United States Patent
`
`[19]
`
`[11] Patent Number:
`
`4,833,706
`
`Hughes-Hartogs
`
`[45] Date of Patent: May 23, 1989
`
`ENSEMBLE MODEM STRUCIURE FOR
`IMPERFECF TRANSMISSION MEDIA
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[54]
`
`[75]
`
`Inventor:
`
`Dirk Hughes-Hartolgs, Morgan Hill,
`Calif.
`
`[73]
`
`Assignee: Telebit Corporation, Mountain View,
`Calif.
`
`[21]
`
`Appl. No.: 140,848
`
`[22]
`
`Filed:
`
`Jan. 5, 1988
`
`[62]
`
`[51]
`[52]
`
`[53]
`
`Related U.S. Application Data
`Division of Ser. No. 2,096, Jan. 12, 1987, Pat. No.
`4,731,816, which is a division of Ser. No. 736,200, May
`20, 1985, Pat. No. 4,679,227.
`
`Int. Cl.‘ ................................... .. H04M 11/00
`U.S. Cl, ...................................... .. 379/98; 370/19;
`375/39
`Field of Search ..................... .. 375/38, 39, 40, 58,
`375/118; 455/63, 68; 340/825.15; 370/16, 19,
`69.1, 70, 100, 106, 108; 379/93, 97, 98
`
`4,206,320
`4,438,511
`
`.................. .. 375/38 X
`6/1980 Keasler et al.
`3/1984 Baran .............................. .. 370/19 X
`
`OTHER PUBLICATIONS
`
`“Information Theory and Reliable Communication”, R.
`Gallager, John Wiley & Sons, New York 1968, pp.
`383-431.
`
`Primary Examt'ner——Jin F. Ng
`Assistant Examiner——Randa1l S. Vaas
`Attorney, Agent, or F1'rm—Townsend and Townsend
`
`ABSTRACT
`[57]
`A high-speed modem that transmits and receives digital
`data on an ensemble of carrier frequencies spanning the
`usable band of a dial-up telephone line. The modem
`includes a system for variably allocating data and power
`among the carriers to compensate for equivalent noise
`and to maximize the data rate. Additionally, systems for
`eliminating the need for an equalization network, for
`adaptively allocating control of a channel, and the
`tracking variations in line parameters are disclosed.
`
`2 Claims, 6 Drawing Sheets
`
`
`
`
`X0
`
`I
`X0 ...x631x63
`
`To
`
`To “Pu '
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`
`C H2
`
`*no23- "o
`1
`
`'
`
`‘as
`
`SAMPLING PERIOD
`
`ARRIS EX. 1009
`
`

`
`U.S. Patent
`
`May 23,1989
`
`Sheet 1 of 6
`
`4,833,706
`
`PARAMETER
`GENERATOR
`
`
`
`
`CONTROL
`AND
`SCHEDUUNG
`UMT
`
`
`
`
`
`DIMTAL
`DATA
`
`GENERATOR
`
`
`
`

`
`U.S. Patent May 23, 1989
`
`Sheet 2 of 6
`
`4,833,706
`
`26
`
`26'
`
`I
`
`ORICINATE
`
`GO OFF HOOK
`DIAL ANSWER MODEM
`
`ANSWER
`
`CO OFF HOOK
`
`TRANSMIT ANSWER
`
`EPOCII COMB
`
`ACCUMULATE NOISE DATA
`FFT ON NOISE DATA
`TO MEASURE AMPLITUDE
`OF NOISE COMPONENTS
`AT EACH CARRIER FRED.
`
`
`
`
`
`RECEIVE ANSWER COMB
`COMPARE RECEIVED
`AMPLITUDES WITH
`TRANSMITTED AMPLITUDES
`
`
`
`
`
`RECEIVE FIRST
`PHASE-ENCODED
`
`SIGNAL
`
`
`
`
`
`TRANSMIT SECOND
`PHASE-ENCODED
`SIGNAL
`
`RECEIVE DATA
`
`
`TRANSMIT DATA ON
`NUMBER OF BITS AND
`
`POWER LEVELS ON EACH
`CARRIER IN ORICINATE-
`ANSWER DIRECTION
`
`
`
`
`
`
`
`
`
`
`RECEIVE ANSWER EPOCH
`COMB
`COMPARE RECEIVED
`AIIIPLITUDES WITH
`TRANSMITTED AMPLITUDES
`
`ACCUMULATE NOISE DATA
`F FT ON NOISE DATA
`TO MEASURE AMPLITUDE
`OF NOISE COMPONENTS
`AT EACH CARRIER FRED.
