High-speed Digital Subscriber Line Research Articles

Analysis of Zero-Forcing Vectoring for VDSL2 and Long Reach VDSL Technologies

A Normal approximation for the downstream bit rate per user in modern Fiber-to-the-Cabinet DSL-based access systems such as Very high-speed Digital Subscriber Line type 2 (VDSL2) and Long Reach VDSL (LR-VDSL) is presented in this paper. It accounts for the positions of subscribers along the main cable from the Cabinet, for the Far-end cross-talk (FEXT) fluctuations, for the transmitter power spectra of subscribers and for zero forcing vectoring. A stochastic model relating the FEXT to residual FEXT after zero forcing vectoring pre-coding is also presented. The proposed Normal model is derived by assuming Log-normal statistics of the signal-to-interference plus noise ratio per sub-carrier. Its validity is assessed by computer calculation by superimposing the cumulative distribution functions obtained from exact bit rate calculation with those obtained with the Normal model. Differences of few Mbit/s have been observed for high percentiles thus proving the effectiveness of the proposed model. We applied the model to assess coexistence among different DSL technologies such as VDSL2 and LR-VDSL sharing the same copper cable from the Cabinet and even to evaluate the downstream bit rate for G.fast with zero forcing vectoring.

GMRES Algorithm for Large-Scale Vectoring in DSL Systems

We propose an iterative crosstalk cancellation scheme based on the generalized minimal residual (GMRES) algorithm for large-scale digital subscriber line (DSL) systems. The proposed scheme does not require channel inversion and stores fewer vectors for crosstalk cancellation. We analyze the convergence of the GMRES algorithm and derive computable bounds on the residual error and signal-to-noise ratio in terms of system parameters at each iteration for upstream DSL systems. We show that the GMRES algorithm typically requires a single iteration for very large vectored systems to achieve crosstalk-free performance for the very high-speed DSL (VDSL) frequencies and only a few more in the highest frequency bands of the G.fast spectrum. This yields significant complexity savings and reduction in memory storage, compared to the zero forcing scheme under certain conditions.

A Worst Case Performance Analysis of Approximated Zero Forcing Vectoring for DSL Systems

In this paper we introduce a Gaussian approximation for the achievable downstream bit rate per user in modern broadband and ultra-broadband digital subscriber loop-based access systems. The considered formulation allows one to account for the main characteristics of the interference scenario (e.g., number and positions of interferers along the cable), the far-end crosstalk (FEXT) fluctuations, bit loading limitation per sub-carrier, and approximated zero forcing in vectoring pre-coding. Formulas are obtained assuming log-normal statistics for the signal-to-interference plus noise ratio per sub-carrier. The validity of the considered approximation has been assessed by computer calculation. A very good agreement between the exact and the approximated bit rates is obtained. The bit rate approximation is then used to analyze the performance of a very high-speed digital subscriber line type 2 (VDSL2, Profile 35b) with vectoring and used to assess G.fast performance degradation when approximated zero forcing in vectoring pre-coding is applied. It is observed for G.fast performance that degradation due to residual FEXT after vectoring pre-coding can be relevant. A significant performance improvement can be achieved at the expense of increased computational complexity of the vectoring pre-coding.

Coexistence in fourth generation digital subscriber lines: Experiment, modeling, and simulation results

The deployment of fourth generation digital subscriber line (DSL) technology (“G.fast”) will be gradual and it may therefore share the cable infrastructure with legacy DSL technologies such as Very high speed DSL transceivers 2 (VDSL2). We perform experiments on coexistence of G.fast with legacy VDSL2, highlighting the practical relevance of out-of-band leakage and aliasing. Furthermore, the differences in transmission parameters (e.g., carrier width and sampling rate) and asynchronous transmission results in inter-carrier and inter-symbol interference (ICSI). Previous work on modeling ICSI in the communication field focused on modeling only a subset of these effects. Hence, we analytically derive a simplified ICSI model, which notably includes the effects of aliasing, leakage, and worst-case symbol misalignment. Our results partially based on simulations show that a) neglecting ICSI potentially leads to significant bit-rate overestimation (e.g., 18% in G.fast rates); and b) a G.fast start frequency of approximately 23MHz may provide sufficient spectral separation with VDSL2 profile 17a transceivers.

