Abstract:
A compensator generating a compensation signal to compensate for nonlinear echo in an output of a current source. The nonlinear echo is a result of transitioning the current source between an ON state and an OFF state. The compensator includes driving, weighting, function, and compensating circuits. The driving circuit receives a first signal that is based on the output of the current source. The weighting circuit is configured to generate a second signal based on weighted versions of the first signal. The function circuit, based on the second signal, (i) updates each of multiple functions, and (ii) selects a first function. The driving circuit generates a driving signal based on the first function selected by the function circuit. The compensating circuit generates the compensation signal based on the driving signal to compensate for the nonlinear echo provided by the output of the current source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is a continuation Application of U.S. patent application Ser. No. 12/215,222 (now U.S. Pat. No. 8,743,939), filed on Jun. 26, 2008, which is a continuation Application of U.S. patent application Ser. No. 10/189,321 (now U.S. Pat. No. 7,409,057), filed Jul. 3, 2002. The entire disclosures of the applications referenced above are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to transmitting and receiving electrical signals through a communications channel, and more particularly to a nonlinear echo compensator for a Class B transmitter line driver. 
     BACKGROUND 
     IEEE section 802.3ab, which is hereby incorporated by reference, specifies physical layer parameters for 1000 BaseT (gigabit) communications channels. The gigabit communications channel employs four twisted pairs of cable. Signals transmitted over the cable are degraded by signal attenuation, return loss, echo, and crosstalk. 
     Referring now to  FIG. 1 , a gigabit Ethernet communications channel  10  is shown. The communications channel  10  includes two nodes  12  and  14  that transmit and receive one gigabit per second (Gbps). The node  12  includes transceivers  16 - 1 ,  16 - 2 ,  16 - 3 , and  16 - 4  and the node  14  includes transceivers  18 - 1 ,  18 - 2 ,  18 - 3  and  18 - 4 . Each transceiver transmits at 250 Mbps. The transceivers  16  and  18  are connected to opposite ends of twisted pairs  20 - 1 ,  20 - 2 ,  20 - 3 , and  20 - 4 . For example, the transceiver  16 - 1  is connected to one end of the twisted pair  20 - 1 . The transceiver  18 - 1  is connected to the opposite end of the twisted pair  20 - 1 . Each transceiver  16  and  18  includes a transmitter  24 , a receiver  26 , and a hybrid circuit  28 . 
     The transmitter  24  of the transceiver  16 - 1  generates a five level pulse amplitude modulated (PAM-5) signal that is transmitted by the transmitter  24  and the hybrid circuit  28  of the transceiver  16 - 1  onto the twisted pair  20 . The hybrid circuit  28  and the receiver  26  of the transceiver  18 - 1  receive the PAM-5 signal. The hybrid circuit  28  enables bi-directional transmission over the same twisted pairs by filtering out the transmit signal at the receiver  26 . 
     Attenuation refers to signal loss of the twisted pair between the transmitter of one receiver and the receiver of another transceiver and is caused by several factors including skin effect. To minimize the effect of attenuation, the lowest possible frequency range that supports the required data rate is typically used. Return loss quantifies the amount of power that is reflected due to cable impedance mismatches. 
     Echo occurs when signals are transmitted and received on the same twisted pair. Echo is caused by residual transmit signals and cable return loss. Crosstalk occurs due to signal coupling between twisted pairs that are in close proximity. For example, the twisted pairs used in 1000 BaseT are affected by crosstalk from adjacent twisted pairs. Near end crosstalk (NEXT) is crosstalk at the transmitter end of the twisted pair. Far-and crosstalk (FEXT) is crosstalk at the receiver end of the twisted pair. Crosstalk is preferably minimized to improve receiver symbol recovery. 
     Referring now to  FIG. 2 , the transceiver  16  includes a transmitter line driver  50  that receives a transmitter signal  52 . The transmitter line driver  50  outputs a multi-level signal to a load such as a matched resistor  54 . A transformer  58  couples the transceiver  16  to a twisted pair  60 . A replica signal generator  64  outputs a replica of the transmitter signal  52  to a summer  66 . A received signal  68  is also input to the summer  66 . 
