Integrated DSL Test, Analysis, and Operations

This paper describes methods of integrating the digital subscriber line (DSL) automated test, data from network elements, analyses, and DSL operations, including work-force administration. Fiber-to-the-node systems using very-high-rate DSL 2 will particularly need the integrated DSL test to increase customer satisfaction and streamline operations to reduce costs. The integrated DSL test extends beyond copper tests to encompass all elements of DSL service, all through the network and at higher layers. An integrated test system queries multiple test points, including network elements, allowing physical and higher layer problems to be identified. Diagnosis, fault isolation, and even suggested reconfigurations are all automated in software. A number of techniques, which are implementable in software, are described, which can show the root cause of many intricate troubles to relatively unskilled technicians. Automated maintenance routines can isolate faults in flowthrough testing, automatically issuing trouble tickets toward the correct work center: Internet service providers, central offices, outside plants, user PCs, etc. New triple-play services have high customer expectations for easy-to-use TV and voice services, with demanding bit rates and reliability requirements that can be ensured with an integrated test.     

Constellations for good diversity performance

The case in which there are several channels available for transmission, and the additive noise on each channel may have different power, is treated. A signal constellation, called the coordinated code, for the signal space formed by the different channels is presented. The coordinated code has good performance when the noise powers are equal, while simultaneously providing diversity for good performance when the noise powers are unequal, and is relatively simple. The maximum-likelihood receiver is presented; it extracts information from the coded signal in inverse proportion to each channel's noise power. The minimum distance of the code is found, and the coding gain over a system that transmits independent bit streams on each channel is derived as a function of the noise statistics. The gain of the code is also found relative to symbol splitting diversity, which transmits a copy of the same message on each channel; this gain is as high as 9 dB. The code is considered in detail for use with a high rate digital subscriber line (HDSL) comprised of two pairs of a local loop, each pair having different amounts of near end crosstalk (NEXT)     

Near-end crosstalk is almost Gaussian

Crosstalk from digital subscriber lines, high rate digital subscriber lines, and asymmetric digital subscriber lines is analyzed. The probability density of crosstalk is estimated. From this estimate the entropy power and chi-squared statistics of crosstalk are computed and compared with that of a Gaussian density     

Single-ended loop-makeup Identification-part II: improved algorithms and performance results

Digital subscriber lines (DSLs) transmit over twisted-pair telephone loops, and knowledge of the loop makeup is vital for determining what types of DSLs may be deployed and at what bit rates they may transmit. This paper expands and applies the algorithm presented in Part I of this study for identifying a loop makeup, including cable gauges, lengths, and bridged taps, via single-ended measurements based on the use of an enhanced time domain reflectometer. Techniques based on the exploitation of the information of the sign of the echo for reducing identification time are described. Loop identification is further improved by retaining multiple hypothesized loop topologies, as the loop estimate is built out constructing multiple paths in a tree search via branch-and-bound techniques. Furthermore, statistical data is used to improve overall loop identification results by accounting for the different likelihood of different cable gauges at different distances from a central office. Test loops derived from a loop database are described, and results of automated loop-makeup identification on these loops indicate the accuracy of the loop identification as a function of the percentage of loops at a wire center.     

A comparison of passband and baseband transmission schemes for HDSL

Using an ideal decision feedback equalizer (DFE), the SNR of quadrature amplitude modulation (QAM) and baseband pulse amplitude modulation (PAM) in the presence of self near-end crosstalk is computed for a large sample of loops within a carrier serving area (CSA). When baud-space feedforward filters are used, PAM has 1-2 dB more SNR than QAM, where the type of PAM is the 2B1Q line code. However, when using fractionally spaced feedforward equalizers (FSEs), the SNRs of 2B1Q and QAM are almost equal for loops at the extreme range of a CSA. Four- and 16-state trellis-coded modulation is applied to PAM and QAM. Coded and uncoded PAM and QAM are simulated with parallel decision feedback estimation. Viterbi receivers and coding gains are computed. QAM has up to 1 dB higher coding gains that PAM. However, the higher coding gains of QAM do not compensate for the lower SNR of uncoded QAM, and coded QAM has worse performance than coded PAM in the presence of self near-end crosstalk. The error rates of PAM and QAM with impulse noise are computed using a collection of measured impulse noise events. Results indicate that QAM has a lower error rate than PAM in the presence of impulse noise     