`
`
`
`TRANSMIT ORICINATE
`COMB
`
`
`
`
`
`TRANSMIT FIRST
`PHASE-ENCODED
`SIGNAL
`
`RECEIVE SECOND
`PHASE-ENCODED
`SIGNAL
`
`
`
`
`
`
` TRANSMIT DATA ON
`NUMBER OF BITS AND
`POWER LEVELS ON EACH
`CARRIER IN ANSWER-
`ORICINATE DIRECTION
`
`
`
`
`
`RECEIVE DATA
`
`I2)
`
`I
`
`(3)
`
`(6)
`
`(T)
`
`(8)
`
`FIC._4.
`
`
`
`

`
`U.S. Patent
`
`May 23, 1989
`
`Sheet 3 of 6
`
`4,833,706
`
`POWER- l.O0
`Eb/No-0.5
`
`POWER-10.0
`Eb/No-.2.5(‘)
`
`
`
` 4 BITS
`
`POWER-32.75
`
`0 BITS
`
`Pom-20.00
`
`Eb/No-4.00
`
`g+s,+5)
`
`FlG....5.
`
`POWER
`
`SIGNAL POWER
`
`
`
`3: AREA LIMITED T0
`-= sous cousmn
`
`FREQUENCY (Hz)
`
`F|G._6.
`
`
`
`

`
`U.S. Patent May 23, 1989
`
`Sheet 4 of 6
`
`4,833,706
`
`
`
`62
`
`63
`
`62.5
`
`63.5
`
`64 POWER m as
`
`5
`
`20"
`I9
`
`M I
`
`?
`I6
`I5
`I4
`I3
`I2
`
`I0
`
`9 8 7 6 5 4 3 2 I °
`
`o as ID 152025 10 5540 45 59 55 an 55
`
`4kHzf
`
`94
`
`(xi)
`
`2kHz
`
`I 592
`I
`I
`I
`
`FIG._7.
`
`‘Xi’
`
`I75)
`
`I512
`
`(yi)
`
`
`
`F]G___8_
`
`RECEWER
`
`
`
`

`
`U.S. Patent
`
`May 23, 1989
`
`Sheet 5 of 6
`
`4,833,706
`
`EPOCH I
`
`(I36 HSEC)
`
`"0
`
`EPOCH 2 (I56 HSEC)
`
`
`
`Yes
`Yo
`V1025
`7:
`Yo
`Xo X65
`X1023
`XI
`Xo
`\___.__,.....jz\__.—.,__/\_____.7———-—I\—-—:w—-—’
`I28 MSEC
`8 HSEC
`I28 MSEC
`8 MSEC
`TIIIIE SERIES
`
`'03
`I04
`
`
`
`
`FIG.....9.
`
`Xo
`
`X1023» Xo
`
`X63
`
`
`
`smum; PERIOD
`
`FIG._ I0.
`
`
`
`

`
`_ U.S. Patent
`
`May 23, 1989
`
`Sheet 6 of 6
`
`4,833,706
`
`
`
`

`
`1
`
`ENSEMBLE MODEM STRUCTURE FOR
`IIVIPERFECI‘ TRANSMISSION MEDIA
`
`4,833,706
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`l0
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`15
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`20
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`25
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`This application is a division of Ser. No. 07/002,096 5
`filed Jan. 12, 1987, now U.S. Pat. No. 4,713,816 issued
`Mar.
`15,
`1988; which is a division of Ser. No.
`06/736,200 filed May 20, 1985, now U.S. Pat. No.
`4,679,227 issued July 7, 1987.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention:
`The invention relates generally to the field of data
`communications and, more particularly, to a high speed
`modem.
`2. Description of the Prior Art:
`Recently, specially designed telephone lines for the
`direct transmission of digital data have been introduced.
