Abstract:
In some implementations, an apparatus includes an echo canceller that generates an echo interference compensation signal that compensates for an echo interference signal in a communication signal, a crosstalk canceller that generates a crosstalk interference compensation signal that compensates for a crosstalk interference signal in the communication signal, and a combiner that generates a combined interference compensation signal based on the echo interference compensation signal and the crosstalk interference compensation signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of and claims the benefit of priority to U.S. application Ser. No. 12/345,440, filed Dec. 29, 2008, issued Sep. 18, 2012, as U.S. Pat. No. 8,270,394, which is a continuation of U.S. application Ser. No. 10/762,153, filed on Jan. 20, 2004, issued Dec. 30, 2008, as U.S. Pat. No. 7,471,670, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The following disclosure relates to electrical circuits. 
     A communication system (e.g., a local area network) allows communication between two or more network devices.  FIG. 1  illustrates an example communication system  100  that includes a network device  102  and a network device  104 . Network devices  102 ,  104  include computers, switches, routers, hubs, gateways, and similar devices (e.g., devices having a network interface card in a network). Though two network devices are illustrated in  FIG. 1  by way of example, communication system  100  can contain a different number of network devices. 
     Referring to  FIG. 2 , communication between network device  102  and network device  104  can be conventionally achieved using a communication line  106 , formed by unshielded twisted pairs (UTP) of wires (or cables), and transceivers  108 - 122 , one transceiver positioned at each end of a UTP. For example, four UTPs  124 - 130  are provided in communication line  106  between network device  102  and network device  104 . Hybrid circuits  132 - 146  (e.g., transformers) can be used at the ends of each UTP  124 - 130  to control access to a corresponding communication channel for full-duplex bidirectional operation. The combination of a hybrid circuit and a transceiver forms one communication channel. Accordingly,  FIG. 2  illustrates four channels of communication, each operating in a similar manner. Each UTP  124 - 130  is connected to a corresponding transceiver through connectors  148 - 162 . 
     A common problem associated with a communication system using multiple UTPs and multiple transceivers is noise in the form of interference signals. The interference signals include echo and near-end crosstalk (NEXT). As a result of these interference signals, the performance of transceivers, in particular the receivers, in a communication system is degraded. 
     An echo interference signal can be produced by each transmitter contained within the same transceiver as a given receiver. Echo interference signals  302 - 316  encountered by respective receivers R 1 -R 8  (of transceivers  108 - 122 ) are shown in  FIG. 3 . Echo interference signals  302 - 316  appear as noise to receivers R 1 -R 8 , which are attempting to detect a direct communication signal (e.g., a data symbol) from a transmitter T 1 -T 8  connected at the opposite end of the communication channel. Accordingly, communication signals received by receivers R 1 -R 8  of transceivers  108 - 122  may experience signal distortion due to echo interference signals  302 - 316 . 
     NEXT is an interference signal that results from capacitive coupling of signals from a near-end transmitter to the input of a receiver. For example, NEXT interference signals  402 - 406  encountered by receiver R 1  of transceiver  108  are shown in  FIG. 4 . NEXT interference signals  402 - 406  appear as noise at the input of receiver R 1 , which is attempting to detect a direct communication signal from transmitter T 5  of transceiver  116 . Each of receivers R 1 -R 8  of transceivers  108 - 122  may encounter the same effect, and accordingly the communication signals received by receivers R 1 -R 8  may also experience signal distortion due to NEXT interference signals.  FIG. 5  shows an example time domain representation of echo and NEXT interference signals encountered by receiver R 1  of transceiver  108 . Echo and NEXT interference signals caused by a reflection due to impedance mismatch at hybrid circuit  132  and connector  148  are identified as the high voltage responses close to zero time. 
