Partial Crosstalk Cancellation for Upstream VDSL

2004

Crosstalk is the major limiting issue in very high bitrate digital subscriber line (VDSL) systems in terms of bit-rate or service coverage. At the central office side, joint signal processing accompanied by appropriate power allocation enables complex multiuser processors to provide near capacity rates. Unfortunately complexity grows with the square of the number of lines within a binder, so by taking into account that there are only a few dominant crosstalkers who contribute to main part of crosstalk power, the canceller structure can be simplified which resulted in a much lower run-time complexity.

2006

Crosstalk is a major limitation to achieving high datarates in next generation VDSL systems. Whilst crosstalk cancellation can be applied to completely remove crosstalk, it is often too complex for application in typical VDSL binders, which can contain up to hundreds of lines. A practical alternative, known as partial cancellation limits the cancellation to crosstalkers that cause severe interference to the other lines within the binder. In real VDSL systems, the crosstalk environment changes rapidly as new lines come online; old lines go offline, and the crosstalk channels change with fluctuations in ambient temperature. Therefore, adaptive crosstalk cancellers are often required. In this paper, we propose a new detection guided adaptive NLMS method for Adaptive Partial Crosstalk Cancellation that detects significant crosstalkers and tracks variations in their crosstalk channels. This exploits the sparse and column-wise diagonal dominant properties of the crosstalk channel matrix and leads to fast convergence, accurate crosstalk channel tracking, with a lower update complexity. The end result is an adaptive Partial Crosstalk Cancellation algorithm that has lower run-time complexity than prior state-of-the-art whilst yielding comparatively high data-rates and reliable service. I. INTRODUCTION Until the promise of a full Fiber Network is fulfilled, Digital Subscriber Lines (DSL) will continue to be an attractive means of providing broadband communication. The twisted line pairs within a DSL cable binder however throw large amounts of electromagnetic coupling between the neighbouring lines. This initiates crosstalk in the adjacent pairs and is a major source of performance degradation as it limits the data rate and the reach at which DSL service is provided [1]. In practice, two main types of crosstalk can cause problems, namely, Near End Crosstalk (NEXT) and Far End Crosstalk (FEXT). NEXT appears at the same end of the binder as the transmission and reception takes place in the same frequency band. FEXT propagates through the binder cable and hence the crosstalk effect can be visible on the other side. NEXT can be avoided

European Transactions on Telecommunications, 2009

Far-end crosstalk (FEXT) is the major limitation to achieve high data rates in very high bit rate digital subscriber line (VDSL) systems. Full crosstalk cancellation (FCC) techniques have been proposed to mitigate the effect of crosstalk. However, these are too complex for application in typical VDSL binders. Considering the fact that there are only a few strongest cross-talkers that contribute to main part of crosstalk, the crosstalk cancellation can be simplified to line selection (LS), tone selection (TS) and joint line-tone selection (JLTS) partial crosstalk cancellation (PCC). These all result in much less online complexity. In this paper, first, we propose a new modified JLTS algorithm with significantly lower initialisation complexity than the optimal JLTS (OJLTS) algorithm and approximately with similar performance. Then a novel joint algorithm is presented that combines the multiuser power control technique with the PCC scheme to achieve near the capacity rates. We also consider dynamic online complexity constraint for each user. This joint algorithm leads to a much larger online complexity reduction (around 60% for long lines) and higher data rates at the expense of slightly higher initialisation complexity. The performance of the proposed algorithm is verified by some simulation results and compared with current algorithms that use flat power spectral densities (PSDs) and other existing joint algorithms.

AEU - International Journal of Electronics and Communications, 2009

Far-end crosstalk (FEXT) is the major limitation to achieve high data-rates in VDSL systems. Full crosstalk cancellation (FCC) techniques have been proposed to mitigate the effect of crosstalk. However, these are too complex. Considering the fact that there are only a few strongest cross-talkers, the crosstalk cancellation can be simplified to line selection (LS), tone selection (TS) and joint line-tone selection (JLTS) partial crosstalk cancellation (PCC). These all result in much less online complexity. In this paper first, we propose a new modified JLTS algorithm with significantly lower initialization complexity than the optimal JLTS (OJLTS) algorithm and approximately with similar performance. Then a novel joint algorithm is presented that combines the multiuser power control technique with the PCC scheme to achieve near the capacity rates. We also consider dynamic online complexity constraint for each user. This joint algorithm leads to much larger online complexity reduction and higher data-rates at the expense of slightly higher initialization complexity. The performance of the proposed algorithm is verified by some simulation results and compared with current algorithms that use flat PSDs and other existing joint algorithms.

