Error correction performance in digital subscriber looptransmission systems

The author describes an error correction system for digital subscriber loop transmission systems which use time compression multiplexing (TCM). An interleaved block code is used to correct the burst errors due to impulse noise from analog telephone circuits. This interleaving method requires no extra hardware and contributes no additional delay. To evaluate the transmission performance of this error correction system, the bit error rate after decoding is derived on the basis of a burst error model for 200 kb/s digital subscriber transmission using the alternate mark inversion (AMI) line code. The experimental results for a 200 kb/s TCM system show that burst errors are substantially reduced     

Cyclostationary crosstalk suppression by decision feedbackequalization on digital subscriber loops

Interference from digital signals in multipair cables has been shown to be cyclostationary under some conditions. This work evaluates the performance of a decision feedback equalizer (DFE) in the presence of cyclostationary interference (CI), intersymbol interference (ISI), and additive white noise (AWN). A comparison between a DFE with CI and one with stationary interference (SI) shows the ability of the DFE to substantially suppress CI. Fractionally spaced and symbol-rate DFE equalizers are also compared and the former is found to yield better performance, especially in the presence of CI. The use of a symbol-rate DFE using an adaptive timing technique that finds the receiver's best sampling phase is proposed for when the fractionally spaced DFE cannot be used because of its complexity. The results also demonstrate the potential benefits of synchronizing central office transmitter clocks, if a fractionally spaced DFE is used at the receiver     

Achievable information rates on digital subscriber loops: limitinginformation rates with crosstalk noise

The capacity and cutoff rates for channels with linear intersymbol interference, power dependent crosstalk noise, and additive white noise are examined, focusing on high speed digital subscriber line data transmission. The effects of varying the level of additive white noise, crosstalk coupling gain, sampling rate, and input power levels are studied in detail for a set of simulated two-wire local loops. A closed-form expression for the shell constrained Gaussian cutoff rate on the crosstalk limited channel is developed and related to the capacity, showing that the relationship between these two rates is the same as on a channel without crosstalk noise. The study also projects achievable rates on a digital subscriber line, inside and outside of a carrier serving area, with a sophisticated but realizable receiver     

Reduction of crosstalk interference in local digital transmission

A technique for the improvement of signal/crosstalk-noise ratio in local digital transmission is proposed. For subscribers' circuits shorter than some maximum permissible length the transmit levels at both ends of the circuits are reduced. The effect of these changes on intersystem compatibility is also shown to be beneficial.     

Application of capacity approaching coding techniques to digital subscriber lines

The use of coding error control is an integral part of the design of modern communication systems. Capacity-approaching codes such as turbo and LDPC codes, discovered or rediscovered in the past decade, offset near-Shannon-limit performance on the AWGN channel with rather low implementation complexity and are therefore increasingly being applied for error control in various fields of data communications. This article describes a generic multilevel modulation and coding scheme based on the use of turbo-like, or LDPC codes for DSL system. It is shown that such codes provide significant gains in performance and allow an increase in data rate and/or loop reach that can be instrumental to the widespread deployment of future DSL services. Such techniques are also suitable for general multilevel modulation system in other application areas.     

Broadcast digital subscriber lines

The adoption of asymmetrical digital subscriber lines (ADSL) technology in conjunction with the broadcast digital subscriber lines (BDSL) proposed in this paper could provide a cost effective solution for the introduction of digital video services in the near term. BDSL provides the same digital broadcast video programs to all subscribers through existing twisted pair telephone loops. In this paper, the basics of telephone loop-based digital subscriber lines are first discussed. Then the potential of BDSL is explored in the twisted pair loop plant near end crosstalk (NEXT), far end crosstalk (FEXT), and white noise environment. The capability of BDSL is examined with different serving areas and transmit power levels. The possibility of using orthogonal quadrature amplitude modulation (QAM) to implement a zero guard-band BDSL system is studied. Computer BDSL performance simulations are then presented. The combination of high-bit-rate digital subscriber lines (HDSL), ADSL, and BDSL can provide telephone subscribers many digital broadband services     

New line code for digital subscriber loop

A new line code is presented which may find applications in the future digital subscriber loop. The line code is well balanced and has a narrow frequency spectrum.     

