The current access telecommunication networks still consist mostly of metallic pairs. These cables are actually used for high-speed digital transmission systems, such as digital subscriber lines (xDSL). These subscriber lines provide affordable and cheap connections mainly for residential use and small business companies [1]. The next generation of xDSL digital subscriber lines, e.g. VDSL2, could provide higher transmission bitrates, but there are several problems related with the usage of metallic lines and cables, which need to be solved first. The major problem, which appears in large metallic networks, is crosstalk [2]. It comes from unbalanced capacitive and inductive couplings between single copper pairs, their quads and multi-quads [3]. These pairs demonstrate towards themselves small irregularities, which are caused by manufacturing tolerances, deformations and other specific reasons [3]. The influence of near-end crosstalk (NEXT) can be easily limited by separating transmission directions using different frequency bands, but the reduction of far-end crosstalk (FEXT) is not so easy and therefore FEXT is a dominant source of disturbance. One of the most promising solutions for the elimination of FEXT is Vectored DMT modulation (VDMT) [4]. This modulation is an upgrade of previous Discrete Multi-tone modulation (DMT) and it offers the cancellation of FEXT crosstalk by coordinating the transmitted DMT symbols [5]. It would be possible to perform the VDMT modulation only for a limited number of the most disturbing pairs in a cable, which would simplify the whole process of coordination [6]. However, this method would require very accurate prediction of crosstalk behavior and realistic modeling of FEXT for all mutual combinations of pairs in a cable. That is why a new advanced method of FEXT modeling is necessary to implement. The present standard FEXT model [7] comes only from the averaged crosstalk values for the whole cable and it uses only one crosstalk parameter given for the whole cable. It is obvious that such model cannot be very accurate and therefore provides only approximate and not very realistic results, as presented in [8]. The accuracy of this model can be sufficient for some specific applications (e.g. summarization of many contributions), but the simple standard FEXT model is not very useful for the precise modeling of perspective VDSL2 lines using VDMT concept. The main problem represents the individual method for modeling of transmission channels and FEXT transmission functions, which is necessary for the implementation of VDMT modulation for all combinations of pairs in a cable.
展开▼
