US4646305A
Expired / Lapsed
US
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High speed data modem using multilevel encoding
Assignee: DOWTY COMMUNICATIONS Inc A CORP OF
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Filing: 1983-09-26
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Granted: 1987-02-24
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Expires: 2003-09-26
Abstract
Abstract A 14.4 kilobit/second modem uses an encoding scheme in which groups of five bits are encoded as one of thirty-two (2 5 ) possible code groups. This is done by using quadrature amplitude modulation and a 6 by 6 space-state constellation which allows a maximum of thirty-six different points to be encoded. Since only thirty-two points are needed the four outer corner points of the constellation are not used. In order to achieve the desired 14.4 KBPS data rate the baud clock must run at 2880 Hz. However, this bandwidth is very close to the maximum bandwidth available on voice-grade telephone lines. Accordingly, data encoding and data recovery techniques must be used which maximize the probabilities of correctly receiving the encoded data signals. These techniques include (1) data scrambling/descrambling; (2) assigning groups of five bits to constellation points, including performing rotational and Gray encoding; (3) a baud clock recovery scheme at the receiver which is performed prior to partial response encoding; and (4) a start-up technique using a three-level partial response ideal reference sequence during initial training at the receiver. In addition, the invention uses passband equalization, class I partial response encoding accomplished by the passband equalizer, and independent inphase and quadrature Viterbi decoders at the receiver.
Inventors
- 👤 Steven A. Tretter
- 👤 Robert P. Redick
- 👤 Richard L. Stuart
- 👤 Paul E. Treynor
- 👤 David Gordon-Smith
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-25 14:56
CPC Classifications
H; H04; H04L; H04L27/00; H04L27/32; H04L27/34; H04L27/38; H04L27/3818; H04L27/3827; H04L25/00; H04L25/38; H04L25/40; H04L25/49; H04L25/497; H04L27/3494
Claims Analysis
Total Claims: 21
Claim 1
Independent Claim
265 words
A system for communicating data in a data stream comprised of binary bits, at data rates up to at least 14,400 bits per second over a band limited communication channel, said system comprising a transmitter for encoding and transmitting bits over said chanel and a receiver for receiving and decoding bits received from said transmitter over said channel, said transmitter comprising means for encoding data bits, said means for encoding comprising: means for grouping sequences of five bits from the bit stream as bauds; means for locating the identical sequence of five bits corresponding to each baud in a constellation of the 32 possible sequences of five bits, said constellation being divided into four quadrants, the first and second bits of each of the eight groupings of bits in each quadrant being identical, and the four groups of the quadrants, each comprising the eight possible sequences of the latter three bits of each said baud, being symmetrical; means for encoding the first two bits of each baud such that identification of the baud with one of said four quadrants is possible at said receiver; means for generating a quadrature amplitude modulated (QAM) signal, said signal varying in dependence on the location of the encoded first two bits and the latter three bits within said constellation; and means for transmitting the generated QAM signal over the band limited communications channel; and said receiver comprising means for receiving the QAM signal from the communications channel, and means for reconstituting the individual bauds therefrom to generate the original bit stream, including means for decoding the first two bits.
Claim 2
Independent Claim
265 words
A system for communicating data in a data stream comprised of binary bits, at data rates up to at least 14,400 bits per second over a band limited communication channel, said system comprising a transmitter for encoding and transmitting bits over said chanel and a receiver for receiving and decoding bits received from said transmitter over said channel, said transmitter comprising means for encoding data bits, said means for encoding comprising: means for grouping sequences of five bits from the bit stream as bauds; means for locating the identical sequence of five bits corresponding to each baud in a constellation of the 32 possible sequences of five bits, said constellation being divided into four quadrants, the first and second bits of each of the eight groupings of bits in each quadrant being identical, and the four groups of the quadrants, each comprising the eight possible sequences of the latter three bits of each said baud, being symmetrical; means for encoding the first two bits of each baud such that identification of the baud with one of said four quadrants is possible at said receiver; means for generating a quadrature amplitude modulated (QAM) signal, said signal varying in dependence on the location of the encoded first two bits and the latter three bits within said constellation; and means for transmitting the generated QAM signal over the band limited communications channel; and said receiver comprising means for receiving the QAM signal from the communications channel, and means for reconstituting the individual bauds therefrom to generate the original bit stream, including means for decoding the first two bits.
