FIELD OF INVENTION

The present invention relates to operating communication systems, in particular communications systems that communicate data signals over pairs of wires. Such communication systems include all Digital Subscriber Line (DSL) systems in which each pair of wires typically comprises a twisted metallic pair (usually copper) as commonly found within telephone access networks throughout the world.

BACKGROUND

DSL technology takes advantage of the fact that although a legacy twisted metallic pair (which was originally installed to provide merely a Plain Old Telephone Services (POTS) telephony connection) might only have been intended to carry signals using differential mode at frequencies of up to a few Kilohertz, in fact such a channel can often reliably carry signals at much greater frequencies. Moreover, the shorter the channel, the greater is the range of frequencies over which signals can be reliably transmitted (especially with the use of technologies such as Discrete Multi-Tone (DMT), etc.). Thus as access networks have evolved, telecommunications network providers have expanded their fibre optic infrastructure outwards towards the edges of the access network, making the lengths of the final portion of each connection to an end user subscriber (which is still typically provided by a metallic twisted pair) shorter and shorter, giving rise to correspondingly greater and greater bandwidth potential over the increasingly short twisted metallic pair connections without having to bear the expense of installing new optic fibre connections to each subscriber.

However, a problem with using high frequency signals is that a phenomenon known as crosstalk can cause significant interference reducing the effectiveness of channels to carry high bandwidth signals in situations where there is more than one metallic pair carrying similar high frequency signals in close proximity to one another. In simple terms, the signals from one wire can "leak" onto a nearby channel carrying similar signals and appear as noise to the other channel. Although crosstalk is a known problem even at relatively low frequencies, the magnitude of this effect tends to increase with frequency to the extent that at frequencies in excess of a few tens of Megahertz (depending on the length of the channels in question), the indirect coupling (e.g. from a near end of a second channel to a remote end of a first channel) can be as great as the direct coupling (e.g. from the near end of the first channel to the remote end of the first channel). In order to alleviate the problems caused by crosstalk (especially Far

End Crosstalk or 'FEXT" as it is known) a technology known as vectoring has been developed in which knowledge of the signals sent over crosstalking channels is used to reduce the effects of the crosstalk. In a typical situation a single DSLAM acts as a co-generator of multiple downstream signals over multiple crosstalking channels and also as a co-receiver of multiple upstream signals from the same multiple crosstalking channels, with each of the channels terminating at a single Customer Premises Equipment (CPE) modem such that no common processing is possible at the CPE ends of the channels. In such a case, downstream signals are pre-distorted to compensate for the expected effects of the crosstalking signals being sent over the neighbouring crosstalking channels such that at reception at the CPE devices the received signals are similar to what would have been received had no crosstalking signals been transmitted on the crosstalking channels. Upstream signals on the other hand are post-distorted (or detected in a manner equivalent to their having been post- distorted) after being received at the co-receiver (the DSLAM) in order to account for the effects of the crosstalk which has leaked into the signals during their transmission.

Such vectoring techniques can deal very successfully with situations where the indirect coupling is significantly weaker than the direct coupling. However as the relative strengths of the direct and indirect coupling approach each other, vectoring is less able to function effectively. SUMMARY OF INVENTION

The present inventors have realised that Twisted Metallic Pair (TMP) connections contained within in a common binder, which are typically used in DSL systems, tend to be affected by crosstalk in non-linear fashion. The present inventors have realised that the degree to which a TMP connection is affected by crosstalk tends to be dependent on the spatial position of that TMP connection in the common binder, with TMP connections that are located closer to a central axis of the binder being generally more affected by crosstalk effects compared to TMP connections that are located closer to an outer edge of the binder. The TMP connections located closer to the central axis of the binder tend to lose more electromagnetic energy via crosstalk to surrounding TMP connections, compared to the amount of energy lost due to crosstalk from the TMP connections that are located closer to the outer edge of the binder. Similarly, the TMP connections located closer to the central axis of the binder tend to receive more crosstalk interference from other TMP connections in the binder compared to the amount received by the TMP connections that are located closer to the outer edge of the binder.

The present invention provides a method for assessing how susceptible a TMP connection within a binder is to the effects of crosstalk, relative to the other TMP connections within that binder. The present invention provides a metric indicative of the susceptibility of a TMP connection within a binder to the effects of crosstalk, relative to the other TMP connections within that binder.

The present invention provides a method for assessing a degree or level to which a TMP connection within a binder is affected by crosstalk, relative to the other TMP connections within that binder. The present invention provides a metric indicative of a degree or level to which a TMP connection within a binder is affected by crosstalk, relative to the other TMP connections within that binder.

In a first aspect, the present invention provides a method of operating a communication system. The communication system comprises a transmitter device connected to a plurality of wire connections, the transmitter device being operable to transmit signals onto the wire connections over a plurality of different channels. The method comprises, for each channel: measuring, for each of the other channels, an electromagnetic coupling between that channel and that other channel; and determining, using the measured electromagnetic coupling measurements for that channel, a score. The score is indicative of how well that channel crosstalk couples with the other channels, i.e. the score indicates how strongly that channel affects the other channels via crosstalk interference from that channel and/or how susceptible that channel is to receiving crosstalk interference from the other channels. The method further comprises operating the communication system in dependence upon the determined scores. Measuring the electromagnetic coupling between a channel and another channel may comprise measuring a value of a channel transfer function between those channels. Measuring a value of the transfer function for a channel may comprise measuring at least one parameter selected from the group of parameters consisting of: a channel response of that channel, an impulse response of that channel, and a frequency response of that channel.

The step of performing an action may comprise allocating, to one or more receiver devices, one or more of the channels based on the determined scores.

Allocating one or more channels may comprises: ranking the plurality of channels based on the score values; and sequentially, in rank order, allocating the channels to the one or more receiver devices. Allocating one or more channels may comprise: allocating a first channel as a direct channel to one or more of the receiver devices, the first channel corresponding to a first score; and allocating a second channel as an indirect channel to one or more of the receiver devices, the second channel corresponding to a second score. The first score may indicates a less strong affect on other channels via crosstalk interference compared to that indicated by the second score. The first score may indicate a lesser susceptibility to receiving crosstalk interference than that indicated by the second score. The step of operating may comprise connecting, using the wire connections, the transmitter device to the one or more receiver devices in accordance with the allocation. The step of operating may further comprise transmitting data from the transmitter device to the one or more receiver devices via the one or more wire connections connected therebetween.

The method may further comprise weighting the determined scores based on one or more parameters selected from the group of parameters consisting of: a length of one or more of the wire connections, a transmission power for a signal that is to be transmitted along a channel, a frequency or frequency band for a signal that is to be transmitted along a channel, and a property of a receiver device that is to receive a transmission via a channel from the transmitter device. The step of operating may be based on the weighted scores.

