Abstract:
The invention relates to a far-end crosstalk canceling circuit for a digital subscriber line transmission system, the transmission system including a plurality of line termination modems receiving discrete multitone symbols from corresponding network termination modems over a plurality of transmission channels. The invention estimates the modulated data actually transmitted by the network termination modems from the frequency components of the discrete multitone symbols received by the line termination modems and to evaluate the far-end crosstalk as a linear combination of these estimates. Far-end crosstalk cancellation for all the line termination modems is also provided in a centralized manner.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to digital subscriber line transmission systems, which allow, in particular, high speed communication on twisted pair telephone lines based on discrete multitone transmission (DMT). The invention relates more specifically to a far-end crosstalk (FEXT) canceller for canceling the crosstalk signal induced by modems located at the far-end of such a transmission system.  
           [0003]    2. Discussion of the Related Art  
           [0004]    [0004]FIG. 1 schematically shows a modem in a conventional DSL transmission system using discrete multitone. The modem includes a transmitter TX and a receiver RX. A serial stream of data X is provided to a mapper circuit  11  mapping each data into a symbol of a constellation, for example of a QAM (Quadrature Amplitude Modulation) constellation. The mapped values are then transformed into a set S of N components by a serial to parallel converter  12 , each component of the set being considered as a frequency domain coefficient. This set of frequency domain coefficients, hereafter also called DMT symbol, is provided to an inverse fast Fourier transform (IFFT) circuit  13  which generates a time domain block of samples and is followed by a parallel/ serial converter (P/S). This time domain block is therefore the sum of N sinusoidal subcarriers of different frequencies, the amplitude thereof being determined by the corresponding frequency domain coefficient received by the IFFT circuit.  
           [0005]    Each time domain block is cyclically prefixed (cp) and suffixed (cs) in a block  19  to eliminate or at least attenuate the Inter Symbol Interference (ISI) and the Inter Carrier Interference (ICI) caused by the channel, and is transmitted onto a telephone line  10  through a hybrid line interface  18 . The line interface  18  also receives incoming time domain blocks from another modem connected to line  10 .  
           [0006]    At the receiving side, the incoming time domain blocks from line  10  are provided to a fast Fourier transform (FFT) circuit  14  through a block  19 ′ that deletes the prefix and suffix and a serial/parallel converter (S/P) which calculates the N frequency domain coefficients for each block. The N frequency domain coefficients are then provided to an equalizer  15  which compensates for the attenuation and phase shift incurred by each frequency component. The equalized values are then serialized by a parallel to serial converter  16  into a stream of N complex numbers R(fj) and then processed by a demapper  17  attributing to each R(fj) the symbol Ŝc of the constellation which comes closest thereto. The demapper  17  further outputs the digital word {circumflex over (X)}c associated with the selected constellation point Ŝc.  
           [0007]    [0007]FIG. 2 schematically shows a DSL transmission system including a central office  20  communicating with a plurality of end-users over telephone lines  25 ,  26 . Modems  21 ,  22 ,  21 ′,  22 ′ have the structure represented in FIG. 1. The end of a telephone line connected to a modem of the central office  20  is called the line termination (LT) side while the end connected to a modem of an end-user is called the network termination (NT) side.  
           [0008]    Ideally, such a DSL transmission system allows the whole frequency band to be used for simultaneous full-duplex transmissions. However, in practice, different sources of noise disturb the transmissions and impede proper reception of data.  
           [0009]    For a given modem, three different sources of noise can be distinguished as illustrated in FIG. 2:  
           [0010]    the self-echo, i.e. for a given modem, the parasitic signal from the transmitter TX leaking to the receiver RX through the hybrid interface;  
           [0011]    the near-end crosstalk (NEXT) arising from signals in adjacent telephone lines  25 ,  26  with opposite transmission directions. More specifically, in the present example, the NEXT generated at the modem  21  is the parasitic signal received by this modem from the modem  22 . In this instance the NEXT is called LT-NEXT because the modem  21  is located on the LT side. Reciprocally, the NEXT generated at modem  21 ′ by the modem  22 ′ is called NT-NEXT;  
           [0012]    the far-end crosstalk (FEXT) arises from signals traveling along the same transmission direction in adjacent telephone lines. More precisely, in the illustrated example, the FEXT generated at the modem  21  is the parasitic signal received by this modem from the modem  22 ′ located on the opposite side, due to the coupling between the telephone lines  25  and  26  sharing a common binder. In this instance the FEXT is called LT-FEXT because the modem  21  is located on the LT side. Reciprocally, the FEXT generated at modem  21 ′ by the modem  22  is called NT-FEXT.  
