Far-echo cancellation in the presence of frequency offset [fullduplex modem]

A design is presented for a full-duplex echo-cancelling data modem based on a combined adaptive reference echo canceller and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of the far-end signal based on receiver decisions. This technique provides a novel approach for full-duplex far-echo cancellation in which the far echo can be cancelled in spite of carrier-frequency offset. To estimate the frequency offset, the system uses a separate receiver structure for the far echo which provides equalization of the far echo channel and tracks the frequency offset in the far echo. The feasibility of the echo-cancelling algorithms is demonstrated by computer simulation with realistic channel distortions and with 4800-b/s data transmission, at which rate the frequency offset in the far echo becomes important     

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     

Echo cancellation in DMT-receivers: circulant decomposition canceler

Asymmetric digital subscriber lines (ADSLs) employ discrete multitone modulation (DMT) as transmission format, where subcarriers are assigned to the up- and/or downstream transmission direction. To separate up- and downstream signals, the ADSL standard allows the use of echo cancellation resulting in improved bit rates, reach, and/or noise margins. In DMT-based modems, typically, the mixed time/frequency (MTF) domain echo canceling scheme, as proposed by Ho et al., is implemented. This technique estimates the echo filter in the frequency domain using the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal will carry data, i.e., will be excited, the MTF adaptation process does not lead to a good estimate for the echo channel, unless extra power on unused echo tones is transmitted. However, transmitting extra power on such tones is often undesired. In this paper, we present an alternative echo canceling scheme referred to as the circulant decomposition canceler (CDC), which works without extra power requirements and with comparable complexity as the method of Ho et al. Similar to MTF echo canceling, the CDC scheme can easily be incorporated into a multirate environment with different transmit and receive rates and can also cheaply be combined with per-tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far-end signal.     

Fast initialization of data-driven Nyquist in-band echo cancellers

A echo canceller fast training scheme for data-driven Nyquist in-band echo cancellers is presented. This scheme simultaneously estimates the desired near and far echo canceller coefficients by sending a special periodic training sequence and correlating a segment of the sequence with the real echo samples. The requirements and the generation of the training sequence are discussed. It is shown that the fast training method can also provide the parameters for the fast initialization of the phase roll compensator in the echo canceller, and thus complete the fast training of the entire echo canceller. Compared to the conventional LMS training and other training methods, this scheme provides accurate estimates of the echo canceller coefficients in a significantly reduced training time     

Fast unbiased echo canceller update during ADSL transmission

This paper presents an efficient method for updating echo canceller (EC) coefficients in asymmetric digital subscriber line transceivers during transmission that removes the dependency of updates on the far-end signal. The new method improves on the frequency-domain EC architecture by Ho et al. (1996), in that it converges faster, improves the stability to perturbation, and is not prone to bias from pilot tones. The new EC updating system architecture is presented for both the central office and remote terminal systems and is fully compliant with the ITU-T G.DMT standard.     

A finite precision analysis of the block-gradient adaptivedata-driven echo canceller

The block-gradient (block LMS) algorithm's finite precision performance in the data-driven echo canceller application is analyzed. From both the analysis and simulation results, it was found that the block LMS requires significantly less precision than the standard LMS algorithm. The analysis also shows how higher convergence and tracking speeds may be traded for an improvement in precision requirements. The authors derive formulae that can be used to accurately compute echo rejection levels as a function of precision, signal powers, step-size, block length, and echo canceller length. The utility of the formulae is demonstrated by showing performance levels for a typical V.32bis full-duplex voiceband modem operating at a transmission rate of 14.4 kbit/s (7200 Hz sampling rate), and for a high-speed digital subscriber line echo canceller operating at a sampling rate of 400 kHz     

An Echo Canceller with Controlled Power for Frequency Offset Correction

In an echo canceller (EC) compensating far-end echos affected by frequency offset, the phase loop gain is highly dependent on the echo and signal powers. Adoption of a normalized gain multiplied by the echo power yields an improved robustness. It is shown that the latter gain depends only on the echo-to-signal ratio. Practical implementation of this idea is realized by generating a first unitary echo replica, on which the DPLL will act, and then controlling the output power. The system has the additional advantage of a reduced binary size for the canceller taps.     

