve is referred to in the art as digital
`subscriber line (“DSL”). DSL refers generically to a public
`netWork technology that delivers relatively high bandWidth
`over conventional telephone company copper Wiring at
`limited distances. DSL has been further separated into
`several different categories of technologies, according to a
`particular expected data transfer rate, the type and length of
`medium over Which data are communicated, and schemes
`for encoding and decoding the communicated data.
`In each case, a DSL system may be considered as a pair
`of communicating modems, one of Which is located at a
`customer site, such as a home or of?ce computer, and the
`other of Which is located at a netWork controller site,
`typically a telephone company central office. Within the
`telephone company system, this modem is connected to
`communicate With some type of netWork, often referred to
`as a backbone netWork, Which is in communication With
`other communication paths by Way of equipment such as
`routers or digital subscriber line access multiplexers
`(DSLAMs). Through these devices, the backbone netWork
`may further communicate With dedicated information
`sources and With the Internet. As a result, information
`accessible to the backbone netWork, such as Internet
`information, may be communicated betWeen the central
`of?ce DSL modem and a customer site having its oWn
`compatible DSL modem.
`Within this general system, it is also anticipated that data
`rates betWeen DSL modems may be far greater than current
`voice modem rates. Indeed, current DSL systems being
`tested or projected range in rates on the order of 500 Kbps
`
`10
`
`15
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`20
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`25
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`30
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`35
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`40
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`2
`to 18 Mbps or higher. According to certain conventional
`techniques, the data communication rates are asymmetrical.
`Typically, the higher rate is provided for so-called doWn
`stream communications, that is from the central of?ce to the
`customer site, With upstream communication from the cus
`tomer site to the central office at a rate considerably loWer
`than the doWnstream rate. Most DSL technologies also do
`not use the Whole bandWidth of the tWisted pair, reserving a
`relatively loW bandWidth channel for voice communication,
`so that voice and data communications may be simulta
`neously carried out over the same line.
`The most publiciZed DSL technology currently under
`development is referred to as Asymmetric Digital Subscriber
`Line, or “ADSL,” and corresponds to ANSI standard
`T1.413. Despite the existence of this standard, debate and
`competition is still present in the art, particularly as to
`Whether devices complying With the standard provide prom
`ise for future Wide scale use and Whether the standard
`requires revision. For example, While the standard currently
`contemplates a modulation technology called Discrete Mul
`titone (DMT) for the transmission of high speed data, an
`alternative data transmission technique referred to as carri
`erless amplitude/phase modulation (CAP) has also recently
`gained favor in the ?eld. In any event, given the current state
`of the art, it is contemplated that ADSL systems Will
`communicate data over a single copper tWisted pair at
`doWnstream rates on the order of 1.5 Mbps to 9 Mbps, and
`With an upstream bandWidth Will range from 16 kbps to 640
`kbps. Along With Internet access, telephone companies are
`contemplating delivering remote LAN access and VOD
`services via ADSL.
`Other DSL technologies being developed include High
`Bit-Rate Digital Subscriber Line (“HDSL”), Single-Line
`Digital Subscriber Line (“SDSL”), and Very-high-data-rate
`Digital Subscriber Line (“VDSL”). HDSL, unlike ADSL as
`described above, has a symmetric data transfer rate, com
`municating at the same speed in both upstream and doWn
`stream directions. Current perceived speeds are on the order
`of 1.544 Mbps of bandWidth, but require tWo copper tWisted
`pairs. HoWever, the operating range of HDSL is more
`limited than that of ADSL, and is currently considered to be
`effective at distances of approximately 12,000 feet or less,
`beyond Which signal repeaters are required. SDSL delivers
`comparable symmetric data transfer speed as HDSL, but
`achieves these results With a single copper tWisted pair
`Which limits the range of an SDSL system to approximately
`10,000 feet. Lastly, VDSL provides asymmetric data transfer
`rates at much higher speeds, such as on the order of 13 Mbps
`to 52 Mbps doWnstream, and 1.5 Mbps to 2.3 Mbps
`upstream, but only over a maximum range of 1,000 to 4,500
`feet.
`Of course, in addition to performance considerations and
`to the distance over Which DSL communications may be
`carried by conventional tWisted-pair infrastructure, the cost
`of the modem hardWare is also a signi?cant factor in the
`selection of a communications technology. It is therefore
`contemplated that a loWer data rate technology may provide
`high-speed data communications, With doWnstream data
`rates exceeding 1 Mbps, over existing tWisted-pair netWorks
`and at cost that is competitive With conventional non-DSL
`modems, such as 56 k, V.34, and ISDN modems.
