Abstract:
A method and system for canceling echo signals originating from a transmitter at a predetermined data rate. The echo signals are received by a receiver at a different data rate. The echo system is configured to manipulate data rates of transmitted signals, and reconstruct echo signals for consideration by the receiver. In view of the band-limited nature of these echo signals, the invention intelligently reduces the computational complexity of reconstructing and canceling echo signals.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to digital communication systems. More particularly, this invention relates to a method and apparatus for echo cancellation in full duplex asymmetric communication systems. 
   2. Description of the Related Art 
   Communication systems are very common in today&#39;s society. These systems have evolved from devices that provide simple half-duplex data transmissions to sophisticated full-duplex systems providing voice, data, and video transmission and reception. Half-duplex communication commonly refers to the communication of information between a transmitter and a receiver in one direction at a time. On the other hand, full-duplex communication commonly refers to the communication of information between the transmitter and receiver in both directions at the same time, i.e., simultaneously. However, with sophistication comes added complexity which often necessitates corrective measures. One complexity that is associated with fill-duplex systems is a phenomenon known as “echo”. The production of echo in a full-duplex system is often attributed to leakage of at least a portion of a transmitted signal into an unintended receiver, such as a receiver portion of a transceiver or a co-located receiver. 
     FIG. 1  shows a functional block diagram of an exemplary modem system. At a near-end of the system  100   a  is a transmitter  110   a  and a receiver  120   a . The transmitter  110   a  and receiver  120   a  are isolated or separated from each other by a hybrid  150   a . As is well known in the art, a hybrid may be defined as a circuit that routes signals from one source (e.g., the transmitter  110   a ) to a desired output port, while preventing the signals from passage to an unintended destination (e.g., the receiver  120   a ). The transmitter  110   a  includes a digital modulator  112   a  that modulates information signals onto a carrier signal in preparation for transmission over a communication medium  170 . The communication medium  170  may be a wired (e.g., telephone lines) and/or a wireless (e.g., airwaves) medium. The transmitter  110   a  further includes a digital-to-analog converter (DAC)  114   a , which converts digital signals received from the digital modulator  112   a  into analog signals prior to transmission over the medium  170 . The digital signal stream has a sampling rate of “fs”. Similarly, the receiver  120   a  includes an analog-to-digital converter (ADC)  124   a , which converts analog signals received from the communication medium  170  into digital signals. The receiver  120   a  further includes a digital demodulator  122   a  that demodulates the digital signals from the carrier signal for further processing at an ultimate destination (not shown in this figure), e.g., a computer, television, or other application device. 
   The far-end portion of the system  100   b  comprises a mirrored-structure of the near-end portion of the system  100   a . More particularly, the system  100   b  further comprises a transmitter  110   b  and a receiver  120   b . The transmitter  110   b  and receiver  120   b  are isolated or separated from each other by a hybrid  150   b . The transmitter  110   b  includes a digital modulator  112   b  that modulates information signals onto a carrier signal for transmission over a communication medium  170 . The transmitter  110   b  further includes a digital-to-analog converter (DAC)  114   b , which converts digital signals received from the digital modulator  112   b  into analog signals prior to transmission over the medium  170 . Similarly, the receiver  120   b  includes an analog-to-digital converter (ADC)  124   b , which converts analog signals received from the communication medium  170  into digital signals. The receiver  120   b  further includes a digital demodulator  122   a  that demodulates the digital signals from the carrier signal for further processing at an ultimate destination (not shown in this figure), e.g., a computer. 
   In practice, at least a portion of signals transmitted from the near-end transmitter  110   a  leak through the hybrid  150   a  into the near-end receiver  120   a . This leakage contaminates the near-end receiver  120   a  in the form of an echo by mixing with or superimposing signals received from the far-end transmitter  110   b . Thus, this superimposition causes interference by the echo signal with the intended information signals. The same is true for the far-end transceiver. 
