Echo cancellation in two-wire, two-way data transmission systems

Abstract

An adaptive echo canceller for two-wire, simultaneous two-way data communication at full bandwidth uses Nyquist-interval, rather than baud-interval, processing to achieve independence from timing discrepancies between near-end and far-end terminals. The entire echo signal, and not merely baud-interval samples thereof, is suppressed. The echo canceller is preferably a transversal structure.

What is claimed is: 
     
       1. An echo cancellation arrangement for a baud-synchronous digital data transmission system .Iadd.comprised of terminals each .Iaddend.having both a transmitter section and a receiver section for simultaneous two-way signaling at full bandwidth over a common signal path.Iadd., said echo cancellation arrangement .Iaddend.comprising at each .[.such.]. terminal, means for sampling incoming received signals at a rate .Iadd.greater than or .Iaddend.substantially equal to twice the highest frequency employed in said signal path,   an adjustable signal processor for compensating for echoes of signals being transmitted by said transmitter section into said receiver section having an input connected to a data source in said transmitter section and an output combined in subtractive relationship with the output signal from said sampling means to form a subtractive output having an error component, said signal processor storing consecutive discrete-level samples from said data source at baud intervals and shifting such samples through a sequence of storage locations at intervals no greater than the reciprocal of twice the highest frequency employed in said signal path and such that an integral number of such shifting intervals occur in each baud interval, .Iadd.and .Iaddend.   means within said signal processor for computing the product of said consecutive samples with the error component of said subtractive output, and   means for .[.recovering digital data from.]. .Iadd.applying .Iaddend.the subtractive output of said signal processor.[.,.]..Iadd.to said receiver section.Iaddend..   
     
     
       2. The arrangement defined in claim 1 in which said adjustable signal processor comprises a synchronously tapped delay medium,   an adjustable gain device for each tap on said delay medium,   means for entering a discrete-level digital data sample into said delay medium at baud intervals and zero-level samples at intervening times,   tap-weight adjustment means for each tap on said delay medium under the control of the error component in said subtractive output, and   means for combining tap signals operated on by said adjustable gain devices.   
     
     
       3. The arrangement defined in claim 1 in which said transmission system operates at baseband frequencies between terminals. 
     
     
       4. The arrangement defined in claim 1 in which said transmission system operates at passband frequencies between terminals and the output of said signal processor is upmodulated to said passband frequency region before being subtractively combined with the output of said sampling means and in which the subtractive output of said signal processor is demodulated from said passband frequency region to the baseband region before application to said signal processor. 
     
     
       5. The arrangement defined in claim 1 in which said adjustable signal processor is adapted to the compensation of both near-end and far-end echo components and comprises first and second synchronously tapped delay media,   an adjustable gain device for each tap on said first and second delay media,   a fixed delay medium comparable in delay to the propagation time differential between near-end and far-end echoes in circuit with said first and second tapped media,   means for entering discrete-level digital data samples into said first delay medium at baud intervals and zero-level signals at intervening times for further propagation through said fixed delay medium and said second tapped delay medium in tandem,   tap-weight adjustment means for each tap on said first and second media controlled by the error component in said subtractive output, and   means for combining tap signals operated on by said tap-weight adjustment means from both of said first and second tapped media.   
     
     
       6. In a two-way data transmission system having a four-wire to two-wire bridge between a common transmission link and each system terminal including separate transmitter and receiver sections, a compensation circuit for transmitter signal components leaking across said bridge between transmitter and receiver sections at each terminal for forming a sampled echo cancellation signal, said compensation circuit storing a plurality of samples of digital data to be transmitted spaced by baud intervals and of zero-order samples at uniform intervening intervals no longer than the reciprocal of twice the highest frequency applied to said transmission link,   sampling means for operating on incoming received signals at a rate .Iadd.greater than or .Iaddend.substantially equal to twice the highest frequency employed on said transmission link to form a high-speed sampled sequence, .Iadd.and .Iaddend.   means for subtracting the sampled echo cancellation signal derived in said compensation circuit from the high-speed sampled sequence derived in said sampling means for forming an output signal substantially free of transmitter signal components for adaptive control of said compensation circuit, and   .[.recovery.]. means for .[.obtaining message data from.]. .Iadd.applying .Iaddend.the output of said .[.substracting.]. .Iadd.subtracting .Iaddend.means .Iadd.to the receiver section of said each terminal.Iaddend..   
     
