Advanced Signal Processing for Digital Subscriber Lines

The recent deployment of digital subscriber line (DSL) technology around the world is rapidly making broadband access for the mass consumer market a reality. The ever-growing customer demand for higher data rates has been fueled by the popularity of applications like peer-to-peer (P2P) filesharing networks and video-streaming and high-definition television (HDTV). DSL technology allows telephone operators to getmaximum leverage out of their existing infrastructure by delivering broadband access over existing twisted-pair telephone lines. At the heart of DSL lies a plethora of signal processing techniques which enable such high-speed transmission to be achieved over a medium originally designed with only voice-band transmission in mind. These advanced signal processing techniques address many challenges that exist in DSL networks today, such as the near-end and far-end crosstalk (NEXT/FEXT), impulse noise, peak-toaverage-power ratio (PAR), intersymbol and intercarrier interference (ISI/ICI), radio-frequency interference (RFI), and so forth. The goal of this special issue is to discuss the state-of-the-art and recent advances in signal processing techniques for DSL.

The recent deployment of digital subscriber line (DSL) technology around the world is rapidly making broadband access for the mass consumer market a reality. The ever-growing customer demand for higher data rates has been fueled by the popularity of applications like peer-to-peer (P2P) filesharing networks and video-streaming and high-definition television (HDTV). DSL technology allows telephone operators to get maximum leverage out of their existing infrastructure by delivering broadband access over existing twistedpair telephone lines. At the heart of DSL lies a plethora of signal processing techniques which enable such high-speed transmission to be achieved over a medium originally designed with only voice-band transmission in mind. These advanced signal processing techniques address many challenges that exist in DSL networks today, such as the near-end and far-end crosstalk (NEXT/FEXT), impulse noise, peak-toaverage-power ratio (PAR), intersymbol and intercarrier interference (ISI/ICI), radio-frequency interference (RFI), and so forth. The goal of this special issue is to discuss the stateof-the-art and recent advances in signal processing techniques for DSL.
The special issue consists of fifteen papers on a range of topics. The first set of papers focuses on the area of dynamic spectrum management (DSM). In a conventional DSL deployment, the transmit spectrum for all modems in a bundle are fixed to a predetermined level. As DSL deployment becomes increasingly heterogeneous, crosstalk produced by modems under a fixed spectrum can be a source of significant interference. Dynamic spectrum management aims to improve the data rates and reaches of conventional DSL systems by adaptively varying transmit power-spectral density according to geographic locations and the crosstalk channel characteristics of the subscribers in each bundle. This issue contains six papers on DSM. In "The worst-case interference in DSL systems employing dynamic spectral management," Brady and Cioffi answer the question of what the worst-case crosstalk interference is for a given DSL line. They characterize the performance of the system under the worstcase noise using a game theory technique. In "Joint multiuser detection and optimal spectrum balancing for digital subscriber lines," Chan and Yu study the optimal spectrum management technique for a scenario in which crosstalk may also be partially cancelled using advanced crosstalk cancellation techniques. In the next three papers, practical spectrum management techniques are investigated. In "Spectrally compatible iterative water-filling," Verlinden et al. study a system in which spectral allocation scheme is constrained by additional spectrum compatibility requirements and propose a new scheme based on an earlier algorithm called iterative water-filling. In "The normalizedrate iterative algorithm: a practical dynamic spectrum management method for DSL," Statovci et al. propose a new low-complexity technique for spectrum balancing and frequency partition in a DSL bundle. In "ADSL transceivers applying DSM and their nonstationary noise robustness," Van den Bogaert et al. report the performance of practical transceivers implementing dynamic spectrum management and study their robustness against nonstationary noise. Finally, from a theoretical perspective, the paper "Analysis of iterative water-filling algorithm for multiuser power control 2 EURASIP Journal on Applied Signal Processing in digital subscriber lines," by Luo and Pang, takes a new look at the iterative water-filling algorithm and gives a novel interpretation of the algorithm based on optimization theory.
The next paper in the special issue deals with crosstalk cancellation. In a DSL deployment, when coordination among the transmit-or receive-modems is possible, further data improvement may be obtained via crosstalk cancellation. In the paper "Alien crosstalk cancellation for multipair digital subscriber line systems," Ginis and Peng give an overview of this area and propose a new crosstalk cancellation technique that takes advantage of the noise correlation among the multiple receivers.
The practical success of dynamic spectrum management and crosstalk cancellation depends very much on how accurately crosstalk channels may be modeled and identified in practice. Two papers of this special issue address this area. In "Crosstalk models for short VDSL2 lines from measured 30 MHz data," Karipidis et al. propose measurement-based crosstalk models for VDSL. In "Error sign feedback as an alternative to pilots for the tracking of FEXT transfer functions in downstream VDSL," Louveaux and Van der Veen propose new ways of identifying the crosstalk channel using a novel feedback scheme.
Equalization and coding continue to be important research topics in DSL. In the area of time-domain equalization (TEQ), the paper "Iterative refinement methods for time-domain equalizer design" by Arslan et al. proposes a new method to reduce the implementation complexity of the TEQ. In the area of error-correcting coding for the DSL system, the paper "Near capacity coding for discrete multitone (DMT) systems with impulse noise" by Ardakani et al. proposes a methodology for the design of the newly emerged low-density parity-check (LDPC) codes for a DMT system, while addressing the practical DSL deployment issue of impulse noise. In "Fine-granularity loading schemes using adaptive Reed-Solomon coding for xDSL-DMT systems," Panigrahi and Le-Ngoc propose a joint design of bit-loading and error-correcting code, and characterize the performance gain made possible by fractional bitloading.
The final set of three papers in this special issue deals with the area of modulation and transmitter design. The design of transmit window to minimize egress is studied in the paper by Cuypers et al. "Intra-symbol windowing for egress reduction in DMT transmitters." The peak-toaverage-power ratio is another important transmitter design issue for DMT systems. This is taken up in the paper "Designing tone reservation PAR reduction" by Andgart et al. DMT is not the only possible multicarrier modulation scheme for DSL. An alternative is proposed and studied in the paper "Cosine modulated multitone for very high-speed digital subscriber lines" by Lin and Farhang-Boroujeny.
The continued growth of digital subscriber line technology worldwide is in part fueled by rapid advances in signal processing techniques. We hope that the readers will enjoy the collection of papers on this timely topic.
Finally, we wish to take this opportunity to acknowledge and to thank all anonymous reviewers, without whom the success of this special issue would not have been possible.