Vectored transmission for digital subscriber line systems

This paper describes the "vectored" transmission technique for digital subscriber line (DSL) systems, which utilizes user coordination at the central office or optical network unit. This method exploits the colocation of the downstream transmitters and of the upstream receivers, in order to achieve far-end crosstalk (FEXT) cancellation and perform multiuser transmission optimization. The performance improvements are particularly pronounced in environments with strong FEXT such as in very high-speed DSL. Discrete multitone is employed for each user with additional constraints on the cyclic prefix length and with the assumption of block-synchronized transmission and reception for downstream and upstream transmission correspondingly. Within each tone, upstream crosstalk is removed by multiple-input-multiple-output decision feedback at the receiving side, while downstream crosstalk is eliminated by analogous preprocessing at the transmitting side. Additionally, the issue of transmission energy allocation in frequency and among users is addressed. Assuming frequency-division duplexing, the corresponding optimization problem is formulated and solved via convex programming both for a fixed upstream-downstream band plan and for a dynamically programmable band plan. The case of power backoff as a means to reduce the impact of crosstalk on alien systems is also treated. Interestingly, the performance of the proposed methods is shown to be very close to known information theory bounds     

Coordinated transmission for two-pair digital subscriber lines

The performance of two-pair dual-duplex systems is substantially improved if the transmission on one pair is coordinated with that on the other, so that the transmitted signals are two-dimensional vector pulses. The advantages of coordination of transmission in an optimal way are quantified. Signal processing gains of 1.8 dB can be achieved in this way by averaging the SNRs on the two pairs constituting the high rate digital subscriber line (HDSL) and by canceling the near-end crosstalk (NEXT) between them. Exploitation of pair-to-pair correlation of NEXT voltages on the constituent pairs provides an additional source of processing gain. Under reasonable assumptions concerning the distribution of NEXT coupling coefficients between cable pairs, there is a better than 50% chance that the signal processing gain achieved by exploiting pair-to-pair NEXT correlation will be greater than 1.5 dB. Coordinated transmission requires the adaptation of four flat gain amplifiers in transmitters to achieve its optimum system configuration     

Distributed multiuser power control for digital subscriber lines

This paper considers the multiuser power control problem in a frequency-selective interference channel. The interference channel is modeled as a noncooperative game, and the existence and uniqueness of a Nash equilibrium are established for a two-player version of the game. An iterative water-filling algorithm is proposed to efficiently reach the Nash equilibrium. The iterative water-filling algorithm can be implemented distributively without the need for centralized control. It implicitly takes into account the loop transfer functions and cross couplings, and it reaches a competitively optimal power allocation by offering an opportunity for loops to negotiate the best use of power and frequency with each other. When applied to the upstream power backoff problem in very-high bit-rate digital subscriber lines and the downstream spectral compatibility problem in asymmetric digital subscriber lines, the new power control algorithm is found to give a significant performance improvement when compared with existing methods     

Comparison of precoding schemes for digital subscriber lines

Precoding at the transmitter side is a practicable method for transmission over intersymbol interference channels. In contrast to decision-feedback equalization no error propagation occurs and coded modulation can be applied as for the intersymbol interference free channel. Tomlinson-Harashima (1971, 1972) precoding and flexible precoding are analyzed and compared. The dualities and differences are discussed. The focus of interest is the application of precoding to fast digital transmission over twisted pair lines, such as high-rate or asymmetric digital subscriber lines. It turns out that flexibility-which is not necessary in the specific application, digital subscriber lines-of flexible precoding is paid with a performance loss compared to Tomlinson-Harashima precoding     

Optimal multiuser spectrum balancing for digital subscriber lines

Crosstalk is a major issue in modern digital subscriber line (DSL) systems such as ADSL and VDSL. Static spectrum management, which is the traditional way of ensuring spectral compatibility, employs spectral masks that can be overly conservative and lead to poor performance. This paper presents a centralized algorithm for optimal spectrum balancing in DSL. The algorithm uses the dual decomposition method to optimize spectra in an efficient and computationally tractable way. The algorithm shows significant performance gains over existing dynamics spectrum management (DSM) techniques, e.g., in one of the cases studied, the proposed centralized algorithm leads to a factor-of-four increase in data rate over the distributed DSM algorithm iterative waterfilling.     

Channelized voice over digital subscriber line

This article presents a promising voice over digital subscriber line (VoDSL) solution: an alternative method that uses physical layer transportation to provide channelized VoDSL (CVoDSL). This article also presents the advantages that the CVoDSL provides to the customers in terms of high-quality services and lower cost, and service providers in terms of market growth and success. It also offers a set of requirements, architectural design, and implementation scenarios that provide end-to-end solutions for CVoDSL.     

