The author gives an overview of progress made in the evolution of technology to provide DS1 rate telephone access in a restricted segment of the loop plant without intermediate repeaters, loop conditioning, or pair selection in assignment. This technology is called the high bit rate digital subscriber line (HDSL). Discussed are background information on electronics in the loop plant and characterization of the tranmission environment in the relevant frequency band. The progress of HDSL study project of the American National Standards Institute (ANSI) is outlined. Analytical and theoretical studies to determine the limits on the transmission capabilities of the loop plant, motivated by the need to determine the feasibility limits of HDSLs, are reviewed. Also discussed is progress in technical work on suitable transmission formats. The possibility of an asymmetrical digital subscriber line (ADSL), transmitting at the DS1 rate from the central office to a remote distribution point, through the entire nonloaded loop plant is discussed. >

High bit rate digital subscriber lines: a review of HDSL progress

Decision feedback equalizers (DFEs) are widely used in modern local network digital transmission systems to remove the intersymbol interference caused by slowly decaying pulse tails. A gradient descent algorithm for adapting a coefficient to model the slowly decaying portion of the tail is described. An equalization strategy is described that exploits prior knowledge of the nature of the subscriber loop channel, together with the new adaptation algorithm, to give reduced complexity DFE structures. The use of this algorithm in FIR and IIR equalizer structures is described. The use of this algorithm in FIR and IIR equalizers is quantitatively compared to a conventional DFE in terms of performance and implementation complexity. An analysis is presented describing the operation of the adaptation algorithm in the presence of noise. Simulation results illustrate the training of the algorithm and its stability in the presence of near-end crosstalk noise. >

Reduced complexity decision feedback equalization for digital subscriber loops

The experience gained with pulse amplitude mod- ulation (PAM) for basic rate ISDN transmission makes this baseband coding scheme a strong contender for early HDSL transceiver implementations. After near-end crosstalk (NEXT), the principal problem with PAM is intersymbol interference (ISI). This paper presents an analytical comparison of various recursive and nonrecursive subtractive equalization techniques which may be used to remove ISI. Properties of subscriber loop responses which may be used in the design of simple and effi- cient equalizers are also explored. For early HDSL transceiver implementations with limited processing power, the best over- all equalization strategy may well be a conventional FIR-filter- based decision feedback equalizer with a distributed partial lin- ear equalizer allowing the best compromise between filter length, residual ISI, NEXT enhancement, and implementation complexity.

Review and Analytical Comparison of Recursive and Nonrecursive Equalization Techniques for PAM

After near-end crosstalk (NEXT), the principal problem with pulse amplitude modulation (PAM) is intersymbol interference (ISI). The author presents an analytical comparison of various recursive and nonrecursive subtractive equalization techniques which may be used to remove ISI. Properties of subscriber loop responses which may be used in the design of simple and efficient equalizers are also explored. Simulation results are presented comparing the performances of many of the described equalizer structures for an 800 kb/s 2B1Q PAM transmission system operating over 12 kft of 24 AWG cable. >

Review and analytical comparison of recursive and nonrecursive equalization techniques for PAM transmission systems

Existing local networks of copper-pair cables provide a medium for digital transmission between subscribers and their local telephone exchange. Transceivers for this medium have undergone a remarkable period of development stimulated by the promise of universal digital communications at rates of 64 kbit/s and above and enabled by rapidly advancing very large scale integrated (VLSI) circuit technology that now allows hitherto unimaginable circuit complexity at low cost, low power consumption and small size. The history of this change and its interaction with standardization processes forms the backdrop of this chapter, which is mainly concerned with the technology of local loop transceivers, especially for the important American National Standards Institute (ANSI) interface standard for integrated services digital network (ISDN) basic rate access. The ANSI standard is outlined and critically appraised and the impact of modern silicon technology trends is assessed on the dichotomy between standardization and functional diversity.

Review of Copper-Pair Local Loop Transmission Systems

The design and performance of high-bit-rate digital subscriber loop (HDSL) transceivers is examined with the intention of devising systems with signal processing and technology requirements that are not vastly dissimilar to those of basic access transceivers. The topology, line codes, postcursor equalization, precursor equalization, and noise prediction of a simple HDSL transceiver are discussed. A design strategy that would allow early development of an HDSL transceiver to provide 800-kb/s access is explored. A useful benchmark against which the performance versus complexity tradeoff of more sophisticated systems may be assessed is described. >

N.G. Cole

Papers

Design issues for early high bit-rate digital subscriber lines