(b) Description of the Related Art Demand for multimedia service supply through a communication network has recently increased, and xDSL (digital subscriber line) methods for providing data rates of from several hundred kbps to several tens of Mbps using conventional copper telephone cables without amplifiers or repeaters have been developed so as to satisfy the demand.

The xDSL has been developed to HDSL (high-data-rate DSL), SDSL (single-line HDSL), ADSL (asymmetric DSL), UADSL (universal ADSL), and VDSL (very-high-bit-rate DSL) for fast data transmission within a short range of from 300 to 1,500m.

Modulation and demodulation methods for the xDSL include: the CAP (carrierless AM/PM) method and the QAM (quadrature amplitude modulation) method, respectively using the SCM (single-carrier modulation) method; and the DMT method using the MCM (multi-carrier modulation) method.

The DMT method divides a total transmission band into a plurality of

narrowband subchannels and transmits them, to thereby increase the transmission period of each subchannel by the number of subchannels, and to compensate for channel distortion through a simple single tap equalizer. Also, by adding a cyclic prefix as a guard interval to a DMT symbol, the method maintains orthogonality between the subchannels and removes inter-symbol interference, thereby providing a simple equalizer configuration at a receiver part. Further, the method can realize high-speed modulation and demodulation processes using the IFFT (inverse fast Fourier transform) and the FFT (fast Fourier transform).

Major factors of attenuating high-speed data rates in the xDSL system environment include : transmission line attenuation by which signals are propagated following a transmission line and the signals are attenuated because of various factors; and crosstalk such as NEXT (near end crosstalk) generated between close transmission lines, and FEXT (far end crosstalk).

Crosstalk occurs when different electrical signals transmitted by various remote transmitters influence each other according to electromagnetic inter- operation between UTPs (unshielded twisted pair) in the same cable. The NEXT is an effect made by signals transmitted from a transmitter within the same section in the same side of one of an ONU (optical network unit) transmit/receive unit and a remote site transmit/receive unit, and the FEXT is an effect made by the signals transmitted from the transmitter on the opposite side.

Among them, the attenuation caused by the transmission line is compensated by an equalizer. The NEXT greatly lowers the performance of the

systems when uplink signals and downlink signals use the same frequency band, and accordingly, the NEXT can be completely removed by use of an FDD (frequency division duplex) that uses different frequency bands respectively for the uplink and downlink signals.

As to techniques for canceling the FEXT, methods applicable to single carrier systems have been proposed, but FEXT canceling methods useful for multi-carrier systems have not yet been proposed because of complexity of the systems.

SUMMARY OF THE INVENTION It is an advantage of the present invention to cancel the FEXT (far end crosstalk) generated from an adjacent transmission line in the DMT system to improve the system performance.

In order to improve the system performance lowered by the FEXT generated from the adjacent transmission line when each signal of a remote site transmitter passes through channel, a technical method for providing an FEXT canceller and a technical method for canceling the FEXT more effectively are provided.

In one aspect of the present invention, a DMT system includes an ONU receiver connected to a first transmission line from a remote site transmitter, for receiving data signals to which FEXT from at least one second transmission line is combined, wherein the ONU receiver comprises: an equalizer for using a tap coefficient of the equalizer to compensate for

transmission line attenuation in the data signals to which the FEXT is combined; and an FEXT canceller for using a tap coefficient of the FEXT canceller and a decision value of the data transmitted through the second transmission line to calculate the influence caused by the FEXT, and wherein the FEXT is cancelled by subtracting the output of the FEXT canceller from the output of the equalizer.

In another aspect of the present invention, a DMT system includes at least one first ONU (optical network unit) receiver connected to a first transmission line from at least one remote site transmitter, and a second ONU receiver connected to a second transmission line different from the first transmission line, for receiving data signals to which FEXT from the first transmission line is combined, wherein the first ONU receiver comprises: a first equalizer for compensating for transmission line attenuation in the data signals received through the first channel; and a first slicer for performing decision- making on the output of the first equalizer to find a decision value, and the second ONU receiver comprises: a second equalizer for using a tap coefficient of the second coefficient to compensate for channel attenuation in the data signals to which the FEXT is combined; and an FEXT canceller for using a tap coefficient of the FEXT canceller and a decision value of the first ONU receiver to calculate the influence caused by the FEXT, and the FEXT is cancelled by subtracting the output of the FEXT canceller from the output of the equalizer.

