The present invention relates to crosstalk mitigation within a wired communication system.

Technical Background of the invention

Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple input Multiple Output (MIMO) wired communication systems, such as Digital Subscriber Line (DSL) communication systems.

As the demand for higher data rates increases, systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say transmission lines that are in close vicinity over part or whole of their length, such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).

Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (vectoring hereinafter).

One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoder before being transmitted over the respective communication channels. The precoder is such that the concatenation of the precoder and the communication channels results in little or no inter-channel interference at the receivers .

A further technique for reducing inter-channel interference is joint signal postcoding (or post-processing): the receive data symbols are jointly passed through a postcoder before being detected. The postcoder is such that the concatenation of the communication channels and the postcoder results in little or no inter-channel interference at the detectors. Postcoders are also sometimes referred to as crosstalk cancellation filters.

Signal vectoring is typically performed at a traffic aggregation point as vectoring primarily implies that data symbols concurrently transmitted over, or received from, the vectored lines are bunched and synchronously passed all together through the precoder or postcoder. For instance, signal vectoring is advantageously performed within a Digital Subscriber Line Access Multiplexer (DSLAM) deployed at a Central Office (CO), or as a fiber-fed remote unit closer to subscriber premises (street cabinet, pole cabinet, building cabinet, etc) . Such a remote unit may also be referred to as a remote DSLAM, or as a Distribution Point Unit (DPU) . Signal precoding is particularly appropriate for downstream communication (toward customer premises) , while signal postcoding is particularly appropriate for upstream communication (from customer premises) .

The choice of the vectoring group, that is to say the set of communication lines, the signals of which are jointly processed, is rather critical for achieving good crosstalk mitigation performances. Within a vectoring group, each communication line is considered as a disturber line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line incurring crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.

Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interfere with each other. Yet, local loop unbundling on account of national regulation policies and/or limited vectoring capabilities may prevent such an exhaustive approach, in which case the vectoring group would only include a sub-set of the interfering lines, thereby yielding limited vectoring gains.

More formally, an NxN MIMO system can be described by the following linear model :

yk = HkXk + zk (1),

wherein the N-component complex vector Xk, respectively yk, is a discrete frequency representation, as a function of the frequency/carrier/tone index k, of the symbols transmitted over, respectively received from, the N vectored channels,

wherein the NxN complex matrix Hk is the channel matrix: the (i ,j)-th component Hij of the channel matrix Hk describes how the communication system produces a signal on the i-th channel output in response to a signal being fed to the j-th channel input; the diagonal elements of the channel matrix describe direct channel coupling, and the off -diagonal elements of the channel matrix (also referred to as the crosstalk coefficients) describe inter-channel coupling,

and wherein the N-component complex vector Zk denotes additive noise over the N channels, such as Radio Frequency interference (RFI) or thermal noise.

Linear signal precoding and postcoding are advantageously implemented by means of matrix products.

in downstream, the linear precoder performs a matrix-product in the frequency domain of a transmit vector Uk with a precoding matrix Pk before actual transmission over the respective subscriber lines, i .e. actual transmit vector is Xk = PkUk in eq. (1), the precoding matrix Pk being such that the overall channel matrix HkPk is diagonal i zed , meaning the off-diagonal coefficients of the overall channel HkPk, and thus the inter-channel interference, mostly reduce to zero.

Practically, and as a fi rst order approximation, the precoder superimposes anti -phase crosstal k pre-compensation signals over the victim line along with the di rect signal that destructively interfere at the receiver with the actual crosstalk signals from the respective disturber lines.

in upstream, the linear postcoder performs a matrix-product in the frequency domain of the receive vector yk with a postcoding matrix Qk to recover the transmit vector Uk (after channel equalization and power normalization) , i .e. detection is performed on y'k = Qkyk, the postcoding matrix Qk being such that the overall channel matrix QkHk is di agonal i zed , meaning the off-diagonal coefficients of the overall channel QkHk, and thus the inter-channel interference, mostly reduce to zero.

The performance of signal vectoring depends critically on the component values of the precoding and postcoding matrix, which component values are to be computed and updated according to the actual and varying channel couplings.

The various channel couplings are estimated by a vectoring controller based on pilot (or probing) signals transmitted over the respective channels. The pilot signals are typically transmitted during dedicated time periods and/or over dedicated tones.

For instance, in the recommendation entitled " Self-FEXT Cancellation (Vectoring) For Use with VDSL 2 Transceivers" , ref. G.993.5 (Vectored VDSL2 hereinafter) , and adopted by the international Telecommunication Union (ITU) in April 2010, the transceiver units send pilot signals on the so-called SYNC symbols. The SYNC symbols occur periodically after every super frame, and are transmitted synchronously over all the vectored lines (super frame alignment). A similar technique has been adopted in G.9701 ITU recommendation (G.fast hereinafter).

On a given disturber line, a subset of the tones of a SYNC symbol (pilot tones hereinafter) are all 4-QAM modulated by the same pilot digit from a given pilot sequence, and transmit one of two complex constellation points, either ' 1 + " corresponding to '+Ι' or ' -1 - j' corresponding to ' -V (Vectored VDSL2) ; or transmit one of three complex constellation points, either ' 1 + j' corresponding to '+Ι' or '-1 - j' corresponding to '-Ι' or Ό + Oj ' corresponding to Ό' (G.fast).

