Patent Description:
In a wired network access communication system such as a Digital Subscriber Line, DSL, communication system, a plurality of communication lines connect an access node, sometimes referred to as distribution point unit, DPU, with remote terminal nodes, sometimes referred to as customer premises equipment, CPE. By using discrete multi-tone modulation, DMT, the data intended for provision to a terminal node is first encoded into frequency domain symbols which are multiplexed onto K different frequency sub-carriers, also referred to as tones. These K frequency domain symbols each corresponding to a different tone, also referred to as tone data values, are then converted to a single time domain symbol for further transmission over the communication line to the respective terminal node. Crosstalk between the different communication lines may cause unwanted interference, i.e. disturb the transmitted time domain symbols. Crosstalk may be mitigated by pre-compensating the useful communication signals for the respective terminal nodes before transmission. The so-obtained pre-compensated communication signals are then jointly transmitted to the respective terminal nodes. This technique is also referred to as vectoring.

Vectoring is a computationally intensive process for which cost and power increases with the number of communication lines and number of tones. A technique for reducing the computational requirements is discontinuous time-frequency operation, DTFO. When using DTFO, the available tones are divided into two sets of size K1 and K2, respectively. All terminal nodes may use the set of K1 tones for data transmission, while only a single selected terminal node can use the set of K2 tones for data transmission. This way, full vectoring is only required on a subset of tones, i.e., only on K<NUM> < K tones, while the data-rate of the selected terminal node can be boosted by also using the set of K2 tones.

<CIT> discloses a method and apparatus for transmitting data from a transmitter device to one or more receiver devices connected to the transmitter device via a respective wire connection, the transmitter device being operable to transmit signals onto the wire connections and a further wire connection at different tones, the method comprising: for each tone, allocating signals transmitted on the further wire connection as supporting signals for a particular wire connection, and measuring electromagnetic coupling between the further wire connection and that particular wire connection; using the measurements, determining a power allocation for transmitting a supporting signal on the further wire connection; for one or more of the tones: transmitting a signal onto the particular wire connection, and transmitting a supporting signal onto the further wire connection at the determined transmission power, thereby to cause crosstalk interference in the particular wire connection.

Embodiments according to the present invention are set out in the appended independent and dependent claims. Amongst others, it is an object of embodiments of the invention to provide a DTFO implementation that provides a high data rate while keeping the needed computational resources low.

The present application discloses an apparatus comprising means for performing:.

The second time domain symbol is thus not only added to the first time domain symbol for the selected terminal node but also to at least one of the first time domain symbols for the other terminal nodes. Although no vectoring operation is needed for the K<NUM> tone data values, i.e. no pre-compensation needs to be applied to the K<NUM> tone data values, a time domain shaping of the transmit symbols in the frequency band of the K<NUM> tones is achieved by the weighting operation. This shaping is further independent from the frequency band represented by the other K<NUM> tones. As a result, when adding the second weighted time domain symbols to the first time domain symbols, a time domain beamforming to the selected terminal node is achieved within the frequency band represented by the K<NUM> tones which boosts the data-rate of the selected terminal node without further affecting the other terminal nodes. Advantageously, all the above steps are performed digitally, i.e. in the digital domain before a further digital to analogue conversion. Moreover, the time domain beamforming requires only memory for up to N<NUM> weighting coefficients for the K<NUM> tones, while (frequency domain) full vectoring requires memory for up to K<NUM>×N<NUM> coefficients for the K<NUM> tones.

The adding in a weighted manner may be performed by adding or subtracting the second time domain symbol from the first time domain symbols. The amount of adding or subtracting, i.e. the weighted manner, may relate to applying a discrete or continuous phase shift, or to applying a multiplication with a single real-valued or complex-valued scalar. In other words, the adding in a weighted manner may be performed according to weighting coefficients.

According to example embodiments, the means are further configured for selecting the selected one of the N terminal nodes based on throughput requirements. For example, a terminal node with a highest throughput requirement may be selected.

According to example embodiments, the means can be further configured for selecting the weighting coefficients based on the selected one of the N terminal nodes. In other words, depending on the selected terminal node, a different set of weighting coefficients is selected. Sets of weighting coefficients associated with the respective terminal nodes may for example be stored in a lookup table and selected therefrom. Alternatively, weighting coefficients may be determined according to a predetermined ruleset or derived from a subset of common weighting coefficients.

