IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 55, NO. 5, OCTOBER 2006 1509 Preprocessing of Signals for Single-Ended Subscriber Line Testing Patrick Boets, Member, IEEE, Tom Bostoen, Leo Van Biesen, Senior Member, IEEE, and Thierry Pollet Abstract—A preprocessing algorithm is proposed to visualize the time-domain one-port scattering parameter of a subscriber line measured at the Central Office. To overcome the high line attenuation and the mismatch between the line and the mea- surement instrument, a preprocessing algorithm is developed to obtain numerically the impulse response of the one-port scattering parameter. The algorithm will search for a quasi-optimal base impedance for the scattering parameter; then, it will de-noise and de-alias the impulse response and will provide an estimate for the first meaningful significant reflection. Index Terms—Impulse response, scattering parameter, time alias, time-domain reflectometry (TDR), transmission lines. I. I NTRODUCTION R ECENTLY, single-ended line testing (SELT) has be- come a new and interesting topic for telecommunication operators [1]–[9]. These operators perform loop testing for prospection, prequalification, and maintenance purposes. SELT is highly demanded by those operators and specifically by the competitive local exchange carriers (CLECs) because they can only access the line via the xDSL modems [10]. CLECs are not allowed to connect dedicated test heads to the line itself, but by using SELT, it becomes possible to provide a line quality measurement service to customers. The idea of SELT is to perform measurements at the Central Office (CO) only to obtain a reasonable estimate of the sub- scriber line (loop) makeup or to identify the channel capacity in bits per second of that loop. To achieve this and, moreover, to give a prediction of the topology of the subscriber line, the one-port scattering parameter S 11 (ω) is used [1]. Although S 11 (ω) contains all the information, the time-domain version of S 11 (ω),which is denoted as s 11 (t), is used to detect peaks in it, which can be viewed as voltage reflections if the loop is excited with a Dirac impulse. These peaks are detected and analyzed whereafter the loop makeup can be recognized by the loop classification expert system [2]. Due to the diversity and complexity of obtaining a correct s 11 (t) curve and detecting the features in s 11 (t), the approach will be multidisciplinary: It includes the one-port measurement, after which, the data needs preprocessing before being passed to the loop topology Manuscript received October 12, 2004; revised May 3, 2006. This work was supported by the Flemish Institute of Science and Technology (IWT), Belgium. P. Boets was with the Department of Fundamental Electronics, Vrije Uni- versiteit Brussel, 1050 Brussels, Belgium. He is now with Banama-Telecom, Brussels, Belgium. T. Bostoen and T. Pollet are with Alcatel Research and Innovation, 2018 Antwerp, Belgium (e-mail: tom.bostoen@alcatel.be; thierry.pollet@alcatel.be). L. Van Biesen is with the Department of Fundamental Electronics, Vrije Universiteit Brussel, 1050 Brussels, Belgium (e-mail: lvbiesen@vub.ac.be). Digital Object Identifier 10.1109/TIM.2006.880290 Fig. 1. Flowchart of the system. The topics in the gray area are discussed in this paper. classification [2] and the loop identification part using the physical cable models [1]. Fig. 1 shows the organization of the loop detection system. For commercial reasons, the measurements at the CO are done with the xDSL modem itself [e.g., asymmetric digital subscriber line (ADSL), ADSL2, ADSL2+, or very-high-data- rate DSL (VDSL)]. The modem transmits discrete multitone (DMT) symbols, and the modem receiver digitizes the response of the loop. Calibration will be necessary because the front- end characteristics are different from port to port on an xDSL multimodem board. The first task of the loop detection system is the measurement and calibration of the DMT signals to obtain S 11 (ω). The next task comprises the preprocessing of the data to derive a valid impulse response s 11 (t). It will be 0018-9456/$20.00 © 2006 IEEE