`However, the vast majority of telephone lines are de-
`signed to carry analdg voice frequency (VF) signals.
`Modems are utilized to modulate VF carrier signals to
`encode digital information on the VF carrier signals and
`to demodulate the signals to decode the digital informa-
`tion carried by the signal.
`Existing VF telephone lines have several limitations
`that degrade the performance of modems and limit the
`rate at which data can be transmitted below desired
`error rates. These limitations include the presence of
`frequency dependent noise on the VF telephone lines, a
`frequency dependent phase delay induced by the VF
`telephone lines, and frequency dependent signal loss.
`Generally, the usable band of a VF telephone line is
`from slightly above zero to about four kHz. The power
`spectrum of the line noise is not uniformly distributed
`over frequency and is generally not determinative.
`Thus, there is no a priori method for determining the
`distribution of the noise spectrum over the usable band-
`width of the VF line.
`Additionally,
`a frequency-dependent propagation
`delay is induced by the VF telephone line. Thus, for a
`complex multi-frequency signal, a phase delay between
`the various components of the signal will be induced by
`the VF telephone line. Again, this phase delay is not
`determinative and must be measured for an individual
`VF telephone line at the specific time that transmission
`takes place.
`Further, the signal loss over the VF telephone line
`varies with frequency. The equivalent noise is the noise
`spectrum component added to the signal loss compo-
`nent for each carrier frequency, where both compo-
`nents are measured in decibels (dB).
`Generally, prior art modems compensate for equiva-
`lent line noise and signal loss by gear-shifting the data
`rate down to achieve a satisfactory error rate. For ex-
`ample, in U.S. Pat. 4,438,511, by Baran, a high speed
`modem designated SM9600 Super Modem manufac-
`tured by Gandalf Data, Inc., is described. In the pres-
`ence of noise impairment, the SM9600 will “gear shift”
`or drop back its transmitted data rate to 4800 bps or
`24-00 bps. The system described in the Baran patent
`transmits data over 64 orthogonally modulated carriers.
`The Baran system compensates for the frequency de-
`pendent nature of the noise on the VF line by terminat-
`ing transmission on carriers having the same frequency ’
`as the frequency of large noise components on the line. 65
`Thus, Baran gracefully degrades its throughput by ceas-
`ing to transmit on carrier frequencies at the highest
`points of the VF line noise spectrum. The Baran system
`
`30
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`
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`
`2
`essentially makes a go/no go decision for each carrier
`signal, depending on the distribution of the VF line
`noise spectrum. This application reflects a continuation
`of the effort initiated by Baran.
`Most prior art systems compensate for frequency
`dependent phase delay induced by the VF line by an
`equalization system. The largest phase delay is induced
`in frequency components near the edges of the usable
`band. Accordingly, the frequency components near the
`center of the band are delayed to allow the frequency
`components at the outside of the band to catch up.
`Equalization generally requires additional circuitry to
`accomplish the above-described delays.
`A further problem associated with two way transmis-
`sion over the VF telephone line is that interference
`between the outgoing and incoming signals is possible.
`Generally, separation and isolation between the two
`signals is achieved in one of three ways:
`(a) Frequency multiplexing in which different fre-
`quencies are used for the different signals. This method
`is common in modem-based telecommunication sys-
`tems.
`
`(b) Time multiplexing, in which different time seg-
`ments are used for the different signals. This method is
`often used in half-duplex systems in which a transmitter
`relinquishes a channel only after sending all the data it
`has. And,
`(c) Code multiplexing, in which the signals are sent
`using orthogonal codes.
`All of the above-described systems divide the space 7
`available according to constant proportions fixed dur-
`ing the initial system design. These constant propor-
`tions, however, may not be suitable to actual traffic load
`problem presented to each modem. For example, a
`clerk at a PC work station connected to a remote host
`computer may type ten or twenty characters and re-
`ceive a full screen in return. In this case, constant pro-
`portions allocating the channel equally between the
`send and receive modems would greatly overallocate
`the channel to the PC work station clerk. Accordingly,
`a modem that allocates channel capacity according to
`the needs of the actual traffic load situation would
`greatly increase the efficient utilization of the channel
`capacity.