Echo cancellation scheme for high speed digital subscriber lines

The authors discuss the implementation of a computationally efficient two stage echo cancellation system for high speed data transmission. In addition to a substantial reduction in hardware complexity, the two stage echo canceller offers the advantage of a speedier convergence compared to the conventional FIR filter     

A CMOS line equalizer for a digital subscriber loop

An adaptive line equalizer for a 200-kb/s digital subscriber loop is developed in the form of a monolithic LSI and implemented using 2.5-/spl mu/m CMOS technology. Most analog portions consist of switched-capacitor circuits successfully designed to minimize power consumption, the amount of hardware, and off-chip components. The main features of the LSI equalizer are an /spl radic/f step equalizer, a five-tap decision-feedback equalizer using /spl Delta/M D/A conversion, a newly developed wave difference method (WDM), tankless timing extraction PLL, and a line driver. Consequently, the LSI can equalize a 52-dB line loss with four bridge taps; it dissipates only 67 mW, and the chip area is 5.7/spl times/5.9 mm/SUP 2/.     

A multi-user power control algorithm for digital subscriber lines

The paper investigates the power control problem in a frequency-selective interference channel. A centralized power control algorithm is developed with the objective of maximizing the achievable rate region given an average power constraint for each user. The proposed algorithm is based on the multi-user discrete bit-loading algorithm that considers the power allocation over frequency and users simultaneously. Simulation results for very high-speed digital subscriber line (VDSL) systems show that the proposed algorithm achieves a larger rate region than that achieved by a power control algorithm based on iterative waterfilling.     

Scatter-glare estimation for digital radiographic systems: comparison of digital filtration and sampling techniques

The scatter and veiling glare distribution in images acquired with a digital subtraction angiography imaging system was estimated using a digital filtration and a beam-stop technique. The digital filtration technique utilizes exposure parameters and image gray levels to estimate scatter-glare intensity based on previous phantom measurements. The beam-stop technique uses an array of lead discs in order to sample scatter-glare intensity for each patient. To test the ability of digital filtration and beam-stop techniques to estimate the scatter-glare intensity, they were applied to images of postmortem swine animal models at different projections and beam energies. The systematic and root-mean-square (rms) percentage errors of these estimates were obtained by comparison to directly measured scatter-glare images using a scanning lead strip technique. The average rms percentage error for the digital filtration and beam-stop techniques were 8.07% and 6.67%, respectively. The changes in scatter-glare intensity due to contrast injection during coronary arteriography and ventriculography were also measured using the beam-stop technique. The maximum changes in scatter-glare intensities during coronary arteriography and ventriculography were 19 and 88%, respectively. The results indicate that the digital filtration technique is more suited for applications such as coronary arteriography and ventriculography where the iodinated contrast material significantly changes the scatter-glare intensity.     

应用超分辨率方向估计实现通信卫星干扰源定位

利用卫星天线多波束特性来获取干扰信号波达方向（DOA）信息,是实现通信卫星干扰源定位的一条新思路。本文研究了在结合卫星多波束天线模型时,利用超分辨率方向估计方法实现通信卫星干扰源精确定位的可毛生,研究表明MN和LP两种算法在用于多波束天线的DOA估计时存在着方向估计模糊,而MV和MUSIC可应用于多波束天线的二维测向;文中还讨论MV、MUSIC应用于卫星干扰源定位的性能情况,并得到一些有益的结论。     

Wide-band digital subscriber access with multidimensional blockmodulation and decision-feedback equalization

The author investigates the potential transmission performance of pair-wire subscriber lines at the higher rate of 800 kb/s, with particular reference to digital subscriber line transmission for ISDN (integrated services digital network) basic access. Block modulation schemes of 1-4 dimensions, at a rate of 2 bits per dimension, are considered. Time-division multiplexing is used to combine the multiple dimensions for transmission over a single-waveform channel, namely, the subscriber line. The channel noise is assumed to be additive and dominated by near-end crosstalk. MMSE (minimum mean-squared error) decision-feedback equalization is used to deal with the noise and the intersymbol interference. Using the theory developed, the potential performance of some simple lines is calculated. The coding gain of a multidimensional modulation scheme is found to be fully preserved after transmission if the equalizer is infinite in length. However, the gain realized can be much lower, or none at all, if the equalizer is only moderate in length. This latter phenomenon is due to the fact that the noise at the decision point is coloured, due to the inability of the equalizer to whiten it sufficiently     

Adaptive crosstalk cancellation for crosspolarised digital transmissions

Cancellation of crosspolarisation for modulated orthogonally polarised signals is feasible. Some networks are presented. The most interesting lie in baseband processing.