     
       7. The two-way transmission system defined in claim .[.5.]. .Iadd.6 .Iaddend.further comprising at each terminal thereof, a carrier wave source,   a transmitter under the control of said carrier wave source for translating data signals to be transmitted to the passband of said transmission link,   an upmodulator under the control of said carrier wave source for elevating the echo-cancellation signal from said compensation circuit to the passband of said transmission system, and   a demodulator under the control of said carrier wave source in circuit between said subtracting means and said compensation circuit for translating the output signal from said combining means to the baseband frequency region.   
     
     
       8. In combination with a digital data transmission system including terminals with transmitter and receiver sections for simultaneous two-way transmission at full bandwidth connected through a hybrid network to a common transmission channel comprising at each terminal a data signal source in the transmitter section,   an adjustable echo canceller having an input connected to said data signal source for an outgoing signal from the transmitter section, delay line taps spaced no further apart than the reciprocal of twice the highest frequency employed on said transmission channel and a summation circuit for selectively weighted signals on said taps for forming an echo cancellation signal,   means for sampling incoming received signals at substantially .Iadd.or greater than .Iaddend.twice the highest .[.usable.]. .Iadd.significant .Iaddend.frequency on said common transmission channel to form a received digital sequence,   means for subtractively combining said echo cancellation signal with said received digital sequence to form a compensated received signal, .Iadd.and .Iaddend.   means for applying said compensated received signal to said echo canceller for multiplication with outgoing digital data samples at the taps thereon for controlling the selective weighting of digital data samples at said taps, and   means for .[.recovering message data from.]. .Iadd.applying .Iaddend.said compensated received signal .Iadd.to the receiver section of said each terminal.Iaddend..   
     
