Impulse response shortening for discrete multitone transceivers

In discrete multitone (DMT) transceivers an intelligent guard time sequence, called a cyclic prefix (CP), is inserted between symbols to ensure that samples from one symbol do not interfere with the samples of another symbol. The length of the CP is determined by the length of the impulse response of the effective physical channel. Using a long CP reduces the throughput of the transceiver, To avoid using a long CP, a short time-domain finite impulse response (FIR) filter is used to shorten the effective channels impulse response. This paper explores various methods of determining the coefficients for this time-domain filter. An optimal shortening and a least-squares (LS) approach are developed for shortening the channel's impulse response. To provide a computationally efficient algorithm a variation of the LS approach is explored. In full-duplex transceivers the length of the effective echo path impacts the computational requirements of the transceiver. A new paradigm of joint shortening is introduced and three methods are developed to jointly shorten the channel and the echo impulse responses in order to reduce the length of the CP and reduce computational requirements for the echo canceller     

Equalization for discrete multitone transceivers to maximize bitrate

In a discrete multitone receiver, a time-domain equalizer (TEQ) reduces the intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Current TEQ design methods such as the minimum mean-squared error (MMSE), maximum shortening SNR (MSSNR), and maximum geometric SNR (MGSNR) do not directly maximize bit rate. We develop two TEQ design methods to maximize the bit rate. First, we partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel SNR definition. Then, we derive a nonlinear function of TEQ taps that measures the bit rate, which the proposed maximum bit rate (MBR) method optimizes. We also propose a minimum-ISI method that generalizes the MSSNR method by weighting the ISI in the frequency domain to obtain higher performance. The minimum-ISI method is amenable to real-time implementation on a fixed-point digital signal processor. Based on simulations using eight different carrier-serving-area loop channels, (1) the proposed methods yield higher bit rates than MMSE, MGSNR, and MSSNR methods; (2) the proposed methods give three-tap TEQs with higher bit rates than 17-tap MMSE, MGSNR, and MSSNR TEQs; (3) the proposed MBR method achieves the channel capacity (as computed by the matched filter bound using the proposed subchannel SNR model) with a five-tap TEQ; and (4) the proposed minimum-ISI method achieves the bit rate of the optimal MBR method     

Overlapped discrete multitone modulation for high speed copper wirecommunications

Multicarrier modulation possesses several properties which make it an attractive approach for high speed copper wire communication networks. Among these properties are the ability to efficiently access and distribute multiplexed data streams, and a reduced susceptibility to impulsive, as well as to narrowband channel disturbances. In digital implementations of multicarrier modulation, subcarrier generation and data modulation are accomplished digitally using orthogonal transformations of data blocks. These implementations are particularly efficient with regard to bandwidth utilization and transceiver complexity. In this paper, we present a form of digital multicarrier modulation which we refer to as overlapped discrete multitone, or discrete wavelet multitone (DWMT), modulation. For DWMT modulation, which is based on the application of M-band wavelet filters, the pulses for different data blocks overlap in time, and are designed to achieve a combination of subchannel spectral containment and bandwidth efficiency that is fundamentally better than with other forms of multicarrier modulation. We show that, as a result of the spectral containment feature, DWMT gives a high level of robustness with regard to noise environments and channel variations that are encountered in practice     

Subsymbol equalization for discrete multitone systems

In conventional discrete multitone (DMT) systems, a multitap time-domain equalizer (TEQ) is used to shorten the channel length, so that the bandwidth efficiency reduction due to cyclic extension is relieved. The TEQ, however, tends to introduce spectral s which degrade the achievable signal-to-noise ratio at corresponding subchannels, thereby decreasing the bandwidth efficiency. Furthermore, computationally expensive joint TEQ initialization and optimal delay (introduced by TEQ) selection is necessary. In this paper, a novel subsymbol-equalization scheme is proposed, and is based on the observation that the high-bit-rate twisted-pair channels rarely contain a zero that is close to the unit circle. Although a delay of a fraction of a DMT symbol period is introduced, the proposed subsymbol-equalization scheme eliminates the necessity of both the channel shortening at the receiver and the cyclic extension at the transmitter. Simulation results demonstrate the computational efficiency (no TEQ) and the bandwidth efficiency of the proposed subsymbol-equalization scheme.     

Subcarrier multiplexing by chaotic multitone modulation

Subcarrier multiplexing by chaotic multitone modulation is investigated. Optical chaotic light can be achieved by injecting multiple subcarriers into a self-pulsating laser diode. Synchronization between two identical chaotic systems (drive and response) can be implemented provided that the conditional Lyapunov exponents are all negative. By adding amplitude modulation (AM) signals to each subcarrier, the two systems become asymptotically synchronized. The AM signals in each subcarrier can be recovered by the introduction of a filtering process where the Lyapunov exponent of the synchronized error function matches the cutoff frequency of a first order low pass filter.     