     SUMMARY 
     In general, in one aspect, this specification describes a transceiver. The transceiver includes a receiver to receive an analog communication signal. The analog communication signal contains an interference signal. The transceiver includes a digital compensation circuit to generate a digital replica of the interference signal contained in the analog communication signal, a converter to convert the digital replica of the interference signal into a corresponding analog replica of the interference signal, and a subtraction circuit to subtract the analog replica of the interference signal from the analog communication signal. 
     Particular implementations may include one or more of the following features. The digital compensation circuit can include an echo canceller to generate a digital replica of an echo interference signal in the analog communication signal. The digital compensation circuit can further include a near-end crosstalk (NEXT) canceller to generate a digital replica of a NEXT interference signal in the analog communication signal. The transceiver can further include an analog-to-digital converter (ADC) to sample the analog communication signal having the analog replica subtracted therefrom, and generate a digital signal that is substantially devoid of the interference signal. The transceiver can further include a first-in-first-out (FIFO) buffer to receive the digital signal and store the digital signal on a first-in-first-out basis. The transceiver can further include a feed forward equalizer (FFE) to receive the digital signals from the FIFO buffer, the FFE operable to filter individual digital signals. The FFE can be a least means square (LMS) type adaptive filter. The transceiver can further include a data detector to detect data from the filtered individual digital signals. The data detector can be a Viterbi detector. The data can be a data symbol. 
     In general, in another aspect, this specification describes a method for reducing interference signals in an analog communication signal. The method includes receiving an analog communication signal through a receiver. The analog communication signal contains an interference signal. The method further includes generating a digital replica of the interference signal contained in the analog communication signal, converting the digital replica of the interference signal into a corresponding analog replica of the interference signal, and subtracting the analog replica of the interference signal from the analog communication signal to substantially cancel the interference signal from the analog communication signal. 
     Particular implementations may include one or more of the following features. The interference signal can be an echo interference signal or a near end crosstalk (NEXT) interference signal. Generating a digital replica of the interference signal can include determining cancellation coefficients that model an impulse response of the interference signal, and multiplying the cancellation coefficients with a communication signal from a transmitter that causes the interference signal. Determining cancellation coefficients can include determining cancellation coefficients using an adaptive filter. The method can further include sampling the analog communication signal having the analog replica subtracted therefrom with an analog-to-digital converter (ADC) to create a digital communication signal. Generating a digital replica of the interference signal can include generating a digital replica of a portion of the interference signal. The portion of the interference signal can include high voltage portions of the interference signal. 
     In general, in another aspect, this specification describes a network device in a communication system. The network device includes a transceiver operable to receive an analog communication signal containing an interference signal. The transceiver includes a receiver to receive the analog communication signal, a digital compensation circuit to generate a digital replica of the interference signal contained in the analog communication signal, a converter to convert the digital replica of the interference signal into a corresponding analog replica of the interference signal, and a subtraction circuit to subtract the analog replica of the interference signal from the analog communication signal. 
     In general, in another aspect, this specification describes a cancellation system for use in a communication system including a communication line. The communication line has a transmitter and a receiver at each end. The cancellation system reduces interference signals in an analog communication signal received by a receiver. The cancellation system includes an echo canceller associated with a receiver. The echo canceller receives a transmitted signal from a transmitter in a same transceiver as the receiver with which the echo canceller is associated. The echo canceller is operable to generate a digital replica echo interference signal. The cancellation systems further includes a converter to convert the digital replica of the echo interference signal into a corresponding analog replica of the echo interference signal, and a subtractor to subtract the replica echo interference signal from an analog communication signal received by the receiver. 
     In general, in another aspect, this specification describes a cancellation system for use in a communication system including a communication line. The communication line has a transmitter and a receiver at each end. The cancellation system reduces interference signals in an analog communication signal received by a receiver. The cancellation system includes a NEXT canceller associated with a receiver. The NEXT canceller receives a transmitted signal from a local transmitter. The NEXT canceller is operable to generate a digital replica NEXT interference signal. The cancellation system further includes a converter to convert the digital replica of the NEXT interference signal into a corresponding analog replica of the NEXT interference signal, and a subtractor to subtract the replica NEXT interference signal from an analog communication signal received by the receiver. 
