Patent Publication Number: US-7215764-B2

Title: Current sensing echo cancellation device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to canceling echoes seen by transceivers when communicating data and/or voice signals, and, more particularly, to a current sensing echo cancellation device. 
   2. Description of the Related Art 
   Echo cancellation techniques are commonly employed in telephony applications to provide enhanced signal quality. Echo cancellation has been used in both voice and data applications. In general terms, a portion of the signal transmitted on a telephone line, otherwise known as a subscriber line, is reflected by the network and an attenuated version is seen as an input signal at the source. That is, an incoming signal received at the near end of the line will include both the signal originating from far end of the line (i.e., the desired message) and a portion of the signal previously sent at the near end of the line (i.e., an echo). This echo is present and deleteriously affects both data and voice transmission quality. 
   Telephone subscriber line cards provide the interface between the subscriber and the central office of the telephone company. Commonly, these line cards include circuitry adapted to remove or reduce the echo component to enhance signal quality. A typical subscriber line is a two-wire twisted pair cable. Within the central office, a four-wire arrangement is used to form a balancing network. The fundamental problem addressed by the echo control circuit of a telephone subscriber line card is the mismatch in the impedance characteristics of the two-wire subscriber loop and the balancing network. Because of the mismatch, some energy from the received far-end signal is reflected back at the 2 to 4 wire hybrid and turned into an unwanted near-end echo. To maximize the attenuation of a near-end echo various techniques have been employed. 
   A typical line card includes a subscriber line interface circuit (SLIC) coupled to the line and a coder/decoder (codec) circuit coupled to the SLIC. The codec receives the analog output of the SLIC and samples the output to generate a digital representation of the signal. The digital information is generally passed to a digital signal processor (DSP) for further processing (e.g., extraction of digital data). Digital data received from the DSP is converted to an analog form by the codec and subsequently sent back to the SLIC for transmission on the subscriber line. 
   One technique for canceling near-end echoes involves using a voltage summing circuit between the codec and the SLIC to scale the voltage of the signal being transmitted and subtracting that scaled voltage from the voltage of the input signal to cancel the echo. If the scaling factor applied to the transmit signal corresponds to the attenuation seen in the transmission line (i.e., the transmit signal travels from the near end to the far end and a portion is reflected back), the echo is reduced. 
   The voltage summing technique has several limitations. First, there is a delay imparted on the echo portion as it travels between the near and far ends. In voice band frequencies and lower frequency data bands this delay can be tolerated. However, as the frequency of the data signal increases (e.g., &gt;100 kHz), this delay is significant and degrades the effectiveness of the echo canceling technique. A second limitation is that during the passage of the transmit signal through the SLIC, through the subscriber line, and back through the SLIC, noise and non-linearities are introduced. The original transmit signal does not contain noise and/or non-linearities. Hence, subtracting the original transmit signal from the receive signal including the echo leaves behind the noise and non-linearities. 
   Another technique employed to cancel echoes involves using digital signal processing algorithms to implement an adaptive digital filter. This technique requires that resources of the DSP be used to perform the adaptive filtering. In applications where the resource demands on the DSP are already great, such as in complex communications algorithms (e.g., asynchronous digital subscriber line), there are not sufficient resources remaining to implement a digital echo cancellation technique without compromising the data handling functions. 
   The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is seen in a passive echo cancellation circuit for reducing echoes on a subscriber line. The echo cancellation circuit includes an output driver adapted to drive a downstream signal on the subscriber line. First sensing circuitry is coupled to the output driver and adapted to sense a current on the subscriber line to generate a first voltage. A balancing network is adapted to receive at least a portion of the downstream signal. Second sensing circuitry is adapted to sense a current in the portion of the downstream signal passing through the balancing network to generate a second voltage. An upstream driver is adapted to subtract the second voltage from the first voltage to generate an upstream signal. 