The method may be performed for each of one or more specified frequency bands.

The method may further comprise determining, for each pair of channels (i, j), a value of:

where: i is an index for the different channels; j is an index for the different channels; m is a frequency index for one or more different tones; ™. is a channel gain in the ith channel caused by the jth channel; and

Ω . . , , L , and βΐ are variables; and

j i.i the scores are determined based on the determined values of μ™ _} ..

The method may further comprise determining, for each pair of channels (i, j), a value of: where: i is an index for the different channels; j is an index for the different channels; m is a frequency index for one or more different tones; ™ is a channel gain in the ith channel caused by the jth channel; and γ™ is a channel gain along the ith channel; and the scores are determined based on the determined values of Γ" ¹ ,. .

The method may further comprise determining, for each of the channel indexed by j ^' e {1,...,&} , and for each tone m , determining a value of a function ϊ , wherein: argmaxl//™ Λ Γ™. } for each of the channel indexed by j ^' e jl,...,&} , and for each tone m , setting a value of a parameter P- · - i , and 0 elsewhere; and for each channel indexed by j ^' e {l,..., k} , determining a value of the score O _j , where:

m

In a further aspect, the present invention provides a method of operating a communication system, the communication system comprising a transmitter device and one or more receiver devices, each of the one or more receiver devices being connected to the transmitter device via a respective wire connection, the transmitter device being operable to transmit signals onto the wire connections over one or more different channels, the method comprising: for each ordered pair of channels, measuring a respective value of a transfer function, the measured value of the transfer function being indicative of crosstalk interference from one of the channels of the ordered pair of channels to the other of the channels of the ordered pair; for each channel, determining, using the values of the transfer function indicative of a transmission onto that channel, a score, the score being representative of a relative sensitivity of that channel to receiving crosstalk interference from the other channels; and performing an action based on the determined scores. In a further aspect, the present invention provides a method of operating a communication system, the communication system comprising a transmitter device connected to a plurality of wire connections, the transmitter device being operable to transmit signals onto the wire connections over a plurality of different channels, the method comprising: for each channel, measuring one or more values of a parameter selected from the group of parameters consisting of: a channel response of that channel, an impulse response of that channel, and a frequency response of that channel, wherein each of the one or more parameter values corresponds to a respective other channel and is dependent on a crosstalk interference transmitted from that other channel onto that channel; for each channel, determining, using the parameter values measured for that channel, a score, the score being representative of how good that channel is at crosstalk coupling with the other channels; and operating a communication system based on the determined scores.

In a further aspect, the present invention provides a method of operating a communication system, the communication system comprising a transmitter device connected to a plurality of wire connections, the transmitter device being operable to transmit signals onto the wire connections over a plurality of different channels, the method comprising: for each channel: measuring, for each of the other channels, an electromagnetic coupling between that channel and that other channel; and determining, using the measured electromagnetic coupling measurements for that channel, a score, the score being indicative of how well that channel performs as a direct or an indirect coupling between the transmitter device and the one or more receivers; and operating the communication system in dependence upon the determined scores.

In a further aspect, the present invention provides apparatus for use with a communication system. The communication system comprising a transmitter device and one or more receiver devices, each of the one or more receiver devices being connected to the transmitter device via a respective wire connection, the transmitter device being operable to transmit signals onto the wire connections over one or more different channels. The apparatus comprises: measurement means configured to measure, for each channel, for each of the other channels, an electromagnetic coupling between that channel and that other channel; and one or more processors configured to determining, using the measured electromagnetic coupling measurements for that channel, a score, the score being indicative of either how strongly that channel affects the other channels via crosstalk interference from that channel, or how susceptible that channel is to receiving crosstalk interference from the other channels, and further configured to operate the communication system based on the determined scores.

In a further aspect, the present invention provides a program or plurality of programs arranged such that when executed by a computer system or one or more processors it/they cause the computer system or the one or more processors to operate in accordance with the method of any preceding aspect.

In a further aspect, the present invention provides a machine readable storage medium storing a program or at least one of the plurality of programs according to the previous aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be better understood, embodiments thereof will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic illustration (not to scale) of a broadband deployment; Figure 2 is a schematic illustration (not to scale) showing a cross section through a bundle of TMP connections of the broadband deployment.

Figure 3 is a schematic illustration (not to scale) showing further specific details of the broadband deployment; Figure 4 is a process flow chart showing certain steps of a method performed by the broadband deployment; and

Figure 5 is a process flow chart showing certain steps of a process performed at a step of the process of Figure 4.

SPECIFIC DESCRIPTION OF EMBODIMENTS

In the below description, reference will be made to modes of communication. Herein, the term "mode" is used to indicate the nature of the manner in which signals are transmitted between transmitter and receiver. In particular, as will be appreciated by persons skilled in the art, there are three principal such modes of communication: differential mode, phantom mode, and common mode. In all three of these modes the signal is transmitted (excited) and received (observed) as the (changing) potential difference (voltage differential) between two voltages (or equivalently between one "live" voltage and one "reference" voltage). In the differential mode the signal is transmitted/observed as the difference in potential between two wires (typically between two wires of a twisted metallic pair). In the phantom mode at least one of the voltages is the average voltage of a pair of wires (note that such average can vary without impacting on a signal carried in the differential mode across that same pair of wires - in this sense the phantom mode can be orthogonal to signals carried in the differential mode if carefully chosen); the term pure phantom mode may be used to specify that both voltages being compared with each other are average voltages, each average voltage being the average or common voltage of at least one pair of wires. Second and higher order phantom modes can also be obtained by using the average voltage of two or more average voltages as one of the voltages to be compared, etc. Finally, the common mode refers to the case where one of the voltages being compared is the "Earth" or ground reference voltage (or something substantially similar for telecommunications purposes). Naturally, it is possible for various mixed modes to also be used for carrying signals. For example, one reference voltage could be a common ground and the other could be the average between the voltages of two wires in a twisted metallic pair (to generate a mixed mode of phantom and common modes). However, in general, reference to a differential mode in this specification is used to refer to a pure differential mode, i.e. it does not include any phantom or common mode component so a mode comprising a comparison between the voltage on a single wire and the average voltage between the voltages of two other wires may be referred to as an impure phantom mode rather than a mixed phantom and differential mode, etc.

Phantom channels can be constructed from different combinations of TMP connections. For instance, a first and a second TMP connection can together generate a single unique phantom channel which has a similar behaviour to that of each directly coupled differential mode channel formed across each pair in terms of channel directivity. However, phantom modes, as mentioned earlier, are due to the variation of the average voltages of the pairs. For more than two coupled pairs, the pairs may couple to each other in the phantom mode in various orthogonal and non-orthogonal manners, e.g. two distinct (but non-orthogonal) phantom mode channels may be exploited which share one common pair. Preferred embodiments of the invention select and construct only orthogonal phantom channels.

