Joint Multiuser Detection and Optimal Spectrum Balancing for Digital Subscriber Lines
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Joint Multiuser Detection and Optimal Spectrum Balancing for Digital Subscriber Lines Vincent M. K. Chan and Wei Yu The Edward S. Rogers Sr. Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON, Canada M5S 3G4 Received 1 December 2004; Revised 27 April 2005; Accepted 8 July 2006 In a digital subscriber line (DSL) system with strong crosstalk, the detection and cancellation of interference signals have the potential to improve the overall data rate performance. However, as DSL crosstalk channels are highly frequency selective and multiuser detection is suitable only when crosstalk is strong, the set of frequency tones in which multiuser detection may be used must be carefully chosen. Further, this problem of tone selection is highly coupled with the transmit power spectra of both direct and interfering signals, so the optimal solution requires the tone selection problem to be solved jointly with the multiuser spectrum optimization problem. The main idea of this paper is that the above joint optimization may be done efficiently using a dual decomposition technique similar to that of the optimal spectrum balancing algorithm. Simulations show that multiuser detection can increase the bit rate performance in a remotely deployed ADSL environment. Rate improvement is also observed when near-end crosstalk is estimated and cancelled in a VDSL environment with overlapping upstream and downstream frequency bands. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Crosstalk noise is a major limiting factor in wideband digital subscriber line (DSL) systems. Current research has focused on dynamic spectrum management (DSM) techniques for mitigating the effect of crosstalk [1]. The goal of DSM is to facilitate cooperation among mutually interfering lines in a binder. Cooperation may be implemented in two different levels. Power spectral density (PSD) level cooperation allows the optimal set of power spectral densities to be computed for each line in the binder so that the effect of mutual interference is minimized. In this case, multiple transmitters in a DSL binder operate independently, but at mutually accommodating PSD levels. The class of algorithms that are capable of computing the best set of PSDs is called spectrum balancing algorithms (e.g., [2, 3]). When cooperation is possible, not only at the PSD level, but also at the transmission signal level, the multiline DSL binder can then be truly designed as a multipleinput multiple-output (MIMO) system where multiuser detection algorithms can be implemented [4]. In this case, each line has the full knowledge of the transmitted signal from neighboring lines, and crosstalk can be completely cancelled. The capacity of a DSL binder with signal-level
cooperation represents the ultimate capacity limit for DSL systems. This paper explores a different form of cooperation that lies between the PSD-level and the signal-level cooperations described above. The algorithms described in this paper are most applicable to DSL conf
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