According to example embodiment, weighting coefficients are selected from one of the groups comprising {<NUM>; <NUM>}; {<NUM>; -<NUM>}; or {<NUM>}. In other words, the second time domain symbol may be added to a first time domain symbol or subtracted from one or more of the first time domain symbols. By using these weighting coefficients, complex scaling circuitry is avoided and only adding or sign switching circuitry is needed. Further, all weighting coefficients may be equal, for example equal to <NUM>. This way, the weighting coefficients are the same for all selected terminal nodes.

According to example embodiments, the second time domain symbol is complex and the means are further configured for performing adding according to complex weighting coefficients. In other words, the conversion of the K<NUM> tone data values from the frequency to the time domain is performed by complex numbers. This way, the weighting coefficients may also be complex thereby achieving a greater flexibility in weighting. For example, the weighting coefficients may be selected such that a rotation of the second time domain symbol is performed in the complex plane. The complex weighting coefficients may for example be selected from the group comprising <NUM>, -<NUM>, j and -j.

According to example embodiments the means are further configured for calculating the N sets of first time domain symbols by performing a frequency to time domain conversion and an upsampling. As the K<NUM> tone data values are only a subset of the tones that are included in the transmitted time domain symbol, there is no need to perform a full resolution frequency to time domain conversion, i.e., from K<NUM> tone data values to <NUM>(K<NUM> + K<NUM>) time domain values. The calculation of this conversion is therefore simplified by first performing a conversion with an accuracy according to the K<NUM> tone data values, resulting into <NUM><NUM> time domain symbols, followed by an upsampling with factor (K<NUM> + K<NUM>)/ K<NUM>, resulting into the final <NUM>(K<NUM> + K<NUM>) time domain values.

According to example embodiments, the means are further configured for adding the second time domain symbol in a weighted manner by performing a bit shifting operation on the second time domain symbol, by performing a sign switching operation on the second time domain symbol, and/or by performing a swapping between a real and imaginary component of the second time domain symbol such that a weighting is achieved and adding the shifted, sign switched, and/or swapped second time domain symbol to the respective one of the first time domain symbols. Digital time domain values may be weighted by a bit shifting operation. For example, for complex weighting coefficients, a weighting by <NUM>, -<NUM>, j and -j may be achieved by such bit shifting operations.

In another embodiment the means comprises at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

The application discloses a second apparatus comprising means for performing:.

The present application relates to an access node for a telecommunication network comprising the apparatus according to the first and/or second example aspect.

The present application relates to a method comprising the steps of:.

The present application relates to a computer program product comprising computer-executable instructions for causing an apparatus to perform at least the following:.

Example embodiments of the present disclosure relate to the calculation of time domain symbols from tone data values for further transmission onto communication lines of a telecommunication network. <FIG> shows an example embodiment of such a telecommunication network <NUM> wherein an access node <NUM> connects to a plurality of N terminal nodes <NUM> to <NUM> over respective communication lines <NUM> to <NUM>. Terminal nodes <NUM> to <NUM> may be located at a client side near the customer premises. Network communication from the access node towards the terminal nodes <NUM> to <NUM> is also referred to as downstream, DS, communication <NUM>. Network communication from the terminal nodes <NUM> to <NUM> towards the access node <NUM> is also referred to as upstream, US, communication <NUM>. Telecommunication network <NUM> uses discrete multi-tone modulation (DMT), to first encode digital data into frequency domain symbols which are multiplexed onto different frequency sub-carriers or tones. These frequency domain symbols each corresponding to a different tone, also referred to as tone data values, are then converted to a time domain symbol for further transmission over a respective one of the communication lines <NUM>-<NUM> to a respective one of terminal nodes <NUM>-<NUM>. To this respect, telecommunication network <NUM> may be referred to as a digital subscriber line, abbreviated by DSL, telecommunication network.

Crosstalk channels between the different communication lines <NUM>-<NUM> may be present, because, for example, different lines are bundled together in a cable bundle or binder <NUM> over at least part of the trajectory between the access node <NUM> and terminal nodes <NUM>-<NUM>. The crosstalk channels cause unwanted interference, i.e. a time domain symbol transmitted from the access node <NUM> to one of the terminal nodes <NUM>-<NUM> may be disturbed by other time domain symbols transmitted from the access node <NUM> to the other terminal nodes. Because of this, the time domain symbol received at a terminal node will also comprise portions of the other time domain symbols. Such interference is also referred to as far end crosstalk, FEXT. Crosstalk may be mitigated by pre-compensating the communication signals for the respective terminal nodes before transmission by the anticipated crosstalk. The so-obtained pre-compensated communication signals are then jointly transmitted to the respective terminal nodes. This technique is also referred to as vectoring.