Analytical Framework for Preliminary Planning of Very High-Speed Digital Subscriber Line Access Networks

In this paper, we introduce an analytical framework for the performance evaluation of the VDSL2-based access systems. It allows for the obtaining of approximations of the achievable bit rate per user, taking into account several factors, such as the bit-loading limitation per sub-carrier; interference scenario, including the number and positions of the active interferers along the cable; crosstalk statistics; and vectoring. A closed-form expression for the maximum sub-carrier frequency that can be loaded with the maximum number of allowed bits is also presented. Formulas are obtained assuming log-normal statistics for the signal-to-interference-plus-noise ratio (SINR) per sub-carrier. The validity of the proposed formulas has been assessed by computer calculations. A very good agreement between the exact and the approximated bit rates has been obtained. The framework can be used for preliminary design of VDSL2 systems in terms of the bit rate coverage, as well as to analyze performance of other access technologies such as ADSL and G.fast.

Future Broadband Access Networks [Point of View

Since the development of practical optical fiber, people have been predicting the widespread introduction of broadband access. The original vision was to replace the copper telephone lines with fiber, but that has not happened. Instead, we see a wide variety of fiberdeeper architectures (where the fiber network is taken closer to the end customer), including very high speed digital subscriber line (VDSL)-based fiber to the node, data over cable service interface specification (DOCSIS)-based hybrid fiber coax (HFC), and passive optical network (PON)-based fiber to the premise. Why such a diversity? And why hasn't the best technology won? The answers to these questions may help us to understand why we have gotten to the present situation, and give us clues to what may come next.

Performance of linear minimum‐output energy receiver for self and alien crosstalk mitigation in upstream vectored very high‐speed digital subscriber line

Linear zero-forcing (ZF) canceller does not perform well in the presence of alien crosstalk (AXT) while the receivers based on minimum-mean-square-error (MMSE) criterion require perfect knowledge of the noise covariance matrices. In this study, the authors consider the use of a constrained linear minimum-output energy (MOE) receiver in the presence of self-crosstalk and AXT in upstream vectored very high-speed digital subscriber line systems, that does not require knowledge of the noise correlation matrices and can be trained using the received signals without the use of training sequences. They derive bounds on the performance of the MOE receiver in the digital subscriber line environment and show that it reaches the MMSE performance for self-crosstalk cancellation. They also show that the performance of the proposed receiver lies in between that of the ZF receiver and the non-linear ZF generalised decision feedback equaliser receiver. An adaptation of the canceller coefficients using MOE algorithm shows comparable performance to that of the least mean squares algorithm. The effect of noise correlation on the capacity has also been highlighted via the Cramer-Rao lower bound. Computer simulations are presented to verify the analytical results and demonstrate the performance of the proposed receiver.

Economic aspects of fibre to the distribution point with G.fast

The use of hybrid fibre- and copper-based access networks can complement the deployment of fibre to the home (FTTH) networks, especially in areas where the deployment of fibre in the final metres is difficult. G.fast is a copper-based access technology that can, in theory, provide a combined downstream and upstream transmission capacity of up to 1Gbps over a distance of 100m. In this article, we analyse the cost of fibre to the distribution point (FTTdp) with G.fast and assess the cost reductions that can be achieved in comparison with FTTH networks. Two types of FTTdp networks have been employed: fibre to the building (FTTdp-Building) and fibre to the street (FTTdp-Street). The results show that, for all the network scenarios studied, the cost of a home passed for FTTH and FTTdp-Building is the same, but the cost of a home passed for FTTdp-Street is on average 17.2% lower than the cost for FTTH. Regarding the cost per home connected, the usage of FTTdp-Building and FTTdp-Street leads to average cost reductions compared with FTTH of 1.1% and 10.8%, respectively. The cost reductions achieved when employing FTTdp are higher in urban areas than in suburban and rural areas. Several measures could be taken by policymakers to promote the deployment of FTTdp networks that work with G.fast: these include the coordination of frequency bands to avoid the interference of G.fast with very high speed digital subscriber line 2 (VDSL2) equipment and the speeding up of permissions for the installation of FTTdp equipment.

Simple and Causal Copper Cable Model Suitable for G.fast Frequencies

G.fast is a new standard from the International Telecommunication Union, which targets 1 Gb/s over short copper loops using frequencies up to 212 MHz. This new technology requires accurate parametric cable models for simulation, design, and performance evaluation tests. Some existing copper cable models were designed for the very high speed digital subscriber line spectra, i.e., frequencies up to 30 MHz, and adopt assumptions that are violated when the frequency range is extended to G.fast frequencies. This paper introduces a simple and causal cable model that is able to accurately characterize copper loops composed by single or multiple segments, in both frequency and time domains. Results using G.fast topologies show that, apart from being accurate, the new model is attractive due to its low computational cost and closed-form expressions for fitting its parameters to measurement data.

