United States Patent [19]
`Hughes-Hartogs
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,679,227
`Jul. 7, 1987
`
`[75]
`
`[54] ENSEMBLE MODEM STRUcruRE FOR
`IMPERFEcr TRANSMISSION MEDIA
`Inventor: Dirk Hughes-Hartogs, Morgan Hill,
`Calif.
`[73] Assignee: Telebit Corporation, Cupertino,
`Qilif.
`[21] Appl. No.: 736,200
`[22] Filed:
`May 20, 1985
`[51]
`Int. Cl.4 .................... H04M 11/00; H04B 15/00;
`H04L 5/00
`[52] U.S. CI •........................................ 379/98; 375139;
`375/58; 370/19; 455163;455/68
`[58] Field of Search ................ 179/2 DP; 375/38, 40,
`375158, 39, 118; 370/16, 19; 455163, 68;
`340/825.15
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,047,153 9/1977 Thirion ............................. 375/40 X
`4,206,320 6/1980 Keasler et aI ..................... 375/38 X
`4,315,319 2/1982 White ................................ 455/63 X
`4,438,511 3/1984 Baran .................................... 370/19
`4,459,701 7/1984 LamiraI et al. ................. 375/118 X
`4,494,238 1/1985 Groth, Jr .......................... 375/40 X
`4,495,619 1/1985 Acompora ........................ 375/58 X
`4,555,790 11/1985 Betts et al. ............................ 375/39
`
`4,559,520 12/1985 Johnston ........................... 370/19 X
`4,573,133 2/1986 White ................................ 455/63 X
`4,574,244 3/1986 Head ..................................... 375/39
`4,601,044 7/1986 Kromer, III et a!. .......... 375/118 X
`
`OTHER PUBLICATIONS
`Johnson, "PC Communications: The Revolution is
`Coming", Telecommunications, vol. 19, No. 10, issued
`Oct. 1985, pp. 58j-58r.
`Information Theory and Reliable Communication, pp.
`383-431, John Wiley & Sons, New York, 1968.
`
`Primary Examiner-Gene Z. Rubinson
`Assistant Examiner-M. Connors
`Attorney, Agent, or Firm-Townsend and Townsend
`
`ABSTRAcr
`[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 for
`tracking variations in line parameters are disclosed.
`
`17 Claims, 13 Drawing Figures
`
`DIGITAL
`DATA IN
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`

`
`u.s. Patent
`
`JuL 7, 1987
`
`Sheet 1 of6
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`
`u.s. Patent
`
`JuL 7,1987
`
`__________ <26
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`ORIGINATE
`
`Sheet 2 of6
`
`4,679,227
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`

`
`u.s. Patent Jut 7, 1987
`Eb/NO+
`
`Sheet 3 of6
`
`4,679,227 .
`POWER -10.0 _
`Eb/No -2.50
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`

`
`u.s. Patent Jut 7,1987
`
`Sheet 4 of6
`
`4,679,227.
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`

`
`u.s. Patent JuL 7, 1987
`
`SheetS of6
`
`4,679;227
`
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`
`u.s. Patent Jut 7, 1987
`
`Sheet 6 of6
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`4,679,227
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`
`1
`
`ENSEMBLE MODEM STRUcrURE FOR
`IMPERFEcr TRANSMISSION MEDIA
`
`4,679,227
`
`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(cid:173)
`signed to carry analog voice frequency (VF) signals.
`Modems are utilized to modulate VF carrier signals to 15
`encode digital information on the VF carrier signals and
`to demodulate the signals to decode the digital informa(cid:173)
`tion carried by the signal.
`Existing VF telephone lines have several limitations
`that degrade the performance of modems and limit the 20
`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 30
`distribution of the noise spectrum over the usable band(cid:173)
`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 35
`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(cid:173)
`nent for each carrier frequency, where both compo(cid:173)
`nents are measured in decibels (dB).
`Generally, prior art modems compensate for equiva(cid:173)
`lent line noise and signal loss by gear-shifting the data
`rate down to achieve a satisfactory error rate. For ex(cid:173)
`ample, in U.S. Pat. No. 4,438,511, by Baran, a high
`speed modem designated SM9600 Super Modem manu- 50
`factured by Gandalf Data, Inc., is described. In the
`presence of noise impairment, the SM9600 will "gear
`shift" or drop back its transmitted data rate to 4800 bps
`or 2400 bps. The system described in the Baran patent
`transmits data over 64 orthogonally modulated carriers. 55
`The Baran system compensates for the frequency de(cid:173)
`pendent nature of the noise on the VF line by terminat(cid:173)
`ing transmission on carriers having the same frequency
`as the frequency of large noise components on the line.
`Thus, Baran gracefully degrades its throughput by ceas(cid:173)
`ing to transmit on carrier frequencies at the highest
`points of the VF line noise spectrum. The Baran system
`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 65
`of the effort initiated by Baran.
`Most prior art systems compensate for frequency
`dependent phase delay induced by the VF line by an
`
`2
`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
`5 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(cid:173)
`sion over the VF telephone line is that interference
`10 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(cid:173)
`quencies are used for the different signals. This method
`is common in modem-based telecommunication sys(cid:173)
`tems.
`(b) Time multiplexing, in which different time seg(cid:173)
`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
`available according to constant proportions fixed dur(cid:173)
`ing the initial system design. These constant propor(cid:173)
`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(cid:173)
`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.
`
`25
`
`40
`
`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
`45 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(cid:173)
`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(cid:173)
`pensating for frequency dependent phase delay and
`preventing intersymbol interference that does not re(cid:173)
`quire an equalization network.
`According to one aspect of the invention, quadrature
`amplitude modulation (QAM) is utilized to encode data
`60 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).
`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(cid:173)
`ity on a given carrier frequency must be increased if the
`equivalent noise component at that frequency increases.
`
`