Vectoring is a computationally intensive process for which cost and power increases with the number of communication lines and number of tones. One technique for reducing the computational requirements is discontinuous time-frequency operation, DTFO. When using DTFO, the available tones are divided into two sets of size K1 and K2, respectively. All terminal nodes <NUM>-<NUM> may use the set of K1 tones for data transmission, whereas only a selected one of the terminal nodes <NUM>-<NUM> can use the set of K2 tones for data transmission. This way, full vectoring is only required on a subset of tones, i.e., only on K<NUM> < K tones, while the data-rate of the selected terminal node can be boosted by also using the set of K2 tones.

The below example embodiments describe, among others, means and circuitries for calculating time domain symbols according to such a discontinuous time-frequency operation, DTFO, mode. Embodiments according to the present disclosure may be used in telecommunication systems operating according to telecommunication protocols supporting DTFO, for example according to the recommendations defined by the ITU-T project G.

<FIG> shows example embodiment of an apparatus <NUM> for calculating N time domain symbols <NUM>, <NUM>. <NUM> from tone data for transmission onto N communication lines. To this end, apparatus <NUM> may be included in an access node <NUM>. In the embodiment depicted in <FIG>, apparatus <NUM> comprises several depicted functional blocks, hereafter also described as "circuitry". In other embodiments all functions of the apparatus can all be performed by one single processor, or other combinations of functional units performed by e.g. co-operating processing modules. In general the apparatus comprises means (<NUM>, <NUM>, <NUM>, <NUM> ) for receiving N sets of K1 tone data values, for receiving a set of K2 tone data values, for pre-compensating N sets of K<NUM> tone data values for crosstalk between N communication lines, calculating from the pre-compensated N sets of K<NUM> tone data values N sets of first time domain symbols, calculating a second time domain symbol from the set of K<NUM> tones values, adding the second time domain symbol in a weighted manner to the first time domain symbols such that the second time domain symbol is added to the first time domain symbol for the selected terminal node and to at least one other of the first time domain symbols for the respective other terminal nodes, and providing the N time domain communication symbols to an output of the apparatus.

The embodiment of <FIG> comprises a further circuitry <NUM> configured to pre-compensate N sets of K<NUM> tone data values <NUM>, <NUM>, <NUM> for crosstalk between the N communication lines. The N sets of K<NUM> tone data values relate to respective N terminal nodes connected to the N communication lines in a digital communication system, e.g. nodes <NUM>-<NUM> of telecommunication system <NUM>. The K<NUM> tones are a subset of a total amount of K tones wherein K = K<NUM> + K<NUM>. The K<NUM> other tones are reserved for a selected one of the terminal nodes. To this end, apparatus <NUM> also receives a single set of K<NUM> tone data values <NUM> for the selected one of the N terminal nodes.

Each tone value may be the result of a preceding bit loading operation during which a certain amount of data bits are loaded onto the respective tone. The number of data bits that are loaded onto a certain tone may be dependent on the channel conditions of the communication line. The number of bits that are loaded onto a certain tone may also be the same for all K<NUM> and/or K<NUM> tone data values. A tone value may be represented by an in-phase, I, and quadrature, Q, component. Each component may then be digitally represented by a binary value.

The K<NUM> tone data values may be associated with a first frequency band B<NUM> available in a communication channel between the access node and respective terminal nodes. The K<NUM> tone data values may be associated with a second other frequency band B<NUM> available in this communication channel. The first frequency band B<NUM> may be lower than this second frequency band B<NUM>, i.e. the first frequency band corresponds to a lower band and the second frequency band to a higher band. In a telecommunication system according to the G. mgfast recommendation, the total available number of tones may range from K = <NUM> to K = <NUM> or even up to K = <NUM>. The following selection may then be made for frequency bands B<NUM> and B<NUM>:.

Circuitry <NUM> calculates N sets of K<NUM> pre-compensated tone data values <NUM>, <NUM>. <NUM> from the N sets of K1 tones values <NUM>, <NUM>. Apparatus <NUM> further comprises a circuitry <NUM> for converting the N sets of K1 pre-compensated tone data values <NUM>, <NUM>. <NUM> to respective N sets of digital time domain symbols <NUM>, <NUM>. Similarly, apparatus <NUM> comprises a circuitry <NUM> for converting the single set of K2 tone data values <NUM> into a single digital time domain symbol <NUM>. A digital time domain symbol is a consecutive series of digital values representing a time-varying communication signal that is to be transmitted onto one of the communication lines.