Hybrid LMS-LMF algorithm for adaptive echo cancellation

The coefficients of an echo canceller with a near-end section and a far-end section are usually updated with the same updating scheme, such as the LMS algorithm. A novel scheme is proposed for echo cancellation that is based on the minimisation of two different cost functions, i.e. one for the near-end section and a different one for the far-end section. The approach considered leads to a substantial improvement in performance over the LMS algorithm when it is applied to both sections of the echo canceller. The convergence properties of the algorithm are derived. The proposed scheme is also shown to be robust to noise variations. Simulation results confirm the superior performance of the new algorithm     

Per-tone echo cancellation for DMT-based systems

A new echo cancellation structure for discrete multitone systems is presented, where each used tone has its own per-tone echo canceller in addition to a per-tone equalizer, which provides an alternative to currently employed time domain and time/frequency domain approaches. The per-tone approach enables us to optimize the signal-to-noise ratio for each tone separately by solving a minimum mean-square error problem for each tone, with implicit so-called joint shortening. Complexity during data transmission is compared for time domain, time/frequency domain, and per-tone echo cancellation. Structures with reduced complexity are derived for an interpolated and a decimated rate setup. Finally, simulation results for an asymmetric digital subscriber line setting demonstrate improved performance over time domain (or time/frequency domain) echo cancellation.     

An Adaptive-Step Sign Algorithm for Fast Convergence of a Data Echo Canceller

This paper proposes an echo cancellation algorithm that can be used for full-duplex digital data transmission over existing twisted-pair cables. The proposed algorithm is a form of the "sign algorithm." A new adaptation method is introduced to change the canceller step size according to the residual echo level by using the correlation between the echo and the error signal. The adaptation method can increase the echo canceller convergence speed by an order of magnitude over the conventional sign algorithm. An echo canceller utilizing the proposed algorithm is suitable for VLSI implementation, because it requires no multibits analog-to-digital converter. Theoretical analysis and parameter optimization are carried out. The rapid convergence property of the algorithm is verified by computer simulation as well as experiments.     

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     

A Phase-Adaptive Echo Canceller with Reduced Sensitivity to Power Variations

In a previous correspondence [1], we have studied an echo canceller (EC) compensating far-end echos affected by frequency offset. It has been shown that the sensitivity of the loop gain versus the powerPof the echo and the powerSof the signal, can be reduced by adoption of a normalized gain multiplied by the echo power. In this correspondence a novel and very simple algorithm is described that further reduces this sensitivity and yields an optimum phase loop gain proportional to(S/ p)^{1/6}.     

Silencing echoes on the telephone network

Echo of a trasmitted signal can occur in the telephone network and is sometimes a disturbing phenomenon. Various methods of echo control have been used for over 50 years but recent introduction of transmission delays in excess of 300 ms, over satellite circuits, has resulted in a general reexamination of suppression methods. This led to development of the echo canceller. Adaptive echo cancellation is accomplished by automatically synthesizing a replica of the echo path response, convolving the incoming signal with the replica and subtracting the resulting echo estimate from the echo path output. Transmission is not thereby interrupted in any way and two-way information transmission proceeds normally with little or no echo. Detailed mathematical formulations for the convergence properties of such devices allow understanding of the effects of various hardware configurations and input signal properties. Echo cancellers may be applied to the network in a number of ways and the development of a VLSI canceller chip promises inexpensive implementation of these applications. Worldwide tests have shown the effectiveness of cancellation methods for satellite transmission.     