2010

As demands for higher data rates increase, DSL systems become incapable of keeping up due to the electromagnetic coupling present in the binders of the telephone lines. This electromagnetic coupling, known as crosstalk, is several orders greater in magnitude than the background noise. The reported techniques focusing on completely removing the crosstalk usually lead to computationally intensive solutions which may be infeasible to implement with existing hardware. For this reason, partial crosstalk removal has been proposed. In this work, we investigate the capability of the partial crosstalk cancellation for the fair or equal rate balancing among the users, subject to a constrained computational resource. Approaches which expend the computational resources in order to achieve fair or equal rates among the users were developed. Specifically, the proposed efficient Max-Min algorithm which is based on a dual optimization framework has a high convergence speed and low complexity for deployment in the xDSL systems.

2008

Perfect crosstalk cancellation techniques have been proposed to mitigate the effect of crosstalk. However, the online complexity of these crosstalk cancellation techniques grows with the square of the number of lines in the binder. Fortunately, most of the crosstalk originates from a limited number of lines on a limited number of tones. As a result, a fraction of the complexity of perfect crosstalk cancellation suffices to cancel most of the crosstalk. This is known as partial crosstalk cancellation. Because the crosstalk profile changes over time, there is additional requirement that partial crosstalk cancellation provide a very low pre-processing complexity. Also, a much lower online complexity can be obtained if the multi-user power control and partial crosstalk cancellation problems are solved jointly. Currently, this joint problem is formulated as a constrained optimization problem and solved by employing Lagrange dual decomposition method. However, it suffers from per-tone exh...

The International Telecommunication Union (ITU) is currently working on a standard for the next digital subscriber line (DSL) generation, called G.fast. It uses frequencies up to 200 MHz and targets bit-rates of up to 1 Gbps over a single twisted-pair line. Crosstalk coupling represents a major limitation on such high frequencies. Hence, in this work we compare the performance of two prominent linear crosstalk cancellation schemes depending on transmission directions, zero-forcing linear equalization (ZFE) for upstream and diagonalizing precoding (DP) for downstream, when applied in a G.fast context. Both schemes require accurate channel state information. Simulation results reveal that even small estimation errors can severely limit the bit-rates achievable by ZFE and DP in some particular scenarios.

IEEE Transactions on Instrumentation and Measurement, 2005

2009

Block crosstalk cancellation techniques in practical multiuser digital subscriber line (DSL) environments may involve a high computational complexity as the channel and noise statistics can vary over time. We follow an adaptive approach by designing a structurally consistent significance-test feature within the normalized least-mean-square (NLMS) adaptive crosstalk canceller, aimed to detect significant crosstalkers within a DSL binder. The proposed detection-guided NLMS adaptive partial crosstalk canceller for DSL targets the dominant crosstalkers across user lines and tones, has low run-time complexity, demonstrates significantly faster convergence, and requires smaller training sequences when compared via simulation to the equivalent standard NLMS adaptive crosstalk canceller.

2015 IEEE International Conference on Communications (ICC), 2015

Crosstalk between physically co-located lines is a pressing issue in VDSL2 access networks. In order to enhance the crosstalk mitigation capabilities of the latest extension to VDSL2, vectoring G.993.5, full control over all lines within the same cable binder is required. However, this is not always possible in practical deployments due to regulatory, structural, or late technology adoption constraints. In these cases a technique to minimize interference from non-controlled lines, known as binder management, aims at rearranging the line configuration within each binder. In this work, we quantify the advantages of binder management in a partially controlled setup. We initially establish a model of a commonly used 50-pair cable binder and provide its far-end crosstalk (FEXT) characterization. We then carry out an extensive simulation study for various degrees of control over the lines and realistic line length distributions to yield tangible metrics on vectoring performance for downstream transmission. Our results show that binder management is of limited use in partially controlled systems. Consequently, we provide an additional comparison study to help DSL providers to evaluate the remaining gains of upgrading to VDSL2-vectoring in such scenarios for different levels of dominance in the cable binder.