On a given victim line, both the real and imaginary part of the received frequency sample before equalization (G.fast), or of the normalized slicer error, which is the difference vector between the received and properly equalized frequency sample and the constellation point onto which this frequency sample is demapped (Vectored VDSL2 and G.fast), are measured on a per pilot tone basis and reported to the vectoring controller for estimation of the various channel couplings.

The successive error samples gathered over a given victim line are next correlated with the pilot sequence used over a given disturber line in order to obtain an estimate of the channel coupling from the given disturber line into the given victim line. To reject the crosstalk contributions from the other disturber lines, the pilot sequences used over the respective disturber lines are mutually orthogonal (e.g., walsh-Hadamard sequences) .

The crosstalk estimates are eventually used for initializing or updating the coefficients of the precoding or postcoding matrix, typically by means of a first or higher-order matrix inversion of the channel matrix (e.g., zero-forcing precoder) or by means of an iterative update (e.g., LMS precoder) .

With the advent of new copper access technologies and the use of even broader spectrum up to and beyond 100 MHz, the crosstalk coupling substantially increases. Consequently, the superimposi tion of the crosstalk precompensation signals on a victim line may cause a violation of the transmit Power Spectral Density (PSD) mask, which defines the allowed amount of signal power for an individual user as a function of frequency, and may also result in signal clipping within the Digital to Analog Converter (DAC) causing severe signal distortions.

A first solution is to scale down the gains of the direct signals, and thus of the corresponding precompensation signals, such that the transmit signals, including both the direct and precompensation signals, remain within the allowed transmit power bounds. The PSD reduction is line and frequency dependent, and may change over time, e.g. when a line joins or leaves the vectoring group. The change in direct signal gains must be communicated to the respective receivers for proper channel equalization.

A second solution is the use of Non-Linear Precoding (NLP) , which applies modulo arithmetic operation to shift a transmit constellation point with excessive power back within the constellation boundary. At the receiver, the same modulo operation will shift the signal back to its original position. NLP is significantly more computationally complex than linear precoding. For example, NLP typically require matrix decompositions, such as QR matrix decomposition, and numerous alternating linear and non-linear operations.

A further and important consequence of the use of such a broad frequency spectrum is the il 1 -conditioning of the channel matrix at numerous tones, i.e. the channel matrix is singular or almost singular, and matrix inversion for achieving zero-forcing condition yields large precoder/postcoder coefficients. Thus significant down-scaling needs to be applied to communication signals for linear precoding, resulting in very poor vectoring gains.

Such channel singularities also cause very large fluctuations of the precoder/postcoder coefficients across frequency, meaning that more memory is required to represent the coefficients numerically, which directly translates into higher costs for the vectoring processor.

Last, channel il 1 -conditioning yields a large distribution of data rates and aggregate transmit power across the subscriber loops. Even with equal loop lengths, we see that the data rate distribution can vary more than 50% from line to line. This is problematic for operators, who are typically interested in delivering common guaranteed data rates (in service classes) to the end-users.

A technique known as tone suppression can circumvent the channel i 11 -condi tioni ng : if, on a particular tone, the NxN channel matrix of an N-users system is ill-conditioned (i.e., has a large condition number), then one looks for a subset of N-M users such that the corresponding (N-M)x(N-M) reduced channel matrix is wel 1 -condi tioned (i.e., has a low condition number). The M suppressed lines do not carry any direct communication signal over this tone (i.e., transmit gain and bit loading are both set to zero). An (N-M)X(N-M) precoder/postcoder is then used to cancel the crosstalk between the N-M remaining users. Tone suppression allows the N-M users to still get good data rates at the expense of the M suppressed users. The set of suppressed lines, and the number of suppressed lines M, may be different across tones. By appropriately selecting which lines to suppress on which tone, the overall data rate can be improved relative to a system that uses full NxN inverse precodi ng/postcodi ng on every tone.

Summary of the invention

It is an object of the present invention to further improve the vectoring gains in the event of the communication channel being ill-conditioned.

in accordance with a first aspect of the invention, a vectoring controller for configuring a vectoring processor that jointly processes Discrete Multi-Tone (DMT) communication signals to be transmitted over, or received from, a plurality of N subscriber lines according to a vectoring matrix is adapted, for given ones of a plurality of tones, to enable the given tone for direct data communication over a first set of N-Mk targeted lines out of the plurality of N subscriber lines, and to disable the given tone for direct data communication over a second disjoint set of Mk supporting lines out of the plurality of N subscriber lines, Mk denoting a non-null positive integer. The vectoring controller is further adapted to configure the vectoring matrix to use an available transmit or receive power at the given tone over the second set of Mk supporting lines for further enhancement of data signal gains at the given tone over the first set of N-Mk targeted lines.

in one embodiment of the invention, the configured vectoring matrix is a rectangular Nx(N-Mk) precoding matrix with at least one non-null off -diagonal precoding coefficient for the respective M matrix rows corresponding to the second set of Mk supporting lines.

in a further embodiment of the invention, the configured rectangular vectoring matrix is a generalized inverse matrix of a (N-Mk)xN reduced channel matrix comprising far-end coupling coefficients from the plurality of N subscriber lines into the first set of N-Mk targeted lines.

