George Ginis

Distributed multiuser power control for digital subscriber lines

This paper considers the multiuser power control problem in a frequency-selective interference channel. The interference channel is modeled as a noncooperative game, and the existence and uniqueness of a Nash equilibrium are established for a two-player version of the game. An iterative water-filling algorithm is proposed to efficiently reach the Nash equilibrium. The iterative water-filling algorithm can be implemented distributively without the need for centralized control. It implicitly takes into account the loop transfer functions and cross couplings, and it reaches a competitively optimal power allocation by offering an opportunity for loops to negotiate the best use of power and frequency with each other. When applied to the upstream power backoff problem in very-high bit-rate digital subscriber lines and the downstream spectral compatibility problem in asymmetric digital subscriber lines, the new power control algorithm is found to give a significant performance improvement when compared with existing methods.

On the relation between V-BLAST and the GDFE

This letter shows that the receiver processing of the Vertical Bell Labs Layered Space-Time (V-BLAST) architecture may he viewed as a generalized decision feedback equalizer (GDFE) as applied to a multiple-input-multiple-output (MIMO) channel. It is proven that, depending on the criterion chosen to reduce the interference, the receiver operations of the V-BLAST system are fundamentally equivalent to either a zero-forcing (ZF) or a minimum-mean-square-error (MMSE) GDFE.

A Near-Optimal Linear Crosstalk Precoder for Downstream VDSL

This letter presents a linear crosstalk precoder for very-high-speed digital subscriber lines (VDSL) that has a low run-time complexity. A lower bound on the data rate of the precoder is developed, and guarantees that the precoder achieves near-optimal performance in 99% of VDSL channels

A Near-Optimal Linear Crosstalk Canceler for Upstream VDSL

Crosstalk is the major source of performance degradation in VDSL. Several crosstalk cancelers have been proposed to address this. Unfortunately, they suffer from error propagation, high complexity, and long latency. This paper presents a simple, linear zero-forcing (ZF) crosstalk canceler. This design has a low complexity and no latency and does not suffer from error propagation. Furthermore, due to the well-conditioned structure of the VDSL channel matrix, the ZF design causes negligible noise enhancement. A lower bound on the performance of the linear ZF canceler is derived. This allows performance to be predicted without explicit knowledge of the crosstalk channels, which simplifies service provisioning considerably. This bound shows that the linear ZF canceler operates close to the single-user bound. Therefore, the linear ZF canceler is a low-complexity, low-latency design with predictable near-optimal performance. The combination of spectral optimization and crosstalk cancellation is also considered. Spectra optimization in a multiaccess channel generally involves a complex optimization problem. Since the linear ZF canceler decouples transmission on each line, the spectrum on each modem can be optimized independently, leading to a significant reduction in complexity

Improved linear crosstalk precompensation for DSL

Crosstalk is the major source of performance degradation in next generation DSL systems such as VDSL. In downstream communications, transmitting modems are co-located at the central office. This allows crosstalk precompensation to be employed. In crosstalk precompensation, the transmitted signal is pre-distorted such that the pre-distortion destructively interferes with the crosstalk introduced by the channel. Existing crosstalk precompensation techniques either give poor performance or require modification of customer premises equipment (CPF). This is impractical since there are millions of legacy CPE modems already in use. We present a novel crosstalk precompensation technique, based on a diagonalization of the crosstalk channel matrix. This technique does not require modification of CPE. Furthermore, certain properties of the DSL channel ensure that this diagonalizing precompensator achieves near-optimal performance.

The ITU-T's new g.vector standard proliferates 100 mb/s dsl

This article explores the recently issued ITUT G.vector (G.993.5) that allows expanded use of 100 Mb/s DSL. A tutorial description on G.vectors crosstalk noise reduction methods leads to specific projections and measurements of expanded DSL 100 Mb/s reach. A discussion on dynamic maintenance to enhance G.vectors practical application then concludes this article.

Partial crosstalk cancellation for upstream VDSL

Crosstalk is a major problem in modern DSL systems such as VDSL. Many crosstalk cancellation techniques have been proposed to help mitigate crosstalk, but whilst they lead to impressive performance gains, their complexity grows with the square of the number of lines within a binder. In binder groups which can carry up to hundreds of lines, this complexity is outside the scope of current implementation. In this paper, we investigate partial crosstalk cancellation for upstream VDSL. The majority of the detrimental effects of crosstalk are typically limited to a small subset of lines and tones. Furthermore, significant crosstalk is often only seen from neighbouring pairs within the binder configuration. We present a number of algorithms which exploit these properties to reduce the complexity of crosstalk cancellation. These algorithms are shown to achieve the majority of the performance gains of full crosstalk cancellation with significantly reduced run-time complexity.

Partial crosstalk precompensation in downstream VDSL

Very high bit-rate digital subscriber line (VDSL) is the latest generation in the ongoing evolution of DSL standards. VDSL aims at bringing truly broadband access, greater than 52 Mbps in the downstream, to the mass consumer market. This is achieved by transmitting in frequencies up to 12 MHz. Operating at such high frequencies gives rise to crosstalk between the DSL systems in a binder, limiting achievable data-rates. Crosstalk is typically 10-15 dB larger than other noise sources and is the primary limitation on performance in VDSL. In downstream transmission several crosstalk precompensation schemes have been proposed to address this issue. Whilst these schemes lead to large performance gains, they also have extremely high complexities, beyond the scope of current implementation.In this paper we develop the concept of partial crosstalk precompensation. The majority of the crosstalk experienced in a DSL system comes from only a few other lines within the binder. Furthermore its effects are limited to a small subset of tones. Partial precompensation exploits this by limiting precompensation to the tones and lines where it gives maximum benefit. As a result, these schemes achieve the majority of the gains of full crosstalk precompensation at a fraction of the run-time complexity. In this paper we develop several partial precompensation schemes. We show that with only 20% of the run-time complexity of full precompensation it is possible to achieve 80% of the performance gains.

The linear zero-forcing crosstalk canceler is near-optimal in DSL channels

Crosstalk is a serious problem in next-generation DSL systems, such as VDSL. Several non-linear crosstalk cancelers and pre-compensators have been proposed to address this. Unfortunately, they all suffer from high complexity, DFE error propagation and/or require modification of CPE. We propose the use of a simple linear zero-forcing crosstalk canceler in upstream transmission and a simple linear diagonalizing crosstalk precoder in downstream transmission. Certain properties of DSL channels ensure that these simple linear designs lead to near-optimal performance. We formulate a bound on the performance of these schemes and show that in 99% of upstream DSL channels, the linear zero-forcing canceler achieves 97% of the theoretical channel capacity. Similarly, in 99% of downstream DSL channels, the linear diagonalizing precoder achieves 91% of the theoretical channel capacity.

Vectored-DMT: a FEXT canceling modulation scheme for coordinating users

This paper describes a modulation scheme achieving far-end crosstalk (FEXT) cancellation, assuming that joint signal processing among the users can be performed at either the receiver or at the transmitter side. The multi-user channel is modeled as a multiple-input-multiple-output (MIMO) system, and the development starts with the zero-forcing generalized decision feedback equalizer (GDFE). Structures are derived, which combine the discrete multi-tone (DMT) transmission technique with either successive cancellation at the receiver or precoding at the transmitter. A promising application of the proposed scheme is in FEXT-limited digital subscriber line (DSL) systems. Computer simulations demonstrate that significant performance improvements can be realized in such scenarios.