     At high frequencies, echo and NEXT interference signals become a significant portion of a received communication signal. The systems and techniques described in this specification remove high voltage portions of echo and NEXT interference signals in a received communication signal prior to the received communication signal being sampled by an analog-to-digital converter. Bit resolution of the analog-to-digital converter is therefore preserved. Specifically, for high throughput systems, the effective number of bits (ENOB) is a critical design parameter. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic block diagram of a conventional communication system. 
         FIG. 2  is a schematic block diagram of a plurality communication channels, each with a transceiver at each end. 
         FIG. 3  is a schematic block diagram of a portion of the communication system of  FIG. 2  depicting echo interference signals. 
         FIG. 4  is a schematic block diagram of a portion of the communication system of  FIG. 2  depicting NEXT interference signals. 
         FIG. 5  is a graph showing echo and NEXT interference signals of a communication channel. 
         FIG. 6  is a schematic block diagram of a communication system. 
         FIG. 7  is a schematic block diagram of a plurality communication channels, each with a transceiver at each end. 
         FIG. 8  is a schematic block diagram of a transceiver structure. 
         FIG. 9  is a schematic block diagram of an echo canceller of  FIG. 8 . 
         FIG. 10  is a schematic block diagram of a NEXT canceller of  FIG. 8 . 
         FIG. 11  is a schematic block diagram of a transceiver structure. 
         FIG. 12  is a schematic block diagram of an echo canceller of  FIG. 11 . 
         FIG. 13  illustrates a process for reducing echo and NEXT interference signals in a communication signal. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     A communication system  600  incorporating features of the systems and methods for reducing echo and NEXT interference signals is generally shown in  FIG. 6 . Communication system  600  includes a network device  602  and a network device  604 . As discussed above Network devices  602 ,  604  include computers, switches, routers, hubs, gateways, and similar devices. Two network devices are shown by way of example—communication system  600  can contain a different number of network devices. Network device  602  communicates with network device  604  through a communication line  606 . 
     Referring to  FIG. 7 , in one implementation, communication line  606  includes four UTPs  724 - 730  that are connected to transceivers  708 - 722  through corresponding connectors  748 - 762 . In one implementation, transceivers  748 - 762  are IEEE 1000Base-TX complaint. Hybrid circuits  732 - 746  are used at the ends of each UTP  724 - 730  to control access to a corresponding communication channel for full-duplex bidirectional operation. 
       FIG. 8  shows one implementation of a transceiver structure  800  of transceiver  708 . Transceivers  710 - 722  can include similar transceiver structures and operate in a similar manner. The transmitter portion  801  of transceiver  708  includes a conventional pulse shaping filter  802  and a digital-to-analog converter (DAC)  804 . Pulse shaping filter  802  receives one or more data symbols (Tx 1 Data) to be transmitted over the first communication channel between transceiver  708  and transceiver  716 . Data symbols Tx 1 Data transmitted by transmitter T 11  pass through pulse shaping filter  802  and are converted into analog signals by DAC  804 . The analog signals gain access to UTP  724  through hybrid circuit  732 . 
     The receiver portion  803  of transceiver  708  includes a digital compensation circuit  805 , an analog-to-digital converter (ADC)  816 , a FIFO  820 , a feed forward equalizer (FFE)  822 , a data detector  826 , and a feedback filter  828 . Digital compensation circuit  805  generates a digital compensation signal to substantially cancel echo and/or NEXT interference signals from a received communication signal (received from transmitter T 15 ) appearing at receiver R 11 . In one implementation, digital compensation circuit  805  includes an echo canceller  806  and a NEXT canceller  808 . 
