Abstract:
A method is provided for reducing crosstalk problems in digital subscriber loop systems. The problems are associated with a second receiver on a second loop responding to a signal from a first transmitter on a first loop. The signal is actually destined for a first receiver and is coupled between the first transmitter and the second receiver as a crosstalk signal. The method includes the step of transmitting a masking signal from a second transmitter to the second receiver for masking said crosstalk signal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
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     BACKGROUND OF THE INVENTION 
     The present invention relates generally to Digital Subscriber Loop (DSL) technology and specifically to a method for masking crosstalk from adjacent loops. 
     Remote access and retrieval of data is becoming increasingly popular in data communication. The proliferation of the Internet has provided a vast network of information that is available to the general public. As the Internet grows and technology advances, this information is becoming increasingly voluminous and the details are become increasingly intricate. What used to comprise mainly of text information has grown to include still and moving images as well as sound. The increase in volume of information to be transferred has presented a need for a high-speed Internet connection, since traditional telephone modems communicate at speeds too slow for efficient communication. 
     One proposal for high-speed communication is the introduction of Digital Subscriber Line (DSL) technology. Currently, there many different DSL standards, including Asymmetric DSL (ADSL), High-speed DSL (HDSL), Very High Speed DSL (VDSL), Single-line DSL (SDSL), Single-line, High-speed DSL (SHDSL) and Integrated Services Digital Network (ISDN) DSL systems. Generically, the term xDSL is used to represent these, and other, standards. One of the most attractive features of xDSL is that it is implemented using an infrastructure that already exists. xDSL shares copper twisted pair lines typically used for telephone communication. 
     Some DSL technologies, including SDSL, ISDN DSL, SHDDL, and HDSL are baseband schemes that cover a band (0 to 4 kHz) dedicated to Plain Old Telephone Service (POTS). Thus, these schemes cannot co-exist with POTS. However, other DSL technologies, including ADSL and VDSL, share the twisted pair with POTS. However, only a small portion of the available bandwidth of the twisted pair line is used for POTS. These schemes takes advantage of the remaining available frequency spectrum for transmitting data and, therefore, can co-exist with POTS. 
     An xDSL modem is a device that modulates and demodulates signals across an xDSL physical interface. A transceiver unit at a remote location (xTU-R) refers to a modem located at a customer&#39;s site, and a transceiver unit at a central location (xTU-C) refers to modem located in a central office (CO) or remote terminal (RT) of a loop provider. Each transceiver typically includes a transmitter and a receiver. Again, the “x” refers generically to transceivers designed for different standards. For example, for ADSL the transceivers are referred to as an ATU-R and an ATU-C. 
     In the many standards of digital subscriber loops, various protocols including activation, initiation, training, and showtime have been designed for initializing communication with between the xTU-C and xTU-R. Activation, for example, is the process of discovery of the xTU-C by the xTU-R, or vice versa, through the use of protocol specific signals. For systems designed to operate with significant loop losses, crosstalk from xDSL systems on adjacent lines can cause significant problems, especially for activation signals. Crosstalk is a disturbance caused by an electric or magnetic fields of one telecommunication signal affecting a signal in an adjacent circuit. 
     Referring to  FIG. 1 , a block diagram illustrating a system affected by crosstalk is shown generally by numeral  100 . The present example refers specifically to a non-overlapped ADSL system. That is, upstream and downstream transmit signals reside in separate, non-overlapped frequency bands. A first ADSL loop  102  connects a first ATU-C  106   a  with a first ATU-R  106   b . A second ADSL loop  104  connects a second ATU-C  108   a  with a second ATU-R  108   b . Typically, both the first and second ATU-Rs  106   b  and  108   b  are designed to be capable of responding to signals that may have experienced significant loop loss. This is true because they are designed to be able to operate on various loop lengths. 
     In the present example, the first loop  102  is longer than the second loop  104 . The first loop  102  is relatively long, thus the two transmitters on that loop, the ATU-C  106   a  downstream transmitter as well as the ATU-R  106   b  upstream transmitter, transmit at full power so as to overcome those loop losses. The second loop  104  is relatively short (for example, having loop losses on the order of 10 dB or less). However, due to the proximity of the loops  102  and  104 , as well as the proximity of the first ATU-C  106   a  and the second ATU-R  108   b , there is significant crosstalk  110  coupling from the transmitter of the first ATU-C  106   a  to the receiver of the second ATU-R  108   b . This crosstalk signal is referred to as Far End Crosstalk (FEXT), since the victim receiver (in the ATU-R  108   b ) is on the far end of the loop from the offending transmitter (in the ATU-C  106   a ). Generally, FEXT reduces as the distance increases between the victim and the offender. The crosstalk coupling loss can be on the order of 70 dB in the ADSL frequency band of interest, which is similar to the loop loss for a long loop. Therefore, the second ATU-R  108   b  may perceive the crosstalk signal from the transmitter of the first ATU-C  106   a  as a signal received from a distant ATU-C at the other end of its own loop, since the ATU-R  108   b  may not have a priori knowledge of the length of its own loop. 
