Abstract:
A system ( 242 ) processes asymmetric digital subscriber line (ADSL) signals. The system ( 242 ) receives a signal that includes an ADSL signal and transient noise ( 230 ). The system ( 242 ) isolates the transient noise ( 230 ) in the signal and retrieves the ADSL signal from the signal using the isolated transient noise ( 230 ).

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 based on U.S. Provisional Application No. 60/369,012, filed Apr. 1, 2002, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to communications systems and, more particularly, to systems and methods for detecting and suppressing transient noise in asymmetric digital subscriber line (ADSL) transmissions. 
     BACKGROUND OF THE INVENTION 
     The Internet has become an increasingly important part of our everyday lives. Millions of people now access the Internet on a daily basis to shop for goods and services, obtain information of interest (e.g., movie listings), and to communicate with friends, family and co-workers (e.g., via e-mail). This demand for Internet access has created a need for high-speed techniques for transmitting data between homes, businesses, and other institutions. One such technique is ADSL. 
     ADSL allows for high-speed data signals and analog voice signals to be simultaneously transmitted and received over existing copper telephone lines. As illustrated in  FIG. 1 , ADSL divides up the available frequencies in a telephone line into a voiceband portion  110 , an upstream data-band portion  120 , and a downstream data-band portion  130 . The voiceband portion  110  carries analog voice signals between the telephone company&#39;s central office and, for example, a customer&#39;s residence. The upstream data-band portion  120  carries data to be uploaded to the Internet from the customer&#39;s residence. The downstream data-band portion  130  carries data to be downloaded from the Internet to the customer&#39;s residence. 
     The lines carrying the ADSL transmissions to and from a customer&#39;s residence or business may be affected by transient noise that may result from bridge taps and line switching. This transient noise may increase the bit error rate of the ADSL transmissions to unacceptable limits. Telephone companies can often predict which lines will be affected by transient noise, but it is difficult to accurately predict transient noise characteristics, such as time of occurrence, rate of occurrence, amplitude in relation to the transmitted signal, duration, and the like. Therefore, it is difficult to eliminate transient noise using conventional telephone hardware and software devices. 
     As a result, telephone companies would often not use telephone lines that exhibited a great deal of transient noise for those ADSL services that are highly affected by transient noise (e.g., user datagram protocol video). The volume of and demand for ADSL service, however, is ever increasing. 
     Therefore, there exists a need for systems and methods that are capable of suppressing transient noise in ADSL transmissions. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this and other needs by providing a mechanism that suppresses transient noise in ADSL transmissions through the use of a multi-scale transformation operation, such as a wavelet transformation operation. 
     In accordance with the purpose of this invention as embodied and broadly described herein, a method for processing ADSL signals is provided. The method includes receiving a signal including an ADSL signal and noise, applying a wavelet transform operation to the signal, applying a threshold to the wavelet transformed signal, discarding a portion of the wavelet transformed signal at or below the threshold to obtain a resulting signal, applying an inverse wavelet transform operation to the resulting signal to obtain a replica signal corresponding to the noise, and subtracting the replica signal from the received signal to obtain the ADSL signal. 
     In another implementation consistent with the present invention, a method for processing an ADSL signal is provided. The method includes receiving a signal including an ADSL signal and transient noise, isolating the transient noise in the signal, and retrieving the ADSL signal from the signal using the isolated transient noise. 
     In yet another implementation consistent with the present invention, a system for processing an ADSL signal is provided. The system includes logic configured to receive a signal including an ADSL signal and transient noise, logic configured to isolate the transient noise in the signal using a multi-scale transformation operation, and logic configured to obtain the ADSL signal from the signal using the isolated transient noise. 
     In still another implementation consistent with the present invention, a system for processing an ADSL signal is provided. The system includes logic configured to receive a signal including an ADSL signal and noise and logic configured to suppress the noise in the signal using a wavelet transformation operation. 
     In a further implementation consistent with the present invention, a noise suppression device includes first logic, second logic, and third logic. The first logic receives a signal including an ADSL signal and noise. The second logic isolates the noise in the received signal. The third logic subtracts the isolated noise from the received signal to obtain the ADSL signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings, 
         FIG. 1  illustrates the possible division of bandwidth of an ADSL line; 
         FIG. 2  illustrates an exemplary system in which systems and methods consistent with the present invention may be implemented; 
         FIG. 3  illustrates an exemplary configuration of the network interface device of  FIG. 2 ; 
         FIG. 4  illustrates an exemplary process for routing ADSL signals in an implementation consistent with the present invention; 
         FIG. 5  illustrates and exemplary corrupt signal that may be received by the network interface device of  FIG. 2 ; 
         FIG. 6  illustrates the signal of  FIG. 5  after applying noise suppression processing consistent with the principles of the present invention; and 
         FIGS. 7 and 8  illustrate graphs of bit error rate versus the ratio of ADSL signal level to transient noise level in an exemplary implementation consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of implementations consistent with the present invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and their equivalents. 