     Echo canceller  806  generates a digital replica of the echo interference signal encountered by receiver R 11  of transceiver  708 . In like manner, NEXT canceller  808  generates a digital replica of the NEXT interference signals encountered by receiver R 11 . In one implementation, the digital replica of the echo interference signal is combined with the digital replica of the echo interference signal through combiner  810 . The combined digital replica of the echo and NEXT interference signals can be converted into a corresponding analog replica of the echo and NEXT interference signals through digital-to-analog converter (DAC)  812 . Signal distortion caused by echo and NEXT interference is cancelled from the received communication signal by subtractor  814  (i.e., subtracting the analog replica of the echo and NEXT interference signals from the received communication signal). 
     In some implementations, echo canceller  806  and NEXT canceller  808  receive as inputs a stream of data symbols generated by local transmitters (e.g., transmitters R 11 -R 14 ). The analog replica echo and NEXT interference signals are subtracted from the received communication signal prior to the received communication signal being sampled by ADC  816 . Echo and NEXT interference signals are, therefore, removed from the received communication signal in the analog domain. 
     ADC  816  samples the received communication signal, that is substantially devoid of signal distortion caused by echo and NEXT interference signals, in accordance with a sample clock signal  818  and generates digital signals at a suitable frequency, for example, at 833 MHz with an 8 bit resolution. Sample clock signal  818  can be provided by a timing recovery circuit (not shown). FIFO  820  receives the digital signals and stores them on a first-in-first-out basis. FIFO  820  forwards individual digital signals to FFE  822  which filters the individual digital signals. In one implementation, FFE  822  is a least means squares (LMS) type adaptive filter which performs equalization and precursor inter-symbol interference (ISI) cancellation. Data detector  826  receives the individual filtered signals and, in combination with combiner  824  and feedback filter  828 , generates an output signal corresponding to a detected data symbol. Data detector  826  can be a symbol-by-symbol detector or a sequential detector which operates on sequences of signals across all four channels, such as a Viterbi detector. 
       FIG. 9  shows an implementation of echo canceller  806 . Echo canceller  806  includes a shift register  900 , an adaptive cancellation filter  902 , and a combiner  904 . Shift register  900  receives data symbols Tx 1 Data. Shift register  900  can have a size (N e ) equal to a length of echo canceller  806 . Adaptive cancellation filter  902  produces echo cancellation coefficients that model impulse responses of the echo interference signal encountered by receiver R 11 . A digital replica of the echo interference signal encountered by receiver R 11  is generated by multiplying the echo cancellation coefficients with data symbols Tx 1 Data and summing the results through combiner  904 . Adaptive cancellation filter  902  can be implemented as an adaptive transversal filter (ATF) using, for example, the LMS algorithm. The digital replica of the echo interference signal can be sent to combiner  810  and to DAC  812 . DAC  812  can be clocked with clock signal  818  to ensure that echo interference signals are properly cancelled out at subtractor  814 . Timing delays that may be associated with the generation of the digital replica of the echo interference signal can be compensated for by appropriate time domain manipulations of the digital replica interference signal. 
     As discussed above ( FIG. 7 ), communication signals transmitted by the transmitters T 11 -T 18  of transceivers  708 - 722  may cause NEXT interference signals in communication signals received by the receivers R 11 -R 18  of transceivers  708 - 722 . Referring again to  FIG. 7 , since each receiver R 11 -R 14  has access to data symbols on the other (e.g., three) channels that may cause the NEXT interference signals, NEXT interference signals can be substantially cancelled. 