Discrete multitone echo cancelation

Multicarrier transmission methods have long been known to optimize the performance of data transceivers on bandlimited communication channels. One form of multicarrier transmission, known as discrete multitone modulation (DMT), is particularly attractive for its ability to be implemented using efficient digital signal processing techniques. Given a basic DMT system, it is possible to increase the aggregate data rate with full-duplex transmission using echo cancelation. However, DMT echo cancelation at first appears difficult because of the computational complexity required in a straightforward implementation to cancel the cross-echoes produced by each carrier into every carrier. This paper presents high-speed echo cancelation techniques for full-duplex data transmission using DMT systems. The techniques estimate the echo with a method of fast convolution that combines a complex multiply per tone with a short convolution in the time domain. In addition, the frequency-domain update of the echo parameters consists of one complex multiply per tap. As a result, these techniques can achieve much lower complexity than that required by traditional single-carrier technique, such as the least mean square (LMS) algorithm. This approach has been implemented for asymmetric digital subscriber line (ADSL) applications     

Optimal Discrete Bit Loading for DMT-Based DSL Systems With Equal-Length Loops

This letter examines a discrete bit-loading problem of upstream and downstream for discrete multitone digital subscriber lines with echo cancellation. Both far-end crosstalk and near-end crosstalk are taken into account. An optimal discrete bit-loading algorithm is developed when the loop lengths of all same-service users in a common binder are the same. Simulation results show that the optimal algorithm achieves a substantially higher data rate than existing suboptimal schemes.     

Performance analysis of an echo-cancellation arrangement thatcompensates for frequency offset in the far echo

Analytical and experimental results are presented for the performance of one echo canceller arrangement. It consists of a data-driven echo canceller having a so-called cross-coupled structure, which is followed by a rotator and a phase-locked loop (PLL). A cross-coupled echo canceller structure without a PLL is analyzed first. Expressions for speed of convergence and achievable echo-return-loss enhancement (ERLE) in the presence of frequency offset are derived. These results are compared in previously published results for a noncross-coupling echo canceller structure. Specifically, it is shown that the cross-coupled structure converges twice as fast as the noncross-coupled structure and provide an achievable ERLE that is about 6 dB better. The joint adaptation of the echo canceller and the PLL is then studied. It is shown that it is always possible to choose design parameters for the echo canceller which are consistent with adaptation requirements under double-talking conditions, provided that the PLL is properly engineered. The sensitivity of the performance of PLL to the power level of the far echo, as well as possible solutions to this problem, are discussed     

数据驱动的频域回波抵消算法

通过对多载波调制系统中数据驱动的频域回波抵消器的分析,提出了一种既容易实现,又能消除出现在回波信道中的带间干扰和块间干扰的数据驱动回波抵消算法;同时也讨论了出现在回波信号中的ICI(interchannelinterference)和IBI(interchannelblocksinterference)的可能性。模拟实验表明,采用这种算法,可以得到较好的回波衰减,大大降低计算复杂度。     

Nonlinear Echo Cancellation of Data Signals

This paper describes a new technique for implementing an echo canceller for full-duplex data transmission (such as in digital subscriber loops and volceband data sets). The canceller can operate in spite of time-invariant nonlinearities in the echo channel or in the implementation of the echo canceller itself (such as in the D/A converters). The basic structure of the linear echo canceller is not changed, but taps are simply added to account for the nonlinearity. The number of taps which must be added depends on the degree of nonlinearity which must be compensated. Numerical results based on computer simulation are given which show that typical nonlinearities encountered in MOS D/A converters can be compensated by a relatively small number of taps added to the linear echo canceller, and substantial improvement in the cancellation results.     

LMS-LMF adaptive scheme for echo cancellation

The authors present a novel algorithm for echo cancellation. The algorithm consists of simultaneously applying the LMS algorithm to the near-end section of the echo canceller, and a controlled mixed LMS-LMF algorithm to the far-end section. This combination results in a substantial improvement in performance of the proposed scheme over the LMS and the LMF algorithms     

Adaptive echo cancellation using least mean mixed-norm algorithm

A novel algorithm for echo cancellation is presented in this work. The algorithm consists of simultaneously applying the least mean square (LMS) algorithm to the near-end section of the echo canceller and the least mean fourth (LMF) algorithm to the far-end section. This new scheme results in a substantial performance improvement over the LMS algorithm and other algorithms