Minimum mean squared error impulse noise estimation andcancellation

Impulses are infrequent bursts of high amplitude noise. A wide-band communications or data acquisition receiver has a fast sampling rate, so it can capture many samples of each impulse waveform. The arrival of an impulse can be identified by its distinct waveform and amplitude. The paper models impulse waveforms as a vector subspace of low dimension. Formulas are derived for the minimum mean squared error (MMSE) estimation of the arrival time and amplitudes of impulses, given that a set of vectors that spans the subspace is known. Formulas are also derived for the adaptive MMSE estimation of a set of vectors that spans the subspace. The values of the mean squared error (MSE) of the amplitude estimates are determined. It is shown how the theory can be used to cancel impulse noise. Correlated impulse noise arriving at a reference input can be used to estimate and cancel the primary input impulse noise. The MMSE coefficients for impulse noise cancellation are derived and presented. Simulations are presented that use the equations and methods derived in the paper for modeling and canceling impulse noise measured on copper telephone loops for asymmetric digital subscriber lines (ADSL)     

Methods of summing crosstalk from mixed sources. I. Theoreticalanalysis

Digital subscriber lines (DSLs) are fundamentally limited by crosstalk. The case where all crosstalk is from the same type of DSL has been studied over the years and accurate models have been standardized. However, crosstalk from multiple different types of DSLs is a relatively new area of study and models of summing mixed crosstalk have only recently been postulated. As more and more types of DSLs are deployed by multiple service providers, it is imperative to gain confidence in the method of modeling worst-case mixed crosstalk and to understand its limitations in order to enable efficient spectrum management of the loop plant. All known crosstalk summation methods are described here in detail and their properties are compared. This paper also presents a new methodology for deriving sound crosstalk summation methods. This new methodology consists of deriving lower bounds to the worst-case method, which is intrinsically too pessimistic. In particular, it has been ascertained that the Minkowski inequality and, to a lesser extent, the Holder inequality, yield new crosstalk summation methods that exhibit appealing properties. It is also shown here that the standardized full service access network (FSAN) method is a particular case of the more general Minkowski-bound method     

A frequency-domain approach to crosstalk identification in xDSLsystems

Crosstalk between multiple services transmitting through the same telephone cable is the primary limitation to digital subscriber line services. From a spectrum management point of view, it is important to have an accurate map of all the services that generate crosstalk into a given pair. If crosstalk is measured via modem-based methods, i.e., while a digital subscriber line (DSL) system is running, what is measured is the crosstalk in the bandwidth of the considered DSL system. For this reason, DSL services running on adjacent pairs may not be detected if their bandwidth is not significantly overlapping with the bandwidth of the disturbed system. This is a major drawback of modem-based system identification techniques since, from a spectrum management point of view, it is important to be able to identify all crosstalkers. We address the important problem of crosstalk identification when the pair under test does not bear DSL services, i.e., via a non-modem-based approach. Crosstalk sources are identified in the frequency domain by finding the maximum correlation with a “basis set” of representative measured crosstalk couplings. The effectiveness of the proposed technique is also verified on the basis of real crosstalk measurements performed on actual cables. Finally, new techniques based on multiple regression and best basis selection are also discussed     

Coaxial cable passive mesh networks

This paper presents a new type of fault-tolerant access network: an all-passive coaxial cable mesh network. The passive mesh network could have any topology, with cycles allowed. A technique for calculating the multipath response of the passive mesh network is presented. Both the delay and attenuation of a coaxial cable are represented by a single transform variable. The mesh network is modeled as a linear system with a state space that represents signal propagation. The channel responses of the individual sections of cable define the entries of a state-transition matrix. Using this theory, expressions are given for the overall mesh-network channel response. These expressions are manipulated to derive equalizer structures. The equalizers are zero-forcing and use decision feedback. It is shown that signals transmitted on any mesh network can be equalized. An example mesh topology, and equalizers for it, are presented. Signal and interference attenuation, and opposite-phase received carrier cancellation, are also discussed. The passive mesh network could be an inexpensive fault-tolerant architecture for residential access to telephony, cable TV, and future services