constraints

In this paper, we develop solutions for the loading of digital subscriber loop (DSL) multicarrier (MC) systems that present constraints both on overall available energy and maximum energy per carrier. In the emerging G.DMT-based systems planned for high-throughput multimedia applications, the constraint on the peak-energy arises from spectral compatibility issues. However, until today, optimal solutions for loading peak-energy constrained MC systems do not seem explicitly developed in the literature. Hence, starting from suitable applications of the Kuhn--Tucker conditions, in this paper, we present the analytical relationships characterizing the optimal solution of the peak-energy-limited loading for the general case of concave "rate-functions," and then, we apply them in the context of the so-called "gap analysis." Thus, a low-complexity iterative algorithm implementing this solution is also developed, and its performance is numerically tested on several ANSI-standard asymmetric DSL (ADSL)-type loops impaired by crosstalk. Furthermore, a version of the presented loading algorithm that guarantees integer bit rates with low computational effort is also presented, and its performance is tested. The carried-out performance comparisons allow us to evaluate the throughput loss induced by peak-energy constraints in emerging ADSL-like services.

A technique is presented for jointly optimizing the signaling in the two directions of transmission on a twisted pair communications channel. It is then applied to twisted pair channel models with monotonic channel response and crosstalk transfer functions. While the signaling strategy presented in this paper can achieve only a lower bound on the true channel capacity, it is a significant improvement over existing signaling schemes. In particular, in contrast with existing schemes, the maximum information rate for the joint signaling strategy increases without bound as the SNR approaches infinity. It is also shown through numerical results that the proposed signaling strategy generalizes naturally to more practical non-monotonic twisted pair channel models incorporating bridge taps and other non-idealities. Finally, the form of the optimal signaling strategy suggests a relatively straightforward implementation using multicarrier modulation.

We present a technique for designing optimal transmit spectra for the HDSL2 service subject to a peak power constraint in the frequency domain. The peak power constraint might be imposed either due to engineering considerations or in order to optimize the transmission under a fixed transmit spectral mask (the OPTIS mask, for example). Using the channel and interference transfer functions, SNR estimates, and the peak constraint, we set up and solve an optimization problem to maximize the capacity. Sizable gains in performance margins (or bit rates) result. Furthermore, by design, the spectra are spectrally compatible with other services. While the technique is quite general --- it does not depend on the exact choice of modulation scheme, for example --- it is also extremely simple and of low computational complexity.

In this paper we study the capacity of multipair DSL systems limited by near end crosstalk. We begin by developing a statistical model for cross spectral matrices of the multipair crosstalk. We then compute the capacity for multiple Gaussian interfering signals based on some simplified analytic models. We analyze two specific cases where the number of disturbers is small and large. We also provide some analysis using measured data to demonstrate the gain in coordinating large number of pairs.

We present a general framework for designing optimal transmit spectra for the HDSL2 service. Using the channel and interference transfer functions and SNR estimates, we set up and solve an optimization problem to maximize the capacity. Sizable gains in performance margins (or bit rates) result. Furthermore, by design, the spectra are spectrally compatible with other services. While the framework is quite general --- it does not depend on the exact choice of modulation scheme, for example --- it is also extremely simple and of low computational complexity. Our results can be used either for dynamically adapting the signaling spectra to account for changing noise or interference conditions or for the design of new fixed transmit spectral masks using worst-case analysis.

Abstract—We assess the capacity potential of very short very-high data-rate digital subscriber line loops using full-binder channel measurements collected by France Telecom R&D. Key statistics are provided for both uncoordinated and vectored systems employing coordinated transmitters and coordinated receivers. The vectoring benefit is evaluated under the assumption of transmit precompensation for the elimination of self-far-end crosstalk, and echo cancellation of self-near-end crosstalk. The results provide useful bounds for developers and providers alike. Index Terms—Capacity, very-high data-rate digital subscriber line (VDSL). I.

In this paper, we present a novel iterative (turbo) receiver with tunable complexity for reliable detection of (uncoded) payload data transmitted over long intersymbol interference (ISI) channels affected by crosstalk, as those typically encountered in emerging HDSL2 services standardized by ANSI T1.418 recommendation. The proposed receiver combines in an original way "minimum mean square error (MMSE) estimation principle," "turbo-processing principle" and "crosstalk-prediction principle" for achieving both suboptimal maximum a posteriori probability channel equalization and reliable soft-mitigation of ISI tail plus crosstalk. More in detail, according to the turbo-processing principle, at each iteration suitable extrinsic informations are extracted from the equalizing and interference-canceling modules and are used as "a priori information" for the next iteration. Several simulation results on typical HDSL-like test-loops confirm superiority of the proposed turbo-detector (TD) over current solutions based on conventional MMSE decision-feedback equalizers and precoders (such as the Tomlinson--Harashima precoder). The numerical tests also point out that performance of the presented TD on typical high bit-rate digital subscriber lines (HDSLs) is not limited by error-floor phenomena, even at error-probabilities below 10 7 .

G.DMT and G.lite Asymmetric Digital Subscriber Line (ADSL) modems and some Very-high-speed Digital Subscriber Line (VDSL) modems rely on discrete multitone modulation (DMT). In an ADSL discrete multitone receiver, a time-domain equalizer (TEQ) reduces intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Previous TEQ design methods such as minimum mean-squared error (MMSE), maximum shortening signal-to-noise ratio (MSSNR), and maximum geometric signal-to-noise ratio (MGSNR) do not directly optimize channel capacity. In this dissertation, I develop a TEQ design method to optimize channel capacity at the output of the TEQ. First, I partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel signal-to-noise (SNR) definition. Using the new subchannel SNR definition, I derive a nonlinear function of TEQ taps that measures channel capacity. Based on the nonlinear function, I propose the optimal maximum-channel-capacity (MCC) TEQ that achieves 99.87% of the matched filter bound on ADSL channel capacity with a 17-tap equalizer. I also derive a computationally-efficient, near-optimal minimum-ISI (min-ISI) method that generalizes the MSSNR method by weighting the ISI in the frequency domain. The frequency domain weighting increases computational complexity for higher bit rate. Based on simulations using eight different carrier-serving-area ADSL channels, (1) the proposed methods yield higher bit rates than the MMSE, MSSNR, and MGSNR methods

A method for approaching the capacity of the twisted pair channel is presented and analyzed. It is then applied to two twisted pair channels that appear in the literature. Although all results represent lower bounds on the true capacity of the channel, the information rate resulting from the application of the optimal signaling strategy presented in this paper is a significant improvement over what exists in the literature. This paper also discusses a straightforward implementation of the optimal signaling strategy using multicarrier modulation. I INTRODUCTION Recent work has shown that twisted pair digital subscriber loops (DSL's) are capable of supporting very high data rates--in excess of 1.6 Mb/s within the standard carrier serving area (CSA), which typically covers a radius of about 12,000 ft around the central office [1, 2, 3, 4]. In this paper we show that further significant increases in the capacity of twisted pair loops can be achieved by jointly optimizing the signaling str...