Alien Crosstalk Cancellation for Multipair Digital Subscriber Line Systems
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Alien Crosstalk Cancellation for Multipair Digital Subscriber Line Systems George Ginis and Chia-Ning Peng Broadband Communications Group, Texas Instruments Incorporated, 2043 Samaritan Drive, San Jose, CA, USA Received 30 November 2004; Revised 18 February 2005; Accepted 4 May 2005 An overview of alien crosstalk cancellation for DSL systems with multiple pairs is here presented. It is shown that when a common crosstalk source affects the receivers of multiple pairs, the noise exhibits a certain correlation among the pairs. In a DMT system, the frequency-domain noise samples are most strongly correlated between pairs when they correspond to the same tone. Thus, noise decorrelation algorithms applied independently for each tone can provide significant performance enhancements. Three possible methods are described for noise decorrelation, one is suitable for two-sided coordination and two are suited for receiver coordination among the pairs. It is theoretically proven that the data-rate performance of these three methods is identical from the perspective of the sum rate over all pairs. Simulation results corresponding to an ADSL2+ two-pair system with a T1 disturber are presented to illustrate the noise correlation property and to indicate the potential performance benefits. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Digital subscriber line (DSL) transmission is typically constrained by crosstalk interference. As DSL technology advances and processing power increases, the interest in techniques for crosstalk reduction has increased. Most of these techniques are based on multiple-input multiple-output (MIMO) system representations, and it is common in the DSL literature to refer to them as vectoring methods and to describe a DSL system employing such methods as a vectored system. This paper gives an overview of a specific class of vectoring methods that aim to suppress interference arising from crosstalk sources that lie outside the vectored system. The effect of such sources is typically referred to as alien or out-of-domain crosstalk. These concepts are further explained in the paragraphs that follow. Vectored systems are comprised of multiple twisted pairs and the corresponding transceiver modules. The multiple twisted pairs are represented as a MIMO channel. In a vectored system, joint signal processing (or MIMO) techniques are employed on one or both sides of the MIMO channel. Vectoring techniques that require joint signal processing at both sides fall under the category of two-sided coordination. Vectoring techniques that only require joint signal processing at one side are described as coordinated reception or coordinated transmission, depending on the side where the joint signal processing takes place.
An alternative categorization of vectoring methods relates to the type of crosstalk that is being cancelled. The crosstalk interference affecting transmission of information on a pair belonging to the vectored system can originate from two kinds of sources. Crosstalk may
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