     Systems and methods consistent with the present invention suppress noise in ADSL transmissions. Through the use of wavelet techniques, transient noise added to ADSL transmissions may be detected and suppressed to thereby reduce the number of bit errors present in the ADSL transmissions without adding significant latency to the transmissions. 
     Exemplary System 
       FIG. 2  illustrates a portion of an exemplary system  200  in which systems and methods, consistent with the present invention, may be implemented. System  200  includes a central office  210  connected to a customer&#39;s premises  240  via an ADSL line  220 . 
     Central office  210  may include a conventional local telephone company switching center that receives and directs telephone calls and ADSL data transmissions to their appropriate destinations. While central office  210  is illustrated as being connected to a single customer  240 , a typical central office  210  may be connected to multiple customers  240 . 
     Central office  210  may include a DSL access multiplexer  215  that receives data signals from, for example, an Internet Service Provider (ISP) and voice signals from, for example, a telephone voice switch, multiplexes the signals, and transmits the multiplexed signals to customer  240  via ADSL line  220 . It will be appreciated that central office  210  may also include a modulator (not shown) that modulates the signals prior to transmission. 
     ADSL line  220  may include any transmission medium capable of transporting broadband ADSL signals to customer  240 . ADSL line  220  may, for example, include one or more conventional copper telephone lines or other similar types of transmission media. 
     It will be appreciated that in some instances, traffic transmitted via ADSL line  220  may be affected by transient noise  230 , resulting in corrupt signals containing a mixture of ADSL signals and transient noise. This transient noise  230  may come from a variety of sources, such as bridge taps, line switching, etc. 
     Customer  240  may include a network interface device  242 , a telephone device  244 , a modem  246 , and a computer device  248 . Network interface device  242  receives the corrupt signals (i.e., the signals resulting after transient noise  230  has been added to the broadband ADSL signals transmitted from central office  210 ) via line  220 , suppresses the transient noise in the corrupt signals, separates the signals in the voiceband range from the signals in the data-band range, and transmits these signals to the appropriate device(s). For example, network interface device  242  may transmit the signals in the voiceband range to one or more telephone devices, such as telephone device  244 , and the signals in the data-band range to one or more modems, such as modem  246 . 
     Telephone device  244  may include one or more devices capable of transmitting and/or receiving analog voice communications, a facsimile device, or the like. Telephone device  244  may, for example, include a plain old telephone system (POTS) telephone. Modem  246  may include one or more devices capable of demodulating the ADSL data signals received by network interface device  242 . Modem  246  may, for example, include one or more conventional ADSL modems. Computer device  248  may include one or more devices capable of transmitting and/or receiving data communications. Computer device  248  may include, for example, a personal computer, a laptop, or the like. 
     The particular components illustrated in  FIG. 2  have been shown for simplicity. It will be appreciated that a typical system may include more or fewer components than illustrated in  FIG. 2 . Moreover, it will be appreciated that central office  210  and customer  240  may include additional or different devices than those illustrated in  FIG. 2  that aid in the reception, processing, and/or transmission of signals. 
       FIG. 3  illustrates an exemplary configuration of network interface device  242  of  FIG. 2  in an implementation consistent with the present invention. As illustrated, network interface device  242  may include an interface  310 , a noise suppression device  320 , and an optional splitter  330 . It will be appreciated that implementations consistent with the present invention are equally applicable to splitterless environments. In a splitterless environment, an RJ11 filter (or some other similar type of filter) may be used to separate the analog voice signals in the ADSL transmission from the data signals. It will be further appreciated that network interface device  242  may include additional devices than those illustrated in  FIG. 3  that aid in receiving, processing, and/or transmitting signals. 
     Interface  310  temporarily stores corrupt signals received via line  220  and passes the corrupt signals to noise suppression device  320 . Noise suppression device  320  receives the corrupt signals from interface  310  and suppresses transient noise  230  in the corrupt signals in order to regain the original ADSL signals transmitted by central office  210 . Noise suppression device  320  may include logic for performing functional operations, such as suppressing noise in received corrupt signals, as described below. This logic may include hardware, such as an application specific integrated circuit or a field programmable gate array, software, or a combination of hardware and software. 
     Splitter  330  may receive the broadband ADSL signals from noise suppression device  320  and separate the analog voice signals from the data signals and routes these signals to the appropriate device(s). For example, splitter  330  may route analog voice signals to telephone  244  and data signals to modem  246 . 
     Exemplary Processing 
     Historically, wavelet techniques have been used in image and signal processing for numerous applications. Specifically, wavelet algorithms have been designed and widely used to remove broadband noise from signals, leaving behind desired transient features. Exemplary implementations consistent with the present invention apply these “de-noising” techniques in reverse, suppressing the interfering transient noise and leaving behind the desired broadband ADSL signal. 
       FIG. 4  illustrates an exemplary process for routing ADSL signals in an implementation consistent with the present invention. Processing may begin with network interface device  242  receiving a corrupt signal from central office  210  via line  220  [act  405 ]. As described above, the corrupt signal includes the broadband ADSL signal transmitted by central office  210  combined with some type of transient noise  230 . The corrupt signal may be represented as follows:
   x[i]=s[i]+n[i],    
where s[i] represents the signal of interest (i.e., the broadband ADSL signal) and n[i] represents transient noise  230 .
 