Coexistence of FTTC and FTTDp network architectures in different VDSL2 scenarios

AbstractVery high speed Digital Subscriber Line type 2 (VDSL2) systems can achieve significant bit rate values depending on the access network architecture and how far‐end crosstalk (FEXT) interference suppression is implemented. The so‐called vectoring technology recently became an implementation reality. It was introduced to dynamically suppress ‘self‐FEXT’ in a cable of twisted pairs on both downstream (DS) and upstream (US) signals. In the DS direction, crosstalk pre‐compensation precedes modulation, and in the US cancellation follows demodulation. However, vectoring allows to effectively eliminate crosstalk if lines belong to the same vectored group. When this condition is not satisfied, in general, even very few uncontrolled interferers can cause significant performance degradation. In this paper, we evaluate by simulation the VDSL2 performance in scenarios of coexistence between Fibre‐To‐The‐Cabinet (FTTC) with vectoring and Fibre‐To‐The‐Distribution point (FTTDp) architectures. We consider two different combinations of VDSL2 band plans assigned to the FTTC and FTTDp networks. We analyse VDSL2 performance in both cases of absence of spectrum management and implementation of the Downstream Power Back‐Off (DPBO) technique. We also study the throughput trade‐off between the two VDSL2 systems due to DPBO. Results show that, to protect VDSL2 FTTC systems, DPBO must be configured on VDSL2 FTTDp, whereas the investment required to bring the optical fibre to the distribution point could be justified by the improved performance achievable with the FTTDp architecture, even with the VDSL2 band profiles currently used. The presented coexistence analysis can be useful with reference to the G.fast spectrum allocations currently under definition. © 2014 The Authors. Transactions on Emerging Telecommunications Technologies published by John Wiley & Sons, Ltd.

Dynamic Bandplanning for Vectored DSL

There are two types of crosstalk in digital subscriber line (DSL) systems, namely near-end crosstalk (NEXT) and far-end crosstalk (FEXT). NEXT is usually much stronger than FEXT. Therefore, high-speed DSL systems transmit in preplanned disjoint downstream (DS) and upstream (US) frequency bands to avoid NEXT. Although easy for implementation, such a fixed bandplan can lead to inefficient bandwidth usage, depending on the DS and US bit rate requirements and the loop topology, particularly in so-called vectored DSL systems, which include signal coordination for FEXT cancellation. In dynamic bandplanning (DBP), each frequency band is allocated to either DS, US, or to both directions depending on the aforementioned parameters. In this paper, we consider optimal DBP for vectored DSL with linear as well as with nonlinear transmitter/receiver structures. We propose an optimal DBP algorithm for systems with disjoint DS and US bands. For systems with overlapping bands, the problem of finding the optimal transmitter/receiver filters is nonconvex and we propose two iterative algorithms based on recent optimum spectrum balancing schemes from the literature. We also study the effect of echo and US NEXT cancellation on the system performance. Finally, simulation results are provided to demonstrate that our algorithms can indeed significantly increase the achievable bit rates.

Examination of Multiplexing VDSL2 over ADSL2+ Line

Today, the second generation of Very High Speed Digital Subscriber Lines (VDSL2) is widely used to replace previous Asymmetric Digital Subscriber Lines (ADSL2+) in access networks in practice. However, the VDSL2 lines can be usually applied only for short distances and also some of the subscribers will probably keep using their old ADSL2+ lines. Due to that, the frequency band of symmetrical pairs with ADSL2+ lines will stay unoccupied above 2.2 MHz and could be possibly used for VDSL2 lines. That is why a new idea about multiplexing VDSL2 line over ADSL2+ is investigated and described in this article. Both of these lines using one single symmetrical pair in a cable must be separated by frequency multiplexing method, when ADSL2+ line occupies frequency band up to 2.2 MHz, while the VDSL2 line would operate from 2.2 MHz to higher frequencies. The method could possibly increase the number of xDSL lines in one cable and could connect more subscribers using existing metallic lines. This article describes the initial idea together with the process of designing possible VDSL2 over ADSL2+ splitter, which was subsequently used for performing real measurements. Thanks to these measurements, simulations of several possible scenarios were performed and are presented in this article as well. DOI: http://dx.doi.org/10.5755/j01.eee.19.8.3116