           [0013]    Echo-cancellers for canceling self-echoes are known e.g. from U.S. patent application number 09/410,636, filed Oct. 1, 1999 and entitled DSL TRANSMISSION SYSTEM WITH ECHO-CANCELLATION, which is incorporated herein by reference.  
           [0014]    There is also known from U.S. Pat. No. 5,887,032, which is incorporated herein by reference, a canceller for canceling out the NEXT interference in an ADSL transmission system on the LT side. This canceller operates in the frequency domain and assumes, for a given subcarrier or tone, that the NEXT interference is proportional to the symbol value emitted by the modem transmitting on the interfering channel. The latter value is scaled by a given coefficient and subtracted from the symbol received by the modem suffering from the NEXT interference.  
           [0015]    Both self-echo cancellation and LT-NEXT cancellation are possible because the signal transmitted by the same modem (in the case of the self-echo) or by a neighboring modem of the central office (in the case of LT-NEXT interference) is directly available.  
           [0016]    FEXT cancellation is however intrinsically more complex than NEXT or self-echo cancellation because the modem transmitting over the interfering channel is now located on the far-end side and the actual values of the interfering symbols are therefore not known.  
         SUMMARY OF THE INVENTION  
         [0017]    An object of the present invention is to design a canceller circuit for a DMT based DSL transmission system capable of significantly removing the FEXT interference and having a simple structure.  
           [0018]    Another object of the present invention is to design an efficient FEXT canceling method in a DMT based DSL transmission system.  
           [0019]    These and other objects are achieved by a far-end crosstalk canceling circuit for a digital subscriber line transmission system, said transmission system comprising a plurality of line termination modems receiving discrete multitone signals from corresponding network termination modems over a plurality of transmission channels, each modem comprising time/frequency transforming means for transforming said discrete multitone signals into a discrete multitone symbol of frequency components and demapping means outputting for each frequency component the symbol of the constellation nearest thereto and the corresponding demodulated data; estimation means, in at least one line termination modem, for estimating the constellation symbols actually sent by the network termination modems, from the frequency components of the discrete multitone symbols received by all modems; calculation means for calculating a linear combination of said estimated modulated data, for subtracting said linear combination from the frequency components of said at least one line termination modem and for applying a resulting difference to the demapping means of said at least one termination modem; error calculation means for calculating the error distance between the constellation symbol from said at least one line termination modem and said difference; and updating means for updating the coefficients of said linear combination as a function of said error distance.  
           [0020]    The invention also provides a far-end crosstalk canceling method for a digital subscriber line transmission system, said transmission system comprising a plurality of line termination modems receiving discrete multitone signals from corresponding network termination modems over a plurality of transmission channels, each line termination modem comprising frequency transforming means for transforming said discrete multitone signals into a discrete multitone symbol of frequency components, and demapping means outputting for each frequency component the symbol of the constellation nearest thereto and the corresponding demodulated data, the method comprising the steps of: estimating, for at least one line termination modem, the constellation symbols actually sent by all the modems from the frequency components of the discrete multitone symbols received by said modems; calculating a linear combination of said estimated symbols, subtracting said linear combination from the frequency components of a discrete multitone symbol and applying the resulting difference to the demapping means of said at least one modem, to obtain a constellation symbol; calculating the error distance between said constellation symbol and said difference; and updating the coefficients of said linear combination as a function of said error distance.  
           [0021]    The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not of limitation with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1, previously described, schematically shows the structure of a modem suitable for use in a DSL transmission system;  
         [0023]    [0023]FIG. 2, previously described, schematically shows the different types of noise occurring in a DSL transmission system;  
         [0024]    [0024]FIG. 3 shows a first and a second embodiment of a FEXT canceller according to the invention;  
         [0025]    [0025]FIG. 4 shows a third embodiment of a FEXT canceller according to the invention;  
         [0026]    [0026]FIG. 5 shows a fourth embodiment of a FEXT canceller according to the invention; and  
         [0027]    [0027]FIG. 6 schematically shows the overall structure of a DSL transmission system comprising a FEXT canceller according to the third or the fourth embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0028]    The invention is based on the idea that the actual value of a symbol causing FEXT interference at the LT side can be obtained from the modem receiving this symbol. The modem receiving the FEXT interfering symbol and the modem receiving the FEXT corrupted symbol being both located at the central office, a connection between the two modems can be realized.  
