Jan Verlinden

Optimal multiuser spectrum balancing for digital subscriber lines

Crosstalk is a major issue in modern digital subscriber line (DSL) systems such as ADSL and VDSL. Static spectrum management, which is the traditional way of ensuring spectral compatibility, employs spectral masks that can be overly conservative and lead to poor performance. This paper presents a centralized algorithm for optimal spectrum balancing in DSL. The algorithm uses the dual decomposition method to optimize spectra in an efficient and computationally tractable way. The algorithm shows significant performance gains over existing dynamics spectrum management (DSM) techniques, e.g., in one of the cases studied, the proposed centralized algorithm leads to a factor-of-four increase in data rate over the distributed DSM algorithm iterative waterfilling.

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.

Optimal multiuser spectrum management for digital subscriber lines

Crosstalk is a major issue in modern DSL systems such as ADSL and VDSL. Static spectrum management, the traditional way of ensuring spectral compatibility, employs spectral masks which can be overly conservative and lead to poor performance. In this paper we present a centralized algorithm for optimal spectrum management (OSM) in DSL. The algorithm uses a dual decomposition to solve the spectrum management problem in an efficient and computationally tractable way. The algorithm shows significant performance gains over existing DSM techniques, e.g. in a downstream ADSL scenario the centralized OSM algorithm can outperform a distributed DSM algorithm such as iterative waterfilling by up to 135%.

A low complexity optimal spectrum balancing algorithm for digital subscriber lines

In modern DSL systems, multi-user crosstalk is a major source of performance degradation. Optimal spectrum balancing (OSB) is a centralized algorithm that mitigates the effect of crosstalk by allocating optimal transmit spectra to all interfering DSL modems. By the use of Lagrange multipliers the algorithm decouples the spectrum management problem into per-tone optimization problems. The remaining issues are then finding the Lagrange multipliers that enforce the constraints and solving the per-tone optimization problems. Finding the optimal Lagrange multipliers can become complex when more than two users are considered. Starting from the single-user case, this paper presents a number of properties, which are then extended to the multi-user case and lead to an efficient search algorithm for the Lagrange multipliers. Simulations show that the number of Lagrange multiplier evaluations is as small as 20-50, independent of the number of users. Secondly, the complexity of the per-tone optimization problems grows exponentially with the number of lines in the binder. For multiple-user scenarios this becomes computationally intractable. This paper presents an efficient branch-and-bound approach for the per-tone optimization problem. Simulations show enormous complexity reductions, especially for a large number of users.

An Efficient Search Algorithm for the Lagrange Multipliers of Optimal Spectrum Balancing in Multi-User XDSL Systems

In modern DSL systems, multi-user crosstalk is a major source of performance degradation. Optimal spectrum balancing (OSB) is a centralized algorithm that optimally allocates the available transmit power over frequencies, thereby mitigating the effect of crosstalk. OSB uses Lagrange multipliers to enforce constraints that are coupled over frequencies. However, finding the optimal Lagrange multipliers can become complex when more than two users are considered. This paper presents a number of properties of the Lagrange multipliers which lead to an efficient search algorithm. Simulations show that the required number of Lagrange multiplier evaluations is independent of the number of users and much smaller compared to the number of evaluations of currently known search algorithms

Multiple Access Channel Optimal Spectrum Balancing for Upstream DSL Transmission

Upstream DSL transmission suffers from in-domain crosstalk as well as out-of-domain or alien crosstalk. Here, the use of multi-user receiver signal coordination e.g. generalized decision feedback equalization, can lead to spectacular performance gains. This paper presents a transmission scheme, referred to as MAC-OSB, which focuses on the weighted rate sum capacity by joint receiver signal coordination and transmit spectrum coordination. The proposed scheme incorporates per-user total power constraints, spectral mask constraints and discrete bit or power loading constraints. Furthermore a low-complexity scheme, referred to as MAC-ISB, is presented which performs similar to MAC-OSB. Simulations show large performance gains over existing methods especially for scenarios with significant alien crosstalk

Partial crosstalk cancellation in a multi-user xDSL environment

In modern DSL systems, crosstalk is a major source of performance degradation. Crosstalk cancellation schemes have been proposed to mitigate the effect of crosstalk. However, the complexity of crosstalk cancellation 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 full crosstalk cancellation suffices to cancel most of the crosstalk. The challenge is then to determine which crosstalk to cancel on which tones, given a certain complexity constraint. This paper presents an algorithm based on a dual decomposition to optimally solve this problem. The proposed algorithm naturally incorporates rate constraints and the complexity of the algorithm compares favourably to a known resource allocation algorithm, where a multi-user extension is made to incorporate the rate constraints.

Echo cancellation for discrete multitone frame-asynchronous ADSL transceivers

In the past, several papers have reported on echo canceller (EC) structures developed for discrete multitone transmission (DMT) based asymmetric digital subscriber lines (ADSL) transmission. The most commonly known scheme, developed by Ho, Cioffi and Bingham, is based on canceling the received echo noise efficiently in times as well as in the frequency domain. The time domain processing part, which is commonly called cyclic echo synthesis (CES), can be minimized by choosing the optimal temporal alignment between the echo transmitter and the EC in the modem. This paper improves previously published work by minimizing the CES complexity without choosing a specific temporal alignment. Moreover, the obtained structure is highly suitable for making the update process of the echo canceller coefficients independent of the far end signal, leading to improved convergence behavior.

Convex Relaxation Based Low-Complexity Optimal Spectrum Balancing for Multi-User DSL

In modern DSL networks, crosstalk between different DSL lines in the same cable bundle is a major source of performance degradation. By balancing the transmit power spectra, also referred to as multi-user power control, the impact of crosstalk can be minimized leading to spectacular performance gains. In this paper a novel low-complexity spectrum balancing algorithm is presented. Its performance is compared to optimal spectrum balancing for multiple-user scenarios and it is seen to yield similar results but with a huge reduction in complexity. Moreover, by the use of a spectrum management center and limited message-passing the algorithm can be executed in a distributed fashion, which is a great asset in current DSL networks.

ADSL Transceivers Applying DSM and Their Nonstationary Noise Robustness

Dynamic spectrum management (DSM) comprises a new set of techniques for multiuser power allocation and/or detection in digital subscriber line (DSL) networks. At the Alcatel Research and Innovation Labs, we have recently developed a DSM test bed, which allows the performance of DSM algorithms to be evaluated in practice. With this test bed, we have evaluated the performance of a DSM level-1 algorithm known as iterative water-filling in an ADSL scenario. This paper describes the results of, on the one hand, the performance gains achieved with iterative water-filling, and, on the other hand, the nonstationary noise robustness of DSM-enabled ADSL modems. It will be shown that DSM trades off nonstationary noise robustness for performance improvements. A new bit swap procedure is then introduced to increase the noise robustness when applying DSM.