Performance Comparison of BPL, EtherLoop and SHDSL technology performance on existing pilot cable circuits under the presence of induced voltage

Pilot cable is originally used for utility protection. Then, pilot cable is further utilized for SCADA communication with low frequency PSK modem in the early 1990. However, the quality of pilot cable communication drops recently. Pilot cable starts to deteriorate due to aging and other unknown factors. It is also believed that the presence of induced voltage causes interference to existing modem communication which operates at low frequency channel. Therefore, BPL (Broadband Power Line), EtherLoop and SHDSL (Symmetrical High-speed Digital Subscriber Line) modem technology are proposed as alternative communication solutions for pilot cable communication. The performance of the 3 selected technologies on existing pilot cable circuits under the presence of induced voltage are measured and compared. Total of 11 pilot circuits with different length and level of induced voltage influence are selected for modem testing. The performance of BPL, EtherLoop and SHDSL modem technology are measured by the delay, bandwidth, packet loss and the long term usability SCADA (Supervisory Control and Data Acquisition) application. The testing results are presented and discussed in this paper. The results show that the 3 selected technologies are dependent on distance and independent on the level of induced voltage.

Measurement system design and implementation for grid-connected marine substation

A marine substation for a grid-connected wave power plant has been designed and constructed at Uppsala University. The measurement system for the substation has been developed from basics, and the procedure is described in this article. Subsea electrical installations set high constraints on the control and data acquisition systems used, and traditional GSM networks and GPS time synchronization are difficult and inefficient. These circumstances exclude many well-proven methods to be utilized. A compactRIO-based system, with integrated real-time controller and Field-Programmable Gate Array chip, is chosen as most suitable for this task. The system is designed to meet the special requirements encountered in this unusual application, including large data sampling, grid connection control and protection systems. The system is communicating to shore via a subsea copper cable and single-pair high-speed digital subscriber line modems. The design was successful and full-fills all the system requirements. The aim of the article is to provide future researchers with a good design and implementation procedure for setting up large measurement systems.

A New Design Framework for Sparse FIR MIMO Equalizers

In this paper, we propose a new framework for the design of sparse finite impulse response (FIR) equalizers. We start by formulating greedy and convex-optimization-based solutions for sparse FIR linear equalizer tap vectors given a maximum allowable loss in the decision-point signal-to-noise ratio. Then, we extend our formulation to decision feedback equalizers and multiple-antenna systems. This is followed by further generalization to the channel shortening setup which is important for communication systems operating over broadband channels with long channel impulse responses. We propose a novel approach to design a sparse target impulse response. Finally, as an application of current practical interest, we consider self far-end crosstalk cancellation on vectored very high-speed digital subscriber line systems for cellular backhaul networks.

On Signal-to-Noise Ratio-Assisted Crosstalk Channel Estimation in Downstream DSL Systems

Downstream (DS) crosstalk channels of digital subscriber line systems can be estimated based on the reported signal-to-noise ratio (SNR). In the case of an already active victim line (VL) impacted by the joining of a new disturber line (DL), sending perturbing signals on the VL and reporting the SNRs allow the estimation of the crosstalk channel from the DL to the VL. This paper generalizes that concept to include full startup, tracking and joining scenarios as well as the impact of different perturbation signal choices. Additionally, the sensitivity of the algorithm to different sources of imperfection is analyzed. Simulation results reveal that starting without precoding, and updating the precoder matrix based on the DS crosstalk channel estimates, the far-end crosstalk-free SNR can be reached in a limited number of iterations (e.g., 36 SNR measurements are needed for the four lines case). The SNR-assisted method is documented in an informative appendix of the draft G.vector recommendation as a backward compatible method for very high speed digital subscriber lines (VDSL2) and, hence, can be used for channel acquisition today.