   To circumvent such echo in full duplex systems, one of two methods may be applied. The first method is frequency division multiplexing (FDM), which may be defined as a multiplexing technique that uses different frequencies to combine multiple streams of signals for transmission over a communications medium. More particularly, forward and reverse streams of signals travelling in opposite directions occupy different portions of the frequency spectrum, with the effect that they can be easily separated in frequency at the receivers through a variety of signal processing techniques. FDM is not bandwidth efficient because it does not make full use of available bandwidth. The second method is echo cancellation, which allows forward and reverse signals to occupy overlapping frequency bands. A copy of the echo signal is reconstructed and subsequently subtracted at the affected receiver. Current echo cancellation techniques have been burdened in many applications by too much computational power and inferior speed performance rendering them undesirable. 
   Therefore, there is a need in the communications technology for a method and system that reduces computational requirement of echo cancellation. The method and system should be adaptable using common efficient and stable techniques without adding implementation hindrances. 
   SUMMARY OF THE INVENTION 
   The invention provides a method of canceling echo signals in a communication system. The communication system comprises a receiver that receives the echo signals at a first data rate, and a transmitter that is configured to transmit signals at a second data rate. In one embodiment, the method comprises increasing the data rate of the transmitted signals from the second data rate to a higher data rate. The method further comprises estimating echo signal components based at least in part on the higher data rate signals. The method further comprises matching the data rate of the estimated echo signal with the first data rate of the receiver. In another embodiment, the method comprises filtering the transmitted signals to substantially remove frequency components above a cut-off frequency that is equivalent to at least one-half of the predetermined data rate. The method further comprises reducing the data rate of the filtered signals from the predetermined data rate to a lower data rate. The method further comprises estimating echo signal components based at least in part on the filtered signals. 
   The invention further provides a system for canceling echo signals received by a receiver that is configured to operate at a first data rate. The echo signals originate from a transmitter that is configured to transmit signals at a second data rate. In one embodiment, the system comprises a filter that is configured to substantially remove from the transmitted signals frequency components above a cut-off frequency that is equivalent to at least one-half of the first data rate. The system further comprises a decimator that is configured to reduce the data rate of the filtered signals from the second data rate to a lower data rate. The system further comprises an echo canceler that is configured to estimate echo signal components based at least in part on the filtered signals at the lower data rate. In another embodiment, the system comprises a first upsampler that is configured to increase the data rate of the transmitted signals from the second data rate to a higher data rate. The system further comprises an echo canceler that is configured to estimate an echo signal based at least in part on the upsampled signals. The system further comprises a second upsampler that is configured to match the data rate of the estimated echo signal with the first data rate of the receiver. 
   In yet another embodiment, the system comprises means for filtering the transmitted signals to substantially remove frequency components above a cut-off frequency that is equivalent to at least one-half of the predetermined data rate. The system further comprises means for reducing the data rate of the filtered signals from the predetermined data rate to a lower data rate. The system further comprises means for estimating echo signal components based at least in part on the filtered signals. In yet another embodiment, the system further comprises means for increasing the data rate of the transmitted signals from the second data rate to a higher data rate. The system further comprises means for estimating echo signal components based at least in part on the higher data rate signals. The system further comprises means for matching the data rate of the estimated echo signal with the first data rate of the receiver. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features, and advantages of the invention will be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a functional block diagram of an exemplary modem system. 
       FIG. 2  is a functional block diagram of a near-end modem system in accordance with the invention. 
       FIG. 3  is a functional block diagram of a far-end modem system in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     FIG. 2  shows a functional block diagram of one embodiment of a near-end modem system  200  in accordance with the invention. The near-end system is sometimes referred to as a remote-terminal (RT) system, whereas a far-end system (see  FIG. 3 ) is sometimes referred to as a central office (CO) system. In this embodiment, the RT system  200  comprises an asymmetric digital subscriber loop (ADSL) system which conforms to ADSL standard promulgated by the American National Standard Institute (ANSI) in T1.413-1998, which is incorporated in its entirety by reference. The communication channel from the RT system  200  to the CO system is commonly referred to as an “upstream” channel, and that from the CO system to the RT system  200  is commonly referred to as a “downstream” channel. 
