A Frequency Domain Identification Algorithm for Single-Ended Line Measurements Carine Neus, Patrick Boets, Leo Van Biesen Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Department of Fundamental Electricity and Instrumentation (ELEC), Email:
[email protected]
3 Frequency domain identification algorithm The main idea is to exploit the information in the periodicity of S11(f), which is present due to constructive and destructive interference of the waves propagating along the line under test. The following processing steps are performed: • Change of impedance base when the measurement device is not matched to the line under test. • Using only the reliable frequency band. Otherwise applying an inverse Fourier-transform will lead to a distorted time-domain signal. • Using the real or complex part of S11(f) instead of the classical polar representation (abs and phase). As shown in Figure 1, the real and complex part contain the same amount of information. • Windowing the frequency domain signal in order to reduce time-domain leakage. • Zero-padding the frequency domain data to improve resolution. This is important especially when only a small frequency band is reliable, resulting in few measurement points.
processed imag(S 11)
SELT measurements can be performed in time or frequency domain. Both domains are also suited for identification. All four combinations of measurement and identification domain are possible, with the use of a direct or inverse Fourier transform where necessary. Till now measurements have mainly been performed by measuring the one-port scattering parameter in the frequency domain (S11(f)) but identification has mainly been attempted in the time domain [1, 2]. This paper proposes a new approach by doing the identification in the frequency domain.
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Subsequently an FFT-operation is applied to reveal the present periodicities. Each periodicity results in a peak, giving us information about the loop make-up. In the example of Figure 1, the line segments of 1000 m and 1500 m can clearly be recognized. It is important to mention that the obtained signal is again in the timedomain but has lost its physical meaning due to the performed processing steps. processed real(S 11)
In order to identify whether a subscriber loop is suitable for a certain Digital Subscriber Line (DSL) service, the transfer function of the loop has to be estimated. Several measurement techniques exist, however Single-Ended Line Testing (SELT) is often preferred by the telecom operators because all necessary measurements are done at the central office, in contrast to Dual-Ended Line Testing (DELT), where a technician needs to be dispatched to the customer’s site.
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1 Introduction
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Figure 1 The periodicity of the processed data is extracted by the FFT; loop make-up: 1000 m in cascade with 1500 m
4 Conclusions A measurement campaign confirms that the new algorithm leads to good loop identification. Once the loop make-up is known, the transfer function can be calculated. This is important for telephone companies providing DSL services, in order to have a tool to characterize and evaluate the capability of a subscriber local loop in carrying DSL services. References [1] P. Boets, T. Bostoen, L. Van Biesen and T. Pollet, “Pre-Processing of Signals for Single-Ended Subscriber Line Testing”, IEEE Trans. Instrum. and Meas., Vol. 55, No. 5, October 2006, pp.1509-1518 [2] S. Galli and K. Kerpez, “Single-Ended Loop MakeUp Identification - Part I: A Method of Analyzing TDR Measurements”, IEEE Trans. Instrum. and Meas., Vol. 55, No. 2, April 2006, pp528-537.