in still a further embodiment of the invention, the generalized inverse matrix is a Moore-Penrose pseudo-inverse of the (N-Mk)xN reduced channel matrix.

in one embodiment of the invention, the configured vectoring matrix is a rectangular (N-Mk)xN postcoding matrix with at least one non-null off -diagonal postcoding coefficient for the respective M matrix columns corresponding to the second set of Mk supporting lines.

in a further embodiment of the invention, the configured rectangular vectoring matrix is a generalized inverse matrix of a Nx (N-Mk) reduced channel matrix comprising far-end coupling coefficients from the first set of N-Mk targeted lines into the plurality of N subscriber lines.

in still a further embodiment of the invention, the generalized inverse matrix is a Moore-Penrose pseudo-inverse of the Nx(N-Mk) reduced channel matrix.

in one embodiment of the invention, the vectoring controller is further adapted to determine the first and second sets of lines on a per tone basis.

in one embodiment of the invention, the plurality of tones comprises ill-conditioned tones.

in one embodiment of the invention, the vectoring controller is further adapted to update respective line metrics currently achieved over the plurality of N subscriber lines given the so-enabled and so-disabled tones and the so-configured vectoring matrices while iterating through the plurality of tones, and to determine the first and second sets of lines based on the so-updated line metrics.

in a further embodiment, the vectoring controller is further adapted to assign at least one subscriber line of the plurality of N subscriber lines, whose updated line metric exceeds a given line metric target, to the second set of Mk supporting lines for at least one subsequent tone iteration.

Alternatively, the vectoring controller is further adapted to assign at least one subscriber line of the plurality of N subscriber lines, whose updated line metric exceeds by a given margin at least one updated line metric of at least one further subscriber line of the plurality of N subscriber lines, to the second set of Mk supporting lines for at least one subsequent tone iteration.

The line metrics may refer to respective aggregate data rates and/or aggregate transmit powers achieved so far over the plurality of N subscriber lines given the so-enabled and so-disabled tones and the so configured vectoring matrices.

Such a vectoring controller typically forms part of an access node providing broadband communication services to subscribers over a copper plant and deployed at a CO or at a remote location closer to subscriber premises, such as a DSLAM or a DPU.

in accordance with another aspect of the invention, a method for configuring a vectoring processor that jointly processes DMT communication signals to be transmitted over, or received from, a plurality of N subscriber lines according to a vectoring matrix comprises, for given ones of a plurality of tones, enabling the given tone for direct data communication over a first set of N-Mk targeted lines out of the plurality of N subscriber lines, and disabling the given tone for direct data communication over a second disjoint set of Mk supporting lines out of the plurality of N subscriber lines, Mk denoting a non-null positive integer. The method further comprises configuring the vectoring matrix to use an available transmit or receive power at the given tone over the second set of Mk supporting lines for further enhancement of data signal gains at the given tone over the first set of N-Mk targeted lines.

Embodiments of a method according to the invention correspond with the embodiments of a vectoring controller according to the invention.

The present invention proposes to boost the data signal gains at a given tone over N-Mk targeted lines (i.e., targeted for direct data communication) by using the transmit power (precoding) or receive power (postcoding) that has been made available over Mk supporting lines (i.e., supporting data communication to the targeted set) on account of no direct data communication taking place over the Mk supporting lines at that given tone.

in this approach, we improve channel conditioning by eliminating certain rows (precoding) or certain columns (postcoding) of the channel matrix, rather than by eliminating both rows and columns. This results in a rectangular (N-Mk)xN (precoding) or Nx(N-Mk) (postcoding) reduced channel matrix compared to the known square (N-Mk)x(N-Mk) reduced channel matrix when the suppressed lines are entirely ignored. Physically, this means that even when the number of users providing data is less than N, namely N-Mk, the precoder still generates N output signals that are transmitted on all N vectored lines, and likewise the postcoder still relies on N input signals received from all N vectored lines.

For instance, suppose that we have an ill-conditioned system of N = 10 lines that is better conditioned when line 10 is not targeted for direct data communication.

When precoding the full system, the signal sent on line 10 uses some of its power budget to send data intended for receiver 10, and some of its budget to cancel crosstalk coming from lines 1 to 9. With the known square tone suppression, no signal at all is sent on line 10. Receiver 10 gets no data, but lines 1 to 9 are relieved from having to use any of their power budget to cancel crosstalk from line 10. With the proposed rectangular conditioning, the full power budget of line 10 is dedicated to sending data intended for lines 1 to 9. Lines 1 to 9 are relieved from having to cancel crosstalk from line 10, and moreover they benefit from this additional transmit power used in their behalf.

When postcoding the full system, the signal received on line 10 is used to remove, from the received signals on lines 1 to 9, crosstalk originating from line 10. in doing so, some received noise on line 10 is introduced into lines 1 to 9. With the known square tone suppression, no signal is sent on line 10. Receiver 10 gets no data, but lines 1 to 9 are relieved from having to remove crosstalk from line 10, and hence are not affected by noise received on line 10. with the proposed rectangular conditioning, the received signal on line 10, which contains crosstalk signals received from lines 1 to 9, may be applied to aiding the reception of data from lines 1 to 9. Lines 1 to 9 are relieved from having to cancel crosstalk from line 10, and moreover they benefit from additional received power from line 10 used in their behalf.

