Emerging high-speed xDSL access services: architectures, issues, insights, and implications

The design and optimization of the high-speed digital subscriber line (HDSL) need powerful computational strategies. Traditional techniques of distributing poles and zeros on Smith charts generally do not work. In the past, such approaches have lead to suboptimal designs for applications where the data capacity sought is considerably less than the Shannon capacity of the lines. Typical subscriber loops are less than perfect and for the current demands on the HDSL at T1/T2 and El rates every possible venue for the HDSL design needs to be investigated, if not exploited. In this Chapter, flexible computational techniques are presented that explore and optimize system components in view of the operating environment of the HDSL and/or the asymmetric digital subscriber line (ADSL) and inherent limitations of system components. The optimization occurs automatically by forcing the computer to track the effects of incremental changes of the subsystem performance (e.g., echo cancelers or equalizers), or the component values (Rs and Cs in the matching circuits) in context to the functional constraints of the (HDSL, ADSL, duplex, dual-duplex, triplex, etc.) line in conjunction with various subscriber loop environments (CSA loops, loops <18 kft., American, Australian, European, ANSI, ETSI, etc.)

This paper presents a study of the effects of bridged taps on very high-speed digital subscriber line (VDSL) transmission systems. It is shown that the short bridged taps; which affect VDSL signals, are much more damaging than the longer bridged taps, which affect other xDSL signals, such as high-rate digital subscriber line (HDSL) and asymmetric digital subscriber line (ADSL). Specifically, short bridged taps introduce a lot of linear distortion in the loop&amp;apos;s transfer function. In addition, they also tend to introduce more overall propagation loss than longer bridged taps. The peak-power-to-average-power ratio (PAR) of the signal increases for the loop with short bridged taps, and thus more AID bits are needed in the implementation of a digital VDSL transceiver as compared to the loop with long bridged taps. The performance of various equalizer structures is discussed in the presence of a severe channel distortion caused by the bridged tap.

A filter-bank modulation technique called filtered multitone (FMT) and its application to data transmission for very high-speed digital subscriber line technology are described. The proposed scheme leads to significantly lower spectral overlapping between adjacent subchannels than for known multicarrier techniques such as discrete multitone (DMT) or discrete wavelet multitone. FMT modulation mitigates interference due to echo and near-end crosstalk signals, and increases the system throughput and reach. Signal equalization in an FMT receiver is accomplished in the form of per-subchannel symbol-spaced or fractionally spaced linear or decision-feedback equalization. The problem of channel coding for this type of modulation is also addressed, and an approach that allows combined removal of intersymbol-interference via precoding and trellis coding is described. Furthermore, practical design aspects regarding filter-bank realization, initial transceiver training, adaptive equalization, and timing recovery are discussed. Finally, simulation results of the performance achieved by FMT modulation for very high-speed digital subscriber line systems, where upstream and downstream signals are separated by frequency-division duplexing, are presented and compared with DMT modulation.

Revisiting Mackay Online

In order to develop an effective and scalable approach for broadband communication over cable, it is essential to understand the realities of today's cable plants, which range from pure-coaxial systems to hybrid fiber/coax (HFC) plants. An effective broadband access system must be able to send high-speed data in both directions over a variety of plant topologies. The upstream data path, occupying the 5-42 megahertz portion of the RF spectrum, is filled with ingress, impulse noise, narrowband interference, wideband Gaussian noise, misalignments, and nonlinear impairments, making high-speed data transmission difficult. Effective upstream communication over this medium requires a new approach. This paper describes the principles of operation of a cable modem access system, based on S-CDMA (Synchronous Code Division Multiple Access). This technology provides the basis for a robust two-way system, optimized for upstream communication over cable. The paper discusses key aspects of operation of an end-to-end cable modem system providing 14 megabit per second two-way communication. The paper discusses the technical and economic advantages of this approach. S-CDMA technology allows cable operators to deploy services in the high-noise portion of the upstream below 20 megahertz, an area where other cable modems cannot operate. S-CDMA also employs a graceful adjustment technique, which allows continuous operation in extreme noise situations. The paper also reviews the economic benefits for cable operators, who can: - Lower plant cleanup costs, since there is no need for high-pass filters, given robust S-CDMA performance. - Operate over cable systems ranging from all-coax to hybrid fiber/coax. - Use the entire upstream spectrum for revenue. - Deploy services rapidly on a mass scale. - Offer multiple classes of tiered services, including premium, guaranteed services for business users, who demand high bandwidth. - Capitalize on high-speed applications, such as telecommuting, corporate Intranets, video conferencing, multimedia, and online games.

The objective of this paper is to provide the reader with a vision of the envisaged work to be performed in the short/long term to fully integrate the UWB (Ultra-WideBand) radio technology into the future heterogeneous networks. In a first stage, the target will be the integration of the available commercial UWB devices with the state-of-the-art network technologies (HSPA (High-Speed Packet Access), WiMAX (Worldwide Interoperability For Microwave Access), ADSL2+ (Asymmetric Digital Subscriber Line 2+)), whereas in a second step, the focus will be on the inclusion of the advanced UWB radios in future wireless networks. The integration process will be carried out on four different fields: user devices, access network equipment, location awareness services and coexistence. Some pre-existing results concerning interworking and coexistence will also be presented. Additionally, the foreseen business benefits, mainly focused on the interests of the operators and the service providers, will be described.