The second ONU receiver further comprises a second slicer for performing decision-making on the value obtained by subtracting the output of

the FEXT canceller from the output of the second equalizer to find a decision value.

The tap coefficient of the FEXT canceller is converged by a factor for determining the convergence rate of the FEXT canceller, a decision error of the second ONU receiver, and a decision value of the first ONU receiver. In detail, the tap coefficient of the FEXT canceller is continuously updated through the subsequent equation to converge to a predetermined value.

FC (n)-, B x E, (n) x b (n) where FC (n) is a tap coefficient of the FEXT canceller, ? is a factor for determining the convergence rate of the FEXT canceller, E2(n) is a decision error of the second ONU receiver, and b (n) is a decision value of the first ONU receiver.

The tap coefficient of the second equalizer is converged by a factor for determining the convergence rate of the second equalizer, a decision error of the second ONU receiver, and a decision value of the second ONU receiver.

In detail, the tap coefficient of the second equalizer is continuously updated through the subsequent equation to converge to a predetermined value.

EQ(n)-ß'#E2(n)#â(n) where EQ (n) is a tap coefficient of the second equalizer, 8'is a factor for determining the convergence rate of the second equalizer, E2 (it) is a decision error of the second ONU receiver, and a (n) is a decision value of the second ONU receiver.

The second ONU receiver further comprises a third slicer for performing decision-making on the output of the second equalizer to find a decision value, and the tap coefficient of the second equalizer is converged by a factor for determining the convergence rate of the second equalizer, a decision error of the second equalizer, and a decision value of the second ONU receiver. In detail, the tap coefficient of the second equalizer is continuously updated through the subsequent equation to converge to a predetermined value.

EQ(n)-ß'#E1(n)#â(n) where EQ (n) is a tap coefficient of the second equalizer, 3'is a factor for determining the convergence rate of the second equalizer, El (n) is a decision error of the second equalizer, and a (n) is a decision value of the second ONU receiver.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: FIG. 1 shows a block diagram of a DMT system for canceling the FEXT according to first and second preferred embodiments of the present invention; and FIGs. 2 and 3 respectively show an operation of an FEXT canceller in

the DMT system according to the first and second preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor (s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.

A DMT system for canceling the FEXT according to first and second preferred embodiments of the present invention will now be described referring to FIG. 1.

FIG. 1 shows a block diagram of a DMT system for canceling the FEXT according to the first and second preferred embodiments of the present invention.

As shown, the DMT system comprises a plurality of remote site transmitters 100 and an ONU (optical network unit) receiver 200. The remote site transmitter 100 comprises a mapper 110, a serial-to-parallel converter 120, an IFFT (inverse fast Fourier transformer) 130, a parallel-to-serial converter 140, and a framer 150. The ONU receiver 200 comprises a plurality of receivers 200a respectively connected to a plurality of transmission lines from the remote site transmitters 100. The receiver 200a comprises a deframer 210, a serial-to-

parallel converter 220, an FFT (fast Fourier transformer) 230, an equalizer 240, an FEXT canceller 250, a parallel-to-serial converter 260, and a demapper 270.

The mapper 110 allocates bit stream type data signals to each subchannel to constellate them. According to the constellation, the data signals are mapped to X and Y coordinate values corresponding to bit values, and the mapped complex values are generally referred to as the original constellation.

The X and Y coordinate values have an (x, y) format and they are generally expressed in the complex format"x+yi". The data signals mapped by the mapper 110 are converted into parallel signals by the serial-to-parallel converter 120, and input to the IFFT 130 to be modulated.

The parallel-to-serial converter 140 converts the data signals modulated by the IFFT 130 into serial signals for serial transmission. The framer 150 adds a prefix for orthogonality between subchannels to the serial signals, performs widowing for preventing inter-symbol interference on them, and transmits result signals.

The signals transmitted from the remote site transmitter 100 are passed through the channel 300 to the ONU receiver 200. In this instance, since each remote site transmitter 100 transmits signals to the ONU receiver 200 through different transmission lines, the FEXT sent from another adjacent transmission line is added to the original data signals, and sent to the ONU receiver 200.

