Multi-User Signal and Spectra Coordination for Digital Subscriber Lines

The appetite amongst consumers for ever higher data-rates seems insatiable. This booming market presents a huge opportunity for telephone and cable operators. It also presents a challenge: the delivery of broadband services to millions of customers across sparsely populated areas. Fully fibre-based networks, whilst technically the most advanced solution, are prohibitively expensive to deploy. Digital subscriber lines (DSL) provide an alternative solution. Seen as a stepping-stone to a fully fibre-based network, DSL operates over telephone lines that are already in place, minimizing the cost of deployment. The basic principle behind DSL technology is to increase data-rate by widening the transmission bandwidth. Unfortunately, operating at high frequencies, in a medium originally designed for voice-band transmission, leads to crosstalk between the different DSLs. Crosstalk is typically 10-15 dB larger than the background noise and is the dominant source of performance degradation in DSL. This thesis develops practical multi-user techniques for mitigating crosstalk in DSL. The techniques proposed have low complexity, low latency, and are compatible with existing customer premises equipment (CPE). In addition to being practical, the techniques also yield near-optimal performance, operating close to the theoretical multi-user channel capacity. Multi-user techniques are based on the coordination of the different users in a network, and this can be done on either a spectral or signal level

Iterative Spectrum Balancing for Digital Subscriber Lines

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 suboptimal 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

Efficient Equalizers for Single and Multi-Carrier Environments with Known Symbol Padding

The use of a cyclic prefix (CP) to mitigate inter-symbol interference is a technique commonly applied in both single and multi-carrier systems. Recently it has been suggested that the CP be replaced by a pre-defined sequence of known symbols. This technique, referred to as 'Known Symbol Padding' (KSP) inserts a short training sequence (TS) at the beginning of each transmission block for equalizer adaption. This allows for fast tracking of changes in the channel and simple synchronization. In this paper we present purely deterministic equalizers for both single and multi-carrier environments with KSP. We show that, through better utilization of the CP overhead, these equalizers exhibit superior performance to those in a conventional CP system. Low-complexity implementations, particularly for the multi-carrier case are also given

Call for Papers - Special Issue on Advanced Signal Processing Techniques 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. DSL allows telephone operators to get maximum 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 advanced signal processing techniques which enable such high-speed transmission to be achieved over a medium originally designed with only voice-band transmission in mind. As DSL networks are deployed, customer demand for ever higher data rates is growing. This has been fueled by the increasing popularity of applications like peer-to-peer (P2P) file-sharing networks, video streaming, and HDTV. Achieving such high data rates will require the development of new, advanced signal processing techniques to address many issues that still exist in DSL networks such as crosstalk, impulse noise, high peak-to-average power ratios (PAPR), intersymbol / intercarrier interference (ISI/ICI), and radio frequency interference (RFI). The goal of this special issue is to discuss the state of the art in signal processing techniques for DSL. Topics of interest include (but are not limited to): - Dynamic spectrum management - Vectoring, bonding, and phantom-mode transmission - Alien crosstalk cancelation - Other multiuser techniques - Turbo/LDPC codes for DSL - Ethernet in the first mile (EFM) - Advanced modulation techniques for DSL - PAPR reduction - Windowing and RFI cancelation - Equalization and echo cancelation - Impulse noise mitigation - Synchronization - Wavelets and filterbank

Real-Time Face Recognition Using Eigenfaces

In recent years considerable progress has been made in the area of face recognition. Through the development of techniques like eigenfaces, computer can now compute favourably with humans in many face recognition tasks, particularly those in which large databases of faces must be searched. Whilst these methods perform extremely well under constrained conditions, the problem of face recognition under gross variations in expressions, view and lighting remains largely unsolved. This paper details the design of a real-time face recognition system aimed at operating in less constrained environments. The system is capable of single scale recognition with an accuracy of 94% at 2 frames per second. A description of the system's performance and the issues and problems faced during its development is given

Performance of Crosstalk Cancellation in VDSL

Crosstalk in DSL leads to significant performance degradation and large loses in data-rate. Several crosstalk cancellation techniques have been proposed to address this problem, however, in the existing literature the analysis of these approaches is based on SNR calculations and the SNR-gap approximation. Furthermore, for crosstalk cancellation techniques based on decision feedback, the effect of error propagation is completely ignored. This makes it hard to predict the performance of crosstalk cancellation in real life, and to see if the significant potential gains can actually be realized. To address this problem, this paper uses Monte-Carlo simulation to investigate the performance of the various crosstalk cancellation techniques that have been proposed. The effect of noise-enhancement in zero-forcing crosstalk cancellers and error-propagation in decision-feedback cancellers is examined. The results confirm that a very simple crosstalk cancellation structure can achieve near-optimal performance

Autonomous Spectrum Balancing for Frequency Selective Interference 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

Simplified Power Allocation for the DSL Multi-access Channel through Column-wise Diagonal Dominance

In the newest generation of DSL systems, crosstalk is the dominant source of performance degradation. Many crosstalk cancellation schemes have been proposed. These schemes typically employ some form of co-ordination between modems and lead to large performance gains. The use of crosstalk cancellation means that power allocation should be viewed as a multi-user problem. In this paper we investigate optimal (i.e. capacity maximizing) power allocation in DSL systems which employ co-ordination to facilitate crosstalk cancellation. By exploiting certain properties of the DSL channel, it is shown that power allocation can be simplified considerably. The result has each user waterfilling against the background noise only, explicitly ignoring the interference from other users. We show this to be near-optimal for upstream DSL when Central Office (CO) modems are co-ordinated. Compared with conventional waterfilling which is done against the background noise and interference, the performance gains are significant

Simplified Power Allocation and TX/RX Structure for MIMO-DSL

In the newest generation of DSL systems crosstalk is the dominant source of performance degradation. Many crosstalk cancellation schemes have been proposed. These schemes typically employ some form of co-ordination between modems and lead to large performance gains. The use of crosstalk cancellation means that power allocation should be viewed as a multi-user problem. In this paper we investigate optimal (ie. capacity maximizing) power allocation in DSL systems which employ co-ordination to facilitate crosstalk cancellation. By exploiting certain properties of the DSL channel it is shown that power allocation can be simplified considerably. The result has each user waterfilling against the background noise only, explicitly ignoring the interference from other users. We show this to be near-optimal for upstream DSL when Central Office (CO) modems are co-ordinated. Compared with conventional waterfilling which is done against the background noise and interference, the performance gains are significant

Dual Optimization Methods for Multiuser OFDM Systems

The design and optimization of orthogonal frequency division multiplex (OFDM) systems typically take the following form: The design objective is usually to maximize the total sum rate which is the sum of individual rates in each frequency tone. The design constraints are usually linear constraints imposed across all tones. This paper explains why dual methods are ideally suited for this class of problems. The main result is the following: Regardless of whether the objective or 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 is much more efficient. This gives 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