To take full advantage of this rectangular channel, we use a wel 1 -designed rectangular Nx(N-Mk) precoder or (N-Mk)xN postcoder for coherent combining of the N-Mk data communication signals traveling through the different propagation paths (inc. the direct and indirect propagation paths) at the N-Mk receivers (precoding) or detectors (postcoding) coupled to the N-Mk targeted lines. Owing to this coherent combining, the power of the useful receive signal (i.e., the data signal) is substantially increased over the N-Mk targeted lines before signal detection, and so is their Signal to Noise Ratio (SNR) and thus their achievable data rate.

Brief Description of the Drawings

The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein:

- fig. 1 represents an overview of an access plant;

- fig. 2 represents further details about an access node;

- fig. 3 represents an overview of rectangular precoding as per the present invention;

- fig. 4 represents an overview of rectangular postcoding as per the present invention;

- fig. 5 represents a method flow chart for determining the sets of targeted and supporting lines for successive tones; and

- fig. 6A , 6B and 6C represent various performance plots for rectangular conditioning versus known techniques (full inverse and square tone suppression).

Detailed Description of the invention

The square brackets hereinafter surround terms that are deemed to be optional, and thus an expression such as e.g. "the [normalized] channel matrix" shall be construed as either "the channel matrix" or "the normalized channel matrix".

There is seen in fig. 1 an access plant 1 comprising a network unit 10 at a CO, an access node 20 coupled via one or more optical fibers to the network unit 10, and further coupled via a copper plant to Customer Premises Equipment (CPE) 30 at various subscriber locations. The transmission media of the copper plant is typically composed of copper Unshielded Twisted Pai rs (UTP) .

As an illustrative example, the copper plant comprises four subscriber lines Li to l_4 sharing a common access segment 40, and then going through dedicated loop segments 50 for final connection to CPEs 30i to 304 respectively.

Within the common access segment 40, the subscriber lines Li to l_4 are in close vicinity and thus induce crosstalk into each other (see the arrows in fig. 1 between the respective subscriber lines) .

The access node 20 comprises a Vectoring Processing Unit 21 (or VPU) for jointly processing the data symbols that are being transmitted over, or received from, the copper plant in order to mitigate the crosstalk and to increase the achievable data rates.

There is seen in fig. 2 further details about an access node 100 and respective CPEs 200.

The access node 100 comprises:

- transceivers 110;

- a Vectoring Processing Unit (VPU) 120; and

- a Vectoring Control Unit (VCU) 130 for controlling the operation of the VPU 120.

The transceivers 110 are individually coupled to the VPU 120 and to the VCU 130. The VCU 130 is further coupled to the VPU 120.

The transceivers 110 individually comprise:

- a Digital Signal Processor (DSP) 111; and

- an Analog Front End (AFE) 112.

The transceivers 110 are coupled to respective transceivers 210 within the CPEs 200 through respective subscriber lines Li to LN, which are assumed to form part of the same vectoring group.

The transceivers 210 individually comprise:

- a Digital Signal Processor (DSP) 211; and

- an Analog Front End (AFE) 212.

The AFEs 112 and 212 individually comprise a Digital -to-Anal og Converter (DAC) and an Analog-to-Digi tal Converter (ADC), a transmit filter and a receive filter for confining the signal energy within the appropriate communication frequency bands while rejecting out-of-band interference, a line driver for amplifying the transmit signal and for driving the transmission line, and a Low Noise Amplifier (LNA) for amplifying the receive signal with as little noise as possible.

in case of Frequency Division Duplexing (FDD) operation where downstream and upstream communications operate simultaneously over the same transmission medium in distinct and non-overlapping frequency bands, the AFEs 112 and 212 further comprise a hybrid for coupling the transmitter output to the transmission medium and the transmission medium to the receiver input while achieving low transmitter- receiver coupling ratio. The AFE may further accommodate echo cancellation filters to reduce the coupling ratio at a further extent.

in case of Time Duplexing Division (TDD) operation where downstream and upstream communications operate over the same frequency band but in distinct and non-overlapping time slots, the hybrid can be advantageously omitted as the transmitter and receiver operate in alternate mode: the receive circuitry is switched OFF (or the receive signal is discarded) while the transmit circuitry is active, and the way around, the transmit circuitry is switched OFF while the receive circuitry is active.

The AFEs 112 and 212 further comprise impedance-matching circuitry for adapting to the characteristic impedance of the transmission medium, clipping circuitry for clipping any voltage or current surge occurring over the transmission medium, and isolation circuitry (typically a transformer) for DC-isolating the transceiver from the transmission medium.

The DSPs 111 and 211 are configured to operate downstream and upstream communication channels for conveying user traffic over the subscriber lines Li to LN.

The DSPs 111 and 211 are further configured to operate downstream and upstream control channels that are used to transport control traffic, such as diagnosis, management or On-Line Reconfiguration (OLR) commands and responses. Control traffic is multiplexed with user traffic over the transmission medium.

More specifically, the DSPs 111 and 211 are for encoding and modulating user and control data into DMT symbols, and for de-modulating and decoding user and control data from DMT symbols.