`By Way of further background, data rate negotiation
`methods are utiliZed in connection With conventional
`modems, to establish the highest data rate at Which com
`munications may be carried out over a channel. For
`example, most modern analog modems execute a data rate
`negotiation With a dial-up host upon connection, to deter
`
`

`
`US 6,219,378 B1
`
`3
`mine the highest common data rate that may be used over the
`current telephone connection.
`BRIEF SUMMARY OF THE INVENTION
`It is an object of the present invention to provide a method
`and system for data communications in Which a remote and
`central of?ce modem may initialize communications in
`order to maximiZe the available data communication rates
`and accuracy.
`It is a further object of the present invention to provide
`such a method and system in Which such initialiZation may
`determine the bit rate capacity at Which each of multiple
`subcarrier channels Within both an upstream and a doWn
`stream spectrum may reliably communicate data.
`It is a further object of the present invention to provide
`such a method and system in Which such initialiZation
`involves the exchange of information betWeen modems that
`is also directed to the setting of equaliZer ?lters.
`It is a further object of the present invention to provide
`such a method and system that is particularly suited in
`modems that may be realiZed in a small number of integrated
`circuit devices.
`Other objects and advantages of the present invention Will
`be apparent to those of ordinary skill in the art having
`reference to the folloWing speci?cation together With its
`draWings.
`The present invention may be implemented into a modem
`system, and into a method of operating the same to initialiZe
`the operating conditions of the modem-to-modem session.
`The modems include analog ?lters at the analog front end
`Which, While reducing the modem cost, also signi?cantly
`increase the channel impulse response and thus necessitate
`relatively long circular pre?xes to eliminate intersymbol
`interference. According to the present invention, folloWing
`initial activation of the session by Way of hand-shaking,
`training of the central office and remote modems is carried
`out in order to establish synchroniZation of the modems With
`one another, and also the parameters by Way of Which a time
`domain equaliZer and a frequency domain equaliZer are set
`in each modem. Training of the time domain equaliZer is
`effected by the communication of a pseudo-random
`sequence from the transmitting modem, folloWed by the
`receiving modem solving a set of linear equations in order
`to ?lter the circular pre?x from the signal. Measurement of
`the signal-to-noise ratio over each subcarrier channel is then
`effected, folloWed by exchange of bit loading information
`for each subchannel betWeen the tWo modems.
`
`BRIEF DESCRIPTION OF THE SEVERAL
`VIEWS OF THE DRAWING
`FIG. 1 is an electrical diagram, in block form, of a digital
`subscriber line communication system into Which the pre
`ferred embodiment of the invention may be implemented.
`FIG. 2 is a plot of frequency versus signal amplitude,
`illustrating the frequency division multiplexing according to
`Which the system of FIG. 1 operates according to the
`preferred embodiment of the present invention.
`FIG. 3 is a plot of frequency versus signal-to-noise ratio,
`and potential bit loading, for each of multiple subchannels
`Within the doWnstream transmission bandWidth in the plot of
`FIG. 2.
`FIG. 4 is a block ?oW diagram illustrating the operating
`functions of the remote modem in the system of FIG. 1 in
`receiving and transmitting messages.
`FIG. 5 is a representation of a time-domain equaliZer
`(TEQ), realiZed by Way of a ?nite impulse response (FIR)
`digital ?lter.
`
`10
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`15
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`20
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`25
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`35
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`40
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`45
`
`50
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`
`60
`
`65
`
`4
`FIG. 6 is a block ?oW diagram illustrating the operating
`functions of the central of?ce modem in the system of FIG.
`1 in receiving and transmitting messages.
`FIG. 7 is an electrical diagram, in block form, of a remote
`modem constructed according to the preferred embodiment
`of the present invention.
`FIG. 8 is a How chart illustrating the operation of the
`initialiZation sequence executed by a remote modem and a
`central office modem according to the preferred embodiment
`of the present invention.
`FIG. 9 is a How chart illustrating the operation of the
`transceiver training processes executed by a remote modem
`and a central of?ce modem in the initialiZation sequence of
`FIG. 8 according to the preferred embodiment of the present
`invention.
`FIG. 10 is a How diagram illustrating the operation of a
`remote modem in establishing one of the coef?cient values
`in the time domain equaliZation ?lter, in the initialiZation
`sequence of FIG. 8 according to the preferred embodiment
`of the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Referring ?rst to FIG. 1, a telecommunication system into
`Which the preferred embodiment of the present invention
`may be implemented Will noW be described in detail.