Reference is also made throughout the below description to direct and indirect coupling and direct and indirect channels. A direct channel is one in which the same physical medium and the same mode of transmission is used for both the transmission of the signal and for the reception of the signal. Thus a normal differential mode transmission across a single TMP from transmitter to receiver would constitute a direct (differential mode) channel between the transmitter and the receiver. By contrast, a channel in which the transmitter transmitted a signal onto a second TMP in differential mode but was received by a receiver from a first TMP in differential mode (the signal having "crosstalked" across from the second to the first pair) is an example of an indirect channel, as is a case in which a signal is transmitted by a transmitter in a phantom mode across the averages of the voltages of the wires in each of a first and second TMP and received (having "crosstalked/mode" converted) by a receiver connected to just the first TMP in differential mode. Moreover, where there are multiple TMP connections emanating from a single transmitter (e.g. an Access Node (AN) or Digital Subscriber Line Access Multiplexor (DSLAM), etc.) in such a way that multiple direct and indirect channels are formed between the transmitter and multiple receivers, the set of TMP connections and their derivative channels (direct and indirect and of various different modes) can be considered as forming a "unified" dynamic shared or composite channel over which a number of virtual channels may be overlaid (i.e. the virtual channels are overlaid over the underlying common shared channel). In this context, a virtual channel can be considered as an overlay channel by which data can be directed to individual receivers even though a single common underlying signal is transmitted onto the underlying common channel; this can be achieved for example by means of a suitable multiple access technique such as Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) or simply be using suitable encryption techniques, etc. Referring now to the Figures, Figure 1 is a schematic illustration (not to scale) of an example broadband deployment in which embodiments of the present invention may be employed.

In this example, the deployment comprises a Distribution Point Unit (DPU) 10 which is connected to three user premises 31 , 32, 33 (which in this example are flats within a single building 30) via respective Twisted Metallic Pair (TMP) connections, namely a first TMP connection 21 , a second TMP connection 22, and a third TMP connection 23. The TMP connections 21 , 22, 23 connect between an Access Node (AN) 16 (which may, for example, be a DSLAM) within the DPU 10 and respective Customer Premises Equipment (CPE) modems 51 , 52 via respective network termination points 41 , 42 within the respective user premises 31 , 32. In this example, as described in more detail later below with reference to Figure 2, the TMP connections 21 , 22, 23 are contained within a common binder 60 over at least a portion of their lengths. In particular, in this example, the TMP connections 21 , 22, 23 are contained within the binder 60 at least between a point at or proximate to where the TMP connections 21 , 22, 23 exit from/enter to the DPU 10 and a point at which the TMP connections 21 , 22, 23 diverge from each other to connect to the respective user premises 31 , 32, 33.

In this example, the DPU 10 additionally includes an Optical Network Termination (ONT) device 14 which provides a backhaul connection from the DPU 10 to a local exchange building via an optical fibre connection such as a Passive Optic-fibre Network (PON) and a controller 12 which co-ordinates communications between the AN 16 and the ONT 14, and which may perform some management functions such as communicating with a remote Persistent Management Agent (PMA). As will be apparent to a person skilled in the art, the illustrated deployment involving an optical fibre backhaul connection from a distribution point and a TMP connection from the distribution point to the "customer" premises is a sort of deployment for which the G.FAST standard is intended to be applicable. In such a situation, the TMP connections 21 , 22, 23 may be as short as a few hundred metres or less, for example possibly a few tens of metres only, and because of this it tends to be possible to use very high frequency signals (e.g. up to a few hundred Megahertz) to communicate over the short TMPs because the attenuation of high frequency signals is insufficient to prevent them from carrying useful information because of the shortness of the channels. However, at such high frequencies crosstalk can become a significant issue. This tends to be the case where the crosstalking channels travel alongside each other for part of their extent (as is the case in the binder 60, as illustrated in Figure 1 ); however, crosstalk is also tends to be an issue at high frequencies (e.g. over 80 MHz) even where the channels only lie close to one another for a very small portion of their total extent (e.g. just when exiting the DPU 10). G.FAST proposes simply using vectoring techniques at all frequencies where there are crosstalking channels in order to mitigate against the crosstalk effects.

Figure 2 is a schematic illustration (not to scale) showing a cross section through the binder 60 and TMP connections contained therein. In this example, the binder 60 is a flexible, electrically non-conductive casing or sheath (e.g. made of plastics) that surrounds, and holds together a bundle of TMP connections. The bundle of TMP connections, which is bound together by the binder 60, comprises the first, second, and third TMP connections 21 , 22, 23 and, additionally, a plurality of further TMP connections (which are not shown in Figure 1 , and some of which are indicated in Figure 2 by the reference numerals 62). The TMP connections 21 , 22, 23, 62 within the bundle are crosstalking channels.

In this example, each further TMP connection 62 is connected at one of its ends to the AN 16 such that a signal may be transmitted (and/or received) along that further TMP connection 62 by the AN 16. Also, each further TMP connection 62 may be, at its end distal with respect to the AN 16, either be connected to a respective receiver (and/or transmitter), or be disconnected from any electronic device.