The high-speed downlink packet access (HSPDA) system is expected to effectively support voice services when Release 7/8 features are adopted in the system. In this letter, we point out that a voice packet can be transmitted at a lower power level than normal in the HSDPA network. We propose a power allocation scheme that adjusts the transmission power of voice packets optimally and allocates the remaining transmission power to data services in order to improve data throughput. The proposed power allocation is analyzed with a Rayleigh fading channel model and compared to conventional fixed power allocation.

According to the characteristics of high-speed railway, we propose a scheme of wireless communication access system based on distributed interference alignment. This system contains two sets of dual-antenna integrated access units inside the train. It can support four users accessing to the high-speed railway wireless access system simultaneously. The integrated access unit can separate expected signal from interference signal. Simulation results show that this inside access system can obtain totally four degrees of freedom and its performance of bit error rate is better than that without adopting this interference alignment technology. Moreover, the performance of downlink bit error rate is slightly better than its uplink's.

The two-channel problem is analyzed for the case in which equal power must be allocated to each transmitter. The performance improvement available using coordinated transmission is described. The scheme provides optimum theoretical performance when the SNR varies between channels and/or the interference is correlated, without varying the transmitted power for each channel. It is shown that a simplified scheme using only rotations in the transmitter and receiver achieves most of the improvement of the general scheme. Variations employing fixed rotations in the transmitter are shown to yield significant performance improvement over conventional schemes with very little cost in complexity. The high speed digital subscriber line using pulse amplitude modulation is chosen as an example.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recently, a novel transmission technique was proposed for delay-spread channels that guarantees a low peak-to-average power ratio while also having low computational complexity. In this paper, we extend the technique to systems where the symbol length or number of parallel channels is large, such as in very high speed digital subscriber line (VDSL) or multiple-input multiple-output (MIMO) systems respectively. While operational complexity is low, the computational cost of initialisation in a VDSL or MIMO channel appears, at first sight, to be prohibitive. We show that initialisation can be carried out efficiently. Having so enabled the technique, we present simulation results in a MIMO-VDSL environment which demonstrate its superiority in comparison to other popular techniques such as V-DMT and DMMT when clipping in the transmit power amplifier is a dominant consideration

This article describes the near-far problem in the upstream direction of VDSL, which results when the lengths of VDSL loops in a binder vary significantly. Methods of upstream power backoff to mitigate the near-far problem are then described. Simulation results are presented, and the performances of the methods are discussed.

Recently, the demand for downlink (DL) high-speed data access has been increasing. For a DL-centered data service, time division duplex-code division multiple access (TDD-CDMA) and adaptive modulation and coding (AMC) are promising techniques. Furthermore, overlay systems not only expand the coverage, but enhance the network and user throughputs. Specifically with TDD-CDMA, each overlay system can re-use the same frequency band, and therefore an efficient frequency usage is achievable while overlay networks interfere with each other. In this paper, we propose a novel time slot configuration over the TDD-CDMA overlay system for high-speed DL access. In our system, micro base stations (microBSs) are equipped with directional antennas to alleviate the strong BS-microBS interference and AMC is also used to gain the synergy effect between the proposed slot configuration and AMC. In the computer simulation results, we show largely increased DL network and user throughputs but slightly decreased uplink throughputs dependent on the probability of LOS between BS and microBS

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.

This paper investigates the effect of radio frequency (RF) interference on the performance of very high-speed digital subscriber line (VDSL) system. It is shown that a blind equalization is extremely difficult in the presence of RF interference. A receiver structure, incorporating an adaptive notch filter (ANF) and hybrid DFE (H-DFE), is proposed to mitigate the effect of RF interference on the VDSL system. The use of an ANF before the DFE yields reliable blind equalization as well as performance improvement. It is also shown that the H-DFE structure employed in the proposed receiver outperforms the conventional DFE or the noise predictive DFE structure in the presence of RF interference.

A polyphase structure for infinite-impulse response (IIR) adaptive filtering is proposed and compared to the direct structure in terms of their reduced error surface. It is shown that the general shape of its surface can make the polyphase structure have higher convergence speed, alleviating the problem of convergence speed in IIR adaptive filters and allowing their computational complexity gain over finite-impulse response (FIR) adaptive filters to be exploited. Benefits regarding filter stability are also achieved with the polyphase structure. An example of a high-speed digital subscriber line (HDSL) application is presented, for which the polyphase structure attains a gain of up to 70 times in convergence speed over an IIR direct structure, leading to roughly the same convergence speed of a FIR structure but with only 12% of its computational complexity. The question of uniqueness of the stationary points of the proposed structure is also discussed. It is pointed out that for white input and sufficient modeling, all stationary points are global minima, a result which does not follow directly from an equivalent property of the direct structure.