Short Description of the Research Projects (Team A)
Design and evaluation of Multiple-Input-Multiple-Output channel models for DSL systems
(Wim Foubert, Leo Van Biesen)
Digital Subscriber Line (DSL) technology provides broadband service over ‘twisted pair’ copper wires of the existing telephone network. Current DSL systems make only use of differential mode (DM) signals, which refers to the electrical voltage difference between the two wires of a twisted pair (see Figure 1).
However, in most countries there are two twisted pairs that enter each house. In next generation DSL technologies, all four wires will be exploited. In this way, two differential mode signals and an additional phantom mode signal can be defined. This new signal is the voltage difference between the references of each twisted pair. In the differential mode, the currents in the two conductors of the twisted pair are in opposite direction. In the phantom mode, they are in the same direction and another twisted pair stands in for the return path. In this way, the phantom mode signal is invisible for the differential mode signal on each pair. The phantom mode signal can also be seen as a differential mode signal but now one uses four conductors instead of two. Using also the phantom mode signal in addition to the differential mode signal, the capacity can be three times higher than the conventional differential mode only capacity !
In order to use also the phantom mode signal, we need reliable transmission line models which take also this signal into account. The derivation of the new models is based on the multiconductor transmission lines (MTL) theory. This theory is only valid for uniform line segments. The figure 2 shows how the behavior of a twisted pair can be approximated. The representation contains a resistor R, an inductor L, a conductance G and a capacitance C. For our model, good approximations for the different physical parameters are indispensable.
This problem is decoupled. First we consider the transversal plane to determine the resistance R and the inductance L. Therefore we use a quasi-stationary approximation since the diameter of the quad is much lower than the wavelengths of the transported signals. An analytical expression exists for the transversal plane, which is called the series impedance and was derived by Belevitch [1]. Beside the skin effect, also the proximity effect is taken into account. This series impedance has been validated with measurements as described in [2]. In the longitudinal plane, the considered line lengths are of the same order of the wavelength. A numerical approximation technique will be used to obtain accurate results for the capacitance matrix. Also this matrix has been validated. Therefore a two-dimensional simulator, called Maxwell is used.
Once we have the complete model for a homogeneous quad, twisting will be introduced. The multiconductor transmission lines theory is only valid for uniform line segments. But, if we want to allow wire twists, strands and variations our cable is not uniform at all. To overcome this problem, the system will be split up in very short line segments. Each segment is considered to be uniform and is represented by a transmission matrix. The overall cable description is obtained by
multiplication of the different matrices. In this way, it’s easy to increase the number of pairs, change the twist rate, stranding or isolation just by changing one or more parameters in the model.
In the next part, the eigenmodes of the quad are investigated. Since we know the four parameters (R, L, G and C), it is possible to calculate the product YZ where Y represents the admittance matrix and Z is the impedance matrix. Diagonalizing this product gives four eigenvalues and the corresponding eigenvectors. Analyzing this, we notice that the two differential modes and the phantom mode are all eigenmodes of the system. Moreover there is no crosstalk between the different modes. This proves that exploiting also the phantom mode signal will strongly increase the available capacity.
References
[1] V. Belevitch, Philips Research Reports “Theory of the proximity effect in multiwire cables”, pp. 16-43, 1977
[2] W. Foubert, P. Boets, L. Van Biesen, C. Neus, “Modeling the Series Impedance of a Quad Cable for
Common Mode DSL Applications”, IEEE Transactions on Instrumentation and Measurement, Vol.59, pp.
259-265, February 2010
Modeling of the channel transfer function and the crosstalk for specific historical connectivity practices in DSL copper networks and assessment of the effect on the achievable data rates
(Carine Neus, Leo Van Biesen, Lieven Mertens*, and Kurt Coulier*)
(*) Belgacom
Digital Subscriber Line technologies like ADSL, ADSL2+, VDSL2 and its evolutions offer high-speed data services (e.g. internet connection, digital television,...) to customers over the existing telephone lines. For practical reasons, the telephone lines are not simply a single straight line between the central office and the customer. Lines are bundled for cost effectiveness, repairs have been made, different line types are cascaded,.... As a consequence, very peculiar topologies can be found in the field.
In this research, we focus on one specific type of connection, namely ‘retour pairs’, which have a different topology than the ‘direct’ pair (see Figure 3). The retour pair passes the customer’s house and returns after a certain distance “r”, typically at the splice at the end of the cable. Hence their name ‘retour pairs’.
Due to historical reasons many customers receive two twisted pairs at home, and in the majority of these cases it is one direct pair and one retour pair.
Due to the characteristics and spectrum usage of VDSL2, the use of the direct pair for service offering is obvious and recommended. But with the ever increasing request for bandwidth driven by strong competition and customer demand, and with the evolution of VDSL2 towards VDSL2 Vectoring, the operators show a need to better understand the behaviour of the retour pair. After all, the use of the retour pair is economically very interesting since civil works might be avoided in some cases, despite the expected degraded overall performance of the retour pair.
This research aims at predicting the achievable data rate on retour pairs. We assume that they will behave differently from a direct pair, as their physical construction is different (i.e. they are folded back on themselves after a certain distance). Basically, two aspects must be studied:
- The channel transfer function needs to be modelled.
- The crosstalk needs to be evaluated.
During 2011, a model has been developed for the channel transfer function of a retour pair. The model has been validated in the ELEC laboratory and in the field (in an operational distribution center). In 2012, the crosstalk in the presence of retour pairs will be evaluated.
This project is performed in cooperation with Belgacom.