In this example, different TMP connections 21 , 22, 23, 62 have different respective spatial positions in the binder 60. Different TMP connections 21 , 22, 23, 62 contained within the common binder 60 tend to be affected by crosstalk interference to different extents, dependent upon their spatial position within the binder 60. In particular, those TMP connections that are closer to a central axis 64 of the bundle of TMP connections tend to be affected by crosstalk to a greater extent than those TMP connections that are closer to an outer edge of the bundle of TMP connections. For example, the first TMP connection 21 may lose more electromagnetic energy via crosstalk to surrounding TMP connections, and/or may receive more crosstalk interference from surrounding TMP connections, compared to the second and third TMP connections 22, 23, which are located relatively closer to the outer edge of the bundle of TMP connections. In some embodiments, the AN 16 operates so as to use signals transmitted on the further TMP connections 62 and/or phantom channels to change (for example, boost or improve) signals transmitted along the first, second, and third TMP channels 21 , 22, 23. Signals transmitted onto the further TMP connections 62 and/or phantom channels will tend to crosstalk onto the conventional differential mode channels provided by the TMP connections 21 , 22, 23, thereby modifying the signals received by the end user receivers (i.e. the termination point and CPE modem combinations 41 /51 , 42/52, 43/53). The present embodiment includes a Phantom Channel - Multiple Optimisation Problem device (PC-MOP) which acts to choose one or more further TMP connections 62 and/or phantom channels to use to change the signals being transmitted along one or more of the TMP connections 21 , 22, 23. This selection by the PC-MOP may be performed such as to try to achieve a particular set of one or more objectives. Figure 3 is a schematic illustration (not to scale) showing further details of the AN 16 and CPE modems 51 , 52, 53 that allow for operation according to below described embodiments. The AN 16 and CPE modems 51 , 52, 53, and connections therebetween shown in Figure 3 may be considered to be a communication system. The AN 16 comprises first, second, and third Data Source, Data Encoder and Serial to Parallel converter (DSDESP) modules 161 1 , 1612 and 1613. These are essentially conventional functions within a DSL modem and will not be further described here except to point out that each one's output is a set of data values di - dM each of which can be mapped to both a set of one or more bits and to a point within a modulation signal constellation associated with a respective tone on which the data value is to be transmitted. For example, if a tone ti is determined to be able to carry 3 bits of data, a corresponding data value will be set to one of 2 ³ = 8 different values (e.g. to a decimal number between 0 and 7) each of which corresponds to a different constellation point within an associated signal constellation having 8 different constellation points. The data values for a single symbol can be thought of as forming a vector of data values (one for each data-carrying tone) and together carry the user data to be transmitted to the end user associated with a respective end user modem 51 , 52, 53 together with any overhead data (e.g. Forward Error Correction data etc.).

The data values leaving each DSDESP module 161 1 , 1612, 1613 are then passed (in an appropriate order) to respective Multiple bit level Quadrature Amplitude Modulation (M-QAM) modulators 1621 , 1622, 1623 which convert each input data value to a respective complex number to x _M ² , and j to x _M ³ each of which represents a complex point within a complex number constellation diagram. For example, a data value = 7 (=1 1 1 in binary) might be mapped by the M-QAM modulator 1621 to the complex number 1 -i for tone 1 where tone 1 has been determined (by say modem 51 ) to be able to carry 3 bits of data each.

Each of these complex numbers to x _M ² , and j to x _M ³ is then entered into a vectoring precoder module 1630 (which in the present embodiment is a single common vectoring precoder module 1630) which performs a largely conventional vectoring operation in order to precode the transmissions to be sent using a combination of predetermined vectoring coefficients and information about the signals to be transmitted onto the other channels within the relevant vector group in a manner, which is well known to those skilled in the art, to compensate for the expected effects of crosstalk from the other channels in the vector group.

In some embodiments, the vectoring precoder module 1630 is operable to additionally precode the transmissions in such a way as to cause them to be pre-compensated for the expected crosstalk effects produced not only by the neighbouring channels operating in a direct differential mode (as per standard vectoring), but also for the effects of crosstalk coming from any signals being transmitted onto one or more phantom channels (or other channels, e.g. the further TMP connections 62, which are not direct differential mode channels). In order to do this, the vectoring precoder module 1630 may receive information about channel estimations of the respective phantom channel(s) (or other channels which are not direct differential mode channels) and also information about any weighting values used to combine signals to be transmitted over the phantom channel(s) (or other channels which are not direct differential mode channels).

An ability of the vectoring precoder module 1630 to receive the weighting values and channel estimation values, which it may use to perform its precoding functions, is illustrated in Figure 3 by the line between the PC-MOP & MICOP & MRC & Management entity module 1690 (which performs general management functions in addition to its specific functions described in greater detail below and, for brevity, may hereinafter be referred to either as the "management entity" or the "PC-MOP module") and the vectoring precoder module 1630. In this embodiment, the PC-MOP module 1690 calculates appropriate values for the channel estimations and the weighting values required by the vectoring precoder module 1630 and the MICOP & MRC precoder module 1640. To do this, the PC-MOP module 1690 may use data reported back to it from the end user modems 51 , 52, 53. The processes and procedures for achieving this are largely conventional and well known to persons skilled in the art and so they are not discussed in great detail herein except to note that it may utilise a backward path from the user modems 51 , 52, 53 to the AN 16. This may be achieved in practice in that the user modems 51 , 52, 53 are transceivers capable of receiving and transmitting signals over the TMPs 21 , 22, 23 as is the AN 16. The receiver parts of the AN 16 and the transmitter parts of the user modems 51 , 52, 53 have simply been omitted from the drawings to avoid unnecessary complication of the figures and because these parts are entirely conventional and not directly pertinent to the present invention. Moreover, each of the user modems 51 , 52, 53 may additionally contain a management entity responsible for performing various processing and communication functions. Any of a multitude of suitable techniques can be employed for obtaining data useful in generating channel estimations. For example, known training signals can be transmitted onto selected channels by the AN 16 during a special training procedure and the results of detecting these by the user modems 51 , 52, 53 can be sent back to the AN 16 in a conventional manner. Additionally, special synchronisation symbols can be transmitted, interspersed with symbols carrying user data, at predetermined "locations" within a "frame" comprising multiple symbols (e.g. at the beginning of each new frame) and the results of attempting to detect these synchronisation symbols can also be sent back to the AN 16 to generate channel estimation values. As is known to persons skilled in the art, different synchronisation signals/symbols can be sent over different channels simultaneously and/or at different times etc. so that different channel estimations (including importantly indirect channels and indirect channels) can be targeted and evaluated, etc.

In this embodiment, the output from the vectoring precoder module 1630 is a set of further modified complex numbers x to x _M ^l , to x _M ² , and J to xl, . These complex numbers are then passed to a Mixed- Integer Convex

Optimisation Problem and Maximal Ratio Combiner (MICOP and MRC) precoder module 1640 (hereinafter referred to as the MICOP and MRC precoder module 1640) which, in the present embodiment, uses weighting values together with channel estimation values provided to it by the PC-MOP module 1690 to calculate, from the modified complex numbers received from the vectoring pre-coder module 1640 (and the weighting values and channel estimation values from the PC-MOP module 1690), further modified (or further pre-distorted) values for the complex numbers to be passed to the IFFTs 1651 - 1652. Thus, MICOP and MRC precoder module 1640 modifies the received numbers to x _M ² , and J to x _M ³ to generate corresponding further modified complex numbers x ^' \ to x _M ^l , x ^' to x _M ² , and to x _M ³ which form (ultimately) the signals to be used in driving the respective TMPs 21 , 22, 23 in direct differential mode.