A coded and shaped discrete multitone system

Shows how coding and constellation shaping may provide significant gains to a discrete multitone (DMT) system transmitting over spectrally-shaped channels. First, the authors present and analyze a concatenated coding scheme consisting of an inner trellis code and outer block code when applied to DMT modulation, and address some of the implementation issues associated with this scheme. Some laboratory test results for a DMT prototype employing the coding scheme are presented. Next, they propose a method for applying Forney's trellis shaper across the tones in a DMT system to realize significant shaping gain. To illustrate the coding and shaping gains achieved, they use scenarios indicative of the newly introduced asymmetric digital subscriber line service. By combining a powerful coding scheme, shaping, and DMT modulation, they arrive at an implementable transceiver that can provide very high data rates over spectrally-shaped channels     

一种新的正交多载波调制技术——滤波多音调制（FMT）

在无线通信中 ,以正交频分复用 (OFDM)为代表的正交多载波调制 (OMCM )技术正在获得深入研究和广泛应用。本文介绍了一种新的正交多载波调制技术———滤波多音调制(FMT) ,并对它与OFDM在无线通信中的特点和性能作了详细比较 ,就其在下一代宽带移动通信的应用前景作了初步探讨。     

Spectral structure of multitone pulse width modulation

The multiple sidetone structure recently reported in pulse width modulation arising from multitone inputs can now be accurately predicted by a newly derived modulation formula. Excellent agreement is demonstrated between measurements and theoretical calculations.<>     

A discrete multitone transceiver system for HDSL applications

A discrete multitone (DMT) transceiver design for high bit rate digital subscriber line (HDSL) access is presented and analyzed. The DMT transmitter and receiver structure and algorithms are detailed, and the computational requirements of DMT for HDSL are estimated. At a sampling rate of 640 kHz, using an appropriate combination of a short finite-impulse-response (FIR) equalizer and a length-512 DMT system, 1.6 Mb/s data transmission is possible within the carrier serving area (CSA) at an error rate of 10^-7 on a single twisted pair. A significant performance margin can be achieved when two coordinated twisted pairs are used to deliver a total data rate of 1.6 Mb/s. In terms of a performance-per-computation figure of merit, the DMT system is an excellent candidate for HDSL implementation     

Data-driven echo cancellation for a multitone modulation system

The problem of echo cancellation in a multitone modulation (MTM) scheme is addressed. A general model for the near-end echo is derived and is used to identify candidate data-driven echo canceller (DDEC) structures. The stability, steady state performance, and associated system complexity of an adaptive DDEC based on the stochastic gradient approach is developed. It is further shown how the symmetry in the derived echo path model can be exploited to enable a reduction in canceller complexity, enhancing convergence speed without sacrificing final SNR. Simulation results are provided that confirm the analytical predictions     

Filtered multitone modulation for very high-speed digitalsubscriber lines

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     

Comparison of single-carrier and multitone digital modulation forADSL applications

Single-carrier modulation such as quadrature amplitude modulation (QAM) or carrierless amplitude modulation-phase modulation (CAP), and DMT are alternative techniques for providing digital communication in a variety of applications, in particular ADSL for communication over the telephone company subscriber lines. Although theory predicts comparable performance under ideal implementations, a definitive comparison of performance over a wide range of conditions will require more experience from field trials. Similarly, accurate comparison of implementation costs must await the greater availability of commercial-grade devices. However, enough is now known about these modulation schemes to compare their inherent similarities and differences in performance and cost. Overall, a present view of single-carrier and multitone modulation indicates comparable performance with some differences depending on the type of degradation. Costs should also be approximately equal, with multitone having some advantage in digital processing, but requiring greater cost in analog circuitry     

Performance limits for filtered multitone modulation in fading channels

In this paper, we investigate the best attainable performance for filtered multitone modulation (FMT) in time-variant frequency-selective fading channels when optimal maximum-likelihood (ML) detection is deployed. FMT generalizes the popular orthogonal frequency division multiplexing (OFDM) scheme through the deployment of subchannel shaping filters. The performance limits are derived by extending to this multichannel context the matched filter bounding technique. We first conduct an exact calculation for the average and the distribution of the matched filter error rate bound. Then, we follow a numerical procedure that overcomes the problems of accuracy and ill conditioning in the implementation of the exact method. This study allows for an analytical treatment of the diversity effect on performance as a function of the time/frequency selectivity of the channel. It is found that FMT is a diversity transform that is capable of yielding coding gains and time/frequency diversity gains as a function of the subcarrier spacing and the subchannel filter shape.     