         [0029]    [0029]FIG. 3 shows a first embodiment of the invention and more specifically a part of the receiver TX of a modem p on the LT side, receiving a FEXT corrupted signal. In this embodiment the blocks  38  and  39  represented with dotted lines do not exist.  
         [0030]    Each modem i on the LT side is connected to a modem c(i) on the NT side through a transmission channel. The blocks  35 ,  36 ,  37  correspond to the blocks  15 ,  16 ,  17  of the receiver RX illustrated in FIG. 1.  
         [0031]    This first embodiment aims at canceling the FEXT interference caused by the signals transmitted by n- 1  modems c(i), i=1 to n, i≢p.  
         [0032]    For clarity purpose, suppose first that a symbol carried by the subcarrier or tone fj is FEXT corrupted by symbols at the same frequency only. If, as illustrated on FIG. 2, H(fj)=(H kl (fj)) the transfer matrix of the n transmission channels from the NT to the LT side, with k,1=1 . . . n, fj being the frequency index with j=1 . . . n, we can write in the frequency domain for the frequency fj:  
         R(fj)=H(fj)*S(fj),  
         [0033]    where R(fj)=Rk(fj), k=1 . . . n, is the vector of the received frequency components and S(fj)=Sk(fj), k=1 . . . n, is the vector of the transmitted DMT symbols from the n modems, for the frequency fj.  
         [0034]    The FEXT interference for a given frequency fj and for a modem p can therefore be written:  
           FEXT                   (   fj   )       =       ∑     1   =   1     n            Hlp        (   fl   )            Sl        (   fj   )             ,     1   ≠   p                           
 
         [0035]    According to the first embodiment of the invention, the complex values S 1 (fj), 1=1 . . . n, 1=≢p are approximated by the symbols Ŝ1(fj), i.e. by the symbols of the constellation coming the closest to the respective received frequency components R 1 (fj), 1=1 . . . n, 1≢p, respectively output by the demappers  37 . This implies that the processing in the modem p is one-symbol delayed with respect to the other modems.  
         [0036]    The complex symbols Ŝ 1 (fj) from the other modems, 1=1 . . . n, 1≢p, are then linearly combined in block  34  and subtracted by a subtractor  31  from the received frequency component Rp(fj) to produce a FEXT-removed complex value Tp(fj). The demapper  37  of modem p outputs a demapped word {circumflex over (X)}p(fj) and the corresponding constellation point Ŝp(fj). The complex value Ŝp(fj) is subtracted from the complex value Tp(fj) to produce an error value. This error value is squared in a circuit  32  and processed in a block  33  to update the coefficients of the linear combination, for example according to the known steepest gradient algorithm. The updated values stored in block  33  will be used for FEXT canceling the next frequency component Rp(fj), i.e. the frequency component Rp(fj) of the next incoming block. After a few iterations, the linear combination coefficients converge towards the values Hlp(fj) of the transfer matrix.  
         [0037]    We have considered above FEXT cancellation at a single tone fj. It is clear however that the processing should be repeated for all the tones j=1 to N, the frequency coefficients Rp(fj) being sequentially output by the parallel to serial converter  36 . The linear combination coefficients for each frequency fj are stored in the memory of block  33 . After a few iterations the memory contains the values Hlp(fj), 1=1 . . . n and 1≢p, j=1 . . . N.  
         [0038]    We have assumed above that the FEXT at the different frequencies could be independently canceled. In a conventional DMT transmission system this can only be regarded as an approximation since the limited duration of the time domain blocks causes a spreading of the frequency components. Generally, the FEXT at a frequency fj depends also upon frequency components transmitted at neighboring frequencies. This problem can be tackled in two different ways.  
         [0039]    Firstly, the crosstalk canceller of FIG. 1 can be adapted so as to take into account intra-frequency crosstalk coefficients Hlp(fi,fj), the modification being straightforward: the linear combination coefficients are now function of a couple of frequencies fi, fj. The processing in modem p has also to be delayed for a full time block since the knowledge of the Ŝ 1 (fj) at all the frequencies is necessary before starting the FEXT cancellation.  