`SUMMARY OF THE INVENTION
`
`The present invention is a high-speed modem for use
`with dial-up VF telephone lines. The modem utilizes a
`multicarrier modulation scheme and variably allocates
`data and power to the various carriers to maximize the
`overall data transmission rate. The allocation of power
`among the carriers is subject to the constraint that the
`total power allocated must not exceed a specified limit.
`In a preferred embodiment, the modem further in-
`cludes a variable allocation system for sharing control
`of a communication link between two modems (A and
`B) according to actual user requirements.
`Another aspect of the invention is a system for com-
`pensating for frequency dependent phase delay and
`preventing intersymbol interference that does not re-
`quire an equalization network.
`According to one aspect of the invention, quadrature
`amplitude modulation (QAM) is utilized to encode data
`elements of varying complexity on each carrier. The
`equivalent noise component at each carrier frequency is
`measured over a communication link between two
`modems (A and B).
`
`

`
`3
`As is known in the art, if the bit error rate (BER) is to
`be maintained below a specified level, then the power
`required to transmit a data element of a given complex-
`ity on a given carrier frequency must be increased if the
`equivalent noise component at that frequency increases.
`Equivalently, to increase data complexity, the signal to
`noise ratio, S/N, must be increased.
`In one embodiment of the present invention, data and
`power are allocated to maximize the overall data rate
`within external BER and total available power con-
`straints. The power allocation system computes the
`marginal required power to increase the symbol rate on
`each carrier from n to n+1 information units. The sys-
`tem then allocates information units to the carrier that
`requires the least additional power to increase its sym-
`bol rate by one information unit. Because the marginal
`powers are dependent on the values of the equivalent
`noise spectrum of the particular established transmis-
`sion link, the allocation of power and data is specifically
`tailored to compensate for noise over this particular
`link.
`
`According to another aspect of the invention, a first
`section of the symbol on each carrier is retransmitted to
`form a guard-time waveform of duration TE+TpH
`where T}; is the duration of the symbol and T12}; is the
`duration of the first section. The magnitude of TPH is
`greater than or equal to the maximum estimated phase
`delay for any frequency component of the waveform.
`For example, if the symbol is represented by the time
`series, xo .
`.
`. x,,_1, transmitted in time T5; then the
`guardtime waveform is represented by the time series,
`xo .
`.
`. x,...1, xo .
`.
`. )_{m_1, transmitted in time TE—§-Tpy.
`The ratio that III bears to n is equal to the ratio that Tpy
`bears to TE.
`At the receiving modern, the time of arrival, T0, of
`the first frequency component of the guard-time wave-
`form is determined. A sampling period, of duration T5,
`is initiated a time To-I-TPH.
`Accordingly, the entire symbol on each carrier fre-
`quency is sampled and intersymbol interference is elimi-
`nated.
`
`According to a still further aspect of the invention,
`allocation of control to the transmission link between
`modems A and B is accomplished by setting limits to
`the number of packets that each modem may transmit
`during one transmission cycle. A packet of information
`comprises the data encoded on the ensemble of carriers
`comprising one waveform. Each modem is also con-
`strained to transmit a minimum number of packets to
`maintain the communication link between the modems.
`Thus, even if one modem has no data to transmit, the
`minimum packets maintain timing and other parameters
`are transmitted. On the other hand, if the volume of data
`for a modem is large, it is constrained to transmit only
`the maximum limited number of packets, N, before
`relinquishing control to the other modern.
`In practice, if modem'A has a small volume of data
`and modem B has a large volume of data, modem B will
`have control of the transmission link most of the time. If
`control is first allocated to modem A it will only trans-
`mit the minimal number, I, of packets. Thus A has con-
`trol for only a short time. Control is then allocated to B
`which transmits N packets, where N may be very large.
`Control is again allocated to modem A which transmits
`I packets before returning control to B.
`Thus, allocation of control is proportional to the ratio
`of I to N. If the transmission of the volume of data on
`modem A requires L packets, where L is between I and
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`N, then the allocation is proportional to the ratio of L to
`N. Accordingly, allocation of the transmission link var-
`ies according to the actual needs of the user.