Worst-case design for optimal channel equalization in filterbank transceivers

A worst-case approach is adopted to tackle optimal channel equalization for multirate filterbank transceivers, which are widely used in data communication networks, such as discrete wavelet multitone (DWMT) systems in digital subscriber lines (DSL), orthogonal frequency multiplexing division (OFMD) in frequency division multiple access (FDMA) systems, applicable to direct-sequence/spread-spectrum (DS/SS) code division multiple access (CDMA) networks. It is assumed that the observation noise is nonwhite with bounded power-norm or root-mean-squared (RMS) value. Our objective is to design the optimal receiving filterbanks that not only achieve the zero-forcing (ZF) condition or channel equalization but also minimize the RMS error between the transmitted symbols and the received symbols in the presence of the worst-case nonwhite noise. All ZF receiving filterbanks will be parameterized, and optimal design for channel equalization will be converted into an equivalent optimal H/sub /spl infin// filtering problem for the augmented receiving filterbanks with RMS error preserved. Our main results cover computation of the optimal RMS error achievable for the worst-case noise and an explicit design algorithm for suboptimal channel equalizers with the performance index arbitrarily close to the optimal one. A simulation example is used to illustrate the proposed optimal channel equalization algorithm.     

滤波多音调制技术的应用

文章详细介绍了滤波多音调制(FMT)这种基于滤波器组调制的新兴多载波调制技术,并将其与现有的其他多载波传输技术(如OFDM)作了对比,指出了有待研究的有关课题,最后对FMT在宽带无线数据传输中的应用作了初步展望。     

On incremental sigma-delta modulation with optimal filtering

The paper presents a quantization-theoretic framework for studying incremental sigma-delta (/spl Sigma//spl Delta/) data conversion systems. The framework makes it possible to efficiently compute the quantization intervals and hence the transfer function of the quantizer, and to determine the mean square error (MSE) and maximum error for the optimal and conventional linear filters for first and second order incremental /spl Sigma//spl Delta/ modulators. The results show that the optimal filter can significantly outperform conventional linear filters in terms of both MSE and maximum error. The performance of conventional /spl Sigma//spl Delta/ data converters is then compared to that of incremental /spl Sigma//spl Delta/ with optimal filtering for bandlimited signals. It is shown that incremental /spl Sigma//spl Delta/ can outperform the conventional approach in terms of signal-to-noise-and-distortion ratio. The framework is also used to provide a simpler and more intuitive derivation of the Zoomer algorithm.     

Low-cost WDM-PON with colorless bidirectional transceivers

This paper presents a low-cost bidirectional (BiDi) wavelength-division-multiplexed passive optical network (WDM-PON) employing colorless uncooled BiDi transceivers (TRxs) and superluminescent diode (SLD)-based broadband light sources (BLSs). The C band is allocated for upstream and the E+ band for downstream in consideration of BiDi packaging, SLD development, and wavelength alignment of dual-window arrayed waveguide gratings (AWGs). The BiDi TRx integrates an uncooled Fabry-Perot laser diode (FP-LD), a p-i-n photodiode (PD), and a 45/spl deg/-angled thin-film filter in a small-form-factor (SFF) package. The SLD-based BLSs provide 13-dBm amplified spontaneous emissions (ASEs) with spectral ripples of < 3 dB and polarization dependencies of <1 dB. Colorless operations over 32 100-GHz-spaced channels are demonstrated from -20 to 80/spl deg/C in 155-Mb/s BiDi transmissions over 25 km.     

Hybrid TDMA/CDMA based on filtered multitone modulation for uplink transmission in HFC networks

We present a hybrid TDMA/CDMA system for uplink transmission in multiple access networks that is based on filtered multitone modulation. We consider an application to uplink transmission in hybrid fiber/coax networks. After discussing the characteristics of the proposed system, we address the problem of initial ranging and power adjustment of unregistered stations. Finally, we introduce a new collision resolution algorithm in conjunction with FMT modulation that uses iterative identification of the competing stations to achieve high throughput on the collision channel. System performance is investigated by simulations.     

A set of optimal discrete linear smoothers

A set of high-resolution low-leakage low-variance discrete spectral windows which have small number of non-zero terms is derived. Inherently, these spectral windows also provide a significant means of high-resolution low-leakage over-shoot-free computationally-efficient discrete-time-data and discrete-image smoothing. For the convenience of the users, a Fortran IV computer program which computes the linear smoothers is included. The first member of the set turns out to be the well-known Hann window. The following twelve members, which have ?60 dB stop-band values, are tabulated. Although the other members of the set can readily be obtained by the given program, the tabulated smoothers are sufficient for most applications.