The following transmit steps are typically performed within the DSPs 111 and 211:

- data encoding, such as data multiplexing, framing, scrambling, error correction encoding and interleaving;

- signal modulation, comprising the steps of ordering the tones according to a tone ordering table, parsing the encoded bit stream according to the respective bit loadings of the ordered tones, and mapping each chunk of bits onto an appropriate transmit constellation point (with respective carrier amplitude and phase), possibly with Trellis coding;

- signal scaling, such as power normalization, transmit PSD shaping and transmit gain scaling;

- inverse Fast Fourier Transform (IFFT);

- Cyclic Prefix (CP) insertion; and

- time-windowing.

The following receive steps are typically performed within the DSPs 111 and 211:

- time-windowing and CP removal;

- Fast Fourier Transform (FFT);

- Frequency EQualization (FEQ);

- signal de-modulation and detection, comprising the steps of applying to each and every equalized frequency sample an appropriate constellation grid, the pattern of which depends on the respective bit loading, detecting the expected transmit constellation point and the corresponding transmit bit sequence, possibly with Trellis decoding, and re-ordering all the detected chunks of bits according to the tone ordering table; and

- data decoding, such as data de-interleaving, error correction decoding, de-scrambling, frame delineation and de- mul ti pi exi ng .

Some of these transmit or receive steps can be omitted, or some additional steps can be present, depending on the exact digital communication technology being used.

The DSPs 111 are further configured to supply transmit frequency samples Uk to the VPU 120 before inverse Fast Fourier Transform (IFFT) for joint signal precoding, and to supply receive frequency samples yk to the VPU 120 after Fast Fourier Transform (FFT) for joint signal post-processing.

The DSPs 111 are further configured to receive pre-compensated transmit samples Xk from the VPU 120 for further transmission, and to receive post-compensated receive samples y'k from the VPU 120 for further detection. Alternatively, the DSPs 111 may receive correction samples to add to the initial frequency samples before further transmission or detection.

The VPU 120 is configured to mitigate the crosstalk induced over the subscriber lines. The VPU 120 comprises a linear precoder configured to multiply a vector Uk of transmit frequency samples with a precoding matrix Pk in order to pre-compensate an estimate of the expected crosstalk, and a linear postcoder configured to multiply a vector of receive frequency samples yk with a crosstalk cancellation matrix Qk so as to post-compensate an estimate of the incurred crosstalk.

in the matrix Pk or Qk, a row i is associated with a particular victim line Li , while a column j is associated with a particular disturber line Lj .

The VCU 130 is basically for controlling the operation of the VPU 120, and more specifically for estimating the crosstalk coefficients between the subscriber lines of the vectoring group, and for initializing and updating the coefficients of the precoding matrix Pk and of the crosstalk cancellation matrix Qk from the so-estimated crosstalk coefficients .

The VCU 130 starts first by configuring the transceivers 110 and 210 with the respective pilot sequences to use for modulation of the pilot tones of the pilot symbols (SYNC symbols). The pilot sequences comprises T pilot digits using {+1, -1} or {+1, 0, -1} as alphabet. The pilot digit that modulates a given tone k during pilot symbol position t over line Li is denoted as w* . Presently, the pilot symbols are not processed through the VPU 120 as one needs first to characterize the channel matrix per se.

The VCU 130 next gathers measurement samples as measured by the transceivers 110 and 210 while the pilot symbols are being transmitted. The measurement sample as measured by the transceiver llOi or 210i over a victim line Li at tone k during pilot symbol position t is denoted as ^

c , k

The VCU 130 correlates T measurement samples

(ei -to+T-i as measured over a given victim line Li during a complete acquisition cycle with the T pilot digits {wj,k}t=to- to+T-i °f tne pilot sequence transmitted over a given disturber line Lj so as to obtain a crosstal k estimate from the disturber line Lj into the victim line Li at frequency index k. As the pilot sequences are mutually orthogonal , the contributions from the other disturber lines reduce to zero after this correlation step.

The channel matrix and/or the normalized channel matrix are determined based on these correlation results. The nominal channel matrix is derived from a measure of the raw receive signals before equalization, whereas the normalized channel matrix is derived from a measure of the slicer errors after channel equalization.

The so-determined channel matrix may exhibit ill -conditioning at specific tones, that is to say one or more row(s) and/or column(s) of the channel matrix corresponding to one or more subscriber lines can be expressed as, or come close to, a linear combination of the other rows or columns, which makes the channel matrix singular or almost singular, and yields very large vectoring coefficients for zero-forcing vectoring.

Typically, the matrix singularity is quantified by means of a condition number, and a matrix is said to be ill -conditioned when that condition number exceeds some given threshold. For instance, the ratio between the largest and lowest singular values of the channel matrix is a good indicator for i 11 -condi tioni ng and can be used as condition number.

The VCU 130 is further configured to characterize the i 11 -condi tioni ng of the so-determined channel matrix at respective tones, and if the channel matrix is characterized as being ill -conditioned at a given tone, then the VCU 130 is further configured to select one or more lines and to disable the given tone for di rect data communication over these lines so as the corresponding reduced channel matrix is wel 1 -condi tioned . The set of lines for which the given tone is kept active for di rect data communication is referred to as the targeted set, whereas the set of lines for which the given tone is disabled for di rect data communication is referred to as the supporting set.