`Additionally, the maximum number of_packets, N,
`need not be the same for each modern, but may be var-
`ied to accommodate known disproportions in the data
`to be transmitted by A and B modems.
`According to another aspect of the invention, signal
`loss. and frequency offset are measured prior to data
`determination. A tracking system determines variations
`from the measured values and compensates for these
`deviations.
`
`According to a further aspect of the invention, a
`system for determining a precise value of To is included.
`This system utilizes two timing signals, at f1 and f2,
`_ incorporated in a waveform transmitted from modem A
`at time T4. The relative phase difference between the
`first and second timing signals at time TA is zero.
`The waveform is received at modem B and a rough
`estimate, TEST, of the time of reception is obtained by
`detecting energy at f1. The relative phase difference
`between the timing signals at time TEST is utilized to
`obtain a precise timing reference, To.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a graph of the ensemble of carrier frequen-
`cies utilized in the present invention.
`FIG. 2 is a graph of the constellation illustrating the
`QAM of each carrier.
`FIG. 3 is a block diagram of an embodiment of the
`invention.
`FIG. 4 is a flow chart illustrating the synchronization
`process of the present invention.
`FIG. 5 is a series of graphs depicting the constella-
`tions for O, 2, 4, 5, 6 bit data elements and exemplary
`signal to noise ratios and power levels for each constel-
`lation.
`
`FIG. 6 is a graph illustrating the waterfilling algo-
`rithm.
`~
`FIG. 7 is a histogram illustrating the application of
`the waterfilling algorithm utilized in the present inven-
`tion.
`
`FIG. 8 is a graph depicting the effects of phase depen-
`dent frequency delay on frequency components in the
`ensemble.
`FIG. 9 is a graph depicting the wave forms utilized in
`the present invention to prevent intersymbol interfer-
`ence.
`
`FIG. 10 is a graph depicting the method of receiving
`the transmitted ensemble.
`
`FIG. 11 is a schematic diagram depicting the modula-
`tion template.
`FIG. 12 is a schematic diagram depicting the quad-
`rants of one square in the modulation template.
`FIG. 13 is a schematic diagram of a hardware em-
`bodiment of the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The present invention is a modem that adaptively
`allocates power between various carrier frequencies in a
`frequency ensemble to compensate for frequency de-
`pendent line noise, eliminates the need for equalization
`circuitry to compensate for a frequency dependent
`phase delay, and provides a duplex mechanism that
`accounts for varying channel load conditions to allocate
`the channel between the send and receive modems.
`
`

`
`5
`Additional features of the invention are described be-
`low.
`
`A brief description of the frequency ensemble and
`modulation scheme -utilized in the present invention is
`first presented with respect to FIGS. 1 and 2 to facilitate
`the understanding of the invention. A specific embodi-
`ment of the invention is then described with reference
`to FIG. 3. Finally, the operation of various features of
`the invention are described with reference to FIGS. 4
`through 13.
`Modulation and Ensemble Configuration
`Referring now to FIG. 1, a diagrammatic representa-
`tion is shown of the transmit ensemble 10 of the present
`invention. The ensemble includes 512 carrier frequen-
`cies 12 equally spaced across the available 4 kHz VF
`band. The present invention utilizes quadrature ampli-
`tude modulation (QAM) wherein phase independent
`sine and cosine signals at each carrier frequency are
`transmitted. The digital information transmitted at a
`given carrier frequency is encoded by amplitude modu-
`lating the independent sine and cosine signals at that
`frequency.
`The QAM system transmits data at an overall bit rate,
`RB. However, the transmission rate on each carrier,
`denoted the symbol or baud rate, R5, is only a fraction
`of R3. For example, if data were allocated equally be-
`tween two carriers then Rs=RB/2.
`In the preferred embodiment 0, 2, 4, 5 or 6 bit data
`elements are encoded on each carrier and the modula-
`tion of each carrier is changed every 136 msec. A theo-
`retical maximum, R3, assuming a 6 bit R5 for each car-
`rier, of 22,580 bit/sec (bps) results. A typical relizable
`Rs, assuming 4 bit Rs over 75% of the carriers, is equal
`to about 11,300 bps. This extremely high R5 is achieved
`with a bit error rate of less than 1 error/ 100,000 bits
`transmitted.