     Since the communications channel transmits and receives on the same twisted pair  60 , the replica transmitted signal is cancelled or subtracted from the received signal  68 . In addition, compensation for NEXT and echo is performed. An output of the summer  66  is input to an optional low pass filter (LPF)  70 . An output of the LPF  70  is input to an analog to digital converter (ADC)  74 . An output of the ADC  74  is input to a summer  78 . A linear echo compensation circuit  82  and NEXT compensation circuit  83  (for NEXT 12 , NEXT 13 , and NEXT 14 ) are also input to the summer  78 . A signal (TA comp ) with NEXT and linear echo compensation is output by the summer  78 . Additional details concerning the transceiver  16  can be found in “Active Resistive Summer for a Transformer Hybrid”, U.S. patent application Ser. No. 09/920,240, filed Aug. 1, 2001, and “A Method and Apparatus for Digital Near-End Echo/Near-End Crosstalk Cancellation with Adaptive Correlation”, U.S. patent application Ser. No. 09/465,228, filed Dec. 17, 1999, which are hereby incorporated by reference. 
     Referring now to  FIG. 3 , the transmitter line driver  50  is shown further and typically includes a plurality of positive current cells  84  and negative current cells  86 . A transmitter driver control  88  selectively switches the positive and negative current cells  84  and  86  on and off to produce positive and negative signal levels. For example, the transmitter line driver for 1000 BaseT employs five symbol levels −2, −1, 0, +1, and +2, which are usually implemented as 0V, +/−0.5V and +/−1V. Future communications systems may include additional symbol levels for increased bandwidth. For example, future signal levels may include 0, +/−2, +/−4, +/−6, and +/−8 signal levels. 
     Referring now to  FIG. 4 , a conceptual illustration of the transmitter line driver  50  is shown. The positive current cells  84  can be thought of as a plurality of individual current sources  90 - 1 ,  90 - 2 ,  90 - 3 , . . . , and  90 - n  that are switched by switches SW P1 , SW P2 , SW P3 , . . . , and SW Pn . The negative current cells  86  can be thought of a plurality of individual current sources  92 - 1 ,  92 - 2 ,  92 - 3 , . . . , and  92 - m  that are switched by switches SW N1 , SW N2 , SW N3 , . . . , and SW Nm . Typically, m=n. Referring now to  FIG. 5 , an exemplary positive current cell  96  is shown. In  FIG. 6 , an exemplary negative current cell  98  is shown. As can be appreciated, other positive and negative current cells can be utilized. 
     When the transmitter line driver  50  is operated in a Class A operating mode, the number of positive current cells that are turned on/off for a transition from a first signal level to a second signal level is equal to the number of negative current cells that are turned off/on. When the transmitter line driver  50  is operated in a Class B operating mode, the number of positive current cells that are turned on/off for a transition from a first signal level to a second signal level is not equal to the number of negative current cells that are turned off/on. The advantage of Class B operation is reduced power consumption as compared with Class A operation. 
     Referring now to  FIG. 7 , Class A operation of the positive and negative current cells  84  and  86  for nine symbol levels is shown. As can be appreciated, when switching between signal level 6 and signal level −4, there are an equal number of positive and negative current cells being turned on and off. In particular, five positive current cells are being turned off and five negative current cells are being turned on. 
     Referring now to  FIG. 8 , exemplary Class B operation of the positive and negative current cells  84  and  86  is shown. As can be appreciated, when switching between signal level 6 and signal level −4, an unequal number of positive and negative current cells are turned on and off. In particular, six positive current cells are turned off and four negative current cells are turned on. While Class B operation provides reduced power consumption, the asymmetry of Class B operation causes nonlinear echo that degrades performance. 
     SUMMARY 
     A nonlinear echo compensator according to the present invention compensates for nonlinear echo in a transceiver including a transmitter line driver with current cells that are operated in an asymmetric low power mode. A mapping circuit generates a pattern dependent driving signal. A canceling circuit communicates with the mapping circuit and compensates for nonlinear echo in a received signal based on the pattern dependent driving signal. 