The VCU 130 distributes targeted and supporting roles to the various subscriber lines Li to LN across different tones . indeed, this is made possible because, when one matrix row or

column is [almost] a linear combination of say two other matrix rows or columns, then any of the three subscriber lines corresponding to these three matrix rows or columns can be selected for the supporting set in order to ' re-orthogonal ize' the channel matrix and make it no longer singular. Owing to the frequency coherence of the channel, the same three lines are likely to be again in the same linear relationship at neighboring tones, and the VCU 130 can select each of these 3 lines in turn. The VCU 130 thus has some degree of freedom for balancing targeted and supporting roles across the lines and achieving some kind of fairness as it will be set forth further in the description.

Also it is noteworthy that, with TDD, the sets of targeted and supporting lines for a given tone can be different in upstream and downstream.

For every line Li, a set of disabled tones STi is determined and sent to the respective transceiver llOi (see "STi = {ki, ...}" in fig. 2), or to a central communication controller that manages the communication parameters of the transceivers 110 and 210. Thereupon, the transceiver llOi configures both the bit loadings bi,k and transmit gains n,k to zero for all tones k in the set STi. The new bit loading and transmit gain values are communicated to the respective peer transceiver 210i (see "bi.ki = 0" and "n,ki = 0" in fig. 2).

It is noteworthy that pilot signals keep on being transmitted over these disabled tones notwithstanding their transmit gain and bit loading, which have been both set to zero. By so doing, the full [normalized] channel matrix can still be estimated on every tone.

Let k be such an ill-conditioned tone: direct data communication at tone k is to be disabled over one or more subscriber lines in order to obtain a wel 1 -conditioned reduced channel matrix at tone k.

Let L = {Li LN} denote the set of all vectored lines. Let Ak denote the set of targeted lines at tone k, that is to say the set of lines for which tone k is kept enabled for direct data communication (i.e., corresponding bit loading and transmit gain are both different from zero), and let Bk denote the set of supporting lines at tone k, that is to say the set of lines for which tone k is disabled for direct data communication (i.e., corresponding bit loading and transmit gain are both set to zero).

We have:

L = k you Bk and Ak a Bk = 0 (2).

Let Mk denote the size of the set Bk, implying that N-Mk is the size of the set Ak on account of the set equations (2). The number Mk of lines that need to be removed from the targeted set is mostly determined by the condition of the [normalized] channel matrix at respective tones, but may also depend on further criteria.

Let Hk denote the NxN [normalized] channel matrix at tone k, and let us arrange the [normalized] channel matrix as

H. HAk wherein HAk denotes a reduced (N-Mk)xN [normalized] HBk

channel matrix comprising the N-Mk rows of the [normalized] channel matrix Hk corresponding to the set of targeted lines Ak and all N columns, and Hek denotes a reduced MkxN [normalized] channel matrix comprising the Mk rows of the [normalized] channel matrix Hk corresponding to the set of supporting lines Bk and all N columns, in other words, the rows of the [normalized] channel matrix Hk have been re-arranged for the supporting lines Bk to occupy the last Mk rows. This rearrangement of rows and columns is to simplify the notation for exposition, and is not necessary in practice as should be clear to those skilled in the art.

Let Pk = [PAk Pek] denote the precoding matrix used at tone k, with PAk denoting a reduced Nx(N-Mk) precoding matrix comprising the N-Mk columns of the precoding matrix Pk corresponding to the set of targeted lines Ak and all N rows, and Pek denoting a reduced NxMk precoding matrix comprising the Mk columns of the precoding matrix Pk corresponding to the set of supporting lines Bk and all N rows.

With zero-forcing precoding and when targeting all lines, we would have Pk = Hk-1.

Let Sk SAk 0 be the transmit gain scaling diagonal

S 0 Bk

matrix at tone k for conformance to a transmit PSD mask and total aggregate power, with SAk and Sek being the diagonal matrices corresponding to the set of targeted lines Ak and the set of supporting lines Bk respectively. The diagonal coefficients of the diagonal matrix Sek are set to zero as there are no data communication signals transmitted over the supporting lines Bk at tone k.

With this notation in mind, the VCU 130 is further configured, for respective ones of the i 11 -configured tones used for downstream communication:

- to determine a set Ak of targeted lines and a corresponding set Bk of supporting lines to obtain a wel 1 -conditioned reduced [normalized] channel matrix HAk; and

- to compute or recompute a rectangular precoding matrix pAk for the targeted lines Ak for which the following three conditions hold:

a) The rows of PAk corresponding to the supporting lines Bk are not forced to zero, and thus precompensation signals are transmitted over the supporting lines Bk.

b) The rows of PAk corresponding to the targeted lines Ak are not the same as the corresponding rows of the full precoding matrix Pk (i.e., when all lines are targeted). This emphasizes that one does not just take the rows of the inverse of the full [normalized] channel matrix Pk = Hk-l.

c) The (re)computed vectoring matrix has a zero-forcing

P A to

design and di agonal izes the overal 1 channel over the targeted lines Ak:

with DAk denoting a diagonal matrix, and X denoting a

'don't care'. The coefficients of the precoding matrix pBk are made irrelevant by setting all gains of the diagonal matrix Sek to zero. These coefficients can be left unchanged, or can be reset to zero too.