     If the transmitted signal is an activation signal, the crosstalk can be falsely detected as a valid activation signal, especially where the crosstalk comes from an xDSL system of the same class. When activation signals are falsely detected, proper initialization of the transceiver falsely detecting the signal can be delayed, sometimes indefinitely. 
     Referring to  FIG. 2 , a graph illustrating a snapshot of the frequency spectrum in a non-overlapped spectra ADSL case is shown generally by numeral  200 . The ATU-R  108   b  sends activation tones  202  in the upstream band, but the ATU-C  108   a  is not sending any tones in the downstream band. In this figure, the crosstalk signals  204  shown in the downstream band may cause the ATU-R  108   b  transceiver to become confused and attempt to activate the line. 
     Therefore, there is a need for a method of inhibiting a transceiver from responding to a crosstalk activation signal. It is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a method for reducing crosstalk problems in digital subscriber loop systems associated with a second receiver on a second loop responding to a signal from a first transmitter on a first loop, wherein the signal is destined for a first receiver and is coupled between the first transmitter and the second receiver as a crosstalk signal. The method includes the step of transmitting a masking signal from a second transmitter to the second receiver for masking the crosstalk signal. The method may also be implemented in hardware or via a computer program. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only with reference to the following drawings in which: 
         FIG. 1  is a block diagram of two ADSL loops where FEXT is a problem (prior art); 
         FIG. 2  is a graph illustrating a frequency spectrum snapshot for a non-overlapped ADSL system (prior art); 
         FIG. 3  is a graph illustrating a frequency spectrum snapshot for a non-overlapped ADSL system in accordance with an embodiment of the present invention; 
         FIG. 4  is a block diagram of two ADSL loops where NEXT is a problem (prior art); 
         FIG. 5  is a block diagram of a termination unit that includes an embodiment of the present invention; 
         FIG. 6  is a block diagram of two ADSL loops including the termination unit of  FIG. 5 ; and 
         FIG. 7  is a flowchart of a method according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For convenience, like numerals in the description refer to like structures in the drawings. A signal is provided for masking a crosstalk signal so that only intended activation signals are detected by a transceiver and initiation can proceed normally. Referring once again to  FIGS. 1 and 2 , the crosstalk can be masked by sending an appropriate broadband random signal, referred to as a masking signal. The masking signal is transmitted across the frequency range of the ATU-R receiver  108   b  likely to be affected. The masking signal is preferably uncorrelated with standard ADSL activation, initialization, and showtime signals, and is transmitted at a level high enough to mask the crosstalk signals. For the ADSL system illustrated in  FIG. 1 , it is convenient to employ a MEDLEY signal as the crosstalk masking signal. MEDLEY is an aperiodic sequence that is also employed during normal ADSL link initialization. When used without a pilot tone or cyclic prefix, the MEDLEY signal is a good approximation of a broadband random signal. 
     Continuing the example described with reference to  FIG. 1 , the first ATU-C  106   a  transmits a valid ADSL downstream signal, potentially including pilot tone, at a nominal power level of −40 dBm/Hz. As previously described, the second ATU-R  108   b , detects the downstream signal from the first ATU-C  106   a , in the form of crosstalk  110 . The coupling loss associated with the crosstalk  110  is 70 dB. Thus, the second ATU-R  108   b  receives the downstream signal from the first ATU-C  106   a  at a power level of approximately −110 dBm/Hz, which is similar to the level that it would expect from a valid ATU-C on a long loop. 
     However, in accordance with the present embodiment, a masking signal is transmitted from the second ATU-C  108   a  to the second ATU-R  108   b . Assuming worst case crosstalk levels of 60-70 dB below nominal transmitter levels, the masking signal is sent at transmit power spectral density (PSD) levels 30-40 dB below nominal transmit levels. Typically, transmitting the mask at such a power level is sufficient to mask the crosstalk signals  110  at the second ATU-R  108   b  and thus inhibit false activation. This is true provided the loop loss on the second loop  104  is modest. Barring poor loop quality, the loop loss will be modest because the loop is typically short. Otherwise, the effect of the FEXT would be less significant. 
     Referring to  FIG. 3 , a graph illustrating a snapshot of the frequency spectrum in a non-overlapped spectra ADSL including a masking signal is illustrated generally by numeral  300 . Assuming that the masking signal is transmitted at a power level of −70 dBm/Hz and that the second loop  104  has a loop loss of 10 dB, the received power level of the masking signal is −80 dBm/Hz. In contrast, the crosstalk  204  is received by the second ATU-R  108   b  at a level of −110 dBm/Hz. Thus, the second ATU-R  108   b  perceives only an elevated, artificial receive noise floor  302 , with no signals correlated to a possible activation signal. The second ATU-R  108   b  is not affected by the correlated crosstalk signal, since it is effectively hidden by the masking signal. 
