Near-end crosstalk mitigation in ADSL systems

Abstract

Near-end crosstalk (NEXT) is one of the major impairments to the current asymmetric digital subscriber line (ADSL) downstream transmission. This paper presents two methods for an ADSL receiver to cancel one (dominant) NEXT signal from other types of services (such as HDSL (high-bit-rate DSL), SDSL (single-pair, symmetric DSL), T1, etc.). 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). The frequency-domain analysis in this paper provides an intuitive explanation of the crosstalk estimation and cancellation, as well as a guidance to select the right frequency bands to observe the crosstalk signal. Moreover, a fast algorithm is proposed for practical implementation. This algorithm avoids matrix inversion and large matrix multiplication in every transmission block. Simulation results show that one of the proposed methods, minimum mean-squares error 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. For example, using a real measured NEXT transfer function, the proposed method can increase the ADSL downstream data rate by 200% for some loops. The methods are extended to estimate and cancel two or more crosstalkers. The amount of improvement depends on the crosstalkers' characteristics and it is generally less than that of a single crosstalker case.