The deframer 210 of the ONU receiver 200 removes the prefix added by the framer 150 for maintaining the orthogonality from the signals, and transmits them to the serial-to-parallel converter 220. The data signals from

which the prefix is removed by the deframer 210 are converted into parallel signals by the serial-to-parallel converter 220, and input to the FFT 230 to be demodulated.

The equalizer 240 comprises a single tap, and it is connected to each subchannel to compensate for transmission line attenuation corresponding to each subchannel. The FEXT canceller 250 including a single tap in the like manner of the equalizer 240 is connected to each subchannel, and receives each output of the receiver 200a connected to another transmission line to compensate for the influence caused by the FEXT.

The influence-compensated data signals are converted into serial signals by the parallel-to-serial converter 260, and input to the demapper 270.

The demapper 270 converts the complex data signals into bit stream signals, and includes a slicer 271 in the front part of the demapper 270. The slicer 271 performs a decision-making process for finding an original constellation that is the nearest to the coordinate value of the input data signals. A decision error is found by subtracting the original constellation that is an output value from the input value of the slicer 271, and the decision error is used for determining a tap coefficient of the FEXT canceller 250.

In general, in order for the DMT system to transmit data, it is required to perform an initialization process which comprises an activation (handshake) process, a training process, and a channel analysis and exchange process. The activation (handshake) process checks whether a transmitter is ready to transmit signals to a receiver. The training process performs symbol

synchronization and equalizer training. The channel analysis and exchange process measures the SNR (signal to noise ratio) for each subchannel to generate a bit table having bit loading information, and allows the transmitter and the receiver to exchange determined various parameters.

The training process performs estimation of tap coefficients that are transfer functions of the equalizer 240 and the FEXT canceller 250.

Referring to FIGs. 2 and 3, a method for canceling the FEXT in the DMT system according to the first and second preferred embodiments of the present invention will be described.

FIGs. 2 and 3 respectively show a detailed operation of an FEXT canceller in the DMT system according to the first and second preferred embodiments of the present invention.

For ease of description, a case of receiving data signals from two transmission lines, which are respectively connected to the two remote site transmitters 100 is assumed, and since the subchannels are independent and they do not interfere with each other, subchannel values of the remote site transmitter 100 and the ONU receiver 200 can be matched individually. For convenience of explanation, a single subchannel is used.

The method for canceling the FEXT according to the first preferred embodiment is different from that of the second preferred embodiment in estimation of the tap coefficients of the FEXT canceller 250. In the method for canceling the FEXT according to the first preferred embodiment, the equalizer 240 and the FEXT canceller 250 use different decision errors to converge the

tap coefficients and cancel the FEXT, and in the method for canceling the FEXT according to the second preferred embodiment, the equalizer 240 and the FEXT canceller 250 use the same decision error to cancel the FEXT.

As shown in FIGs. 2 and 3, transmit signals a (n) and b (n) from the remote site transmitters 100 are transmitted to the ONU receiver 200 through respective transmission lines. In this instance, the transmit signals b (n) passed to another transmission line through which the transmit signals a (n) are transmitted, and become FEXT.

EQ (n) and FC (n) respectively indicate the transfer functions of the equalizer 240 and the FEXT canceller 250, and the transfer functions are updated until they are converged. CHa and CHb respectively represent the transfer functions of the transmission line attenuations in the transmission lines that transmit the transmit signals a (n) and b (n). FX shows the transfer function when the transmit signals b (n) are changed into the FEXT of the transmit signals a (n), and the above-noted transfer functions are values that rarely change with respect to time.

First, referring to FIG. 2, a method for canceling the FEXT according to the first preferred embodiment of the present invention will now be described.

As shown in the DMT system, a receiver of the ONU receiver 200 comprises equalizers 241 and 242 connected to the data signals a (n) and b (n) from the respective remote site transmitters, and an FEXT canceller 251 connected to an output end of the equalizer 241, for canceling the FEXT caused by b (n). Slicers 271 and 272 are provided at output ends of the

equalizers 241 and 242 to decide an original constellation from the output values of the equalizers 241 and 242. A slicer 273 for deciding the original constellation from the value from which the FEXT is removed by the FEXT canceller 251 at the output of the equalizer 241 is further provided.

The tap coefficient E!a (n) that is a transfer function of the equalizer 241 is updated to a new tap coefficient EQa (ra+1) as per the following Equation 1.