Effect of PLC Signal Induced into VDSL System by Conductive Coupling

We investigated the effect of a high-speed power line communication (PLC) signal induced into a very high-speed digital subscriber line (VDSL) system by conductive coupling based on a network model. Four electronic devices with AC mains and telecommunication ports were modeled using a 4-port network, and the parameters of the network were obtained from measuring impedance and transmission loss. We evaluated the decoupling factor from the mains port to the telecommunication port of a VDSL modem using these parameters for the four electric and electronic devices. The results indicate that the mean value of the decoupling factor for the differential and common mode signals were more than 88 and 62dB, respectively, in the frequency range of a PLC system. Taking the following parameters into consideration; decoupling factor Ld, the average transmission signal powers of VDSL and PLC, desired and undesired (DU) ratio, and transmission loss of a typical 300-m-long indoor telecommunication line, the VDSL system cannot be disturbed by the PLC signal induced into the VDSL modem from the AC mains port in normal installation.

A blind detection-guided approach for normalised multi-modulus crosstalk estimator in sparse multi-user DSL channels

In high-speed digital subscriber lines (DSL), far-end crosstalk is the main limiting factor on data rates. However, most of the crosstalk is due to the neighbouring twisted pairs in the binder. Therefore, the crosstalk channel matrix is sparse. Using Level 3 of Dynamic Spectrum Management, users are co-ordinated at the central office to cancel the crosstalk. Means for estimating the crosstalk canceller matrix are of critical importance for the cancellation to prove effective. Preferably, the estimation procedure should have low overhead both in computation and bandwidth. Normalised least mean squares (NLMS) based adaptive crosstalk cancellers (Gujrathi et al. (2009) [1]) have a low computational overhead but use a training sequence to ensure they converge adequately. However, using a training sequence consumes some amount of bandwidth which can be avoided if an unsupervised or blind algorithm like a normalised multi-modulus algorithm (NMMA) is used instead. A limitation of NMMA is that its convergence time is often longer than that of the NLMS algorithm. Furthermore, this is made worse as the number of canceller coefficients is made larger. In application to adaptive crosstalk cancellation within the multi-user DSL binder-channel, we argue that the convergence time can be significantly decreased by using an activity detector to exclude canceller coefficients below an appropriate minimum. In this paper, we present an activity detector design using a thresholding criterion based on the least squares technique, Akaike's information criterion (Homer et al. (1998) [2]) and Donoho's universal thresholding principle (Donoho (1995) [3]). This enables us to identify the significant crosstalkers within a DSL binder for each user. We further incorporate this strategy within the blind estimation NMMA and propose an enhanced crosstalk canceller. Our simulations indicate this multi-modulus detection-guided crosstalk canceller demonstrates improved convergence speed and has a steady state error close to that of the standard (non-detection-guided) canceller.

Energy Consumption in Optical IP Networks

As community concerns about global energy consumption grow, the power consumption of the Internet is becoming an issue of increasing importance. In this paper, we present a network-based model of power consumption in optical IP networks and use this model to estimate the energy consumption of the Internet. The model includes the core, metro and edge, access and video distribution networks, and takes into account energy consumption in switching and transmission equipment. We include a number of access technologies, including digital subscriber line with ADSL2+, fiber to the home using passive optical networks, fiber to the node combined with very high-speed digital subscriber line and point-to-point optical systems. In addition to estimating the power consumption of today's Internet, we make predictions of power consumption in a future higher capacity Internet using estimates of improvements in efficiency in coming generations of network equipment. We estimate that the Internet currently consumes about 0.4% of electricity consumption in broadband-enabled countries. While the energy efficiency of network equipment will improve, and savings can be made by employing optical bypass and multicast, the power consumption of the Internet could approach 1% of electricity consumption as access rates increase. The energy consumption per bit of data on the Internet is around 75\bm muJ at low access rates and decreases to around 2-4 \bm muJ at an access rate of 100 Mb/s.

Validating a method for classifying digitally modulated signals

In order to design a flexible measurement instrument for digital telecommunication signals, a method has been proposed in a precedent paper. The method can automatically recognize the classical band-pass single carrier modulations such as ASK (Amplitude Shift Keying), PSK (Phase Shift Keying), FSK (Frequency Shift Keying), QAM (Quadrature Amplitude Modulation), and the multicarrier OFDM (Orthogonal Frequency Division Multiplexing), such as the DMT (Discrete MultiTone) required by the ADSL (Asymmetric Digital Subscriber Line) and VDSL (Very high-speed Digital Subscriber Line) standards, and by the HomePlug 1.0 PLC (PowerLine Carrier) transmission. This paper presents the first experimental results obtained by applying the proposed method on simulated signals in different environments in order to evaluate its performances and application range. In particular it has been validated in presence of additive white Gaussian noise, colored noise and multipath fading showing good classification percentages, also in very noisy environments.