`According to this system, a user in a home or of?ce
`environment (shoWn as user environment H) operates user
`unit 8, such as a personal computer or Workstation, or Which
`alternatively may be an entertainment unit in the video-on
`demand (VOD) context. User unit 8 is connected to remote
`modem 10, the construction and operation of Which Will be
`described in detail hereinbeloW, Which is connected to
`tWisted-pair Wire facility TWP by Way of a conventional
`phone jack PJ. One or more telephones (not shoWn) may
`also be connected into tWisted pair Wire facility TWP by Way
`of phone jack P], such that voice communications may
`alternatively or additionally be communicated over tWisted
`pair Wire facility TWP.
`TWisted pair Wire facility TWP is implemented by Way of
`conventional telephone Wiring betWeen a home or office user
`environment H and central of?ce CO. As is fundamental in
`the communications ?eld, central of?ce CO is a location that
`is operated by a telephone carrier or service provider, and
`Which provides sWitching of telephone calls, both incoming
`and outgoing, relative to the user environments H that it
`services. Central office CO is typically provided relatively
`close to its associated user environments H; in this example,
`for purposes of achieving high-speed data communications
`according to the ADSL or MDSL protocols to be described
`herein, distance d betWeen central of?ce CO and user
`environment H is preferably no greater than 18,000 feet, due
`to the attenuation of high speed communications over con
`ventional tWisted pair Wire facility TWP. Alternatively, if
`user environment H is more than this speci?ed distance from
`central of?ce CO, one or more signal repeaters (not shoWn)
`may be included Within tWisted pair Wire facility TWP to
`boost the signals along their respective paths, particularly
`from central of?ce CO to user environment H.
`According to this embodiment of the invention, central
`of?ce CO includes ADSL modem rack 20, Which includes
`multiple ones of central of?ce modems 20k,one of Which is
`in communication (either directly, or through a sWitch) With
`tWisted pair Wire facility TWP associated With user envi
`ronment H. The construction and operation of each of
`modems 20k Will be described in further detail hereinbeloW.
`
`

`
`US 6,219,378 B1
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`5
`ADSL rack 20 is connected to a conventional router 22, and
`in turn via channel service unit (CSU) 24 to Internet service
`provider ISP via ?ber optic line FO. Internet service provider
`ISP may be at a separate location, operated separately from
`central of?ce CO or, if the telephone service provider that
`operates central of?ce CO also provides Internet access,
`Internet service provider ISP may simply be a separate
`function Within central of?ce CO or elseWhere in its net
`Work. As illustrated in FIG. 1, Internet service provider ISP
`generally includes multiplexer 26 to service multiple central
`of?ces CO, and router 28 by Way of Which user units 8 may
`be placed in connection With other Internet servers for
`communication of information thereWith.
`According to the preferred embodiment of the present
`invention, high-speed data communication from central
`of?ce CO to home environment H over tWisted pair Wire
`facility TWP of limited length d is preferably accomplished
`by Way of frequency division multiplexing (FDM). In this
`example, Which Will noW be described relative to FIG. 2,
`so-called doWnstream communication from central of?ce
`CO to home environment H is effected at high frequency,
`Within bandWidth DOWN illustrated in FIG. 2. For example,
`as shoWn in FIG. 2, doWnstream bandWidth DOWN ranges
`from approximately 180 kHZ to approximately 640 kHZ. In
`contrast to the doWnstream transmission, upstream commu
`nication from home environment H to central of?ce CO over
`tWisted pair Wire facility TWP is effected at loWer frequency,
`Within a loWer frequency bandWidth UP (FIG. 2) Which, in
`this example, ranges from approximately 20 kHZ to approxi
`mately 140 kHZ. The actual bandWidths illustrated in FIG. 2
`are consistent With the so-called Medium Data Subscriber
`Line (MDSL) technology utiliZed in the preferred embodi
`ment of the invention described herein; by Way of reference,
`full ADSL communication occupies approximately tWice the
`bandWidth illustrated in FIG. 2.
`This frequency division betWeen upstream and doWn
`stream communications is selected considering that most
`home and office users Will be more frequently doWnloading
`larger blocks of information from the Internet via central
`of?ce CO, relative to the amount of information (typically
`email) that these users Will be uploading to the Internet.