     In other features, the mapping circuit receives a multi-level signal and maps the multi-level signal to the pattern dependent driving signal. The mapping circuit includes a symbol weighting circuit that generates a weighted signal. The symbol weighting circuit generates the weighted signal by summing a first product of a current symbol and a first weighting factor with a second product of a prior symbol and a second weighting factor. The mapping circuit includes a function generator that generates the pattern dependent driving signal based on the weighted signal and a scaling circuit that scales the pattern dependent driving signal. 
     In still other features, a coefficient generator generates a first compensator coefficient based on a sum of a prior compensator coefficient and a product of an error signal and a sign function of the pattern dependent driving signal. The coefficient generator generates first, second and third compensator coefficients. 
     In other features, the canceling circuit includes a first multiplier that has a first input that receives the pattern driving signal and a second input that receives the first compensator coefficient. The first multiplier generates a first product. A second multiplier has a first input that receives the pattern driving signal and a second input that receives the second compensator coefficient. The second multiplier generates a second product. A third multiplier has a first input that receives the pattern driving signal and a second input that receives the third compensator coefficient. The third multiplier generates a third product. 
     In still other features, the canceling circuit further includes a first unit delay that receives the third product of the third multiplier. A first summer has a first input that receives the second product of the second multiplier and a second input that communicates with the first unit delay. A second unit delay communicates with an output of the first summer. A second summer has a first input that communicates with the second unit delay and a second input that receives the first product of the first multiplier. 
     Further areas of applicability will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to be limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram illustrating an exemplary gigabit communications channel according to the prior art; 
         FIG. 2  is a functional block diagram illustrating a transceiver with a transmitter line driver and linear echo, NEXT and replica transmitter signal compensation according to the prior art; 
         FIG. 3  is a functional block diagram of the transmitter line driver of  FIG. 2  according to the prior art; 
         FIG. 4  is a conceptual electrical schematic of the transmitter line driver according to the prior art; 
         FIG. 5  is an electrical schematic of an exemplary positive current cell in the transmitter line driver according to the prior art; 
         FIG. 6  is an electrical schematic of an exemplary negative current cell in the transmitter line driver according to the prior art; 
         FIG. 7  is a table illustrating Class A operation of the transmitter line driver according to the prior art; 
         FIG. 8  is a table illustrating Class B operation of the transmitter line driver according to the prior art; 
         FIG. 9  illustrates ideal current cell rise and fall transition characteristics; 
         FIG. 10  illustrates actual current cell rise and fall transition characteristics; 
         FIG. 11  is a functional block diagram illustrating a transceiver with a transmitter line driver and linear and nonlinear echo, NEXT and transmitter signal compensation according to the present disclosure; 
         FIG. 12  illustrates a nonlinear echo compensation circuit according to the present disclosure; 
         FIG. 13  illustrates a mapping circuit of  FIG. 12  in further detail; 
         FIG. 14  illustrates a least means squared (LMS) circuit according to the present disclosure; and 
         FIG. 15  illustrates mean squared error (MSE) as a function of sample phase for a first transceiver with linear echo compensation and a second transceiver according to the present disclosure with linear and nonlinear echo compensation. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and is in no way intended to limiting. For purposes of clarity, the same reference numerals will be used in the drawings to identify similar elements. 
     Referring now to  FIG. 9 , rise h r  and fall h f  characteristics of an ideal current cell is shown. As can be appreciated, the ideal rise h r  and fall h f  characteristics are symmetric such that h r +h f =1. In  FIG. 10 , rise h r  and fall h f  characteristics of typical current cells are not ideal. For some time periods, h r +h f ≠1. The nonlinear echo compensation circuit for the Class B driver according to the present disclosure compensates for nonlinear echo that is introduced as a result of this asymmetry. The transmitter line driver of the transceiver according to the present disclosure can be operated in the Class B mode with reduced power consumption and without sacrificing performance. 
