David W. Lin

Video on phone lines: technology and applications

This paper reviews the telephone loop plant characteristics, current DSL (digital subscriber line) technologies, recent efforts in video coding standards, and the interrelationship between DSL technologies and visual communications over subscriber lines. In overview of the loop plant characteristics we examine its physical makeup and transmission properties, where for the latter we discuss frequency and time responses of wire-pair lines and the impairments of echo, crosstalk, impulse noise, and radio frequency interference. We trace the historical development of various DSL technologies and comment on possible future evolution. Transmission technologies used in the ISDN basic-access DSL, the high bit-rate DSL, and the asymmetric DSL are portrayed. And the issue of spectrum compatibility among different transmission systems is explained. Several important video coding standards are briefly described, including ITU-Ts H.261 and ISOs JPEG and MPEG series, which are either completed or emerging. The synergistic relationship between these standards and the DSL technologies is elucidated. As a result, DSL technologies provide the potential of delivering certain broadband services well in advance of direct fiber access for telephone subscribers. >

Minimum mean-square error echo cancellation and equalization for digital subscriber line transmission. I. Theory and computation

An integral part of an ISDN (integrated services digital network) is the provision of full-duplex digital transmission capability over voice-grade metallic subscriber lines with all the associated disturbances from echo, intersymbol interference, and crosstalk, among other things. The author presents a theory for analyzing the optimal performance, in the MMSE (minimum-mean-squared error) sense, of full-duplex transceiver structures incorporating echo cancelers and decision-feedback equalizers. The theory augments previously published results by allowing a colored input signal and a fractionally spaced multitap MMSE forward filter in the decision-feedback equalizer. The properties of and efficient ways of calculating the MMSE solutions for various types of line codes, including the precoded partial-response codes and the block codes, are investigated. The author discusses how the MMSE is related to the ubiquitous SNR (signal-to-noise ratio) measure and the concerns associated with using it to gauge the transmission performance. >

Joint carrier recovery and multimodulus blind decision-feedback equalization under high-order QAM

The multimodulus algorithm (MMA) for blind equalization has good performance, but can only tolerate small carrier frequency offsets under high-order QAM. We propose a structure for joint carrier recovery (CR) and blind decision-feedback equalization (DFE), where the DFE is MMA-based. It turns out that a key point is the design and analysis of reduced-constellation phase detectors (PD) that can facilitate a wide frequency lock range and make the CR loop converge in severe intersymbol interference and noise. Based on this, we design a multistage startup procedure for joint blind DFE and CR. Simulations show that the procedure can yield fast convergence in large carrier frequency errors under high-order QAM.

A tutorial on digital subscriber line transceiver for ISDN

An integral part of ISDN (integrated-services digital network) is the provision of full-duplex digital transmission capability at the basic access information rate of 144 kb/s over voice-grade metallic subscriber lines with all the associated disturbances from echo, intersymbol interference, and crosstalk, among other things. The authors provide an overview of the transmission environment and discuss the structure of transceivers for such use. Line coding, equalization, echo cancellation, and timing recovery are discussed at some length. A few numerical examples are given to illustrate the theoretically optimal transmission performance that can be attained with minimum mean-squared error equalization. Considerations regarding VLSI implementation of transceivers are briefly addressed.<<ETX>>

A hybrid variable step-size adaptive blind equalization algorithm for QAM signals

We develop an adaptive decision-feedback equalization algorithm that combines blind adaptation and decision-directed LMS in a dynamic manner according to the amount of equalizer output error. By observing how the mean-square blind equalization error depends on the adaptation step size, we obtain a way of continuously varying the adaptation speed of the overall algorithm with the equalizer output error as well as a way to shift the relative emphasis between blind and decision-directed LMS operation. We also describe the way to estimate the amount of equalizer output error in the algorithm. Simulation results show that the proposed algorithm can achieve faster convergence at a lower complexity than some recently proposed hybrid adaptation algorithms

Receiver optimization for dispersive channels employing coded modulation, with application in high rate digital subscriber line transmission

The authors consider decision-feedback sequence estimation of coded modulated signals transmitted over dispersive channels, and examine its application in high-bit-rate (>144-kb/s) transmission over metallic subscriber lines. To mitigate the adverse effect of colored noise, they consider a scheme called noise prediction. Formulas for optimal receiver filters under a minimum mean-squared-error criterion are derived. A key step in the optimization process is the solution of a max-min problem. Because of the complexity in solving this problem, a suboptimal solution is considered which leads to a convenient lower bound on the potential transmission performance. The derived theory is applied to a few example subscriber lines. Numerical results show the potential of substantial signal-to-signal-ratio gain over pure decision-feedback equalization.<<ETX>>

Variable-step-size multimodulus blind decision-feedback equalization for high-order QAM based on boundary MSE estimation

We consider blind decision-feedback equalization (DFE) under high-order modulation, which presents more difficult operating requirements than lower-order modulation. We base our design on the multimodulus algorithm (MMA). To attain fast convergence speed and low steady-state mean-square error (MSE), we consider varying the adaptation step size according to the presently achieved MSE. For this we investigate the properties of the MSE under blind MMA-based DFE and, based on the results, propose a method to estimate its value. The estimate is obtained by analyzing those equalizer filter outputs whose values fall outside the boundary of the modulations constellation. Simulation results demonstrate the effectiveness of the proposed scheme.

An efficient FSE/DFE-based HDSL equalizer with new adaptive algorithms

We consider the design of an efficient FSE/DFE-based HDSL equalizer. For this, several variants of the conventional LMS algorithm and their delayed versions are investigated. In addition, two new adaptive algorithms are also studied which yield lower computational complexity but similar performance when compared to conventional algorithms. We also propose a way to initialize the DFE coefficients for fast convergence. Extensive simulation is conducted to evaluate various algorithms performance and to facilitate hardware design. Due to time constraint, the present design employs a conventional algorithm. It is verified with the Verilog and Opus VLSI CAD tools. Layout design of the equalizer chip has been taped out for foundry fabrication.<<ETX>>

On optimal bit loading for multitone ADSL

Bit loading is a key issue in digital subscriber line transmission employing discrete multitone (DMT) modulation. We demonstrate that most existing algorithms are suboptimal. We present a solution based on the Lagrange multiplier approach. Numerical results are presented to illustrate the solutions performance.

High bit rate digital subscriber line transmission with noise-predictive decision-feedback equalization and block coded modulation

The author presents a proposal for solving the problem of reduced performance of coded modulation due to colored channel noise by whitening the noise with an added noise predictor in the decision-feedback equalizer (DFE). This also has the benefit of increasing the nominal SNR. The resulting equalizer has a structure encompassing as a special case the noise-predictive DFE proposed earlier by others. Several numerical examples are provided to illustrate the digital subscriber line transmission performance with the new noise-predictive DFE. It is shown that the added noise predictor improves the transmission performance in two ways: first, it decorrelates the noise, and second, it decreases the noise power by its prediction gain.<<ETX>>