   Another aspect of the present invention is seen in a method for canceling echoes on a subscriber line. The method includes driving a downstream signal on the subscriber line; sensing a current on the subscriber line to generate a first voltage; providing a portion of the downstream signal to a balancing network; sensing a current in the portion of the downstream signal passing through the balancing network to generate a second voltage; and subtracting the second voltage from the first voltage to generate an upstream signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
       FIG. 1  is a simplified block diagram of a line card in accordance with the present invention; and 
       FIG. 2  is a circuit diagram of an echo cancellation circuit in a subscriber line interface chip of the line card of  FIG. 1 . 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
   Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
   Referring first to  FIG. 1 , a simplified block diagram of a line card  10  is provided. The line card  10  interfaces with a subscriber line  20 . In an actual implementation, the line card  10  interfaces with a plurality of subscriber lines  20 , but for clarity and ease of illustration, only one is shown. A subscriber line interface circuit (SLIC)  30  is coupled to the subscriber line  20 . The SLIC  30  includes an echo cancellation circuit  35  adapted to cancel or at least reduce near-end echo signals received over the subscriber line  20 . Hereinafter, signals received by the line card  10  over the subscriber line  20  are referred to as upstream signals (i.e., designated by an arrow  37 ), and signals transmitted by the line card  10  on the subscriber line  20  are referred to as downstream signals (i.e., designated by an arrow  38 ). The SLIC  30  includes an amplifier  39  for amplifying the analog upstream signal generated by the echo cancellation circuit  35 . 
   The SLIC  30  supplies the analog upstream signal to a coder/decoder (codec)  40 . The codec  40  receives the analog upstream signal and generates a digital upstream signal that is subsequently passed to a digital signal processor  50 . The DSP  50  also provides a digital downstream signal for eventual transmission on the subscriber line  20 . The codec  40  receives the digital downstream signal, converts it to an analog downstream signal, and provides the analog downstream signal to the SLIC  30 , which sends the downstream analog signal over the subscriber line  20 . The operation of the echo cancellation circuit  35  in canceling or reducing the echo signal is described in greater detail below in reference to  FIG. 2 . The echo cancellation circuit  35  uses a passive technique for canceling echoes prior to the amplifier  39 . 
   In the illustrated embodiment, the line card  10  is adapted to implement an asynchronous digital subscriber line (ADSL) modem for high bandwidth data transfer. The ADSL protocol is described in ANSI T1.413 Issue 2, entitled, “Interface Between Networks and Customer Installation—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.” The construct and operation of the SLIC  30 , codec  40 , and DSP  50  for implementing their respective signal and/or data handling functions are well known to those of ordinary skill in the art, and for clarity and ease of illustration, they are not described in greater detail herein. 
   Turning now to  FIG. 2 , a circuit diagram of the echo cancellation circuit  35  is provided. The portion of the echo cancellation circuit  35  inside the dashed box represents those components residing on the SLIC  30 . Transmit drivers  100 ,  110  receive differential components, V D + and V D −, of the downstream signal provided by the codec  40  (shown in  FIG. 1 ). The outputs of the transmit drivers  100 ,  110  are coupled to sensing resistors  120 ,  130 , respectively. The voltage drop over the sensing resistors  120 ,  130  is proportional to the current of the downstream signal. Subscriber line output terminals  140 ,  150  (V AD  and V BD ) of the SLIC  30  are coupled to protection resistors  160 ,  170 . In the illustrated embodiment, the sensing resistors  120 ,  130  have a resistance, R S , of about 15 ohms, and the protection resistors  160 ,  170  have a resistance, R PROT , of about 35 ohms. Hence, the total output resistance of the line card  100  is 100 ohms, as is typically provided for in industry standards. 
   A subscriber line load (Z LINE )  180  is shown to model the characteristics of the subscriber line  20 . Typically, the subscriber line load  180  is modeled as a resistance (100 ohms) in series with the combination of a second resistance (˜900 ohms) in parallel with a capacitance (having a break frequency of about 1 kHz). At higher frequencies (i.e., data band frequencies), the effects of the second resistance and capacitance tend to cancel each other, leaving an apparent load impedance of about 100 ohms. A voltage source  190  is shown to model the upstream signal, V U , generated at the far end of the subscriber line  20 . 
