Single Ended Loop Topology Estimation using FDR and Correlation TDR in a DSL Modem

The broadband capability of a DSL is dependant on the copper access network. Single Ended Loop Testing (SELT) is the most preferred and economical way in estimating the copper loop topology. The combined use of complementary code based Correlation Time Domain Reflectometry (CTDR) and Frequency Domain Reflectometry (FDR) for accurate loop topology estimation is presented in this paper. The advantage of the proposed method is that the measurement is done by reusing most of the firmware modules in a typical DSL broadband modem without affecting other pairs in the bundle. Since the measurement is done in real time the effect of cross talk and AWGN have also to be considered. In this proposed method approximate loop estimation is obtained from CTDR measurements. An optimization algorithm is then used to predict a more accurate loop topology from the CTDR predicted loop. Employing FDR measured data and the FDR data of the predicted topology, an objective function is defined. The objective function is then minimized using Nelder-Mead multivariable optimization method to get an accurate loop estimate. Tests carried out on typical ANSI loops shows good prediction capability of the proposed method. No prior knowledge of the network topology is required in this process. Keywords— Central Office, Digital subscriber line (DSL), Frequency domain Reflectometry (FDR), Correlation time domain Reflectometry (CTDR), Loop qualification, Optimization.