   As shown in  FIG. 2 , the RT system  200  comprises a transmitter  210  that is configured to transmit signals over a communication medium  270  via a hybrid  250 . The RT system  200  further comprises a receiver  220  that is configured to receive signals over the communication medium  270  via the hybrid  250 . The RT system  200  further comprises an echo canceler (EC) subsystem  230  that is configured to cancel or minimize echo signals. As shown in  FIG. 2 , the echo path is represented pictorially by an echo channel  252 , and it is understood that such pictorial representation is intended to describe the general direction of echo flow and, thus, is not intended to describe the physical path of echo signals. Each of these subsystems is described in detail below. 
   The transmitter  210  comprises a digital modulator  212  that receives and modulates a digital information signal onto a carrier signal. The output of the digital modulator  212  is a modulated discrete-time sequence signal having a data or sampling rate of fs/K, where fs is the downstream transmission data or sampling rate and K is a constant integer. The modulated discrete-time sequence signal is fed into an up-sampler  216  to be upsampled by a factor K. Upsampling by a factor K of a discrete-time sequence signal may be accomplished mathematically by inserting K-1 sequence points with zero amplitude between each of the samples or sequence points of the modulated sequence, and increasing the sampling rate by a factor of K. Upsampling is basically the reverse process of downsampling. 
   In an ADSL system, the downstream sampling rate of transmitted signals is K times greater than that of the upstream sampling rate, where K=8. More particularly, the sampling rate of the downstream channel is typically about fs=2.208 MHz. The sampling rate of the upstream channel is typically about fs/8=276 kHz. Since, both the upstream and downstream channels are band-limited, the echo channel  252  is also band-limited. As will be appreciated and understood by those of ordinary skill in the art, a continuous-time signal may be sampled into a discrete-time sequence without a loss of information when the sampling rate (i.e., Nyquist frequency or sampling rate) is at least two times the highest frequency component of the original continuous-time signal. In this embodiment, it is desirable to sample signals of the echo channel  252  at a minimum rate possible (i.e., fs/K), so that the corresponding EC subsystem  230  uses a smaller number of tap weights than that of higher sampling rates. Ideally, the number of tap weights of the EC subsystem  230  is substantially equal to the number of discrete samples of the impulse response h(t) of the echo channel  252 . For a fixed duration of the impulse response h(t), the number of tape weights required by the EC subsystem  230  depends on the sampling rate. Thus, it is desirable to drive the EC subsystem  230  by an input signal having the least sampling rate possible. 
   As noted above, the sampling rate of the modulated signal (i.e., output of the digital modulator  212 ) is fs/K. Thus, the echo channel of such an ADSL system is band-limited to a frequency band that is one-half (½) of the sampling rate (i.e., fs/K) of the upstream channel. Accordingly, in one embodiment, the echo channel  252  is band-limited to fs/2K (e.g., 2.208 MHz/16=138 kHz), since a bandwidth of 138 kHz is sufficient for signals of the transmitter  210 . However, in practice the echo channel  252  may still have appreciable or significant energy in frequency bands that are greater than fs/2K. The presence of frequency components above fs/2K may hamper or reduce the accuracy of performance of the EC subsystem  230  that operates at a sampling rate of fs/K. Thus, it is desirable to introduce an additional band limiting filter (e.g., low pass filter)  218  into the echo path at the RT system  210 . Accordingly, the RT system  210  further comprises a band limiting filter  218  that is configured to attenuate echo signals having frequency components above fs(J/2K), where J is a positive integer less than K and K/J is also an integer. In one embodiment, it is desirable to select J=2, which yields an EC subsystem that is usually less than 100 taps long in ADSL implementation. The transmitter  210  further comprises a digital-to-analog converter (DAC)  214  which receives output signals from the band limiting filter  218  to convert digital signals to analog form for transmission. The DAC  214  feeds converted analog signals into a transmit filter  244 , which filters undesired frequencies from analog signals before transmission via the hybrid  250  to the communication medium  270 . 