     Network interface device  242  may then apply a wavelet transform to the corrupt signal [act  410 ]. The wavelet transform breaks up the corrupt signal into a series of scale space representations by correlating the signal with quadrature mirror filters based on a series of dilated or compressed wavelets. It will be appreciated that the wavelet transform compresses transient energy into relatively few wavelet coefficients while the broadband ADSL signal is generally spread evenly across the wavelet bases. Taking the wavelet transform of the corrupt signal yields:
 
 X   w   [k]=W{x[i]}=S   w   [k]+N   w   [k],  
 
where W{ } represents the wavelet transform operation and S w [k] and N w [k] represent the wavelet transform of the signal and noise components, respectively.
 
     Network interface device  242  may estimate the corrupt signal as: 
               Signal   =         ∑       n   2     +   1     n     ⁢              X   w     ⁢   n     0.6745            n       ,         
where n represents the number of frequency bins for the broadband ADSL transmission [act  415 ]. Network interface device  242  may then apply a hard threshold to X w  at a point above the estimated signal level [act  420 ]. The threshold may, for example, be set using:
 τ= A   * Signal * √{square root over (2log( n ))}, 
where A is a constant optimized for the basis set used in the wavelet transform. Based on the hard threshold τ, network interface device  242  may consider all values of X w  above the threshold τ as transient noise  230 .
 