   The downstream drivers  100 ,  110  drive the downstream analog signal that is transmitted on the subscriber line  20 . Due to impedance mismatches in the subscriber line  20 , a portion of the downstream signal is reflected back and received as part of the upstream signal by the line card  10 . The output terminals of the downstream drivers  100 ,  110  are coupled to scaled sensing resistors  200 ,  210 , respectively. The scaled sensing resistors  200 ,  210  have a resistance, NR S , equal to an integer multiple of the sensing resistors  120 ,  130 . The voltage drop over the scaled sensing resistors  200 ,  210  represents a scaled version of the downstream signal current. In the illustrated embodiment, the scaling factor, N, is 100, although other scaling factors may be used. The effect of the scaling factor is to limit the power of the scaled downstream signal. For example, if the scaling factor is  100 , the power diverted from the output drivers  100 ,  110  is only about 1%. 
   Balancing output terminals  220 ,  230  (V HA  and V HB ) of the SLIC  30  are coupled to a balancing network  235  including a scaled protection resistor  240  having a resistance,  2 NR PROT , of about 7 kohms and a balancing load (Z BAL )  250 . The scaled protection resistor  240  and the balancing load  250  model the characteristics of the subscriber line, albeit on a scaled down basis. In the illustrated embodiment, the balancing load  250  is a capacitor having a capacitance of about 1 nF with a series resistance of about 10 kohms, although other capacitance and resistance values may be used depending on the expected characteristics of the subscriber line  20 . The scaled protection resistor  240  and the resistive component of the balancing load  250  may be combined in a single resistor (e.g., 17 kohms). 
   Although the balancing network  235  is illustrated as being external to the SLIC  30 , it is contemplated that the balancing network  235  may be entirely or partially incorporated in the SLIC  30 . In the illustrated embodiment, the balancing network  235  is external to the SLIC  30  to more closely simulate the conditions seen by the downstream signal as it exits through the output terminals  140 ,  150  to the subscriber line  20 . 
   An upstream differential driver  260  receives as input signals the voltage at the output terminal of the sensing resistor  120  and the voltage at the output terminal of the scaled sensing resistor  200 . Likewise, another upstream differential driver  270  receives as input signals the voltage at the output terminal of the sensing resistor  130  and the voltage at the output terminal of the scaled sensing resistor  210 . The differential drivers  260 ,  270  have differential outputs. As will be shown mathematically below, if the balancing load  250  closely mirrors the subscriber line load  180 , the output signals of the sensing resistors  120 ,  130  represent the combination of the upstream and downstream signals and the output signals of the scaled sensing resistors  200 ,  210  represent a scaled version of only the downstream signal as affected by the subscriber line  20  characteristics. Hence, the differential upstream drivers  260 ,  270  cancel the downstream echo, leaving the upstream signal, V U + and V U −. 
   A small portion of the echo signal may still be present, because the balancing load  250  does not mirror the subscriber line characteristics perfectly. However, the effect of the echo is greatly reduced. The signals generated by the differential upstream drivers  260 ,  270  are illustrated by the following equations. For simplicity, only the equations for the positive component V U + are shown. Corresponding equations exist for the negative component, V U −. 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   If the balancing network  235  closely mirrors the subscriber line  20 , the V D + and V D − terms in equation 3 go to zero, leaving the upstream signal. 
   The echo cancellation circuit  35  operates in a current sensing mode. In general terms, the technique employed in the echo cancellation circuit  35  includes driving a downstream signal on the subscriber line; sensing a current on the subscriber line to generate a first voltage; providing a portion of the downstream signal to a balancing network; sensing a current in the portion of the downstream signal passing through the balancing network to generate a second voltage; and subtracting the second voltage from the first voltage to generate an upstream signal. 
   Current sensing is more accurate than previously employed voltage summing methods. Also, because the sensing is conducted inside the SLIC  30  at the transmit drivers  100 ,  110 , the delay problem associated with the prior technique is substantially reduced. Also, because the downstream signal and the scaled downstream signal used for echo cancellation are subjected to the same circuitry, noise and non-linearities are equally imposed. Hence, the echo cancellation circuit  35  also substantially reduces these factors. The passive echo cancellation technique described herein has numerous advantages. Because the echo cancellation is performed prior to the amplification stage (i.e., in the amplifier  39 ), distortion is reduced due to the smaller signal amplitude resulting from the echo cancellation. The passive echo cancellation technique also reduces noise in the upstream signal. The passive nature of the circuit also results in a reduced cost and reduced power consumption due to a reduction in the amount of circuitry required. 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.