`
`In FIG. 1, a plurality of vertical lines 14 separates
`each ensemble into time increments known hereafter as
`“epochs.” The epoch is of duration TE where the mag-
`nitude of TE is determined as set forth below.
`The QAM system for encoding digital data onto the
`various carrier frequencies will now be described with
`reference to FIG. 2. In FIG. 2 a four bit “constellation”
`20 for the nth carrier is depicted. A four bit number may
`assume sixteen discrete values. Each point in the con-
`stellation represents a vector (x,,, y,,) with x,. being the
`amplitude of the sine signal and y,, being the amplitude
`of the cosine signal in the above-described QAM sys-
`tem. The subscript n indicates the carrier being modu-
`lated. Accordingly, the four bit constellation requires
`four discrete y,, and four discrete x,. values. As de-
`scribed more fully below, increased power is required
`to increase the number of bits transmitted at a given
`carrier frequency due to the equivalent noise compo-
`nent at that frequency. The receive modem, in the case
`of four bit transmission, must be able to discriminate
`between four possible values of the x,, and y,, amplitude
`coefficients. This ability to discriminate is dependent on
`the signal to noise ratio for a given carrier frequency.
`In a preferred embodiment, packet
`technology is
`utilized to reduce the error rate. A packet includes the
`modulated epoch of carriers and error detection data.
`Each packet in error is retransmitted until correct. Al-
`ternatively, in systems where retransmission of data is
`undesirable, epochs with forward error correcting
`codes may be utilized.
`'
`Block Diagram
`
`10
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`4,833,706
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`6
`FIG. 3 is a block diagram of an embodiment of the
`present invention. The description that follows is of an
`originate modem 26 coupled to an originate end of a
`communication link formed over a public switched
`telephone line. It is understood that a communication
`system also includes an answer modem coupled to the
`answer end of the communication link. In the following
`discussion, parts in the answer modem corresponding to
`identical or similar parts in the originate modem will be
`designated by the reference number of the originate
`modem primed.
`Referring now to FIG. 3, an incoming data stream is
`received by a send system 28 of the modem 26 at data
`input 30. The data is stored as a sequence of data bits in
`a buffer memory 32. The output of buffer memory 32 is
`coupled to the input of a modulation parameter genera-
`tor 34. The output of the modulation parameter genera-
`tor 34 is coupled to a vector table buffer memory 36
`with the vector table buffer memory 36 also coupled to
`the input of a modulator 40. The output of the modula-
`tor 40 is coupled to a time sequence buffer 42 with the
`time sequence buffer 42 also coupled to the input of a
`digital-to-analog converter 43 included in an analog
`I/O interface 44. The interface 44 couples the output of
`the modem to the public switched telephone lines 48.
`A receive ‘system 50 includes an analog-to-digital
`converter (ADC) 52 coupled to the public switched
`telephone line 48 and included in the interface 44. The
`output from the ADC 52 is coupled to a receive time
`series buffer 54 which is also coupled to the input of a
`demodulator 56. The output of the demodulator 56 is
`coupled to a receive vector table buffer 58 which is also
`coupled to the input of a digital data generator 60. The
`digital data generator 60 has an output coupled to a
`receive data bit buffer 62 which is also coupled to an
`output terminal 64.
`A control and scheduling unit 66 is coupled with the
`modulation parameter generator 34,
`the vector table
`buffer 36, the demodulator 56, and the receive vector
`table buffer 58.
`
`An overview of the functioning of the embodiment
`depicted in FIG. 3 will now be presented. Prior to the
`transmission of data, the originate modem 26, in cooper-
`ation with the answer modem 26’, measures the equiva-
`lent noise level at each carrier frequency, determines
`the number of bits per epoch to be transmitted on each
`carrier frequency, and allocates power to each carrier
`frequency as described more fully below.
`The incoming data is received at input port 30 and
`formatted into a bit sequence stored in the input buffer
`32.