     Network interface device  242  discards all wavelet coefficients that are equal to or below the threshold τ [act  425 ]. That is, network interface device  242  discards the wavelet coefficients relating to the broadband ADSL signal, thereby leaving those wavelet coefficients (i.e., those above the threshold τ) that correspond to transient noise  230 . Network interface device  242  may then apply an inverse wavelet transform to the remaining group of wavelet coefficients [act  430 ]. The resulting signal is a replica of the transient noise  230  added to the broadband ADSL signal as it was transmitted over line  220 . 
     Network interface device  242  may subtract the resulting signal from the original received signal (i.e., the received corrupt signal) to obtain a replica of the original broadband ADSL signal that was transmitted by central office  210  [act  435 ]. In this manner, network interface device  242  may suppress transient noise  230  in a received corrupted signal. It will be appreciated that the above-described processing may be performed on ADSL transmissions without adding significant latency. 
     Network interface device  242  may separate the analog voice signal from the data signal in the broadband ADSL signal in a well-known manner [act  440 ] and pass the analog voice signal and data signal on to the appropriate device [act  445 ]. For example, network interface device  242  may pass the voice signal to a telephone device, such as telephone  244 , and the data signal to a modem, such as modem  246 . 
       FIG. 5  illustrates an exemplary corrupt signal  510  that may be received by network interface device  242 . As illustrated, the spike  520  occurring at approximately the 0.24 millisecond mark is a result of transient noise being added to the broadband ADSL signal.  FIG. 6  illustrates a signal  610 , which is the signal  510  of  FIG. 5  after application of the noise suppression operation of the present invention. In this implementation, A was set to 1.2. The number of frequency bins for the ADSL signal transmission was 256. As evident from signal  610  illustrated in  FIG. 6 , the noise suppression processing of the present invention effectively removes transient noise added to a signal transmitted over an ADSL line. 
     To quantify the performance of the above-described processing, line qualification tests from ANSI T1.413 were modified. In this case, two random blocks of ADSL data were corrupted with T1.413 simulated transients. The level of the transient with respect to the ADSL signal was varied from −2 dB to 10 dB. At each transient level, the above-described processing was applied to 20 different ADSL signals (corrupted with a transient). The original and wavelet processed signals were decoded and the bit error rate for both signals was measured. The bit error rate (BER) may be defined as: 
     
       
         
           
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     Graphs of the BER versus the ration of ADSL signal level to transient noise level (SNR) for two ANSI T1.413 transients are illustrated in  FIGS. 7 and 8 . Each point in the graph represents the average BER of 20 iterations. In  FIG. 7 , line  710  represents an unprocessed signal corrupted by sample transient number  1  from ANSI T1.413 and line  720  represents the signal  710  after application of noise suppression processing consistent with the principles of the present invention. Similarly in  FIG. 8 , line  810  represents an unprocessed signal corrupted by sample transient number  2  from ANSI T1.413 and line  820  represents the signal  810  after application of noise suppression processing consistent with the principles of the present invention. It is evident from the graphs in  FIGS. 7 and 8  that significant improvement in BER is possible using noise suppression processing consistent with the present invention. For example, as evident from  FIG. 7 , greater than 60% improvement in BER is possible using the above-described noise suppression processing. 
     CONCLUSION 
     Systems and methods consistent with the present invention suppress transient noise in ADSL transmissions through the use of wavelet techniques. By using wavelet techniques, transient noise added to ADSL transmissions may be detected and suppressed to thereby reduce the number of bit errors in the ADSL transmissions without adding significant latency. 
     The foregoing description of exemplary embodiments of the present invention provides illustration and description, but is not limited to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. For example, while the above description focused on the use of wavelet processing for performing noise suppression, the present invention is not so limited. Other multi-scale transformation operations may alternatively be used. 
     Additionally, the above description described the network interface device as performing noise suppression processing consistent with the present invention. It will be appreciated that the noise suppression processing may be performed by another device at the customer&#39;s premises. Moreover, the central office may also perform noise suppression processing consistent with the present invention on traffic received from the customer. 
     The above implementations can be implemented in software, hardware, or a combination of software and hardware. Thus, the present invention is not limited to any specific combination of hardware circuitry and software. 
     While a series of acts has been described with regard to  FIG. 4 , the order of the acts may be varied in other implementations consistent with the present invention. Moreover, non-dependent acts may be implemented in parallel. No element, act, or instruction used in the description of the present application should by construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. 
     The scope of the invention is defined by the claims and their equivalents.