Digital Subscriber Lines (DSL) is point-to-point transmission technologies over a transmission medium of twisted pairs. The DSL provides an efficient and economic access to a broadband network. The DSL are also commonly referred to as xDSL, where “x” representing diffident kinds of digital subscriber line technologies. The xDSL generally includes Integrated Services Digital Network Digital Subscriber Lines (IDSL, ISDN DSL), Asymmetric Digital Subscriber Lines (ADSL), High-bit-rate Digital Subscriber Lines (HDSL), Very high data rate Digital Subscriber Lines (VDSL), etc. The various digital subscriber lines technologies are different from each other mainly in terms of signal transmission rate and distance. Except for the baseband transmission based IDSL, the xDSL generally coexists with the Plain Old Telephone Service (POTS) on the same twisted pair, where the POTS occupies the baseband part and the xDSL occupies the high frequency band for transmission independent of each other.
With the development of applications, requirements for the xDSL bandwidth become increasingly strict and the frequency band used for the xDSL also becomes higher accordingly, so that the crosstalk, especially the crosstalk at the high frequency band, becomes increasingly exacerbated. In the xDSL, a Digital Subscriber Line Access Multiplexer (DSLAM) is usually used to provide an access service for multiple branches of xDSL signals. Because the xDSL adopts frequency division multiplexing for uplink and downlink channels, a near-end crosstalk may not endanger considerably the system performance. However, a far-end crosstalk may influence seriously the transmission performance of lines. When an xDSL service is required to be enabled for multiple branches of users over a bundle of cables, some lines may have a low speed, unstable performance and even may be unable to function due to the far-end crosstalk, which may eventually result in a low line activation ratio of the DSLAM.
FIG. 1 illustrates a schematic diagram of a far-end crosstalk, where x1, x2 and x3 denote signal transmitting points, y1, y2 and y3 denote corresponding far-end signal receiving points, solid line arrows denote normal signal transmission, and dotted line arrows denote a crosstalk caused by a signal transmitting points to far-end receiving points corresponding to other signal transmitting points. As can be seen from FIG. 1, signals to be transmitted at the points x2 and x3 are crosstalk sources for those at the point x1, and naturally, signals to be transmitted at the point x1 are crosstalk sources for those at the points x2 and x3. Therefore, for clarity, a branch of signals to be transmitted is described as a reference object while regarding other signals as their crosstalk sources hereinafter. Such descriptions can be adaptive to respective branches of signals. Distinguishing names used for signals are merely for convenience, but not intended to differentiate the signals substantively.
In order to address the problem of the degraded channel performance due to the far-end crosstalk, a method of coordinated signal processing was proposed in the industry to cancel a far-end crosstalk among respective branches of signals by use of the feature of coordinated transmission and reception at the DSLAM end. At present, the signals are processed with a fixed filter in the frequency domain based upon such a principle that crosstalk cancellation calculations are performed on the premise that a channel transmission matrix has been pre-known. For coordinated reception of signals, this method frequency domain filters respective frequency points of received signals according to the pre-known channel transmission matrix, and then estimates input channel signals in a general decision feedback equalization method. The essence of the method lies in that: because the channel transmission matrix is known, the relationship between crosstalk components in the received signals and a crosstalk source may be deduced, so that received signals corresponding to the crosstalk source can be used to approximately simulate the crosstalk source, thereby implementing a crosstalk cancellation at the coordinated receiving end. On the other hand, for coordinated transmission of signals, the method is similar to that for coordinated reception except that the signals are pre-coded in the frequency domain before transmission instead of processing the signals undergoing a crosstalk, to pre-cancel a crosstalk which may occur. Therefore, the receiving end receives the signals from which the crosstalk has been canceled.
The above method has a disadvantage in that the channel transmission matrix has to be pre-known, but the matrix may be difficult to be obtained accurately and conveniently. In addition, the matrix per se features slowly time-varying and may be susceptible to a transmission environmental factor. Consequently, the above solution may be difficult to implement in practice.