Apparatus <NUM> further comprises a circuitry <NUM> that is configured to add the time domain signal <NUM> to the respective N time domain symbols <NUM>, <NUM>. <NUM> in a weighted manner. In a weighted manner may be understood as that the circuitry <NUM> is further configured to i) add the time domain symbol <NUM> to the one of the time domain symbols <NUM>, <NUM>. <NUM> that pertains to same terminal node, i.e. the terminal node for which the data bits loaded onto the K<NUM> tone data values is destined; and to ii) add the time domain symbol <NUM> to at least one other of the time domain symbols <NUM>, <NUM>. In case there is no boosting applied for a selected terminal node, no addition may be performed at all, i.e. steps i) and ii) are skipped.

According to an example embodiment, circuitry <NUM> may further comprise N adder circuitries <NUM>, <NUM>. <NUM> for adding N weighted time domain symbols <NUM>, <NUM>. <NUM> to the respective N time domain symbols <NUM>, <NUM>. Circuitry <NUM> then further comprises a weighting circuitry <NUM> that is configured to weight the time domain symbol <NUM> according to N weighting coefficients thereby producing the N weighted time domain symbols <NUM>, <NUM>.

The so-obtained N digital time domain symbols <NUM>, <NUM>. <NUM> may then be further processed in a transmission pipeline for transmission onto N respective communication lines to the N respective terminal nodes. By the weighted addition of the time domain symbol <NUM>, an improved delivery of the symbol to the selected terminal node is achieved while keeping the processing and memory requirements to a minimum. The circuitry <NUM> has the effect that a beamforming effect is achieved towards the selected terminal node within the frequency band corresponding to the K<NUM> tone data values. Furthermore, the pre-compensating circuitry <NUM> and conversion circuitries <NUM>, <NUM>. <NUM> are independent from the signal <NUM>, i.e. these circuitries' processing requirements are only based on the K<NUM> tone data values.

The weighting coefficients used by circuitry <NUM> may be different for each selected terminal node, i.e. for each selected terminal node a different set of N weighting coefficients are needed. These different sets may be stored locally in circuitry <NUM>, for example in a lookup-table wherein circuitry <NUM> retrieves the set of N applicable weighting coefficients based on an identification of the selected terminal node. The weighting circuitries may also be directly provided by another circuitry <NUM>. Or, this circuitry <NUM> may update the weighting coefficients stored in the lookup-table from time to time, e.g. when the weighting coefficients have changed.

The weighting coefficients may further be determined by circuitry <NUM> by globally optimizing the coefficients for a maximum data throughput for each of the selected terminal nodes. In <FIG> this circuitry is depicted as not forming part of the apparatus <NUM>. In other embodiments this circuitry can be part of the apparatus.

For this determination of the weighting coefficients, a locally-optimal approach may be followed wherein the best coefficient is determined for one line while keeping the other coefficients constant. When the optimal coefficient for this line is found, the circuitry <NUM> proceeds to a next iteration wherein the best coefficient is determined for a next communication line. The circuitry may repeat such iteration several times over all communication lines until a convergence is reached.

In an embodiment circuitry <NUM> may correspond to a vectoring control entity, VCE, which is also not part of apparatus <NUM>. Such a vector controlling entity may be configured to determine the vectoring coefficients for use in circuitry <NUM> for the pre-compensation of the tone data values <NUM>, <NUM>. These vectoring coefficients may be derived from a channel estimation, i.e. an estimation of cross-talk channels between the N communication lines for the K<NUM> tones. Such vectoring control entity receives performance measurements from the remote terminal nodes for deriving the vectoring coefficients. Likewise, the VCE may also retrieve performance measurements on the K<NUM> tones from the selected terminal node. Alternatively, the VCE may also derive the weighting coefficients from channel measurements on the K<NUM> tones of the communication lines.

