SINGLE-ENDED LINE TESTING - A WHITE BOX APPROACH Patrick Boets and Leo Van Biesen Department of Fundamental Electricity and Instrumentation, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, pboets@vub.ac.be Tom Bostoen Department of Research and Innovation, ALCATELL, F. Wellesplein 1 B-2018 Antwerp,Belgium, tom.bostoen@alcatel.be Daniel Gardan Department of R&D/RTA Caract´ erisation du R´ eseau Cuivre, France Telecom, Avenue Pierre Marzin 2, 22307 Lannion Cedex - France, daniel.gardan@rd.francetelecom.com ABSTRACT A measurement, modelling and identification system is proposed to qualify a subscriber line with the constraint that only measurements can be conducted at the Central Office. The system uses the one-port scattering parame- ter as a base measurement of the loop. The time domain version of this scattering parameter will be pre-processed so that the features, which are the start, maximum and end positions of a reflection, are clearly visible and hence de- tectable. These features are used by the interpreting expert system which performs a topology estimation of that loop. Once the topology is known, a loop model, based on the physical properties of a twisted pair line, is build and the loop can be identified using a Maximum Likelihood Esti- mator. Next, the end-to-end transfer function will be calcu- lated. The results and observations of a measurement cam- paign using a France Telecom cable plant will illustrate the proposed Single-Ended Line Testing approach. KEY WORDS Channel Estimation, System Capacity Analysis 1 Introduction Operators today are performing loop qualification to pro- vide, support and troubleshoot xDSL service. Up till now line testing happened by placing measurement equipment at the Central Office (CO) and the Customer Premises (CP) side. This requires an expensive truck roll at the CP lo- cation but the measurement is quite accurate. Recently, Single-Ended Line Testing (SELT) has become an new and interesting topic [1, 2, 3, 4, 5, 6, 7]. The idea is to perform measurements at the Central Office only in order to obtain a reasonable estimate of the line quality. Therefore, the ob- jectives of SELT are the prediction of the end-to-end trans- fer function of the loop, the knowledge of the loop topology (topography, line types and line lengths), the identification of the disturbers at the receiver and even cable fault detec- tion is possible. Once the channel topology and disturbers are known, the capacity in bits/s (e.g. for ADSL or VDSL) can be predicted. SELT measurements can be performed by stand-alone equipment but it is commercially more in- teresting by letting the broadband modem itself do the tests. In this paper, a well-founded model based approach for SELT is given. For the moment, it operates under the following constraints: i. It is assumed that the load impedance is high enough to be considered as an open line end. Most terminals such as telephone, fax and minitel have a high impedance but some electronic POTS systems and ADSL-modems are better matched to the line. ii. The CP location is known in case bridged taps exist in the net- work. iii. a priori knowledge about the cable properties must be available. iv. Cable faults must be removed before applying SELT, because faults are not yet included in the models. The noise level identification at the CP side is not treated in this paper but the authors refer to [2, 5] 2 Description of the approach 2.1 Measurement Quantities at CO In order to determine the channel capacity of the Device Under Test (DUT), two loop properties have to be iden- tified. Firstly, the end-to-end Power Transfer Function (PTF) in a predefined termination impedance, e.g. 100Ω for ADSL. Secondly, the Power Spectral Density (PSD) at the receiver must be measured or predicted. An estimation of the PTF should be obtained from a testhead independent quantity. This quantity must contain fundamental information about the loop but may not de- pend on the nature of the excitation signal - if one uses plain Time Domain Reflectometry (TDR) then the trace depends on the shape of the injected pulse too. Possible information carrying quantities are: the one-port scattering parameter S 11 (ω) or the input impedance Z in (ω) of the loop. The quantities are mostly represented in the frequency domain but their time domain versions are valid descriptions too. The one-port scattering parameter S 11 (ω) was chosen for the description of the loop’s behavior and is defined as (see also figure 1): S 11 (ω)= b(ω) a(ω)     Z base (1) with a(ω) the incident voltage wave and b(ω) the reflected voltage wave both given in the base impedance Z base and 422-039 393