     The sampling point of the ADC  74  is determined by the received signal and not by the transmitted signal. In some cases, the sampling point occurs when the difference between h r  and 1−h f  is greater than zero. The replica transmitter signal does not have nonlinear echo characteristics because the replica transmitter signal is not generated by the transmitter line driver, which is the source of the nonlinear echo. 
     Referring now to  FIG. 11 , a transceiver  100  according to the present disclosure receives a transmitter signal  52 . The transmitter line driver  50  supplies a multi-level signal to a load such as the matched resistor  54  based on the transmitter signal  52 . The transformer  58  couples the transmitter line driver  50  to the twisted pair  60 . The replica signal generator  64  outputs a replica of the transmitter signal  52  to the summer  66 . The received signal  68  is also input to the summer  66 . 
     The output of the summer  66  is input to the LPF  70 . An output of the LPF is input to the ADC  74 . The output of the ADC  74  is input to the summer  78 . The linear echo compensation signal from the linear echo compensation circuit  82  and the NEXT compensation signal from the circuit  83  (canceling NEXT 12 , NEXT 13 , and NEXT 14 ) are also input to the summer  78 . A non-linear echo compensation signal from a compensator  104  according to the present disclosure is also input to the summer  78 . A signal (TA comp ) with linear and nonlinear echo compensation and NEXT compensation is output by the summer  78 . 
     Referring now to  FIG. 12 , the nonlinear echo compensator  104  is shown to include a mapping circuit  114  and a canceller circuit  118 . A transmitted signal TA 1 (k+L) is input to a variable delay  120  that provides a delay of L clock cycles. The delayed transmitter signal is input to the linear echo compensation circuit  82  and the mapping circuit  114 . The mapping circuit  114  outputs a pattern dependent driving signal δ k  to the canceller circuit  118 . The pattern dependent driving signal is input to first inputs of first, second and third multipliers  122 ,  124  and  126 . Another input of the multiplier  122  receives a third compensator coefficient h 2  from unit delay  130 . As can be appreciated, unit delays can be implemented as a register or in any other suitable manner. A second input of the multiplier  124  receives a second compensator coefficient h 1  from unit delay  132 . A second input of the multiplier  126  receives a first compensator coefficient h 0  from unit delay  134 . 
     An output of the multiplier  122  is input to unit delay  140 . An output of the unit delay  140  is input to a first input of a summer  142 . An output of the multiplier  124  is input to a second input of the summer  142 . An output of the summer  142  is input to unit delay  146 . An output of the unit delay  146  is input to a first input of a summer  148 . An output of the multiplier  126  is input to a second input of the summer  148 . An output of the summer  148  is input to unit delay  150 . An output of the unit delay  150  is input to a summer  154 . 
     An output of the linear echo compensation circuit  82  is input to unit delay  158 . An output of the unit delay  158  is input to the summer  154 . Transmitter signals from other twisted pairs are input to variable delay circuits  160 ,  162  and  164 . Outputs of the variable delay circuits  160 ,  162  and  164  are input to NEXT compensation circuits  166   168  and  170 . Outputs of the NEXT compensation circuits  166 ,  168  and  170  are summed by a summer  174  and input to the summer  154 . The transmitter signal TA 1 (k) is input to ADC  180  and output to a summer  184 . An output of the summer  154  is input to an inverting input of the summer  184 , which outputs the compensated signal (TA comp )  186 . 
     Referring now to  FIG. 13 , the mapping circuit  114  is illustrated in further detail. The mapping circuit  114  includes a weighting circuit  201 . The transmitter signal is input to unit delay  202  and a first input of the multiplier  204 . A second input of the multiplier  204  receives a first constant scale factor. An output of the unit delay  200  is input to a first input of a multiplier  208 . A second input of the multiplier  208  is connected to a second constant scale factor. Outputs of the multipliers  204  and  208  are input to a summer  212 . An output of the summer  212  is input to unit delay  216 , which outputs a signal b k+1  to a function generator  220 . The function generator  220  outputs the pattern dependent driving signal (before delay and scaling) as follows:
 
