Method and apparatus for reducing interference in a twisted wire pair transmission system

The present invention provides a method and apparatus for eliminating or reducing local area and broad area interference in a twisted pair transmission system. The apparatus of the present invention comprises a detection device, such as an antenna, for example, for detecting electromagnetic interference coupled into a twisted pair line, a sampling/scaling device which samples and scales the detected signal, and a combiner device which combines a signal correction component with the signal received over the twisted pair by a differential receiver. The sampling/scaling device preferably comprises an analog-to-digital converter (ADC) which converts the detected analog signal into a digital signal and a digital signal processor which receives the digital signal from the ADC and processes the signal to generate a correction signal. The correction signal is then subtracted from the signal received by the differential receiver. The sampling/scaling device may be fixed or adaptive. If the interference is stationary in time, a fixed sampling/scaling device is suitable for use with the present invention. If the interference is non-stationary, preferably the sampling/scaling device will be adaptive such that the amount of scaling performed can be altered in accordance with changes in the interference signal detected by the detector.

TECHNICAL FIELD OF THE INVENTION
 This invention generally relates to the field of communications and, more
 particularly, to a method and apparatus which detects interference in a
 twisted wire pair caused by sources external to the transmission system
 and which eliminates or reduces the effects of the interference.
 BACKGROUND OF THE INVENTION
 Twisted pairs of copper telephone wire, commonly referred to as twisted
 pairs, have been in existence since the late nineteenth century when
 Alexander Graham Bell first invented them. In fact, most of the
 transmission lines in the U.S. telephone network, commonly referred to as
 the telephone loop plant, are twisted pairs. Although much of the loop
 plant has been or is currently being upgraded with optical fiber, it is
 expected that twisted pair technology will remain in place for many years
 due to the high cost of upgrading and due to the ability to achieve high
 transmission rates over twisted pairs.
 Digital subscriber line (DSL) technology, first developed to support basic
 rate integrated services digital network (ISDN) transmission over twisted
 pair, has made it possible to achieve high transmission rates of video,
 audio and data over twisted pairs. Today, there are many variations of DSL
 technology in use, such as asymmetric digital subscriber line (ADSL),
 high-bit-rate DSL (HDSL), and rate-adaptive DSL (RADSL), which are
 collectively classified under the rubric "xDSL". All of these technologies
 correspond to line coding schemes which can be used to transmit and
 receive voice, video, and data over existing copper twisted pairs.
 Twisted pairs attenuate signals in proportion to the length of the line and
 the frequency. In order to compensate for this characteristic, telephone
 companies design lines in accordance with certain standards designed to
 maximize performance, such as requiring that wires which extend over
 certain distances be a certain gauge and requiring that loading coils be
 installed on lines in many cases. Although, attenuation generally is the
 dominant factor affecting performance of twisted pairs, cross talk also
 affects performance. Cross talk corresponds to interference in a twisted
 pair caused by an adjacent twisted pair. Cross talk increases with
 frequency and with the number of cross-talking pairs. ADSL was developed
 to reduce the effects of cross talk. In accordance with ADSL technology,
 data is transmitted upstream at one rate and downstream at a much higher
 rate than the upstream rate, with the upstream and downstream data being
 separated through the use of frequency division multiplexing.
 Another factor which affects the performance of twisted pairs is near and
 far end echo resulting from impedance differences created by telephone
 loop hybrid circuits. Telephone loop hybrid circuits are necessary in
 order to enable full duplexing of the signals transmitted between the end
 user and the central office. Echo cancellation techniques allow the echo
 to be calculated and canceled out to improve signal quality. Echo
 cancellation technology has now been widely implemented and most modems
 manufactured today contain an echo cancellation component.
 Another factor which affects the performance of twisted pairs is noise or
 interference created by sources which are external to the telephone
 network. This interference may emanate from local area sources, such as
 electrical wiring within the customer premises, or from broad area
 sources, such as power lines which run along side the telephone lines
 within the loop plant and from radio transmitting stations. It would be
 beneficial to provide a method and apparatus capable of eliminating or
 reducing local area and broad area interference in a twisted pair
 transmission system in order to maximize performance of twisted pairs.
 SUMMARY OF THE INVENTION
 The present invention provides a method and apparatus for eliminating or
 reducing local area and broad area interference in a twisted pair
 transmission system. The apparatus of the present invention comprises a
 detection device, such as an antenna, for example, for acquiring
 representative electromagnetic interference coupled into a twisted pair
 line, a sampling/scaling device which samples and scales the acquired
 signal, and a combiner device which combines a signal correction component
 with the signal received over the twisted pair by a differential receiver.
 In accordance with the preferred embodiment of the present invention, the
 sampling/scaling device comprises an analog-to-digital converter which
 converts the detected analog signal into a digital signal and a digital
 signal processor which receives the digital signal from the ADC and
 processes the signal to generate a correction signal. The correction
 signal is scaled in amplitude and phase such that, when subtracted from
 the received signal, it will cancel the additive interference that was
 present in the received signal. The correction signal is then subtracted
 from the signal received by the differential receiver from the twisted
 pair. The sampling/scaling device may be fixed or adaptive. If the
 interference is stationary in time, a fixed sampling/scaling device is
 suitable for use with the present invention. If the interference is
 non-stationary, preferably the sampling/scaling device will be adaptive
 such that the amount of scaling performed can be altered in accordance
 with changes in the interference signal detected by the detector.
 The detector may be any device capable of detecting the interference, such
 as, for example, a capacitor, an antenna, or an induction coil, or a
 combination of two or more of these. The type of detector used will depend
 on the type of interference being coupled into the twisted pair
 transmission system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 functionally illustrates the interference canceler apparatus of the
 present invention for detecting and eliminating interference coupled into
 a twisted pair transmission system 2 from a source external to the system.
 A differential transmitter 14 located at the central office 4 outputs
 signals onto twisted pair 5 to be sent to a differential receiver 15
 located at the customer premises 6. During the transmission of the signal
 to the customer premises 6, which may be a home or office, for example,
 electromagnetic interference may be coupled into the twisted pair 5. The
 interference may emanate from broad area sources (not shown), which may
 be, for example, power lines running along side the twisted pair 5 from
 the central office 4, or from local area sources (not shown), which may
 be, for example, electrical wiring inside of the customer premises 6 or a
 radio transmitting station.
 A detector 9 detects the broad or local area interference being coupled
 into twisted pair 5 and generates an analog signal relating to the
 detected interference signal. This analog signal is then input to a
 sampling/scaling device 10 which samples and scales the signal to produce
 an interference cancellation signal. The interference cancellation signal,
 or correction signal, on line 11 is scaled in amplitude and phase such
 that, when subtracted from the received signal on line 7, it will cancel
 the additive interference that was present in the received signal. The
 interference cancellation signal on line 11 is subtracted from the
 received signal on line 7 by adder 12 to eliminate the interference to
 produce output signal 13.
 Preferably, the interference canceler of the present invention is
 incorporated into a modem, such as a DSL modem. However, it will be
 apparent to those skilled in the art that it is not necessary for the
 interference canceler to be incorporated into a modem and that the
 interference canceler may be implemented separately from a modem. In
 accordance with the first embodiment of the present invention, the
 sampling/scaling device 10 and adder 12 are implemented in analog
 hardware. The signal received by differential receiver 15 is an analog
 signal. The differential receiver 15 may be a component in, for example, a
 DSL modem or some other data communication equipment (DCE). The analog
 sampling/scaling device 10 may comprise typical filter and amplification
 circuitry for sampling and scaling the received analog signal.
 In accordance with the first embodiment, the detector 9 may be implemented
 as any type of analog component or circuit designed to detect magnetic
 and/or electrical interference. The detected interference is then sampled
 and scaled by sampling/scaling device 10 and subtracted by adder 12 from
 the signal received by differential receiver 15 to eliminate or reduce the
 interference. The adder 12 can be any analog circuit or device capable of
 combining the interference cancellation signal produced by the
 sampling/scaling device 10 with the signal output from the differential
 receiver 15.
 The detector 9 may be, for example, a dipole antenna constructed to receive
 a preselected bandwidth of signals. Alternatively, it may be desirable to
 construct the detector 9 as a broadband or narrowband antenna or antenna
 array. It may also be desirable to construct the detector 9 as a scanning
 antenna array which is capable of scanning for interfering signals over a
 plurality of frequency ranges. Alternatively, it may be desirable to
 implement the detector 9 as a capacitively-coupled circuit located in
 proximity to the twisted pair 5 which detects electrical interference
 coupled into the twisted pair 5. Alternatively, it may be desirable to
 implement the detector 9 as an induction coil located in proximity to the
 twisted pair 5 which detects magnetic interference coupled into the
 twisted pair 5. It will be apparent to those skilled in the art that the
 present invention is not limited with respect to the components used to
 detect and eliminate interference coupled into twisted pair 5.
 FIG. 2 is a block diagram functionally illustrating the preferred
 embodiment of the interference canceler apparatus of the present
 invention. In accordance with this embodiment, the interference canceler
 is implemented in digital circuitry. The sampling/scaling device 10
 receives the interference signal from the detector 9 and inputs it to ADC
 16 which samples the analog signal and converts it into a digital signal.
 The digital signal is then output to a signal processor 19 which can be
 fixed or adaptive. The signal processor 19 preferably is a digital signal
 processor (DSP) which performs one or more mathematical algorithms on the
 input signal to produce an interference cancellation signal.
 It will be apparent to those skilled in the art that the present invention
 is not limited with respect to the types of operations performed by the
 signal processor 19. It will also be apparent to those skilled in the art
 that the present invention is not limited with respect to the type of
 signal processor 19 selected for use with the present invention. It should
 be noted that the signal processor 19 can be implemented solely in
 hardware, such as, for example, in a state machine or in an application
 specific integrated circuit (ASIC), or it can be implemented in a
 combination of hardware and software, such as, for example, in a
 microprocessor which is programmed with software to perform the necessary
 operations.
 It will be apparent to those skilled in the art that many variations and
 modifications can be made to the present invention without departing from
 the spirit and scope of the present invention. For example, although the
 present invention has been described with respect to its use for detecting
 and eliminating interference in twisted pairs, it will be apparent to
 those skilled in the art that the present invention may be useful for
 detecting and canceling interference in other types of transmission
 environments.