     When the second ATU-C  108   a  does transmit a standard activation signal, it will transmit at its nominal level of about 30-40 dB above the artificial noise floor. Since the loop loss is only 10 dB, the ATU-R  108   b  will have little problem detecting the activation signal and permitting proper activation and initialization. 
     In order to reduce unnecessary power consumption, as well as for other reasons, it is preferable that the ATU-C  108   a  transmits the masking signal only when it is suspects that the ATU-R  108   b  is responding to a crosstalk activation signal. How this is determined is dependent on the xDSL protocol used. For the example illustrated in  FIG. 1  of an ADSL ATU-R  108   b  responding to downstream far-end crosstalk, the ATU-C  108   a  on the victim system, or second loop  104 , determines from an upstream receive signal sequence that the victim ATU-R  108   b  has advanced in the activation sequence in the absence of its own required corresponding signal. The ATU-C  108   a  infers that this has occurred because the ATU-R  108   b  has misinterpreted crosstalk as a valid activation signal or response. As a result, the ATU-C  108   a  transmits the masking signal to the ATU-R  108   b.    
     Unable to achieve a valid activation response, the ATU-R  108   b  returns to the start of activation, but is now no longer able to detect the crosstalk signal which is now masked by the masking signal generated by the victim system ATU-C  108   a . As a result, the ATU-R  108   b  continues to search for a valid activation signal/response that may be detected above the masking signal. 
     Although the above description refers specifically to ADSL technology, it will be apparent to a person skilled in the art that utilization of a crosstalk masking signal may be used in other xDSL systems. 
     Yet further, although the above description refers specifically to the use of the MEDLEY signal, at a reduced transmit PSD level, as the masking signal, other signals are also applicable. Any broadband signal, bandlimited to the ADSL downstream band for spectral compatibility, that is uncorrelated to the expected activation signals may be adequate to serve as the crosstalk masking signal. MEDLEY is one such a signal and also happens to be easily generated by any ATU-C transmitter. 
     In the non-overlapped ADSL case illustrated in  FIG. 1 , false activations are due to far-end crosstalk into ATU-R receivers on short loops only. Referring to  FIG. 4 , a block diagram of an xDSL system  400  having overlapped spectra is illustrated generally. The systems includes a loop  402  coupling an xTU-C  406   a  and an xTU-R  406   b , and a loop  404  coupling an xTU-C  408   a  and an xTU-R  408   b . For such a case, the crosstalk encountered is near-end crosstalk (NEXT)  410 , which can occur at significant levels even where the victim loops are long. In such systems, the effectiveness of crosstalk-masking is limited to victim systems on short to medium length loops where the loop losses on the crosstalk masker do not prevent it from being able to mask the crosstalk at the victim receiver. 
       FIG. 5  is a block diagram of an improved transceiver unit (TU)  500  according to an embodiment of the present invention. The TU  500  sends and receives information on the loop. The TU  500  includes a transmitter  502 , a receiver  504 , a processor  506 , and a memory  508 . The transmitter  502  and receiver  504  transmit and receive the xDSL signals on the loop. The processor  506  controls the TU  500 , processing information and generating the various control signals. The memory  508  stores data used by the TU  500  and the processor  506 , and can also store programs executed by the processor  506 . Further functions and features of the TU  500  are as described above in more detail. 
       FIG. 6  is a block diagram of an improved system  600  according to an embodiment of the present invention.  FIG. 6  is similar to  FIG. 1  with the addition of the TU  500  from  FIG. 5 . Operation of the system  600  is discussed below regarding  FIG. 7 . 
       FIG. 7  is a flowchart of a method  700  according to an embodiment of the present invention. The steps of the method  700  are described below with reference to  FIG. 6 . In step  702 , a DSL signal is transmitted on the loop  102  from the ATU-C  106   a  to the ATU-R  106   b . In step  704 , the DSL signal is perceived as crosstalk  110  on the loop  104  by the ATU-R  108   b . The ATU-R  108   b  then responds to the crosstalk  110 . In step  706 , the ATU-C  500  detects the response by the ATU-R  108   b . In step  708 , the ATU-C  500  transmits the masking signal on the loop  104 . The ATU-R  108   b  then no longer responds to the crosstalk because the crosstalk is masked. More details regarding these steps have already been provided above and are not repeated. 
     The method  700  may be implemented by the TU  500  of  FIG. 5 . In such a case, a computer program implementing the method may be stored in the memory  508  and executed by the processor  506 . The memory  508  may be a random access memory or any other type of storage (including magnetic memory, floppy disk, optical disk, flash memory, read-only memory, etc.) according to design factors. The processor  506  may be a general-purpose processor or may be a specialized processor for DSL applications. The computer program may be stored on other types of computer-readable media (including magnetic memory, floppy disk, optical disk, flash memory, read-only memory, etc.) prior to being loaded onto the TU  500 . 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.