`
`4,679,227
`
`3
`Equivalently, to increase data complexity, the signal to
`noise ratio, SIN, 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- 5
`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(cid:173)
`tem then allocates information units to the carrier that
`requires the least additional power to increase its sym- 10
`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(cid:173)
`sion link, the allocation of power and data is specifically
`tailored to compensate for noise over this particular 15
`link.
`According to another aspect of the invention, a first
`section of the symbol on each carner is retransmitted to
`form a guard-time waveform of duration TE+ TpH
`where TE is the duration of the symbol and TpH is the 20
`duration of the first section. The magnitUde of T PH 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 ... Xn-h transmitted in time TE, then the 25
`guard time waveform is represented by the time series,
`xo ... Xn-l, xo· .. Xm-b transmitted in time TE+ TpH.
`The ratio that m bears to n is equal to the ratio that T PH
`bears to TE.
`At the receiving modem, the time of arrival, To, of 30
`the first frequency component of the guard-time wave(cid:173)
`form is determined. A sampling period, of duration TE,
`is initiated a time To+TpH.
`Accordingly, the entire symbol on each carrier fre(cid:173)
`quency is sampled and intersymbol interference is elimi- 35
`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 40
`..... 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(cid:173)
`strained to transmit a minimum number of packets to
`maintain the communication link between the modems. 45
`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 50
`relinquishing control to the other modem.
`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- 55
`mit the minimal number, I, of packets. Thus A has con(cid:173)
`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
`N, then the allocation is proportional to the ratio of L to
`N. Accordingly, allocation of the transmission link var- 65
`ies according to the actual needs of the user.
`Additionally, the maximum number of packets, N,
`need not be the same for each modem, but may be var-
`
`4
`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 in(cid:173)
`cluded. This system utilizes two timing signals, at f1 and
`f2,
`incorporated in a waveform
`transmitted from
`modem A at time T A. The relative phase difference
`between the first and second timing signals at time T A 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 fl. 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(cid:173)
`cies utilized in the present invention.
`FIG. 2 is a graph ofthe 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(cid:173)
`tions for 0, 2, 4, 5, 6 bit data elements and exemplary
`signal to noise ratios and power levels for each constel(cid:173)
`lating
`FIG. 6 is a graph illustrating the waterfilling algo(cid:173)
`rithm.
`FIG. 7 is a histogram illustrating the application of
`the waterfilling algorithm utilized in the present inven(cid:173)
`tion.
`FIG. 8 is a graph depicting the effects of phase depen(cid:173)
`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(cid:173)
`ence
`FIG. 10 is a graph depicting the method of receiving
`the traasmitted ensemble.
`FIG. 11 is a schematic diagram depicting the modula(cid:173)
`tion template.
`FIG. 12 is a schematic diagram depicting the quad(cid:173)
`rants of one sare in the modulation template.
`FIG. 13 is a schematic diagram of a hardware em(cid:173)
`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(cid:173)
`pendent line noise, eliminates the need for equalization
`60 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.
`Additional features of the invention are described be(cid:173)
`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
`
`