       FIG. 10  shows an implementation of NEXT canceller  808  that substantially cancels NEXT interference signals caused by transmitters T 12 -T 14  ( FIG. 8 ). Next canceller  808  includes shift registers  1000 - 1004 , NEXT adaptive filters  1006 - 1010 , and combiners  1012 - 1018 . Shift registers  1000 - 1004  receive data symbols TxData 2 , TxData 3 , and TxData 4  from transmitters T 2 -T 4 , respectively. Each NEXT adaptive filter  1006 - 1010  generates NEXT cancellation coefficients that model impulse responses of the NEXT interference signal caused by given transmitters T 12 -T 14 , respectively. In one implementation, NEXT adaptive filters  1006 - 1010  are implemented as ATFs, each using the LMS algorithm. Individual digital replicas of NEXT interference signals caused by transmitters T 12 -T 14  are generated by multiplying the NEXT cancellation coefficients with a respective one of data symbols TxData 2 , TxData 3 , and TxData 4 , and summing the results through combiners  1012 - 1016 . Combiner  1018  sums the individual digital replicas of NEXT interference signals to produce a digital replica of the total NEXT interference signals encountered by receiver R 11 . In one implementation, the digital replica of the total NEXT interference signals is combined with the digital replica of the echo interference signal by combiner  810  and sent to subtractor  814  through DAC  812 . Alternatively, any number of the individual digital replicas of the NEXT interference signals can be sent directly to DAC  812 . 
       FIG. 11  shows an alternative implementation of a transceiver structure  1100  of transceiver  708 . In the implementation shown in  FIG. 11 , echo canceller  1102  and/or NEXT canceller  1104  removes high voltage responses (e.g., caused by a reflection due to impedance mismatch at hybrid circuit  132  and connector  148 ) within a received communication signal before the received communication signal is sampled by ADC  816 . The remainder of the echo and NEXT interference signals (contained in the received communication signal) that is not removed by echo canceller  1102  and NEXT canceller  1104 , is removed by a conventional echo canceller  1106  and a conventional NEXT canceller  1108  in the digital domain (i.e., after the received communication signal has been sampled by ADC  816 ). 
       FIG. 12  shows an implementation of echo canceller  1102 . NEXT canceller  1104  has the same principle operation of echo canceller  1102 . Echo canceller  1102  includes a shift register  1200 , a programmable delay  1202 , an adaptive cancellation filter  1204 , and a combiner  1206 . Data symbols Tx 1 Data are passed into shift register  1200  after a pre-determined amount of time. The pre-determined amount of time is adjustable through programmable delay  1202  to ensure that a generated replica echo interference signal arrives substantially coincident with a received echo interference signal at subtractor  1110 . Adaptive cancellation filter  1204  produces echo cancellation coefficients that model high voltage impulse responses of the echo interference signal. Adaptive cancellation filter  1204  can be implemented as an adaptive transversal filter (ATF) using, for example, the LMS algorithm. A digital replica of the high voltage echo interference signals encountered by receiver R 11  is generated by multiplying the echo cancellation coefficients with data symbols Tx 1 Data and summing the results through combiner  1206 . The digital replica of the high voltage echo interference signals can be sent to subtractor  1110  through DAC  812 . 
       FIG. 13  shows a method  1300  for reducing echo and NEXT interference signals in a received communication signal. A communication signal is received by a receiver (step  1302 ). A digital replica of an interference signal (or a portion thereof) is generated (step  1304 ). In one implementation, a digital replica of all interference signals encountered by a receiver are generated, including echo and all NEXT interference signals encountered by a receiver. In another implementation, a digital replica of a number of the interference signals less than all of the interference signals can be generated in step  1104 . The digital replica of the interference signal(s) are converted into a corresponding analog replica interference signal(s) (step  1306 ). The analog replica interference signal(s) are subtracted from the received communication signal (step  1308 ). In one implementation, the analog replica interference signal(s) are subtracted from the received communication signal to substantially remove the interference signal(s) from the received communication signal. After the replica interference signal(s) have been removed from the received communication signal, the received communication signal is then sampled by an ADC for digital processing (step  1310 ). 
     Various implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, instead of in an adaptive manner, the echo cancellers and NEXT cancellers described above can generate corresponding replica interference signals deterministically. In addition, the number of transmitters and receivers per transceiver can be different. Accordingly, other implementations are within the scope of the following claims.