`
`The modulator 34 encodes a given number of bits into
`an (xn, y,,) vector for each carrier frequency utilizing
`the QAM system described above. For example, if it
`were determined that four bits were to be transmitted at
`frequency f,. then four bits from the bit stream would be
`converted to one of the sixteen points in the four bit
`constellation of FIG. 2. Each of these constellation
`points corresponds to one of sixteen possible combina-
`tions of four bits. The amplitudes of the sine and cosine
`signals for frequency 11 then corresponds to the point in
`the constellation encoding the four bits of the bit se-
`quence. The (x,,, y,,) vectors are then stored in the vec-
`tor buffer table 36. The modulator receives the table of
`(x,,, y,,) vectors for the carriers in the ensemble and
`generates a digitally encoded time series representing a
`wave form comprising the ensemble of QAM carrier
`frequencies.
`
`

`
`7
`In a preferred embodiment the modulator 40 includes
`a fast Fourier transform (FFT) and performs an inverse
`FFT operation utilizing the (x,y) vectors as the FFT
`coefficients. The vector table includes 1,024 indepen-
`dent points representing the 1,024 FFT points of the 512
`frequency constellation. The inverse FF I operation
`generates 1,024 points in a time series representing the
`QAM ensemble. The 1,024 elements of this digitally
`encoded time series are stored in the digital time series
`buffer 42.. The digital time sequence is converted to an
`analog wave form by the analog to digital converter 43
`and the interface 46 conditions the signal for transmis-
`sion over the public switched telephone lines 48.
`Turning now to the receive system 50, the received
`analog waveform from the public switched telephone
`lines 48 is conditioned by the interface 46 and directed
`to the analog to digital converter 52. The analog to
`digital converter 52 converts the analog waveform to a
`digital 1,024 entry time series table which is stored in
`the receive time series buffer 54. The demodulator 56
`converts the 1,024 entry time series table_ into a 512
`entry (x,,,y,,) vector table stored in the receive vector
`table buffer 58. This conversion is accomplished by
`performing an FFT on the time series. Note that infor-
`mation regarding the number of bits encoded onto each
`frequency carrier has been previously stored in the
`demodulator and digital data generator 60 so that the
`(x,y) table stored in the receive vector table buffer 58
`may be transformed to an output data bit sequence by
`the digital data generator 60. For example, if the (x,,, y,.)
`vector represents a four bit sequence then this vector
`would be converted to a four bit sequence and stored in
`the receive data bit buffer 62 by the digital data genera-
`tor 60. The receive data bit sequence is then directed to
`the output 64 as an output data stream.
`A full description of the FFT techniques utilized is
`described in a book by Rabiner et al., entitled Theory
`and Applications of Digital Signal Processing, Prentice-
`Hall, Inc., N.J., 1975. However, the FFT modulation
`technique described above is not an integral part of the
`present invention. Alternatively, modulation could be
`accomplished by direct multiplication of the carrier
`tones as described in the above-referenced Baran pa-
`tent, which is hereby incorporated by reference, at col.
`10, lines 13-70, and col. 11, lines 1-30. Additionally, the
`demodulation system described in Baran at col. 12, lines
`35-70, col. 13, lines 1-70, and col. 14, lines 1-13 could
`be substituted.
`The control and scheduling unit 66 maintains overall
`supervision of the sequence of operations and controls
`input and output functions.
`Determination of Equivalent Noise
`As described above, the information content of the
`data element encoded on each frequency carrier and the
`power allocated to that frequency carrier depends on
`the magnitude of the channel noise component at that
`carrier frequency. The equivalent
`transmitted noise
`component at frequency fn, N(f,,), is the measured (re-
`ceived) noise power at frequency f,, multiplied by the
`measured signal loss at frequency f,.,. The equivalent
`noise varies from line to line and also varies on a given
`line at different times. Accordingly, in the present sys-
`tern, N(f) is measured immediately prior to data trans-
`mission.
`
`The steps of a synchronization technique utilized in
`the present system to measure NO) and establish a trans-
`mission link between answer and originate modems 26
`and 26' are illustrated in FIG. 4. Referring now to FIG.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`4,833,706
`
`8
`4, in step 1 the originate modem dials the number of the
`answer modem and the answer modem goes off hook.
`In step 2 the answer modem transmits an epoch of two
`frequencies at the following power levels:
`(a) 1437.5 Hz. at -3 dBR; and
`(b) 1687.5 Hz at ——3 dBR.