Also, the MICOP and MRC precoder module 1640 may additionally generate one or more new sets of complex numbers, for example x ^{' 4} to x _M ^A , x to x _M ⁵ , x ^' to x _M ⁶ , and so on, for forming (ultimately) the signals to be used to drive a respective one or more of the further TMPs 62, or a respective (single ended) phantom mode channel, to be accessed via the MPAD module described below. (In Figure 3, only one of these new sets of complex numbers, namely x ^{' 4} to x ^' _M ⁴ , is depicted for reasons of clarity and ease of understanding of the Figures. However, it will be appreciated by the skilled person that, in practice, multiple of these new sets of complex numbers may be generated by the MICOP and MRC precoder module 1640, and may then be sent to a respective IFFT module.)

Any appropriate way of generating the sets of complex numbers ( x{ to x _M ^l , etc.) may be performed. For example, the method of transmitting data in differential and phantom modes that is described in WO 2016/139156 A1 , which is incorporated herein in its entirety, may be implemented. Once these values have been calculated by the MICOP and MRC precoder 1640 they are passed to the respective IFFT modules 1651 -1654, with super-script 1 values going to IFFT 1651 , superscript 2 values going to IFFT 1652, and so on. The next two steps of the processing are conventional and not relevant to the present invention. Thus, each set of generated values (e.g. x ^' \ to x _M ^l ) is formed by the respective IFFT module into a quadrature time domain signal in the normal manner in Orthogonal Frequency Division Multiplexing (OFDM)/DMT systems. The time domain signals are then processed by a suitable Analogue

Front End (AFE) module 1661 to 1664 again in any suitable such manner including any normal conventional manner. After processing by the AFE module 1650, the resulting analogue signals are passed to a Multiple Phantom Access device (MPAD) module 1670. In overview, the MPAD module 1670 provides switchable access to centre taps of any of the TMPs such that any of the possible phantom channels associated with the connected channels can be driven by the incoming signal arriving from AFE 1664 as well as directly passing on the signals from AFEs 1661 -1663 directly to TMPs 21 -23 for driving in the normal direct differential mode. During transmission over the TMP connections 21 , 22, 23 the signals will be modified in the normal way according to the channel response of the channel and due to external noise impinging onto the connections. In particular, there will typically be crosstalking (including, for example, far-end crosstalking) between the three direct channels (the direct channels being one from the transmitter 16 to the modems 41 -43 via the TMPs 21 -23), the further channels provided by the further TMP connections 62, and the phantom channels. However, the effect of the precoding is to largely precompensate for the effects of the crosstalk. Additionally, the targeted receivers may benefit from increased SNR of the received signal destined for them arriving via crosstalk from one or more of the further TMP connections 62, and/or the phantom channel. After passing over the TMP connections 21 , 22, 23 the signals are received by the modems 41 -43 at a respective Analogue Front End (AFE) module 5150, 5250, 5350 which performs the usual analogue front end processing. The thus processed signals are then each passed to a respective Fast Fourier Transform (FFT) module 5140, 5240, 5340 which performs the usual conversion of the received signal from the time domain to the frequency domain. The signals leaving the FFT modules 5140, 5240, 5340, y{ to y _M ^l , y to y _M ² , and y to y _M ³ , are then each passed, in the present embodiment, to a respective Frequency domain EQualiser (FEQ) module 5130, 5230, 5330. The operation of such frequency domain equaliser modules 5130, 5230, 5330 is well-known in the art and will not therefore be further described herein. It should be noted however, that any type of equalisation could be performed here, such as using a simple time-domain linear equalizer, a decision feedback equaliser, etc. For further information on equalisation in OFDM systems, the reader is referred to: "Zero-Forcing Frequency- Domain Equalization for Generalized DMT Transceivers with Insufficient Guard Interval " by Tanja Karp, Steffen Trautmann, Norbert J. Fliege, EURASIP Journal on Applied Signal Processing 2004:10, 1446-1459.

Once the received signal has passed through the AFE, FFT and FEQ modules, the resulting signals to x _M ^l , x ^' to x _M ² , and to x _M ³ tend to be similar to the complex numbers originally output by the M-QAM modules 1621 -1623, except that there may be some degree of error resulting from imperfect equalisation of the channel and the effect of external noise impinging onto the channels during transmission of the signals between the AN 16 and the modems 51 , 52, 53. This error will in general differ from one receiving modem to the next. This can be expressed mathematically as x ^' _m ^l = x _m ¹ + e _m ¹ etc. Provided the error however is sufficiently small the signal should be recoverable in the normal way after processing by the M-QAM demodulator modules 5120-5320 where a corresponding constellation point is selected for each value x _m ^l depending on its value (e.g. by selecting the constellation point closest to the point represented by the value x _m ^l unless trellis coding is being used, etc.). The resulting values to d _M ^l , dl to d _M ² , and d to d^ should mostly correspond to the data values dl to d _M ^l , dl to d _M ² , and dl to d _M ³ originally entered to the corresponding M-QAM modules 1621 , 1622, 1623 respectively within the AN 16. These values are then entered into a respective decoder (and received data processing) module 51 10, 5210 and 5230 which reassembles the detected data and performs any necessary forward error correction etc. and then presents the recovered user data to whichever service it is addressed to in the normal manner, thus completing the successful transmission of this data.

Following now from the above overview of Figure 3, a more detailed explanation is provided of the non-conventional elements within the embodiment illustrated in Figures 1 to 3 and described briefly above.

In this embodiment, the PC-MOP module 1690 is a component configured to determine, for each channel (e.g. for each of the TMP connections 21 , 22, 23, 62 and phantom channels), a value of a metric. The PC-MOP module 1690 may be configured to perform and/or process measurements of the channel transfer functions to determine the metric values. The metric is a crosstalk assessment metric indicative of how well that channel crosstalk coupling with other channels. A value of the metric determined for a channel is indicative of a degree or level to which that channel is affected by crosstalk, relative to the other channels. The determined values may be representative of an amount of signal leaking to and/or from that TMP connection 21 , 22, 23, 62 or phantom channels. The determination of the metric values is described in more detail later below with reference to Figure 4.

Also, in this embodiment, the PC-MOP module 1690 is a component configured to, based on the determined metric values, allocate one or more of the channels (i.e. to one or more of the TMP connections 21 , 22, 23, 62 and phantom channels) to the end user modems 51 , 52, 53. The channels are then connected to the end user modems 51 , 52, 53 according to that allocation. For example, in this embodiment, as described in more detail later below with reference to Figure 4, based on respective metric values determined for the first, second, and third TMP connections 21 , 22, 23, the PC-MOP module 1690 specifies that those first, second, and third TMP connections 21 , 22, 23 are to be used as direct channels from the transmitter 16 to the modems 51 , 52, 53 respectively. In other words, the PC-MOP module 1690 allocates, based on the determined metric values, the first, second, and third TMP connections 21 , 22, 23 respectively to the three modems 51 , 52, 53. The TMP connections 21 , 22, 23 are then connected or attached to the modems 51 , 52, 53 respectively, according to the allocation. Also for example, in this embodiment, as described in more detail later below with reference to Figure 4, based on the determined metric values, the PC-MOP module 1690 allocates, to one or more of the further TMP connections 62 and/or the phantom channels, a role of crosstalking with one or more of the first, second, and third TMP connections 21 , 22, 23, for example to increase the SNR of signals thereon.