Determining the selected terminal node from the N terminal nodes may be done by the dynamic resource allocation (DRA) component and may be based on different metrics such as the throughput requirements of the terminal nodes. The node with the highest throughput requirement may then be selected as the selected node and have its data-rate boosted. Throughput requirements may be further based on physical metrics such as the actual instantaneous throughput or a throughput demand from the terminal nodes. Throughput requirements may also be based on economic metrics such as a subscription model wherein users may pay extra for having their data rate boosted. Selection of the selected terminal node may vary in time, for example on a symbol by symbol basis, such that several terminal node may benefit from a boosted data rate.

<FIG> illustrates a further embodiment <NUM> of the above described apparatus <NUM>. Circuitry <NUM> is further embodied as an N × N vectoring processor <NUM> for pre-compensating the N sets of K<NUM> tone data values <NUM>, <NUM>. <NUM> for crosstalk thereby obtaining the N sets of K1 pre-compensated tone data values <NUM>, <NUM>. Frequency to time domain conversion circuitries <NUM>, <NUM>. <NUM> are further embodied by a respective first circuitry <NUM>, <NUM>. <NUM> that applies a <NUM> times K<NUM> point inverse fast Fourier transform, IFFT, to obtain respective time domain symbols <NUM>, <NUM>. To ensure real-valued time domain symbols, such a <NUM><NUM>-point IFFT operation includes a mirror copy of the K<NUM> complex-valued pre-compensated tone data values, resulting in <NUM><NUM> IFFT input values with a complex conjugate symmetric structure. These symbols <NUM>, <NUM>. <NUM> are then upsampled by respective upsampling circuitries <NUM>, <NUM>. <NUM> according to an upsampling factor of (K<NUM> + K<NUM>)/K<NUM> thereby obtaining digital time domain symbols <NUM>, <NUM>. <NUM> having <NUM> times K, <NUM>, time values. Similarly, circuitry <NUM> comprises an IFFT circuitry <NUM> as further embodiment of circuitry <NUM>. IFFT circuitry <NUM> is further configured to calculate a <NUM> times K point IFFT on <NUM> input values, which are formed as follows:.

thereby obtaining the time domain symbol <NUM> having <NUM> times K, <NUM>, time values. The time domain symbol <NUM> is then either added to or subtracted from the respective time domain symbols <NUM>, <NUM>. <NUM>, i.e. the time symbol <NUM> is first weighted by a weighting coefficient with value <NUM> or -<NUM> and then added by adding circuitry <NUM>, <NUM>. The subtraction may for example be performed by a digital sign switching circuitry. The so-obtained time domain symbols <NUM>, <NUM>. <NUM> may then be further processed by further circuitries of the transmission pipeline, e.g. by adding a cyclic extension by respective circuitries <NUM>, <NUM>. <NUM>, and by digital and analogue front ends, DFEs and AFEs, <NUM>, <NUM>. Certain tones may further be dropped during frequency domain processing by means of loading said tones with a zero-valued frequency symbol, e.g. the tone associated with the zero frequency, DC, and Nyquist frequency.

The frequency-domain representation of the communication signal received from a circuitry <NUM> by the selected terminal node n for a certain tone k selected from the K<NUM> tones may be written as follows: <MAT> wherein.

The term time-domain thereby refers to the fact that the weighting coefficients Dm,n are applied after the frequency to time domain conversion circuitry <NUM>.

According to other example embodiments different time-domain beamforming schemes may be achieved by choosing other weighting coefficients. For example, weighting coefficients may be selected from the set {<NUM>, <NUM>}, {<NUM>, <NUM>, -<NUM>} or even all have a value <NUM>. In the last example, the coefficients do not need to be changed upon change of the selected terminal node. The coefficients may also have a real scalar value thereby achieving a scaling of the time domain symbol <NUM> in circuitry <NUM> by performing a multiplication with the respective coefficients in circuitry <NUM>.