   The receiver  220  comprises an anti-aliasing filter  248  that receives signals via the hybrid  250  from the communication medium  270 . The anti-aliasing filter  248  feeds its output signals, which are in analog form, into an analog-to-digital converter (ADC)  224 . The ADC  224  converts analog signals into a digital or discrete-time sequence at a sampling rate fs. The ADC  224  feeds its digitized sequence into a subtractor  246  and into the EC subsystem  230  for further processing. 
   The EC subsystem  230  comprises an echo canceler (EC) finite impulse response (FIR) filter  234  in the time-domain. In one embodiment, the EC FIR filter  234  may comprise a transversal filter. To provide an input signal having a sampling rate of fs(J/K) into the EC FIR filter  234 , the output signal of the digital modulator  212  is fed into an upsampler  232  that is configured to upsample the modulated signal by a factor J from its original sampling rate fs/K. The upsampler  232  outputs the upsampled signal with a sampling rate of fs(J/K). After proper training (see description below), the EC FIR filter  234  is configured to reconstruct from the upsampled signal a signal that is substantially identical to the echo signal, i.e., output signals of the ADC  224  having a sampling rate of fs, after being decimated by a factor K/J. Decimation by a factor of K/J may be accomplished mathematically by extracting every K/Jth sample from the digital sequence. 
   Pursuant to the ANSI standard, a predetermined period of time is dedicated to properly train the EC FIR filter  234  at the RT system  200  and CO system  300 . During such period, only one modem system (e.g., RT system  200 ) transmits signals while the other modem system (e.g., CO system  300 ) remains silent. Thus, any signals received by the transmitting system represent mainly echo signals. In this embodiment, the EC FIR filter  234  feeds the reconstructed echo signal into a subtractor  236 , which subtracts the reconstructed echo signal from the actual echo signal received from the decimator  238 . The subtractor  236  may generate an “error” signal which is fed back into the EC FIR filter  234  to dynamically adapt the EC FIR filter  234  to changing echo conditions. The dynamic adaptation of the EC FIR filter  234  may be necessary because of possible changes in impedance characteristics of the communication medium and variation in echo conditions from one installation to another. With a sampling rate of fs(J/K), the error signal may be used to update the EC FIR filter  234  using any variant of the family of least means square (LMS) adaptive algorithms. The adaptation may be used during the training period of the EC FIR filter  234 , or during full-duplex operation to track and compensate for any variations in echo channel characteristics. 
   Since echo signals have a limited bandwidth of fs(J/2K), the reconstructed echo signal of the EC FIR filter  234  may be interpolated by a factor of K/J to accurately represent the echo signals of the echo channel  252  at each sample point. Such interpolation may be carried out using multi-rate signal processing techniques. More particularly, interpolation by K/J may be accomplished by first upsampling the reconstructed echo signal by a factor of K/J using the upsampler  240 . In the frequency domain, upsampling causes the frequency band to shrink by a factor of K/J in frequency, and the shank frequency band is replicated (K/J-1) times in the frequency domain. Interpolation is accomplished by passing the upsampled signal through an interpolation (low pass) filter  242  that is configured to remove the replicated (K/J-1) frequency bands produced by the upsampler  240 . The output of the interpolation filter  242  is fed into a subtractor  246 , which subtracts the output of the interpolation filter  242  from output signals of the ADC  224  at a common sampling rate fs to cancel the echo signals. During full-duplex operation, the output of the subtractor  246  is fed into the digital demodulator  222  for demodulation in accordance with the implemented demodulation scheme. 
