Block crosstalk cancellation techniques in practical multi-user 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.

DSM is an exciting new technology with the potential to significantly improve data-rates, service stability and power consumption in DSL. Developing DSM from a theoretical technique into a commercial product is a challenging task and requires many practical issues to be addressed. This paper describes our experience in developing a DSM Level 2 prototype. By focusing on low complexity, practical algorithms, we have managed to achieve gains in data-rate of up to 120% and substantial reductions in power consumption. This was achieve in a real implementation on the Huawei SmartAX DSLAM using unmodified, standard compliant DSL chipsets. This demonstrates the substantial gains that DSM can achieve in real DSL systems.

This is an overview of the ongoing FAST Copper project, which is aimed at substantial improvements in rate, reach, reliability, and quality in copper-last-mile broadband access through fiber/DSL deployment, engineering innovations, and fundamental research. The project is funded by NSF, and is currently pursued jointly by Princeton University, Stanford University, and Fraser Research Lab. In this article, we outline the motivations, challenges, and research issues associated with the project, and report some of the recent results by the Princeton team in each of the four dimensions: frequency, amplitude, space, and time.

Most previous work in statistical multiplexing only considered the case where the link transmission rates are fixed. In this paper, we consider statistical multiplexing in networks with adaptive transmission rates, with focus on DSL broadband access networks. This requires a jointly optimized allocation of buffer space and transmission bandwidth to traffic flows, which takes the flow traffic characteristics, the user QoS requirements, and the user interactions at the physical layer into consideration. Using the effective bandwidth concept, we propose a class of alternate maximization (AM) algorithms (AM-D and AM-M), which solve the statistical multiplexing problem for both delay insensitive data traffic and delay sensitive multimedia traffic. With low complexity as a design goal, the AM algorithms incorporate our recently proposed autonomous spectrum balancing (ASB) algorithm, which was originally designed for DSL physical layer spectrum management. Our numerical results show that the AM algorithms combines the gain due to statistical multiplexing and that due to spectrum management.

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

For frequency selective interference channels where interference is treated as noise, distributively attaining the boundary of the rate region is an open problem, and is particularly important for broadband DSL access. This paper develops, analyzes, and simulates a new algorithm for power allocation in frequency selective interference channels called autonomous spectrum balancing (ASB). It utilizes the concept of a "reference line", which mimics a typical victim line in the interference channel. Compared with the state-of-the-art iterative watefilling and optimum spectrum balancing methods, the ASB algorithm is completely autonomous, has linear complexity in both the number of users and tones, and gives close to near-optimal performance. Convergence of a version of ASB is proven for any number of users

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.

Digital subscriber line (DSL) deployment is evolving to ever higher bit rates resulting in the use of broader spectra. The DSL flavor using the broadest spectrum today is known as VDSL. The reach performance of VDSL is upstream limited as the upstream bands use the highest frequencies. These high frequencies experience more channel attenuation, resulting in smaller signal-to-noise ratio (SNR) values. In this paper, a way of increasing the upstream reach performance is described. After investigating the VDSL transceiver, the reach performance of a VDSL modem is identified to be dominated by the echo power. Therefore, limiting the downstream echo yields a better upstream receive signal. This results in trading off downstream bit rate for more upstream bit rate. The way to optimally limit the echo is derived and high performance gains can be achieved. Finally, the optimal solution is approximated with a near-optimal solution with considerably less complexity. The near-optimal solution performs very well compared to the optimal solution.

Dynamic spectrum management (DSM) is an important technique for mitigating crosstalk in DSL. One of the first DSM algorithms proposed, iterative waterfilling (IW), has a low complexity and demonstrates the spectacular performance gains that are possible. Unfortunately IW tends to be highly sub-optimal in mixed CO/RT deployments and upstream VDSL. Another DSM algorithm, optimal spectrum balancing (OSB), uses a weighted rate-sum to find the theoretically optimal transmit spectra. Unfortunately its complexity scales exponentially with the number of lines in the binder N. Typical binders contain 25-100 lines, for which OSB is intractable. This paper presents a new iterative algorithm for spectrum management in DSL. The algorithm optimizes the weighted rate-sum in an iterative fashion, which leads to a quadratic, rather than exponential, complexity in N. The algorithm is tractable for large N and can be used to optimize entire binders. Simulations show that the algorithm performs very close to the theoretical optimum achieved by OSB.

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.

The design and optimization of orthogonal frequency division multiplex (OFDM) systems typically take the following form. The design objective is to maximize the total data rate which is the sum of individual rates in each frequency tone. The design constraints are usually linear constraints imposed across all tones. The paper shows that, regardless of whether the objective and the constraints are convex, the duality gap for this class of problems is always zero in the limit as the number of frequency tones goes to infinity. As the dual problem typically decouples into many smaller per-tone problems, solving the dual problem is much more efficient. This observation leads to an efficient method to find the global optimum of non-convex optimization problems for the OFDM system. Multiuser optimal power allocation, optimal frequency planning and optimal low-complexity crosstalk cancellation for vectored DSL are used to illustrate this point.

Dynamic spectrum management (DSM) is a new technique for multiuser power allocation and/or detection in digital subscriber line (DSL) networks. In DSM, transmit spectra are adapted based on the direct and crosstalk channels seen by the modems within a network. This allows binder bit rate performance maximisation. At the Alcatel Research and Innovation labs, we have recently developed a DSM testbed which allows the performance of DSM algorithms to be evaluated in practice. The paper describes the development of this DSM testbed. With this testbed we have evaluated the performance of a DSM algorithm known as iterative water-filling. It has been demonstrated that iterative water-filling can yield data-rate gains of up to 500% in the real world. Practical issues on the implementation of iterative water-filling are also discussed.

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.

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%.

We investigate power allocation and crosstalk cancellation for MIMO-DSL (a.k.a. bonded-DSL) systems. Exploiting a property of the DSL channel, namely column-wise diagonal dominance, allows us to simplify the power allocation and crosstalk cancellation problems. This leads to a significant reduction in initialisation and run-time complexity whilst maintaining near-optimal performance. We. develop a bound which relates the deviation of the simplified power allocation to the degree of column-wise diagonal dominance. Reliable transmission at near-capacity data-rates is also demonstrated through simulation.

Crosstalk is the major source of performance degradation in VDSL. A number of crosstalk cancellation techniques have been proposed to address this. Whilst these schemes lead to large performance increases they also have high run-time complexities, a problem which grows rapidly with the number of lines within a binder. Since the majority of crosstalk typically comes from only a few dominant crosstalkers, it is possible to do partial crosstalk cancellation. We present a low-complexity, partial crosstalk cancellation technique for VDSL based on line selection. We derive the optimal line selection technique, and several low-complexity selection algorithms which give near-optimal performance in most scenarios. These techniques lead to significant reductions in runtime complexity whilst giving similar performance to full crosstalk cancellation.