Apparatus, including the PC-MOP module 1690, for implementing the above arrangement, and performing the method steps to be described later below, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and/or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media.

What follows is a mathematical explanation of the functioning of certain elements of the above described system. This mathematical explanation is useful in the understanding of embodiments of crosstalk assessment methods, which embodiments are described in more detail later below with reference to Figure 4. Considering the system shown in Figures 1 to 3 and described in more detail earlier above, the system comprises a bundle of k TMPs and/or phantom channels 21 , 22, 23, 62. The full transmission characteristics for a single frequency of the channel may be represented as the following channel transfer matrix:

where /i. indicates a channel transfer function, a value of which is dependent on crosstalk transmissions from the jth TMP/phantom channel onto the ith TMP/phantom channel. In other words, h _{t j} is a measure of the electromagnetic coupling of the jth channel to the ith channel. For example, h _j may be indicative of the extent of coupling between the jth channel and the ith channel. Values of h _{t j} may be dependent on an attenuation on the amplitude of signals on the ith channel caused by the jth channel. Values of h _j may be dependent on a delay and/or phase shift on the phase of signals on the ith channel caused by the jth channel.

In this embodiment, the channel transfer matrix H is known i.e. measured. For example, the channel transfer matrix H may be determined using known channel estimation techniques.

Values of h _j may be measured, for example, using test signals, by the MPAD 1670. Measurement of the h . values may comprise, or be regarded as equivalent to, measuring one or more of the following parameters selected from the group of parameters consisting of: a channel response of the ith channel under conditions in which the jth channel (e.g. only the jth channel) is transmitting crosstalk interference onto the ith channel, an impulse response of the ith channel under conditions in which the jth channel (e.g. only the jth channel) is transmitting crosstalk interference onto the ith channel, and a frequency response of the ith channel under conditions in which the jth channel (e.g. only the jth channel) is transmitting crosstalk interference onto the ith channel.

In this embodiment, h _j indicates the channel transfer function for transmissions on the jth TMP/phantom channel to the first TMP connection 21 . Also, h _{2 j} indicates the channel transfer function for transmissions on the jth

TMP/phantom channel to the second TMP connection 22. Also, h _{3 j} indicates the channel transfer function for transmissions on the jth TMP/phantom channel to the third TMP connection 23.

The bundle of k TMPs are contained in the common binder 60.

The maximum capacity of a channel (which may also be referred to as Shannon's capacity limit) may be expressed as: where: C is the maximum capacity of the channel (in bits/second) for the given channel; B is the bandwidth of the channel (in Hertz); S is the signal power (in Watts); and N is the noise power (in Watts). The ratio S/N is called the Signal to Noise Ratio (SNR).

In this embodiment, determining the metric values and allocating the TMP connections 21 -23, 62 to be direct or crosstalk couplers may be based on maximum capacity of the channel (Equation 1 ). The metric values may be based on a combinatorial optimisation model, which includes maximising the following function:

where: i=1 , ...k, and i is an index for the different channels receiving crosstalk interference from channel j; j=1 , ...,k, and j is an index for the different channels transmitting of crosstalk interference onto channel i; m is a frequency index for M different tones, m=1 , ... M ;

Cj is a cost function associated with the jth channel transmitting crosstalk interference; p™j is a binary allocation factor, where p™ =1 if the jth channel transmits crosstalk interference onto the ith channel, and p^ =0 if the jth channel does not transmit crosstalk interference onto the ith channel ; s™j is a power level of crosstalk interference transmitted from the jth channel onto the ith channel, at the mth tone; and ™ is a channel gain in the ith channel caused by crosstalk interference from the jth channel. In this embodiment, ™. is a ratio of power coupling coefficient to the noise level at the mth tone, i.e.

, where n is a noise level of the ith channel for given

tone m. The channel gain 7™. can also include other factors, for example one or more factors selected from the group of factors consisting of: coding gain, margin, gap value, and deterministic noise signals.

In this embodiment, the above objective function (equation (2)) is ised with respect to the following constraints:

(3)

< 1, Vm (4)

(5) where: P _T is the maximum transmitting power (also known as the

Aggregate Transmit power (ATP)) permitted for the bundle of k TMPs and/or phantom channels; p _m is a transmission power mask for tone m. In this embodiment, the power mask is an upper threshold for power for signal transmission at tone m above which power value transmission is not permitted. The transmission power mask(s) may be set, for example, by an official regulatory body; b^ is a lower bound for the channel capacity. In this embodiment, b^ is set to zero (0). However, in other embodiments, b^ may have a different appropriate value; is an upper bound for the channel capacity. In this embodiment, is a bit limit for the TMPs and/or phantom channels.

Values for b . and/or b may result from hardware limitations.

The above objective function (equation (2)) is a concave objective function that is to be maximised subject to the constraints in equations (3)-(6). Since the objective function (equation (2)) is concave, its optimisation is tractable. The optimisation of the objective function (equation (2)) proceeds with the Lagrangian as follows:

where Ω _; ■ , , μ ^™ . , βΖ _* , and β ^™ _* are Lagrangian multipliers.

To solve (7) and to show its optimality, in this embodiment the Karush Kuhn Tucker (KKT) conditions are satisfied. These conditions are as follows:

1 . Feasibility of the primal constraints as well as the multipliers, i.e. Ω^ , μ and βΐ≥0 .

2. The gradient of the Lagrangian (equation (7)) with respect to s , and the gradient of the Lagrangian (equation (7)) with respect to p both become zero.

Differentiating the Lagrangian (equation (7)) with respect to s , i.e. d£/d s = 0, and rearranging for s gives the optimal power formula:

,. + ¾)1 ^η 2 (Watt/Hz) (8)

i

Differentiating the Lagrangian (equation (7)) with respect to p , i.e.

6LI6 p = 0, and rearran ing for μ gives:

(9)

What will now be described with reference to Figure 4 is an embodiment of a method in which the TMP connections 21 , 22, 23, 62 and phantom channels are assessed based on how strongly they crosstalk with each other. In this embodiment, this method is performed by the MPAD 1670 and/or the PC- MOP 1690. However, in other embodiments, the method is performed by a different entity instead of or in addition to the MPAD 1670 and/or the PC-MOP 1690.