     FIG. 3  is a functional block diagram of the CO modem system  300  in accordance with the invention. As will be apparent from the following description, the CO system  300  is similar to the RT system  200 . More particularly, the transmit portion of the CO system  300  comprises a digital modulator  312  connected to a DAC  314  and a transmit filter  344 . The structure and operation of each of these transmitter components is substantially similar to its respective component in the transmitter  210  (FIG.  2 ). The receiver portion of the CO system  300  comprises an anti-aliasing filter  348  connected to an ADC  314 , which is configured to convert analog signals received from the RT system  200  over the communication medium  270  into digital form, i.e., discrete-time sequence. The ADC  314  forwards the discrete-time sequence to a band limiting (low pass) filter  318  to band limit received echo signals to fs(J/2K). The output of the band limiting filter  318  is fed into a decimator  316  to decimate the discrete-time sequence by a factor of K, thereby yielding an output discrete-time sequence having a data or sampling rate of fs/K. The output of the decimator  316  is fed into a subtractor  346  for further processing, as described below. 
   As described in connection with the RT system  200 , the CO system  300  also comprises an EC subsystem that is configured to cancel echo signals at the CO system  300 . As shown in  FIG. 3 , the echo path is represented by an echo channel  352 . Because of the band limiting filter  318  and other band limiting effects inherent in ADSL systems, the echo channel  352  is practically band limited to fs(J/2K). Only the frequency portion of the transmit signal (from the digital modulator  312 ) that coincides with the bandwidth of the echo channel is expected to appear as echo at the receiver of the CO system  300 . Thus, frequency components of the transmit signal in the frequency band of 0-fs(J/2K) Hz are sufficient to reconstruct the echo signal by the EC subsystem. Accordingly, the EC subsystem comprises a decimation filter  342  that is configured to filter out frequency components outside fs(J/2K) of the transmit signal received from the digital modulator  312 . As will be understood by one of ordinary skill in the art, the normalized upper cut-off frequency in designing the decimation filter  342  is π/(K/J). The output of the decimation filter  342  is a discrete-time sequence having a sampling rate of fs that is fed into a decimator  340 . The decimator  340  decimates the filtered discrete-time sequence by a factor of K/J to produce a discrete-time sequence having a sampling rate of fs(J/K). 
   The output sequence of the decimator  340  is fed into an EC FIR filter  334  that is configured to reconstruct an echo signal based on the transmit signal of the digital modulator  312 . To compress the sampling rate from fs(J/K) down to fs/K, it is desirable to perform a decimation by a factor of J on the echo signal that is reconstructed by the EC  334 . Accordingly, the output of the EC FIR filter  334  is fed into a decimator  332  to decimate the reconstructed echo signal by a factor of J and provide a discrete-time sequence having a sampling rate of fs/K. The output of the decimator  332 , which represents the final reconstructed echo signal, is fed into the subtractor  346 , which removes the reconstructed echo signal from the output sequence of the decimator  316 , thereby canceling the echo signal produced by the echo channel  352 . 
   As discussed in connection with the EC FIR filter  234 , the output of the subtractor  346  may be used to produce an error signal to be fed back into the EC FIR filter  334  for adaptation. The error signal is used in an adaptation algorithm, such as one of the LMS-type algorithms known in the art. Since the error signal comprises a discrete-time sequence at a sampling rate of fs/K, the adaptive update in the LMS algorithm is carried out at the same rate of fs/K. 
   In light of the above description, it will be apparent to those of ordinary skill in the art to implement some or all of the components of the RT system  200  and CO system  300  using conventional software programming, dedicated hardware circuitry, or a combination of both. For example, the EC subsystems may be implemented using a programmable device, such as a microprocessor or an application specific integrated circuit (ASIC), that is programmed with instructions that performs the above-described functions. 
   In view of the foregoing, it will be appreciated that the invention overcomes the long-standing need for a method and system for efficiently canceling echo in modem systems, such as ADSL systems. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiment is to be considered in all respects only illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather by the foregoing description. All changes that fall within the meaning and range of equivalency of the claims are to be embraced within their scope.