<FIG> illustrates a further embodiment <NUM> of the above described apparatus <NUM>. Circuitry <NUM> is further embodied as an N × N vectoring processor <NUM> for pre-compensating the N sets of K<NUM> tone data values <NUM>, <NUM>. <NUM> for crosstalk thereby obtaining the N sets of K1 pre-compensated tone data values <NUM>, <NUM>. Frequency to time domain conversion circuitries <NUM>, <NUM>. <NUM> are further embodied by a respective first circuitry <NUM>, <NUM>. <NUM> that applies a <NUM> times K<NUM> point inverse fast Fourier transform, IFFT, including a mirror copy of the K<NUM> pre-compensated tone data values, to obtain respective time domain symbols <NUM>, <NUM>. These symbols are then upsampled by respective upsampling circuitries <NUM>, <NUM>. <NUM> according to an upsampling factor of (K<NUM> + K<NUM>)/K<NUM> thereby obtaining digital time domain symbols <NUM>, <NUM>. <NUM> having <NUM> times K, <NUM>, time values. Circuitry <NUM> further comprises a complex IFFT circuitry <NUM> as further embodiment of circuitry <NUM>. IFFT circuitry <NUM> is further configured to calculate a <NUM> times K point complex IFFT from the K<NUM> tone data values <NUM> thereby obtaining the time domain symbol <NUM> having <NUM> times K, <NUM>, complex time values. i.e. time values represented by complex numbers. Circuitry <NUM> then applies complex weighting coefficients to the complex time domain symbol <NUM>. The so-obtained weighted complex time domain symbols values are then converted to real values by taking two times the real component of these complex values. The resulting weighted real time domain symbols <NUM>, <NUM>. <NUM> are then added to the respective time domain symbols <NUM>, <NUM>, <NUM> thereby obtaining the output time domain symbols <NUM>, <NUM>. These time domain symbols may then be further processed by further circuitries of the transmission pipeline, e.g. by adding a cyclic extension by respective circuitries <NUM>, <NUM>. <NUM>, and by digital and analogue front ends, DFEs and AFEs, <NUM>, <NUM>.

By the embodiment of the apparatus via the shown means <NUM>, a more general form of time-domain beamforming is obtained because this circuitry uses complex valued weighting coefficients. To achieve this, IFFT circuitry <NUM> calculates complex-valued time-domain symbols <NUM>. According to an embodiment, this is done by discarding the mirror copy of the complex-valued tone data values <NUM>. To explain this, define the real-valued symbol sequence {x<NUM>,. , xK-<NUM>} as the output of a K-point IFFT complex-valued symbol sequence {X<NUM>,. ,XK-<NUM>}, together with the discrete Fourier transform symmetry given by <MAT>, ∀k = <NUM>. K - <NUM> and <MAT> (according to a zero-valued DC and Nyquist frequency). Then for every l ∈ [<NUM>, K - <NUM>] the following holds: <MAT> From this it follows that complex-valued time-domain beamforming may be implemented by:.

When using complex weighting coefficients, the coefficients may be chosen such that a rotation is performed in the complex plane, i.e. by selecting the weighting coefficient as Dm,n = ejθ and θ∈[<NUM>,2π]. In addition to rotating, the coefficients may also achieve a scaling, i.e. by selecting the weighting coefficient as Dm,n = Aejθ with A being real scalar and θ∈[<NUM>,2π].

To further simplify the rotation, a set of predefined complex weighting coefficients may be selected such as for example Dm,n = {<NUM>, -<NUM>, j, -j}. In such case, the weighting circuitry <NUM> may be further simplified by implementing a combination of a bit shifting operation, a sign switching operation or a swapping operation between the real and imaginary value of the complex time domain symbol for the four multiplications with the weighting coefficients as follows: <MAT> <MAT> <MAT> and <MAT> wherein.

As used in this application, the term "circuitry" as well as "means" may refer to one or more or all of the following:.

This definition of circuitry and/or means applies to all uses of this term in this application, including in any claims.

Claim 1:
Apparatus (<NUM>) comprising means (<NUM>, <NUM>, <NUM>) for performing:
- pre-compensating (<NUM>, <NUM>, <NUM>) N sets of K<NUM> tone data values (<NUM>, <NUM>, <NUM>) for crosstalk between N communication lines (<NUM>-<NUM>); the N sets of K<NUM> tone data values pertaining to respective N terminal nodes (<NUM>-<NUM>) of a digital communication system;
- calculating (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) from the pre-compensated N sets of K<NUM> tone data values (<NUM>, <NUM>, <NUM>) N sets of first time domain symbols (<NUM>, <NUM>, <NUM>);
- calculating (<NUM>, <NUM>, <NUM>) a second time domain symbol (<NUM>, <NUM>, <NUM>) from a set of K<NUM> tone data values (<NUM>, <NUM>, <NUM>); the K<NUM> tone data values pertaining to a selected one of the N terminal nodes;
- adding (<NUM>, <NUM>, <NUM>) the second time domain symbol in a weighted manner to the first time domain symbols such that the second time domain symbol is added to the first time domain symbol for the selected terminal node and to at least one other of the first time domain symbols for the respective other terminal nodes.