`
`5
`the understanding of the invention. A specific embodi(cid:173)
`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.
`
`4,679,227
`
`Modulation and Ensemble Configuration
`Referring now to FIG. 1, a diagrammatic representa(cid:173)
`tion is shown of the transmit ensemble 10 of the present
`invention. The ensemble includes 512 carrier frequen(cid:173)
`cies 12 equally spaced across the available 4 kHz VF
`band. The present invention utilizes quadrature ampli(cid:173)
`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(cid:173)
`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, Rs, is only a fraction
`of RB. For example, if data were allocated equally be(cid:173)
`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(cid:173)
`tion of each carrier is changed every 136 msec. A theo(cid:173)
`retical maximum, RB, assuming a 6 bit Rs for each car(cid:173)
`rier, of 22,580 bit/sec (bps) results. A typical realizable
`Rs, assuming 4 bit Rs over 75% of the carriers, is equal
`to about 11,300 bps. This extremely high Rsis achieved
`with a bit error rate of less than I error/loo,OOO 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 TEwhere the mag(cid:173)
`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(cid:173)
`stellation represents a vector (xn,Yn) with Xn being the
`amplitUde of the sine signal and y n being the amplitude
`of the cosine signal in the abovedescribed QAM system.
`The subscript n indicates the carrier being modulated. 45
`Accordingly, the four bit constellation requires four
`discrete Yn and four discrete Xn values. As described
`more fully below, increased power is required to in(cid:173)
`crease the number of bits transmitted at a given carrier
`frequency due to the equivalent noise component at that 50
`frequency. The receive modem, in the case of four bit
`transmission, must be able to discriminate between four
`possible values of the Xn and Yn 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(cid:173)
`ternatively, in systems where retransmission of data is 60
`undesirable, epochs with forward error correcting
`codes may be utilized.
`
`Block Diagram
`FIG. 3 is a block diagram of an embodiment of the 65
`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
`
`5
`
`6
`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
`10 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(cid:173)
`tor 34. The output of the modulation parameter genera(cid:173)
`tor 34 is coupled to a vector table buffer memory 36
`15 with the vector table buffer memory 36 also coupled to
`the input of a modulator 40. The output of the modula(cid:173)
`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
`20 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
`25 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
`30 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
`35 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-
`40 ation with the answer modem 26', measures the equiva(cid:173)
`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,Yn) 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 fre-
`quency fn 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
`55 points corresponds to one of sixteen possible combina(cid:173)
`tions of four bits. The amplitudes of the sine and cosine
`signals for frequency n then corresponds to the point in
`the constellation encoding the four bits of the bit se-
`quence. The (xn,Yn) vectors are then stored in the vec(cid:173)
`tor buffer table 36. The modulator receives the table of
`(xn,yn) vectors for the carriers in the ensemble and gen-
`erates a digitally encoded time series representing a
`wave form comprising the ensemble of QAM carrier
`frequencies.
`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-
`
`

`
`7
`dent points representing the 1,024 FFT points of the 512
`frequency constellation. The inverse FFT 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 5
`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(cid:173)
`sion over the public switched telephone lines 48.
`Turning now to the receive system 50, the received 10
`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 lime series table which is stored in 15
`the receive time series buffer 54. The demodulator 56
`converts the 1,024 entry time series table into a 512
`entry (xn,Yn) 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- 20
`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 25
`may be transformed to an output data bit sequence by
`the digital data generator 60. For example, if the (xn,Yn)
`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- 30
`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 aI., entitled Theory
`and Applications of Digital Signal Processing, Prentice- 35
`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
`accompished by direct multipication of the carrier tones
`as described in the above-referenced Baran patent, 40
`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 f n• N(fn), is the measured (re(cid:173)
`ceived) noise power at frequency fn multiplied by the
`measured signal loss at frequency fn . The equivalent
`noise varies from line to line and also varies on a given
`line at different times. Accordingly, in the present sys(cid:173)
`tem, N(t) is measured immediately prior to data trans(cid:173)
`mission.
`The steps of a synchronization technique utilized in
`the present system to measure N(t) and establish a trans(cid:173)
`mission link between answer and originate modems 26
`and 26' are illustrated in FIG. 4. Referring now to FIG.
`4, in step 1 the originate modem dials the number of the
`answer modem and the answer modem goes off hook.
`
`4,679,227
`
`8
`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, 0 dBR= -9 dBm, m
`being a millivolt. These tones are used to determine
`timing and frequency offset as detailed subsequently.
`The answer modem then transmits an answer comb
`containing all 512 frequencies at -27 dBR. The origi(cid:173)
`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
`(xn,Yn) values for each frequency of the received code
`and compares those values to a table of (xn,Yn) values
`representing the power levels of the transmitted answer
`code. This comparison yieds the signal loss at each
`frequency due to the transmission over the VF tele(cid:173)
`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(cid:173)
`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-
`45 chronization process.
`In step 5 the originate modem generates and t