Equation 1 EQa (n + 1) = EQa (n)-ß #E2(n) x a (n) where ? is a step size for determining a convergence rate of the equalizer 241, E2 (n) is a decision error of the equalizer 241, which is a difference between an output value and an input value of the slicer 271, and a (n) is an estimate of a (n) by the ONU receiver 200, that is, a value decided by the slicer 273.

The tap coefficient FC(n) which is a transfer function of the FEXT canceller 251 is updated to a new tap coefficient FC (n+1) as per the following Equation 2.

Equation 2 FC (n + 1) = FC(n)-ß'#E1(n) x b (n) where ß' is a step size for determining the convergence rate of the FEXT canceller 251, E, (n) is a decision error which is a difference between an output value and an input value of the slicer 273, and b (n) is an estimate of b (n) by the ONU receiver 200, that is, a value decided by the slicer 272.

The tap coefficients EQa (n) and FC (n) of the equalizer 241 and the FEXT canceller 251 are updated for each symbol to converge into EQa and FC according to Equations 1 and 2. Therefore, when a sufficient time passes and the tap coefficients of the equalizer 241 and the FEXT canceller 251 are respectively converged, the FEXT caused by another transmission line is added to the transmit signal a (n) transmitted from the remote site transmitter 100 to generate al (n) which is a signal before input to the equalizer 241 of the ONU receiver 200 as expressed in Equation 3.

Equation 3 al (n) = a (n) x CHa + b (n) x FX Next, the transmission line attenuation is compensated by the equalizer 241, and hence, al (n) becomes a2 (n) as shown in Equation 4.

Equation 4 a2 (n) = a (n) x CHa x EQa + b (n) x FX x ES.

Since the FEXT operating as noise to a2 (n) is generated by the signal b (n) transmitted from another transmission line, the FEXT is cancelled using b (n) that is an estimate of b (n). That is, the signal a3 (n) before input to the slicer 273 is given as Equation 5.

Equation 5 a3 (n) = a (n) x CHa x EQa+b(n) x FX x EQa-#(n) x FC Since CHa is a transfer function of transmission line attenuation, and

EQa is a transfer function for compensating transmission line attenuation, it is satisfied that EQa = CHa-1. Since b (n) is an estimate of b (n), it is satisfied that #(n)#b(n), and since FC that is a transfer function of the FEXT canceller 251 converges to FX#EQa, it is satisfied that FC #FX#EQa. Therefore, Equation 5 is given as Equation 6, and only a (n) with the cancelled FEXT is provided, thereby canceling the FEXT.

Equation 6 a3 (n) # a (n) x CHa x CHa-l + b (n) x F X x EQa-b (n) x FX x EQa = a (n) Next, referring to FIG. 3, a method for canceling the FEXT according to the second preferred embodiment of the present invention will be described.

Differing from the first embodiment, the method for canceling the FEXT according to the second preferred embodiment allows the equalizer 241 and the FEXT canceller 251 to use the same decision error. Therefore, no slicer for finding the decision error E2 (n) of the equalizer 241 is needed. That is, the decision error E2 (n) used for the equalizer 241 has the same value as the decision value EI (n) used for the FEXT canceller 251. Accordingly, the respective tap coefficients EQa (n) and FC (n) of the equalizer 241 and the FEXT canceller 251 are updated to new tap coefficients EQa(n+1) and FC (n +1) according to Equation 7, and converged to EQa and FC.

Equation 7

EQa (n + 1) = EQa (n)-ß x E1 (n) x â(n) FC (n + 1) = FC ()-'x Ei () x b (n) The subsequent process is progressed as described in Equations 3 through 6 to cancel the FEXT. By using the same decision error at the equalizer 241 and the FEXT canceller 251, the tap coefficient FC of the FEXT canceller 251 converges more closely to FX#EQa.

In the first and second preferred embodiments, it is assumed that data signals are received from the transmission lines connected to two remote site transmitters, and a single subchannel is used, but the present invention is not restricted to this. A process for canceling the FEXT generated when receiving the data signals from a plurality of remote site transmitters can be easily understood by a skilled person from the above-noted descriptions, and hence, no description is provided.

According to the present invention, by adding an FEXT canceller to the DMT system, the FEXT caused by adjacent transmission lines can be cancelled, and accordingly, the data rates and the SNR are improved to enhance the system performance.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.