     
       9. The combination set forth in claim 8 in which said transmission channel operates in a baseband frequency region. 
     
     
       10. The combination set forth in claim 8 in which said transmission channel operates in a passband frequency region and said echo cancellation signal is upmodulated to passband before application to said combining means and said compensated received signal is demodulated to baseband before application to said echo canceller. .Iadd. 11. The arrangement defined in claims, 1, 2, 3, 4 or 5 wherein said receiver section comprises means for recovering digital data from the subtractive output of said signal processor. .Iaddend..Iadd. 12. The two-way transmission system defined in claims 6 or 7 wherein said receiver section of said each terminal comprises recovery means for obtaining message data from said output of said subtracting means. .Iaddend..Iadd. 13. The combination set forth in claims 8, 9 or 10 wherein said receiver section of said each terminal comprises means for recovering message data from said compensated received signal. .Iaddend. .Iadd. 14. An arrangement for use in conjunction with data communications circuitry which accepts near-end baseband data having a predetermined baud rate, transmits signals representing said baseband data, and receives signals which include echoes of said transmitted signals, said arrangement comprising, means for forming a succession of samples of said received signals at a rate which is greater than or substantially equal to twice the highest significant frequency in said received signals and which is l times greater than said baud rate, l being a predetermined number,   means for storing l sets of coefficients,   means for forming a succession of echo cancellation samples and for combining each echo cancellation sample with a respective one of said received signal samples to form a succession of compensated samples, each one of l successive echo cancellation samples being equal to the sum of the products of (a) the coefficients of a different one of the l coefficient sets with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l samples, and   means for repetitively updating the values of said coefficients in response to at least ones of said compensated samples such that the energy in said compensated samples originating from said echoes is minimized. .Iaddend. .Iadd. 15. An arrangement for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said arrangement comprising   means for forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes,   means for forming a plurality of samples of an echo cancellation signal, the value of each one of l successive ones of said cancellation signal samples being a function of (a) a plurality of elements of said baseband data associated with said l samples and (b) a predetermined one of l sets of coefficients, l being a predetermined number, and   means for combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the values of said coefficients being such that said compensated samples are substantially free of said echo components. .Iaddend..Iadd. 16. A method for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said method comprising the steps of   forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes,   forming a plurality of samples of an echo cancellation signal, the value of each one of l successive ones of said cancellation signal samples being a function of (a) a plurality of elements of said baseband data associated with said l samples and (b) a predetermined one of l sets of coefficients, l being a predetermined number, and   combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the values of said coefficients being such that said compensated samples are substantially free of said echo components. .Iaddend. .Iadd. 17. An arrangement for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said arrangement comprising   means for forming a plurality of samples of the received signal at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes,   signal processing means for forming a plurality of samples of a cancellation signal, said signal processing means including means for storing l sets of coefficients and means for forming as each one of l successive cancellation signal samples the sum of the products of (a ) the coefficients of a different one of the coefficient sets with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l cancellation signal samples, l being a predetermined number, and   means for combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the magnitude of each cancellation signal sample being substantially equal to the magnitude of the echo component of the respective received signal sample. .Iaddend. .Iadd. 18. The invention of claim 17 wherein said signal processing means further includes means for updating the values of at least individual ones of said coefficients such that over time, the difference between the magnitude of each cancellation signal sample and the magnitude of the echo component of the respective received signal sample is minimized. .Iaddend..Iadd. 19. The invention of claim 18 wherein said updating means includes means for combining with the values of said individual ones of said coefficients respective updating terms, each updating term being a function of a respective one of said compensated samples. .Iaddend..Iadd. 20. The invention of claim 18 wherein said updating means includes means for combining with the values of said individual ones of said coefficients respective successions of updating terms, each updating term being a function of (a) a respective compensated sample and (b) the signal with which the coefficient being updated was multiplied in the formation of said respective compensated sample. .Iaddend..Iadd. 21. The invention of claim 18 wherein said received signals represent far-end data and wherein said arrangement further comprises means for processing said compensated samples to recover said far-end data therefrom. .Iaddend..Iadd. 22. The invention of claims 14 or 17 wherein said received signals represent far end data and wherein said arrangement further comprises means for recovering said far-end data from said compensated samples. .Iaddend. .Iadd. 23. A method for use in conjunction with data communications circuitry which accepts near-end baseband data having a predetermined baud rate, transmits signals representing said baseband data, and receives signals which include echoes of said transmitted signals, said method comprising the steps of   forming a succession of samples of said received signals at a rate which is greater than or substantially equal to twice the highest significant frequency in said received signals and which is l times greater than said baud rate, l being a predetermined number,   forming a succession of echo cancellation samples   combining each echo cancellation sample with a respective one of said received signal samples to form a succession of compensated samples, each one of l successive echo cancellation samples being equal to the sum of the products of (a) the coefficients of a different one of l sets of coefficients with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l samples, and   repetitively updating the values of said coefficients in response to at least ones of said compensated samples in such a way as to minimize the energy in said compensated samples originating from said echoes. .Iaddend. .Iadd. 24. A method for use in an arrangement which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said method comprising the steps of   forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes,   forming a plurality of samples of a cancellation signal including the step of forming as each one of l successive cancellation signal samples the sum of the products of (a) the coefficients of a different one of l sets of coefficients with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l cancellation signal samples, l being a predetermined number, and   combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the magnitude of each cancellation signal sample being substantially equal to the magnitude of the echo component of the respective received signal sample. .Iaddend. .Iadd. 25. The invention of claim 24 wherein said cancellation signal forming step includes the further step of updating the values of at least individual ones of said coefficients such that over time, the difference between the magnitude of each cancellation signal sample and the magnitude of the echo component of the respective received signal sample is minimized. .Iaddend..Iadd. 26. The invention of claim 25 wherein said updating step includes the step of combining with the values of said individual ones of said coefficients respective updating terms, each updating term being a function of a respective one of said compensated samples. .Iaddend..Iadd. 27. The invention of claim 25 wherein said updating step includes the step of combining with the values of said individual ones of said coefficients respective successions of updating terms, each updating term being a function of (a) a respective compensated sample and (b) the signal with which the coefficient being updated was multiplied in the formation of said respective compensated sample. .Iaddend..Iadd. 28. The invention of claim 25 wherein said received signals represent far-end data and wherein said method comprises the further step of processing said compensated samples to recover said far-end data therefrom. .Iaddend. .Iadd. 29. The invention of claims 23 or 24 wherein said received signals represent far end data and wherein said method comprises the further step of recovering said far-end data from said compensated samples. .Iaddend.