δ k+1   =|b   k+1   |−|b   k | if  b   k+1   ≧b   k  
 
δ k+1   =|b   k   |−|b   k+1 | if  b   k+1   &lt;b   k  
 
     The pattern dependent driving signal that is output by the function generator  220  is input to unit delay  224 . An output of the unit delay  224  is input to a scaling circuit  228 . One exemplary scaling circuit  228  includes a multiplier  230  having a first input coupled to the unit delay  224  and a second input coupled to a constant value. The scaling circuit  228  preferably offsets the effects of the weighting circuit  201 , although other scaling may be performed. In the exemplary weighting circuit  201 , the signal TA 1 (k) is multiplied by  6  and the signal TA 1 (k−1) is multiplied by 2. The scaling circuit  228  multiplies by ⅛. 
     Referring now to  FIG. 14 , a least mean squared (LMS) circuit  250  is illustrated. The LMS circuit  250  includes a compensator coefficient generator  254 . An error signal  255  is input to a selector switch  256 . A receiver error signal  258  is also input to the selector switch  256 . The selector switch  256  selects one of the error signals  255  or  258 . The switch  256  preferably selects the output of the compensator (the summer  78  in  FIG. 11 ) as the error signal when a remote transceiver has not sent signals. Ideally, the output of the summer  78  is zero since the receiver should not detect a signal. When an incoming signal is received, the switch  256  selects the error signal at the output of the followed detector, which eliminates the effect of the incoming signal in the error signal. 
     An output of the selector switch  256  is input to a multiplier  260 . Another input of the multiplier  260  is coupled to a scaling factor or loop gain (μ). An output of the multiplier  260  is input to the compensator coefficient generator  254 . A sign function of the transmitted signal is input to a variable delay  264 . An output of the variable delay is input to a multiplier  266 . An output of the multiplier  260  is input to the multiplier  266 . An output of the multiplier  266  is input to a summer  270 . An output of the summer  270  is fed back through a unit delay  274  to the summer  270 . An output of the summer  270  is input to unit delay  276 . An output of the unit delay  276  provides a linear echo compensation signal (AA 0 ). 
     A sign function of the pattern dependent driving signal is input to a variable delay  280  of the compensator coefficient generator  254 . An output of the variable delay  280  is input to a multiplier  282 . An output of the multiplier  260  is also input to the multiplier  282 . An output of the multiplier  282  is input to a summer  284 . An output of the summer  284  is input to a limiter  286 , which limits the signal input between upper and lower limits. For example, the limiter  286  may limit the signal to +/− 1/32. An output of the limiter  286  is input to unit delay  288  and to unit delay  290 . An output of the unit delay  290  is input to the summer  284 . An output of the unit delay  288  provides the first compensator coefficient h 0  as follows:
 
 h   0   ←h   0   +μ*e   k−L *sign(δ k )
 
     An output of the variable delay  280  is input to unit delay  300 . An output of the unit delay  300  is input to a multiplier  302  and unit delay  304 . An output of the multiplier  260  is also input to the multiplier  302 . An output of the multiplier  302  is input to a summer  306 . An output of the summer  306  is input to a limiter  308 . An output of the limiter  308  is input to unit delays  310  and  312 . An output of the unit delay  312  is input to the summer  306 . An output of the unit delay  310  provides the second compensator coefficient h 1  as follows:
 
 h   1   ←h   1   +μ*e   k−L *sign(δ k−1 )
 
     An output of the unit delay  304  is input to a multiplier  320 . An output of the multiplier  260  is also input to the multiplier  320 . An output of the multiplier  320  is input to a summer  322 . An output of the summer  322  is input to a limiter  324 . An output of the limiter  324  is input to unit delays  326  and  328 . An output of the unit delay  328  is input to the summer  322 . An output of the unit delay  326  provides the third compensator coefficient h 2  as follows:
 
 h   2   ←h   2   +μ*e   k−L *sign(δ k−2 )
 
     Referring now to  FIG. 15 , mean squared error is shown as a function of sample phase. The mean squared error for transceivers with linear and nonlinear echo compensation according to the present disclosure is significantly lower than the mean squared error for transceivers with linear echo compensation. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while the embodiments disclosed herein have been described in connection with particular examples thereof, other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.