`Recognition of these different uses and traf?c requirements
`for upstream and doWnstream communication permits the
`division of the overall bandWidth in the manner illustrated in
`FIG. 2. As a result, modems 10, 20k may readily commu
`nicate With one another, With such noise effects as echoes
`being readily ?lterable by simple bandpass ?ltering, given
`the Wide disparity betWeen upstream and doWnstream fre
`quencies. For example, an upstream echo of doWnstream
`data Will be at the higher, doWnstream, frequency When
`received at central of?ce 20, thus permitting this echo to be
`easily ?ltered from the loWer frequency signal. Additionally,
`the frequency division multiplexing illustrated in FIG. 2 also
`facilitates the ?ltering of near-end crosstalk (NEXT), in
`much the same manner as echo cancellation.
`According to the preferred embodiment of the invention,
`the frequency division multiplexed communication is car
`ried out by Way of discrete multi-tone modulation (DMT), in
`Which the bandWidth is uniformly divided into multiple bins,
`or subchannels, each having a relatively narroW bandWidth,
`for example 5 kHZ. FIG. 3 illustrates an example of this
`division, for a portion of the doWnstream frequency band
`Width DOWN of FIG. 2. In the system of FIG. 1, the
`transmission characteristics of tWisted pair Wire facility
`TWP, as terminated at remote modem 10 and central of?ce
`modem 20k, varies With frequency, such that each subchan
`nel may have a different signal-to-noise ratio as illustrated in
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`10
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`15
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`25
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`35
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`45
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`55
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`65
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`6
`FIG. 3. According to the preferred embodiment of the
`present invention, the various subchannels are “loaded” to
`carry data at a WordWidth that depends upon their signal
`to-noise ratio. In other Words, subchannels having a higher
`signal-to-noise ratio are capable of carrying more bits than
`are subchannels With a poorer signal-to-noise ratio; as such,
`FIG. 3 illustrates that the number of “bits per tone” corre
`sponds to the signal-to-noise ratio of the subchannels.
`It is contemplated that the attenuation behavior of the
`system over frequency cannot generally be determined a
`priori, as this behavior is highly determined by characteris
`tics of each individual installation, and thus does not nec
`essarily folloW a simple-order model. As such, upon estab
`lishing a connection, the transmitting portions of remote
`modem 10 and central of?ce 20k are set during the initial
`iZation procedure according to the preferred embodiment of
`the invention, as Will be described in detail hereinbeloW.
`This initialiZation procedure includes one modem sending a
`probe signal to determine the signal-to-noise ratio (SNR) of
`each frequency bin, and the receiving modem returning a
`signal, back to the transmitting modem, indicating the
`measurement result and the bit rate loading for each sub
`channel.
`Referring noW to FIG. 4, the functional construction and
`operation of remote modem 10 according to the preferred
`embodiment of the present invention Will noW be described
`in detail. As illustrated in FIGS. 1 and 4, remote modem 10
`interfaces With user unit 8 over interface 9, and With phone
`jack P] by Way of analog front end
`18; bidirectional
`communication is thus effected by Way of interfaces 9, 18,
`With transmit and receive paths disposed therebetWeen as
`Will noW be described. The functional blocks of FIG. 4 are
`processes that are preferably carried out by Way of program
`mable hardWare, as Will be described in further detail
`hereinbeloW, or alternatively by Way of custom hardWare for
`one or more of the speci?c functions.
`On the transmit side, modem 10 receives digital data from
`user unit 8 via interface 9, and packets the data in a physical
`layer frame, Which is then modulated to multiple DMT
`subcarriers, or subchannels, by Way of an inverse Fast
`Fourier Transform (IFFT) process, for application to tWisted
`pair Wire facility TWP via phone jack PJ. Framing and How
`control process 11 receives the digital data from interface 9,
`and buffers the data according to a positive ?oW control
`method in Which modem 10 receives a reply from modem
`20k for a previously sent data packet prior to sending the
`next data packet. Preferably, framing and How control
`process 11 formats each packet of data to include a header
`?eld in advance of the data for synchroniZation, and a
`circular redundant code (CRC) ?eld folloWing the data ?eld
`to permit error detection.