Figure 4 is a process flow chart showing certain steps of an embodiment of a process of allocating and connecting TMPs to end user receivers. ln this embodiment, b^ = 0. Thus, also ΐ = 0. However, in some embodiments, b^ may have a different, non-zero value. Also, in some embodiments, ? may have a different, non-zero value.

At step s2, the index i is initialised to one, i.e. i = 1 .

At step s4, for each of the TMP connections 21 -23, 62 and phantom channels operating as a transmitter of crosstalk interference (i.e. for each j e {1,..., £} ), and also for each tone m e {Ι,.,.,Μ} , a value of //" is computed.

Computation of the values of //" is described in more detail later below with reference to Figure 5.

At step s6, for each of the TMP connections 21 -23, 62 and phantom channels operating as a transmitter of crosstalk interference (i.e. for each channel indexed by j e {1,..., £} ), and also for each tone m e {Ι,.,.,Μ} , a value of

Γ™. is computed. In this embodiment, Γ™. is given by:

where: ™ is a channel gain in the ith channel caused by the jth channel ; and

Y _j is a channel gain along the jth channel.

In this embodiment, the gains and y _j are determined or measured by the MPAD 1 670 or by state of the art channel estimation in DSL devices, e.g. vectoring control entity (VCE) channel feedback in G.fast.

At step s8, for each of the TMP connections 21 -23, 62 and phantom channels operating as a transmitter of crosstalk interference (i.e. for each channel indexed by j e {1,..., £} ), and also for each tone m e {Ι,.,.,Μ} , a value of the function ϊ is determined, where: =argmax{ . A^.}

(11) where the wedge symbol (Λ) denotes the logical conjunction (AND) operator.

In this embodiment, ϊ is the index identifier of the TMP connection 21- 23, 62 or phantom channel that most strongly receives crosstalk interference from the jth TMP connection 21-23, 62 or phantom channel, at the mth tone. In other words, ϊ is an identifier of the channel acting as a receiver of crosstalk which is most strongly affected by, or crosstalk coupled to, signals transmitted on the jth channel at the mth tone. At step s16, which follows step s8, for each of the TMP connections 21-

23, 62 and phantom channels operating as a transmitter of crosstalk interference (i.e. for each channel indexed by j ^' e {1,...,£}), and also for each tone me {Ι,.,., } , a value of the binary allocation factor/?™ . is set equal to one, i.e.:

Of. = 1 , and elsewhere

At step s18, for each of the TMP connections 21-23, 62 and phantom channels operating as a receiver of crosstalk interference (i.e. for each channel indexed by ie {!,...,£}), a value of the function O _j is determined, where:

O _j may be referred to as an orthogonality factor. In this embodiment, the function O . is a metric indicative of a how good, over the entire frequency range of interest, the jth channel is as a direct coupler.

In some embodiments, a different score value is determined instead of or in addition to O _j is determined. An example different score value is a value of the function O. , is determined, where:

i ,m

In this embodiment, the function O. . is a metric indicative of a how good, over the entire frequency range of interest, the jth channel is as an indirect coupler to the ith channel.

A further example of a different score value is a score value for a channel that indicates of how susceptible that channel is to receiving crosstalk interference from the other channels.

At step s20, the channels indexed by J— 1,..., * are ordered (i.e. ranked) based on their respective orthogonality factors 0 (or other score values). In this embodiment, the channels are ranked in order of decending 0 values. However, in other embodiments, TMP connections 21 -23, 62 and phantom channels are ordered, sorted, or ranked in a different appropriate way, for example in order of ascending O . value. Relatively higher values of 0 indicate that the corresponding channels are better direct couplers compared to the channels corresponding to lower O . values. Thus, in this embodiment, the TMP connections and phantom channels are ranked in order, from best direct couplers to worst direct couplers. In some embodiments, the channels may be ranked depending on how good they are as an indirect coupler. In some embodiments, the channel that may be regarded as the "best" direct coupling channel may be identified using the following formula:

J arg max

best _ direct _ coupling _ candidate ΑΟΛ

In some embodiment, the channel that offers the maximum FEXT contributions to other channels may be identified using the following formula: _ candidate

At step s22, the TMP connections 21 -23, 62 and/or phantom channels are allocated to the end user receivers (i.e. the termination point and CPE modem combinations 41 /51 , 42/52, 43/53) based on the ranking generated at step s20. Equivalently, the channels are allocated to the end user receivers based on their respective orthogonality factor values O _j . The TMP connections

21 -23, 62 are subsequently connected to the end user receiver according to the determined allocation.

In some embodiments, the TMP connections are sequentially allocated to end user receivers in ranking order, from highest O _j value to lowest O _j value.

In this embodiment, the TMP connections corresponding to the highest orthogonality factor values O _j are deemed to be most suitable for direct coupling to the end user receivers. This is because the relatively higher O _j values indicate that these TMP connections are affected, to a relatively lesser extent, by crosstalk interference, compared to other channels corresponding to relatively lower 0 values. In this embodiment, the TMP connections corresponding to the three highest orthogonality factor values 0 are the first, second, and third TMP connections 21 , 22, 23. The further TMP connections 62 correspond to relatively higher O . values than the first, second, and third TMP connections 21 , 22, 23 and are thus ranked lower. Thus, the first, second, and third TMP connections 21 , 22, 23 are allocated, as direct channels, to the first, second, and third modem combinations 41 /51 , 42/52, 43/53 respectively. Also, the first, second, and third TMP connections 21 , 22, 23 are directly connected, to provide direct couplings, to the first, second, and third modem combinations 41 /51 , 42/52, 43/53 respectively.

In this embodiment, the channels corresponding to the highest orthogonality factor values O _i . are deemed to be most suitable for crosstalk coupling to the end user receivers. This is because the relatively higher O _i . values indicate that these channels are relatively more highly crosstalk coupled with the other channels, compared to other channels corresponding to relatively lower O values. In some embodiments, one or more TMP connections and/or

•j

phantom channels corresponding to relatively higher orthogonality factor values O (e.g. one or more of the further TMP connections 62) are allocated, as indirect channels (i.e. crosstalk couplers), to one or more of the modem combinations 41 /51 , 42/52, 43/53. In other words, one or more of the lower ranked further TMP connections 62 (which correspond to relatively higher O _i values) may be allocated as an indirect channel for an end receiver. Preferably, the highest ranked (i.e. highest O value) TMP channels are allocated as indirect channels as these tend to be the most effective crosstalk couplers. At step s24, the AN 1 6 transmits signals along the TMP connections 21 -

23, 62 and phantom channels allocated, and connected, at step s22.

In particular, the AN 1 6 may transmit signals using differential mode transmission along a direct channel provided by one or more of the first, second, and third TMP connections 21 -23, to the modem combinations 41 /51 , 42/52, 43/53 respectively.