Claim 3
Dependent Claim
33 words
The system of claim 1 wherein said first two bits of each baud in said bit stream are encoded in accordance with the value of the first two bits of the preceding baud.
Claim 4
Dependent Claim
48 words
The system of claim 2 wherein said first two bits are encoded according to the following rules: EA(I)=EA(I-1).XOR.AS(I).XOR.(EB(I-1).AND.BS(I)); and EB(I)=EB(I-1).XOR.BS(I), wherein the quantity (EB)I-1).AND.BS(I)) is a carry bit, AS(I) and BS(I) are the current first two bits and EA(I-1) and EB(I-1) are the previously encoded first two bits.
Claim 5
Dependent Claim
40 words
The system of claim 1 where said constellation is substantially as follows: ##EQU5## wherein the bits marked A and B correspond to said first two bits and the bits marked C, D, and E correspond to said latter three bits.
Claim 6
Independent Claim
206 words
A method for communicating data in a data stream comprised of binary bits, at data rates up to 14,400 bits per second over a band limited communication channel, comprising a transmitting step and a receiving step, said transmitting step comprising the steps of: grouping sequences of five bits of the bit stream as bauds; locating the identical sequence of five bits of each baud in a constellation of the 32 possible groupings of five bits, said constellation being divided into four quadrants, the first and second bits of each of the eight groupings of bits in each quadrant being identical, and the four groups of the eight possible arrangement of the latter three bits of each said baud being symmetrical in each quadrant; encoding the first two bits of each baud such that identification of the baud with one of said four quadrants is possible at said receiver; generating a quadrature amplitude modulated (QAM) signal responsive to the location of each of said bauds within said constellation; and transmitting the QAM signal over the band limited communications channel; and said receiving step comprising the steps of receiving the QAM signal from the communications channel and recovering the individual bauds therefrom to generate the original bit stream.
Claim 7
Independent Claim
206 words
A method for communicating data in a data stream comprised of binary bits, at data rates up to 14,400 bits per second over a band limited communication channel, comprising a transmitting step and a receiving step, said transmitting step comprising the steps of: grouping sequences of five bits of the bit stream as bauds; locating the identical sequence of five bits of each baud in a constellation of the 32 possible groupings of five bits, said constellation being divided into four quadrants, the first and second bits of each of the eight groupings of bits in each quadrant being identical, and the four groups of the eight possible arrangement of the latter three bits of each said baud being symmetrical in each quadrant; encoding the first two bits of each baud such that identification of the baud with one of said four quadrants is possible at said receiver; generating a quadrature amplitude modulated (QAM) signal responsive to the location of each of said bauds within said constellation; and transmitting the QAM signal over the band limited communications channel; and said receiving step comprising the steps of receiving the QAM signal from the communications channel and recovering the individual bauds therefrom to generate the original bit stream.
Claim 8
Dependent Claim
34 words
The method of claim 5 wherein said encoding of the first two bits of each baud is performed in a manner responsive to the values of the first two bits of the preceding baud.
Claim 9
Dependent Claim
53 words
The method of claim 6 wherein said first two bits are encoded according to the following rules: EA(I)=EA(I-1).XOR.AS(I).XOR.(EB(I-1).AND.BX(I)); and EB(I)=EB(I-1).XOR.BS(I), wherein the quantity (EB)I-1).AND.BS(I)) is a carry bit, AS(I) and BS(I) are the first two bits of the current baud, and EA(I-1) and EB(I-1) are the first two bits of the preceeding baud.