One possible concrete precoder computation is to use the Moore-Penrose pseudo-inverse of the reduced [normalized] channel matrix HAk as follows:

Ak : the H A K ( the H ak the H ak ) (4)

However, this is not the only choice, but a whole family of generalized inverses can serve as precoder. For instance, the following generalized inverse could be used as zero-forcing precoder too:

Pak = WH Ak ( H Ak WH Ak) 5 )

wherein W denotes a NxN diagonal matrix with real diagonal coefficients .

in general , an Nx(N-Mk) precoder has N (N-Mk) degrees of freedom, of which (N-Mk)2 should be used to ensure that all crosstalk is canceled (DA is diagonal), and the remaining (N- Mk)Mk degrees of freedom are available to try to maximize the power of the receive communication signals on the N-Mk targeted lines, that is to say to make diagonal values of the diagonal matrix DAk as large as possible while meeting the various power constraints.

A few words regarding the use of the channel matrix or the normalized channel matrix for the computation of the precoding matrix.

The channel matrix and the normalized channel matrix only differ by left-multiplication with a diagonal matrix Fk comprising the inverses of the diagonal elements of Hk:

Gk = FkHk (5) ,

wherein Hk and Gk denote the now-differentiated channel matrix and normalized channel matrix respectively.

By virtue of eq. (4) or (5) , the corresponding precoding matrices will only differ by right-multiplication with a diagonal matrix:

P A k = G Ak ( G Ak G Ak) 1

= (F A Th A k) ((F A Th Ak ) (F Ak H Ak )) = H A k F Ak (F Ak H Ak H Ak F Ak ) (6),

= H Ak F Ak F Ak (F A Th Ak H Ak ) = H Ak (hl Ak H Ak ) F Ak

wherein the normalized channel matrix Gk has been decomposed into GAk and Gek as per the channel matrix Hk, and wherein FAk denotes a diagonal matrix comprising the diagonal coefficients of the diagonal matrix Fk corresponding to the N-Mk targeted 1 i nes .

Some power normalization is however required at transmit side through pre-mul ti pi i cati on with the scaling matrix SAk in order to achieve the required transmit power, and thus the use of the nominal or normalized channel matrix for the

computation of the precoding matrix will eventually yield the same effective precoding matrix.

Rectangular precoding is illustrated in fig. 3.

N transceivers TU-Ci to TU-CN at network side supplies a downstream transmit vector Uk for every transmit DMT symbol period for further precoding, transmission, reception and detection by N respective transceivers TU-Ri to TU-RN at subscriber side. The transmit vector Uk, comprising respective transmit frequency samples ui,k to UN,k after constellation mapping and scaling, is input to a precoder with precoding matrix Pk to yield a precoded signal Xk = [xi,k . . . XN,k]T = PkUk for further transmission over the respective subscriber lines Li to LN.

Let us assume for convenience that, at tone k, one line is being moved from the targeted set Ak to the supporting set Bk (i .e. , Mk = 1) in order to obtain a wel 1 -conditioned reduced channel matrix, and let us further assume that this supporting line is line LN. Thus no downstream data is conveyed over line LN at tone k, or equivalently UN,k = 0 (see crosshatched area in transmit vector Uk) , and corresponding transmit gain and bit loading are both set to zero. As a consequence, the corresponding Nth column of the precoding matrix Pk is irrelevant as it does not play any role in signal precoding (see crosshatched area in precoding matrix Pk) . The corresponding precoding coefficients PIN to PNN are preferably (but not necessarily) set to zero.

Yet, one or more off -diagonal coefficients of the Nth row of the precoding matrix Pk are set to a value different from 0 (see boldened coefficients in precoding matrix Pk) , meaning that crosstalk precompensation signals keep on being transmitted over line LN at tone k despite tone k being disabled for direct data communication over line LN (i .e. , XN,k≠0 despite UN,k=0). The effective precoding matrix has thus a rectangular shape.

Next, the precoded vector xk goes through the channel matrix Hk, thereby yielding the receive vector yk = [yi,k . . . yN,k]T for reception and detection by the transceivers TU-Ri to TU-RN. AS no communication takes place over tone k for line LN, the transceiver TU-RN discards the receive frequency sample yN,k and the corresponding Nth row of the channel matrix Hk is

irrelevant (see crosshatched area in channel matrix Hk and vector yk) .

A new rectangular Nx(N-l) precoding matrix PAk is computed and enforced into the columns of the precoding matrix Pk corresponding to the targeted lines Ak. The new rectangular

precoding matrix PAk makes an opportunistic use of the available transmit power over the supporting line LN (on account of no data communication signal being sent on that line) for insertion over that line of appropriately scaled and rotated replicas of the N-l communication signals ui,i≠N (or a part thereof), and further for coherent combining of these N-l communication signals UI.IDN traveling over the various propagation paths at the transceivers TU-Ri , ≠N , which then undergo a substantial increase of their receive signal power compared to full precoding (all lines being targeted with considerable signal down-scaling) or square tone suppression (one or more lines being suppressed without any signal transmission over the suppressed lines) .

The data signal gain ai,k over line Li at tone k is given by:

N

and now comprises an additional term H N,kPN ,k on account of the rectangular conditioning. This term will reinforce the power (or magnitude) of the useful signal ui,k provided the precoding coefficient PN .k is appropriately chosen.