`The power is measured relative to a reference, R,
`where, in a preferred embodiment, OdBR= -9 dBm, in
`being a millivolt. These tones are used to determine
`timing and frequency offset as detailed subsequently.
`The answer modern then transmits an answer comb
`containing all 512 frequencies at -27 dBR. The origi-
`nate‘ modem receives the answer comb and performs an
`FFT on the comb. Since the power levels of the 512
`frequencies were set at specified values, the control and
`scheduling unit 66 answer modem 26 compares the
`(x,,,y,,) values for each frequency of the received code
`and compares those values to a table of (x,,,y,,) values
`representing the power levels of the transmitted answer
`code. This comparison yields the signal loss at each
`frequency due to the transmission over the VF tele-
`phone lines.
`During step 3 both the originate and answer modems
`26 and 26' accumulate noise data present on the line in
`the absence of any transmission by either modem. Both
`modems then perform an FFT on the accumulated
`noise signals to determine the measured (received) noise
`spectrum component values at each carrier frequency.
`Several epochs of noise may be averaged to refine the
`measurement.
`
`In step 4 the originate modem transmits an epoch of
`two frequencies followed by an originate comb of 512
`frequencies with the same power levels described above
`for step 2. The answer modem receives the epoch and
`the originate comb and calculates the timing, frequency
`offset and signal loss values at each carrier frequency as
`described above for the originate modem in step 2. At
`this point the originate modem 26 has accumulated
`noise and signal loss data for transmission in the answer
`originate direction while the answer modem has accu-
`mulated the same data relating to transmission in the
`originate answer direction. Each modem requires data
`relating to transmission loss and receive noise in both
`the originate-answer and answer-originate directions.
`Therefore,
`this data is exchanged between the two
`modems according to the remaining steps of the syn-
`chronization process.
`In step 5 the originate modem generates and transmits
`a first phase encoded signal indicating which carrier
`frequencies will support two bit transmission at stan-
`dard power levels in the answer-originate direction.
`Each component that will support two bits in the an-
`swer-originate direction at a standard power level is
`generated as a -28 dBR signal with 180° relative phase.
`Each component that will not support two bit transmis-
`sion in the answer-originate direction at the standard
`power level is coded as a -28 dBR, 0° relative phase
`signal. The answer modem receives this signal and de-
`termines which frequency carriers will support two bit
`transmission in the answer-originate direction.
`In step 6 the answer modem generates and transmits
`a second phase encoded signal indicating which carrier
`frequencies will support two bit transmission in both the
`originate-answer and answer-originate directions. The
`generation of this signal is possible because the answer
`modem has accumulated noise and signal loss data in the
`originate-answer direction and has recieved the same
`data for the answer-originate direction in the signal
`
`

`
`9
`generated by the originate modem in step 5. In the
`signal generated by the originate modem, each fre-
`quency component that will support two bits in both
`directions is coded with 180° relative phase and all
`other components are coded with 0° relative phase.
`A transmission link now exists between the two
`
`modems. In general, 300 to 400 frequency components
`will support two bit transmission at a standard power
`level, thereby establishing about a 600 bit/epoch rate
`between the two modems.
`In step 7 the originate
`modern sends data on the number of bits (0 to 15) and
`the power levels (0 to 63 dB) that can be supported on
`each frequency in the answer-originate direction in
`ensemble packets formed over this existing data link.
`Accordingly, both the originate and answer modem
`now have the data relating to transmission in the an-
`swer-originate direction. The steps for calculating the
`number of bits and power levels that can be supported
`on each frequency component will be described below.
`In step 8 the answer modem sends data on the number
`of bits and power levels that can be supported on each
`frequency in the originate-answer direction utilizing the
`existing data link. Thus, both modems are apprised of
`the number of bits and power levels to be supported on
`each frequency component in both the answer-originate
`and originate-answer directions.
`The above description of the determination of the
`equivalent noise level component at each carrier fre-
`quency sets forth the required steps in a given sequence.
`However, the sequence of steps is not critical and many
`of the steps may be done simultaneously or in different
`order, for example, the performance of the FFT on the
`originate code and the accumulati