Claim 10
Dependent Claim
40 words
The method of claim 5 wherein said constellation is substantially as follows: ##EQU6## wherein the bits marked A and B correspond to said first two bits and the bits marked C, D, and E correspond to said latter three bits.
Claim 11
Dependent Claim
105 words
A modem clock recovery circuit for recovering a modem clock signal from a signal modulated according to a quadrature amplitude modulation (QAM) method, said modulated signal being transmitted at a nominal data rate, comprising: means for selecting spectral components about the upper and lower Nyquist frequencies of said modulated signal; means for summing said spectral components; means for squaring the output of said means for summing, said means for squaring having an output signal including frequency components at the sum, difference and second harmonic frquencies; and means for selecting the difference component of the output of said means for squaring as said modem clock signal.
Claim 12
Dependent Claim
105 words
A modem clock recovery circuit for recovering a modem clock signal from a signal modulated according to a quadrature amplitude modulation (QAM) method, said modulated signal being transmitted at a nominal data rate, comprising: means for selecting spectral components about the upper and lower Nyquist frequencies of said modulated signal; means for summing said spectral components; means for squaring the output of said means for summing, said means for squaring having an output signal including frequency components at the sum, difference and second harmonic frquencies; and means for selecting the difference component of the output of said means for squaring as said modem clock signal.
Claim 13
Dependent Claim
21 words
The modem circuit of claim 9 comprising means for slicing the output of said selecting step to a desired logic level.
Claim 14
Dependent Claim
28 words
The modem circuit of claim 9 further comprising means for generating a phase-locked loop signal responsive to the difference component of the output of said means for squaring.
Claim 15
Dependent Claim
40 words
The modem circuit of claim 9 wherein said spectral components are selected by plural second order bandpass filter means, each having a bandwidth of approximately 40 Hz at the half power points, and wherein said nominal frequency is 2,880 Hz.
Claim 16
Dependent Claim
24 words
The modem circuit of claim 9 further comprising transversal equalizer means for converting the received QAM signal to produce a partial response QAM signal.
Claim 17
Independent Claim
90 words
A method for recovering a modem clock signal from a signal modulated according to a quadrature amplitude modulation (QAM) method, said modulated signal being transmitted at a nominal data rate, comprising the steps of: selecting spectral components about the upper and lower Nyquist frequencies of said modulated signal; summing said spectral components; squaring the output of said summing step, in order to provide frequency components at the sum, difference and second harmonic frequencies; and selecting the difference component of the output of said squaring step as said modem clock signal.
Claim 18
Independent Claim
90 words
A method for recovering a modem clock signal from a signal modulated according to a quadrature amplitude modulation (QAM) method, said modulated signal being transmitted at a nominal data rate, comprising the steps of: selecting spectral components about the upper and lower Nyquist frequencies of said modulated signal; summing said spectral components; squaring the output of said summing step, in order to provide frequency components at the sum, difference and second harmonic frequencies; and selecting the difference component of the output of said squaring step as said modem clock signal.
Claim 19
Dependent Claim
25 words
The method for recovering a modem clock signal of claim 14 comprising the additional step of slicing said difference component to a desired logic level.
Claim 20
Dependent Claim
32 words
The method for recovering a modem clock signal of claim 14 comprising the step of generating a phase-locked loop signal responsive to said difference component of the output of said squaring circuit.
Claim 21
Dependent Claim
49 words
The method for recovering a modem clock signal of claim 14 wherein the step of selecting said spectral components is performed by plural second order bandpass filter means each having a bandwidth of approximately 40 Hz at the half power points, and wherein said nominal frequency is 2,880 Hz.
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USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1990-02-23 | 1990-08-25 | 1991-02-24 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1994-02-23 | 1994-08-25 | 1995-02-24 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1998-02-23 | 1998-08-25 | 1999-02-24 | $7700.00 | Window Closed / Lapsed |