The crosstalk among the twisted pairs copper wires in the same or neighboring bundles in telephone cables is a major impairment in the current digital subscriber line (DSL) systems. It is invaluable for a service operator to obtain the crosstalk transfer functions because this information can be used to facilitate the provisioning, maintenance, and diagnosis of the DSL systems, and to significantly improve the DSL systems' performance (e.g., the data rate and the line reach).; This dissertation proposes a novel idea of an impartial network maintenance center that identifies the crosstalk coupling functions among the twisted pairs in these DSL systems. The crosstalk identification technique includes the following four major procedures: (1) the transmitted and received signals from each DSL modem for a pre-defined time period are collected and sent to the network maintenance center; (2) the signals are resampled according to the clock rate of the receiver of interest; (3) the signals' timing differences are estimated by crosscorrelation; (4) the crosstalk coupling functions are estimated using the least-squares method. Simulation results show that the proposed methods can identify the crosstalk functions accurately and are consistent with theoretical analysis.; Near-end crosstalk (NEXT) from other types of systems (such as HDSL, SDSL, T1, etc) is the major impairment to current asymmetric DSL (ADSL) downstream transmission. This dissertation presents two methods for an ADSL receiver to cancel one (dominant) NEXT signal. The methods exploit the fact that the crosstalk signal has a large excess bandwidth and its spectra in the main lobe and in the excess band are strongly correlated. The principal idea is then to estimate the crosstalk in some frequency bands (e.g., excess band) and cancel it in other frequency bands (e.g., main lobe). Simulation results show that one of the proposed methods, MMSE estimation and cancellation, is very effective to cancel one (dominant) NEXT and the improvement is significant in terms of the data rate and the line reach for the ADSL service.
展开▼