The postcoding use case is very similar to the precoding use case.

Let us arrange the NxN [normalized] channel matrix as

Hk = [HAkHBk] » wherein HAk now denotes a reduced Nx(N-Mk) [normalized] channel matrix comprising the N-Mk columns of the [normalized] channel matrix Hk corresponding to the set of targeted lines Ak and all N rows, and Hek now denotes a reduced NxMk [normalized] channel matrix comprising the Mk columns of the [normalized] channel matrix Hk corresponding to the set of supporting lines Bk and all N rows, in other words, the columns of the [normalized] channel matrix Hk have been re-arranged for the supporting lines Bk to occupy the last Mk columns.

CLAIMS

1. A vectoring controller (130) for configuring a vectoring processor (120) that jointly processes Discrete Multi-Tone DMT communication signals (UL .UN; yi..yN) to be transmitted over, or received from, a plurality of N subscriber lines (LL .LN) according to a vectoring matrix (Pk; Qk) , and adapted, for given ones (k) of a plurality of tones:

- to enable the given tone for direct data communication over a first set of N-Mk targeted lines (Ak) out of the plurality of N subscriber lines, and to disable the given tone for direct data communication over a second disjoint set of Mk supporting lines (Bk) out of the plurality of N subscriber lines, Mk denoting a non-null positive integer,

- to configure the vectoring matrix to use an available transmit or receive power at the given tone over the second set of Mk supporting lines for further enhancement of data signal gains at the given tone over the first set of N-Mk targeted lines.

2. A vectoring controller (130) according to claim 1, wherein the configured vectoring matrix is a rectangular

Nx (N-Mk) precoding matrix ( PAk ) with at least one non-null off-diagonal precoding coefficient (PNI, PN for the respective Mk matrix rows corresponding to the second set of Mk supporting 1 i nes .

3. A vectoring controller (130) according to claim 2, wherein the configured rectangular vectoring matrix is a generalized inverse matrix of a (N-I%)XN reduced channel matrix (HA comprising far-end coupling coefficients from the plurality of N subscriber lines into the first set of N-Mk targeted lines.

4. A vectoring controller (130) according to claim 3, wherein the generalized inverse matrix is a Moore-Penrose pseudo-inverse of the (N-Mk)xN reduced channel matrix.

5. A vectoring controller (130) according to claim 1, wherein the configured vectoring matrix is a rectangular

(N-Mk)xN postcoding matrix ( QAK ) with at least one non-null off-diagonal postcoding coefficient (QIN, QIN) for the respective Mk matrix columns corresponding to the second set of Mk supporting lines.

6. A vectoring controller (130) according to claim 5, wherein the configured rectangular vectoring matrix is a generalized inverse matrix of a NX(N-MI<) reduced channel matrix

(HA comprising far-end coupling coefficients from the first set of N-Mk targeted lines into the plurality of N subscriber 1 i nes .

7. A vectoring controller (130) according to claim 6, wherein the generalized inverse matrix is a Moore-Penrose pseudo-inverse of the Nx(N-Mk) reduced channel matrix.

8. A vectoring controller (130) according to claim 1, wherein the vectoring controller is further adapted to determine the first and second sets of lines on a per tone basi s .

9. A vectoring controller (130) according to claim 1, wherein the plurality of tones comprises ill-conditioned tones.

10. A vectoring controller (130) according to claim 1, wherein the vectoring controller is further adapted to update respective line metrics (LMn) currently achieved over the plurality of N subscriber lines given the so-enabled and so-disabled tones and the so-configured vectoring matrices while iterating through the plurality of tones, and to determine the first and second sets of lines based on the so-updated line metrics .

11. A vectoring controller (130) according to claim 10, wherein the vectoring controller is further adapted to assign at least one subscriber line of the plurality of N subscriber lines, whose updated line metric exceeds a given line metric target (i_MTn), to the second set of Mk supporting lines for at least one subsequent tone iteration.

12. A vectoring controller (130) according to claim 10, wherein the vectoring controller is further adapted to assign at least one subscriber line of the plurality of N subscriber lines, whose updated line metric exceeds by a given margin at least one updated line metric of at least one further subscriber line of the plurality of N subscriber lines, to the second set of Mk supporting lines for at least one subsequent tone iteration.

13. A vectoring controller (130) according to any of claims 10 to 12, wherein the line metrics are aggregate data rates .

14. An access node (100) for providing broadband communication services to subscribers, and comprising a vectoring controller (130) according to any of claims 1 to 13.

15. A method for configuring a vectoring processor (120) that jointly processes Discrete Multi-Tone DMT communication signals (UL . UN; VI. .VN) to be transmitted over, or received from, a plurality of N subscriber lines (LL . LN) according to a vectoring matrix (Pk; Qk) , and comprising, for given ones (k) of a plurality of tones:

- enabling the given tone for direct data communication over a first set of N-Mk targeted lines (Ak) out of the plurality of N subscriber lines, and disabling the given tone for direct data communication over a second disjoint set of Mk supporting lines (Bk) out of the plurality of N subscriber lines, Mk denoting a non-null positive integer,

- configuring the vectoring matrix to use an available transmit or receive power at the given tone over the second set of Mk supporting lines for further enhancement of data signal gains at the given tone over the first set of N-Mk targeted lines.