         [0040]    Secondly, if the modems are synchronous Zipper modems as described in the international application WO97/06619, which is incorporated herein by reference, the assumption made above is fully valid, i.e. the FEXT at a frequency fj is independent from the frequency components transmitted at the frequencies fi, i≢j. Indeed, in such modems, the cyclic extension added to each time domain block before transmission eliminates any inter-frequency crosstalk.  
         [0041]    [0041]FIG. 4 shows a second embodiment of the invention. In this embodiment, the FEXT second interference is canceled in a centralized manner by a crosstalk canceller  40  operating now for all the LT modems or at least for all the LT modems cross-linked by the same FEXT. The crosstalk canceller 40 receives the sets of frequency components Ri, i=1 to n (Ri=Ri(fj), j=1 to N) from FFT circuits  44  and uses Ri to approximate Si(fj). At time t, the vector R constituted by the Ri&#39;s is multiplied by the matrix H −1   t−1  which is an estimate of the inverse of the transfer matrix at time t− 1 . The resulting vector is split up in n sets (H −1   t−1  *R) i , each having N frequency components. Each set is parallel to serial converted by converters  46  and the frequency components (H −1   t−1 *R) j  (fj) are then demapped by demappers  47 . The demappers  47  output the nearest constellation symbols Ŝi(fj) and the digital words {circumflex over (X)}i(fj) associated therewith. For each line i, the N consecutive symbols Ŝi(fj) belonging to the same time domain symbol are converted back by serial to parallel converters  48  and fed back to the crosstalk canceller circuit  40 . The resulting vector Ŝ t  of N*n components is built and the vector H −1   t−1 *R is subtracted therefrom, giving an error vector of norm e t . The coefficients of the matrix H −1   t−1  are then updated, for example according to the steepest gradient algorithm, to produce at time t+ 1  an updated matrix H −1   t . The previous processing steps are iterated.  
         [0042]    In contrast with the first and second embodiments, it should be emphasized that equalization is directly provided by the crosstalk canceller itself since it is taken into account by the diagonal coefficients of the matrix H −1   t . In this embodiment the equalization coefficients and the crosstalk coefficients are co-estimated instead of being sequentially estimated. This leads to a more accurate evaluation of both groups of coefficients.  
         [0043]    [0043]FIG. 5 shows a fourth embodiment of the invention.  
         [0044]    No assumption as to inter-frequency FEXT has been made with respect to the third embodiment. However, as discussed above, if the modems are of the synchronous Zipper type, the inter-frequency FEXT is negligible and the matrix H -1   t  has the simple form of a block matrix exhibiting blocks H −1   t (fj), j=1 to N along its diagonal. In such an instance, FEXT cancellation can be performed sequentially for each frequency in a more simple way, as illustrated by FIG. 5. In contrast with FIG. 4, parallel to serial converters  56  sequentially provide the frequency components Ri(fj) to the crosstalk canceller  50 . The product of the vector R(fj) constituted by the Ri(fj)&#39;s is multiplied by the matrix H −1   t−1 (fj) which is an estimate of the inverse of the transfer matrix at time t− 1  and frequency fj. The N components (complex scalar values) of the resulting vector are demapped by demappers  57  and the respective closest constellation symbols Ŝi(fj), i=1 to N are fed back to the FEXT canceller. The error calculation and the updating of the coefficients are a simple transposition of those set out in the description of the third embodiment.  
         [0045]    [0045]FIG. 6 shows the overall structure of a DSL transmission system with a FEXT canceller  60  according to the third or the fourth embodiment connected to n LT modems M 1  . . . Mn. Each modem is connected to a bi-directional transmission line  61 , an input Din and an output Dout for inputting the digital words X(i) to be transmitted and outputting the received words {circumflex over (X)}i(fj). In addition, each modem has an input  63  for inputting the values (H −1 *R) i  and an output  62  for outputting the nearest constellation symbols Ŝi(fj).  
         [0046]    Although the embodiments have been described with an adaptation of the linear combination coefficients/matrix coefficients for each time domain block received, it should be understood that this adaptation can be made at a much lower rate, depending upon the characteristics of the transmission channels.  
         [0047]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.  
         [0048]    What is claimed is: