Method, system and device for xDSL crosstalk cancellation

A method and system for xDSL crosstalk cancellation is provided. The method includes dividing xDSL signals into a plurality of signal sets; and connecting signals from a same signal set to a same processing unit to be processed. A digital subscriber line access multiplexer (DSLAM) includes a line switching control module and at least one processing unit.

BACKGROUND

1. Field of the Disclosure

The present invention relates to the field of digital subscriber line (DSL) technology, and more particularly to a method, system, and device for crosstalk cancellation of multi-pair xDSL.

2. Discussion of the Related Art

A pass-band transmission xDSL adopts discrete multi-tone modulation (DMT) technology for modulation and demodulation. A system for providing multiple DSL access is referred to as a digital subscriber line access multiplexer (DSLAM), a connection relation of which is shown inFIG. 1. The subscriber end xDSL transceiver120includes a subscriber end transceiver unit121and a splitter/integrator122. In an uplink direction, the subscriber end transceiver unit121receives and amplifies a DSL signal from a computer110, and sends the amplified DSL signal to the splitter/integrator122. The splitter/integrator122integrates the DSL signal from the subscriber end transceiver unit121with a plain old telephone service (POTS) signal from a telephone terminal130. The integrated signal is transmitted through multiple unshielded twisted pairs (UTPs)140and received by a splitter/integrator151in a center office end xDSL transceiver150. The splitter/integrator151splits the received signal, sends the POTS signal to a public switched telephone network (PSTN)160and sends the DSL signal to a center office end transceiver unit152of the center office end xDSL transceiver150. The center office end transceiver unit152re-amplifies the received xDSL signal and then sends it to a network management system (NMS)170. In a downlink direction, signal is transmitted in a sequence reverse to the above processes.

As a frequency band adopted in the xDSL technology is continuously increased, the crosstalk becomes increasingly severe, especially in high frequency bands. Referring toFIG. 2, because uplink and downlink channels of the xDSL adopt frequency division multiplexing technology, a near-end crosstalk (NEXT) does not cause significant influences to the system performance; however, a far-end crosstalk (FEXT) brings severe impacts on the transmission performance of the lines. When xDSL services are activated in a bundle of cables upon being requested by a plurality of subscribers, certain lines may suffer from a low transmission rate and an instability problem; even the xDSL services may not be activated due to FEXT, which results in a low line activation rate of the DSLAM. For example, according to current technical standards for xDSL, theoretically, VDSL2 (vectored-DSL) can provide an uplink-downlink symmetrical rate of up to 100 Mbps. However, an obvious problem may occur during the actual deployment due to FEXT and high frequency signal attenuation.

Currently, a vectored-DSL technology has been proposed in the industry, which mainly uses the DSLAM terminals to perform joint transmitting and receiving, so as to cancel the interference of FEXT by means of signal processing, thereby eventually enabling each signal to be free of FEXT interference.

FIG. 3shows a situation where a center office end jointly sends and subscriber ends respectively receive downlink vectors. The process of receiving downlink vectors is described as follows.1. A matrix HTis expressed as HT=Qi·Riaccording to QR decomposition. Herein, R is an upper triangular matrix; Q* is a unitary matrix, i.e., QQ*=Q*Q=1, in which the superscript * represents a conjugate transpose; HTis a transpose matrix of H. Accordingly, H=RTQT.

2. It is assumed that xi=QiT*xi′, and xi′=Ri−Tdiag(RiT){tilde over (x)}i, in which diag represents a diagonalizable matrix.

If yi=Hixi+Ni=RiTQiTQiT*Ri−Tdiag(RiT){tilde over (x)}i+Ni=diag(RiT){tilde over (x)}i+Ni, as for a noiseless channel, an output is turned to be ŷ=diag(RiT){tilde over (x)}i, which is a diagonal matrix, thereby canceling the crosstalk.

FIG. 4shows a situation where the subscriber ends respectively send and the center office end jointly receives uplink vectors. The process of receiving uplink vectors is described as follows.1. The matrix H is expressed as Hi=Qi·Riaccording to QR decomposition. Herein, R is an upper triangular matrix; Q is a unitary matrix, i.e., QQ*=Q*Q=1, in which the superscript * represents a conjugate transpose.2. An uplink receiving end is:
Yi=Hixi+Ni(1),

Both sides of Equation (1) are multiplied by Q*, so as to obtain the following equation:
Ŷi=Q*(Hixi+Ni)  (2).
Accordingly,
Ŷi=Q*·Q·Rixi+Q*·Ni=Rixi+Q*·N(3).

As seen from Equation (3), as for a noiseless channel, an output is Ŷi=Rixi,1≦i≦L, which is an upper triangular matrix.3. An output value is estimated through using generalized decision feedback equalization (GDFE).

It can be seen that, the Lthoutput is a value without crosstalk and can be estimated by using a simple decoder, so as to obtain the Lthoutput value. By means of subtracting the Lthestimated result from the (L−1)thoutput, the crosstalk of the (L−1)thtone caused by the Lthtone is cancelled. Through simple estimation, the (L−1)thoutput value can be obtained, and so forth. Therefore, the first output value is obtained by subtracting the previously estimated value, and the ISI (Inter Symbol Interference) is thus cancelled.

The shared channel H inFIGS. 3 and 4may be expressed as a matrix:

H(f)=[Hkm(f)]k=1 . . . L,m=1 . . . L, in which Hkm(f) is a propagation equation from a pair m to a pair k. In practice, k is equal to m and both pairs are equal to the number of channels involved in a crosstalk effect on each other in the shared channel, which is set as L herein. Thus, H is a L×L channel transmission matrix. A processor processes the L×L channel transmission matrix, so as to cancel the interference of FEXT.

A typical DSL bundle generally consists of 50 to 100 twisted pairs. If it intends to cancel all of the crosstalk, the processor generally needs to process a H matrix of 50×50 or 100×100, which exceeds the current computation complexity constraints of digital signal processing at the centre office (CO) end.

SUMMARY

A method for xDSL crosstalk cancellation is provided, which includes the following steps: A plurality of xDSL signals is divided into two or more signal sets. Signals belonging to the same signal set are connected to the same processing unit in a digital subscriber line access multiplexer (DSLAM) to be processed. An xDSL system is also provided, which includes a line switching control module and a DSLAM. The line switching control module is adapted to divide a plurality of xDSL signals into two or more signal sets and connect signals belonging to the same signal set to a same processing unit in the DSLAM. The DSLAM includes at least one processing unit. Each of the processing units is respectively adapted to process signals belonging to the same signal set. A digital subscriber line access multiplexer (DSLAM) is also provided, which includes a line switching control module and a plurality of processing units. The line switching control module is adapted to divide a plurality of xDSL signals into two or more signal sets and transmit signals belonging to the same signal set to the same processing unit. Each of the processing units is adapted to process the signals belonging to a same signal set.

In the embodiments of the present invention, signals carried by all pairs in a bundle are divided into several signal sets, and signals in each signal set are processed by one processing unit, so that numbers of rows and columns in a channel transmission matrix processed by each processing unit are far less than those in the prior art. Through using the technical solutions of embodiments of the present invention, the computing operation of the transmission matrix on the processor is simplified, and the crosstalk cancellation is also achieved.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the embodiments of the present invention, first, a plurality of signals carried by all pairs in a bundle are divided into several signal sets based upon the principles that the signal sets are mutually exclusive and the crosstalk generated between the signals in each divided signal set is rather significant. Then, signals belonging to the same signal set are connected by a line switching control module to the same processor in a digital subscriber line access multiplexer (DSLAM) to be processed. Numbers of rows and columns in a channel transmission matrix processed in each processor are far less than those in the prior art.

A method for crosstalk cancellation provided in an embodiment of the present invention includes the following steps.

1. Signals carried by all pairs in a bundle are divided into a plurality of signal sets.

In a DSL system, as for a particular subscriber, most crosstalk is only generated from few other subscribers in the system. Therefore, M signal sets may be selected from a bundle (assuming that a bundle consists of L signals) by a selection algorithm based on lines, tones, or combination of lines and tones, and are stored at a DSLAM end. The selected M signal sets meet the following requirements: Crosstalk between the signals in each signal set is rather significant, the signal sets are mutually exclusive, each signal set contains L/M signals, and M is exactly divisible by L. In addition, because channels performance slowly changes as time elapses and as the subscribers log in or log out, crosstalk between the subscribers changes dynamically. After the signals have been divided into the signal sets according to the above method, if the crosstalk between the subscribers changes, a channel monitoring system is used to monitor and collect crosstalk information, so as to obtain information about crosstalk variation between the subscribers and transmit the information to a line switching control module. The line switching control module re-divides the signals carried by the pairs in a bundle into a plurality of signal sets according to the information about crosstalk variation. The information about crosstalk variation includes crosstalk variation information caused by the subscribers logging in or log out and/or by the channels performance slowly changing as time elapses.

The signals in a bundle may be divided into a plurality of signal sets by a variety of algorithms such as a greedy algorithm. The signals are divided in such a way that the divided signal sets are mutually exclusive, and crosstalk between the signals in each signal set is rather significant.

2. Signals belonging to the same signal set are connected, by the line switching control module, to the same processor in the DSLAM for a computing operation. The number of rows and the number of columns in a channel transmission matrix processed in each processor are all L/M.

In an embodiment of the present invention, the line switching control module may be a switching matrix. The switching matrix generally refers to a matrix that has a plurality of output options in the case of a plurality of inputs, so as to form a matrix structure as shown inFIG. 5. In other words, each output can be connected to different input signals through switching.

A system in an embodiment of the present invention includes a DSLAM and a switching matrix.

The switching matrix is adapted to divide a plurality of xDSL signals into two or more signal sets and transmit signals belonging to the same signal set to the same processor in the DSLAM to be processed.

The DSLAM includes two or more processors. Each processor is adapted to process signals belonging to the same signal set.

Detailed descriptions are given below with reference to the accompanying drawings.

In the embodiments of the present invention, it is assumed that a bundle consists of 24 pairs of lines for transmitting 24 signals, the 24 signals are divided into 6 signal sets, each signal set contains 4 signals, the signal sets are mutually exclusive, and crosstalk between the signals in each signal set is rather significant.

The Switching Matrix Adopts a Relay Matrix

The 24 signals are divided into 6 signal sets by a greedy algorithm. If crosstalk between a subscriber1(pair1), and a subscriber2(pair2), a subscriber7(pair7), as well as a subscriber8(pair8) is rather significant, the subscriber1, the subscriber2, the subscriber7, and the subscriber8constitute a signal set. Similarly, a subscriber3, a subscriber4, a subscriber5, and a subscriber15constitute another signal set. Correspondingly, the other 16 signals of the 24 signals are divided into the other 4 signal sets.

Referring toFIGS. 6 and 7, a DSLAM61sends a control signal through a network management system (NMS)62. Upon receiving the control signal, a relay matrix63switches a selection switch thereof. Signals from the pair1, the pair2, the pair7, and the pair8of a main distribution frame (MDF)64are A/D converted and then computed by a processor (processor1). Similarly, the pair3, the pair4, the pair5, and the pair15are connected to another processor (processor2) for computation processing. Other signals belonging to the same signal set are connected to another processor (not shown inFIG. 6) to be processed. The number of rows and the number of columns for a channel transmission matrix processed in each processor are both4. To the contrary, downlink signals are processed and D/A converted, and then pass through the relay matrix63, so as to be connected to the MDF64.

In this embodiment, the relay matrix is disposed separately from the DSLAM. Alternatively, it may also be disposed within the DSLAM in specific applications.

The Switching Matrix Adopts a Digital Matrix

An algorithm for dividing xDSL signals into two or more signal sets is the same as that in Embodiment 1.

In this embodiment, a digital matrix is used to select the signals. The digital matrix has a larger capacity and a better maintainability than the analog matrix (e.g. relay matrix). At the DSLAM end, signals belonging to the same signal set are connected to the same processor for operational processing through the digital matrix.FIG. 8shows principles of implementing line switching by using a digital matrix. Subscriber line signals1,2,3, . . . n from an MDF (not shown inFIG. 8) are A/D converted by an analog front end (AFE)81and connected to a digital matrix82. The digital matrix82selects the signals by a multiplexer as shown inFIG. 9, and connects the selected signals that belong to the same signal set to the same processor for operational processing. Referring toFIG. 8, signals carried by a pair1, a pair2, a pair7, and a pair8are connected to one processor (processor1) for operational processing. Signals carried by a pair3, a pair4, a pair5, and a pair15are connected to another processor (processor2) for operational processing. Other signals belonging to the same signal set are connected to a same processor for operational processing. The number of rows and the number of columns for a channel transmission matrix processed in each processor are 4. To the contrary, after the operational processing by the processors, downlink signals pass through the digital matrix82, are D/A converted in the AFE and then output to the MDF (not shown).

In this embodiment, the digital matrix is disposed within the DSLAM. Alternatively, it may also be disposed separately from the DSLAM in specific applications.

The Switching Matrix Adopts a Digital Matrix and a Relay Matrix

Referring toFIG. 10, the system in this embodiment of the present invention may also adopt both a digital matrix101and a relay matrix102to implement signal switching. Through using both the digital matrix and the relay matrix, the transmission matrix on the processor is simplified into two transmission matrixes. For example, a single transmission matrix of 100×100 may be made into 5 matrixes of 100×20 and 5 matrixes of 20×20, so as to simplify this large matrix.

The switching matrix (relay matrix or digital matrix) in the above embodiments may be configured as an independent entity, and may also be integrated into the MDF.

Dynamically Adjust the Signal Sets When Crosstalk between the Subscribers Changes

FIG. 11shows a system structure of this embodiment. A line switching control module adopts a digital matrix111and is disposed within a DSLAM1101.

The digital matrix111divides a plurality of xDSL signals into two or more signal sets according to updated crosstalk information and transmits signals belonging to the same signal set to the same processor in the DSLAM1101to be processed.

The process of dividing the signals into a plurality of signals sets may be implemented by a variety of algorithms such as a greedy algorithm. The signals are divided in such a way that the divided signal sets are mutually exclusive, and crosstalk between the signals in each signal set is rather significant. The DSLAM1101further includes a channel monitoring system112. The channel monitoring system112is adapted to monitor crosstalk information between the subscribers through a shared channel H and transmit information about crosstalk variation between the subscribers to the digital matrix111. The information about crosstalk variation includes crosstalk variation information caused by the subscribers going online or offline and/or the channels performance slowly changing as time elapses.

In particular, the channel monitoring system112monitors the online and offline states of the subscribers. The channel monitoring system112tracks and processes the channels (for example, by sending an “abuse” signal) and obtains crosstalk information between the subscribers in real time according to the online and offline states of the subscribers. When a subscriber goes online, changed crosstalk information is transferred to the digital matrix111, and the digital matrix111divides signals in a bundle into a plurality of signal sets by a certain algorithm (for example, a greedy algorithm) according to the changed crosstalk information and outputs the signals belonging to the same signal set into the same processor to be processed. When a subscriber goes offline, changed crosstalk information is transferred to the digital matrix111, and the digital matrix111divides signals in a bundle into a plurality of signal sets according to the changed crosstalk information and outputs the signals belonging to the same signal set into the same processor to be processed.

The line switching control module in the system of the above embodiments may also adopt a relay matrix configured independent from the DSLAM.

The processors in all the embodiments mentioned above may also be circuit modules that can jointly process signals from a certain number of lines and can achieve crosstalk cancellation. The circuit modules may also be referred to as processing units.

In addition, the line switching control module can also adjust the division of signal sets in real time according to the condition of crosstalk variation, thereby achieving a better crosstalk cancellation effect.