Also, the AN 1 6 may transmit signals along one or more of the indirect channels provided by a further TMP connection 62 or phantom channel, to be received by one or more of the modem combinations 41 /51 , 42/52, 43/53. Signals transmitted along indirect channels may change (for example, boost or improve) signals transmitted along the direct channels 21 -23. Thus, an embodiment of the crosstalk assessment method is provided.

Referring back to step s4 of the method of Figure 4, Figure 5 is a process flow chart showing certain steps of a process performed at step s4, i.e. computation of the values for μ™ . The process shown in Figure 5 and described in more detail below, is described as being performed for a single channel indexed by j ^' e {1,...,£} , and for a single tone m e {Ι,.,.,Μ} . However, it will be appreciated that, in practice, the process of Figure 5 is performed for each of the TMP connections 21 -23, 62 and phantom channels operating as a transmitter of crosstalk interference (i.e. for each channel indexed by j ^' e {1,...,£} ), and also for each tone m e {Ι,.,.,Μ} .

In this embodiment, b^ = 0. Thus, also β! = 0. However, in some embodiments, b^ may have a different, non-zero value. Also, in some embodiments, ? may have a different, non-zero value.

At step s30, β _* and 77™ are initialised to zero, and /?" is initialised to one. That is to say:

am m

TJ _j - = 0

i,j ' ^■ >

At step s32, a value of Ω. . is computed. In this embodiment, Ω. . is computed by using equations (8) and (3), with the parameters initialised as in step s30. In particular, equation (8) is plugged into equation (3) to give:

M is the number of different tones; and

Δ , is a frequency spacing between adjacent tones. At step s34, a value for 77™. is computed. In this embodiment, 77™. is computed by using equation (8) and the value for Ω. . computed at step s32. In particular, equation (8) is set equal to p _m (i.e. the transmission power mask for tone m), and the rearranged for 77™. to give: m ± ij β _{( 1 6)}

At step s36, a value for β _* is computed. In this embodiment, β , is computed using equation (8) and the equation lo^ i^ + ^s i,j Yi,j ) ⁼ ^ max . In particular, equation (8) is plugged into 1°β2 ^ ^s i,j7 j ) ⁼ ^ max and rearranged for β2 _> to give:

2^ (1 ^η 2)(Ω. , + 7™, )

β m'"ax = - 1* -

(17)

At step s38, a value for /" is computed using equation (9) and the computed and the values for Ω _; . , and β _^ computed at steps s32, s34, and s36 respectively.

After step s38, the method proceeds with step s6.

Thus, the process performed at step s4, i.e. a process of computing the values for is provided.

Advantageously, the above described method and apparatus tend to provide that channels that are relatively least affected by crosstalk interference are allocated, and directly coupled to, end user receivers. These channels may be used to provide for differential mode transmission to the end user receivers. Thus, signals received by the end user receivers in differential mode tend to suffer less from detrimental crosstalk effects. Advantageously, the above described method and apparatus tend to provide that channels that are relatively strong crosstalk couplers are allocated to end user receivers as indirect channels. Signals on these indirect channels (e.g. in phantom mode, common mode, or mixed mode) may be used to improve signal quality on the direct channels.

Advantageously, it tends to be possible to implement the above described methods and apparatus for G.fast and XG.fast at all frequencies.

Advantageously, any complex functionality for implementing certain preferred embodiments of the invention can reside solely in the access network (e.g. at an AN or DSLAM, etc.) rather than requiring any special Customer Premises Equipment (CPE), in certain preferred embodiments of the invention.

It should be noted that certain of the process steps depicted in the flowcharts of Figures 4 and 5 and described above may be omitted or such process steps may be performed in differing order to that presented above and shown in Figures 4 and 5. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally- sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally.

It should be noted that the described embodiments are couched in terms of the downstream direction of data only (i.e. from an Access Node/DSLAM to Customer Premises Equipment (CPE) devices). However, in a practical implementation the "transmitter" of the above embodiments (e.g. the Access Node) also, naturally, functions as a receiver for upstream transmissions from the various CPE devices (which are also therefore in practice operating as transceivers rather than just receivers). Embodiments of the invention may operate in an entirely conventional manner in the upstream direction.

In the above embodiments, the DPU is connected to three user premises via respective TMP connections which connect between the AN within the DPU and respective CPE modems within the respective user premises. However, in other embodiments, the DPU is connected to a different number of user premises (e.g. more than three) via a different number of TMP connections. In some embodiments, the DPU is connected to a different number of CPE modems via respective one or more TMP connections. In some embodiments, one or more user premises comprises multiple CPE modems.

In the above embodiments, the TMP connections and phantom channels are ranked and allocated based on computed values of the metrics O _j and/or O _i . However, in other embodiments, the TMP connections and phantom channels are ranked and allocated based on a different measure or assessment of their crosstalk couplings.

In the above embodiments, the TMP connections and phantom channels are ranked and allocated based on a measure or assessment of their crosstalk coupling strengths, e.g. values of the metric O _i and/or O _t . . However, in other embodiments, the ranking and allocation is dependent upon one or more different parameters in addition to the measure of crosstalk coupling. For example, the ranking and allocation may additionally be dependent upon one or more parameters selected from the group of parameters consisting of: a length of one or more of the TMP connections, a transmission power for signals transmitted along one or more of the TMP connections and/or phantom channels, a frequency or frequency band for signals transmitted along one or more of the TMP connections and/or phantom channels, and a transmission mode for transmitting signals along one or more of the TMP connections and/or phantom channels. For example, the metric O _j values (or other appropriate metric values) of the channels may be weighted depending upon the respective cable lengths, e.g., such that longer channels are weighted more highly (such that they are ranked relatively lower) than shorter channels. This may be done to account for signal attenuation along the channels. For example, long channels tend to experience higher levels of signal attenuation compared to relatively shorter channels.

In the above embodiments, the process of allocating channels to end user receivers and connecting those channels accordingly is performed over a frequency band defined by tones me {l,...,M} . In some embodiments, the frequency band defined by m e {l,..., } covers substantially an entirety of a transmission spectrum. In other embodiments, the frequency band defined by m e {l,..., } does not cover an entirety of the transmission spectrum. For example, in some embodiments, the frequency band defined by m e {l,..., } only covers a strict subrange of the transmission spectrum. In some embodiments, the process of allocating channels to end user receivers and connecting those channels accordingly is performed for each of a plurality of different strict subranges of the transmission spectrum. Thus, different score values (i.e. metric values) may be determined for different strict subranges of the transmission spectrum for a common channel. Thus, a channel may be allocated (and perform) a different role for different frequency signals.