`By Way of an aside, it is useful to transmit “dummy” data,
`such as a recurring scrambled pseudo-random sequence,
`over facility TWP When in an idle mode, in order to maintain
`synchroniZation betWeen modems 10, 20k. This idle mode is
`particularly prevalent for the transmit side of remote modem
`10, considering that most of the data communication
`betWeen modems 10, 20k Will be in the doWnstream direc
`tion. In any case, modems 10, 20k must be able to distinguish
`dummy data from actual data, preferably by Way of the
`header ?eld. These idle periods may also be used to actively
`check the SNR for various subchannels, in similar manner as
`during initialiZation. Additionally, if certain subchannels are
`found to be consistently unusable, framing and How control
`process 11 can generate “stuf?ng” Words to ensure that the
`bad subchannels are not used; the receiving modem must be
`aWare of these bad subchannels, so that the stuf?ng Words
`
`

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`US 6,219,378 B1
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`10
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`15
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`7
`(Which are themselves usually modi?ed by the severe
`attenuation) are ignored.
`According to the DMT approach, digital data is encoded
`to correspond to a point in an amplitude-phase “constella
`tion”. Discussion of the encoding of DMT data may be
`found in Ciof?, “A Multicarrier Primer”, Tutorial submitted
`to Standards Committee T1 ofIEEE (1991); ChoW, et al., “A
`Discrete Multitone Transceiver System for HDSL
`Applications”, Journal on Selected Areas in
`Communications, Vol. 9, No. 6 (IEEE, Aug. 1991), pp.
`895—908; and Bingham, “Multicarrier Modulation for Data
`Transmission: An Idea Whose Time Has Come”, IEEE
`Communications Magazine (May, 1990), pp. 5—14, all incor
`porated herein by this reference. As is knoWn in this art,
`DMT associates each possible digital value (depending upon
`the bit loading for the particular subchannel or subcarrier)
`With an amplitude and phase combination. For eXample, if a
`subcarrier has been assigned to a bit loading of four, the
`constellation for that subcarrier includes siXteen possible
`amplitude-phase combinations, each associated With one of
`the siXteen possible digital values; if a subcarrier has been
`assigned to a bit loading of eight (Which is the maXimum
`according to the MDSL approach used in this embodiment
`of the invention), tWo-hundred ?fty-siX amplitude-phase
`combinations are present in the constellation, each associ
`ated With one of the tWo-hundred ?fty-siX possible digital
`values presented by the eight bits. The smaller constellations
`are preferably a subset of the largest (eight-bit) constellation,
`for ease of encoding. HoWever, the less-populated constel
`lations Will have less poWer than the more heavily populated
`subchannels, and as such gain scaling of the subchannels is
`preferred to amplify the less-populated subchannels.
`Furthermore, it is preferred to encode subcarriers as a group,
`for ef?ciency of operation Where modem 10 is implemented
`by Way of a pipelined digital signal processor (DSP); this
`grouping combines multiple sub-carriers into 16-bit Word
`units, such that each sub-carrier is con?ned Within a Word
`boundary. Some subcarriers Will have their bit loading
`reduced by one or more bits as necessary to maintain this
`grouping. It is also preferred to pre-group subcarriers as part
`of the initialiZation process to generate a pre-stored macro of
`the subcarrier grouping, eliminating the need for conditional
`call and conditional branch operations in the DSP code.
`In the How of FIG. 4, the ordering of the received data
`according to subchannel and its unpacking is performed in
`process 12, While the mapping of the data into the constel
`lation points Within each subcarrier is performed as part of
`bit mapping and gain scaling process 13, preferably through
`use of a look-up table. Scaling of the amplitudes of the
`various subchannels is also carried out in process 13. The
`grouping and encoding of processes 12,13 effectively con
`vert each of the data Words into the frequency domain, as the
`output of process 13 is a sequence of amplitude and phase
`values (encoded by the constellations), With the order in the
`sequence corresponding to the frequency of the associated
`subchannels.
`Clipping control process 14 and IFFT process 15 are then
`neXt performed to generate time-domain signals correspond
`ing to the encoded subcarriers. IFFT process 15 in modem
`10 thus generates thirty-tWo tones (for the relatively loW
`frequency upstream signals) for this communication, pref
`erably by a siXty-four tone IFFT in Which the upper thirty
`tWo tones are set to Zero, providing better frequency cut-off.
`Clipping control process 14 is done prior to IFFT, simply by
`monitoring over?oW ?ags in the status register of the DSP;
`upon detection of an over?oW, process 14 sets certain bits
`(referred to as operation and maintenance bits (OAM) bits)
`
`8
`plus a pilot tone, indicating that the transmitter is repairing
`the clipped frame over the neXt tWo frames; these tWo frames
`Will be combined and decoded by the receiving modem 20k.
`FolloWing IFFT process 15, process 16 adds a circular
`pre?x to interframe portions of the sequence as a guard time,
`Which permits the time-domain equaliZer ?lter of the receiv
`ing modem 20k to have the appropriate impuls