Antoni Fertner

Improvement of bit-error-rate in decision feedback equalizer by preventing decision-error propagation

A technique for reducing the influence of a phenomenon known as error propagation through the decision feedback equalizer is proposed. An error threshold may be defined to compare with the decision estimation error. If the estimation error is less than the threshold, an assumption is made that the DFE is functioning correctly; otherwise, the decision is classified as unreliable and replaced by a temporary decision. The temporary decision is chosen to prevent further, self-sustained incorrect decisions. The estimation error caused by adopting the temporary decision is used to determine the correct decision, which replaces the temporary decision. Statistical analysis indicates that the performance of the algorithm critically depends on the value of the first postcursor and the accompanying noise level.

Frequency-domain echo canceller with phase adjustment

The frequency-domain filter is an efficient realization of an echo canceller that provides a simple and elegant solution to the problem of jitter compensation. An essential requirement for a low-error digital transmission system is that the echo canceller retains high accuracy while the sampling instant is being adjusted. One of the most attractive features of discrete Fourier transform is its capacity to reconstruct time-shifted signals using phase shifts in the frequency-domain. The signals involved, however, are not always alias-free (nor are they in telecommunications), which seriously degrades the echo cancellation. This paper introduces a de-aliasing technique, which results in an algorithm that is largely insensitive to the spectral content of cancelled signals. The de-aliasing technique allows preservation of high-performance echo cancellation during rapid changes in the sampling phase of the input signal. Computer simulations confirm the theoretically predicted behavior and accuracy.

Computationally efficient methods for analysis and synthesis of real signals using FFT and IFFT

The discrete Fourier transform (DFT) and the inverse discrete Fourier transform (IDFT) are used in a wide variety of signal processing applications. Even with the increased speed of modern processors, there is an ongoing need to further develop more efficient methods for computing DFT and IDFT, with a particular effort to reduce the number of complex multiplications. The properties of certain complex sequences are extraordinarily useful in the sense that they lead to data manipulation schemes that result in the sequences to which traditional but much shorter fast Fourier transform (FFT) algorithms may be applied. This is achieved by exploiting a certain regularity in the complex data. The index-reversed complex conjugate sequence and the mirror symmetric complex conjugate sequence were defined. A significant reduction in the number of complex computations is achieved if a sequence in either domain exhibits such symmetry.

Symbol-rate timing recovery comprising the optimum signal-to-noise ratio in a digital subscriber loop

The paper investigates a symbol-rate timing recovery algorithm that is based on the correlation between an error from the decision feedback equalizer and the arriving signal samples. The mean-square error due to uncancelled precursor intersymbol interference is applied as a criterion to choose optimal timing instants. Various signal and error combinations may be used to approximate its minimum as a function of the steady-state location of the sampling instants. The timing function is selected on the premise that the estimated correlation function passes through zero only once, at the desired sampling phase. We propose a semianalytical framework for analyzing the relationship among the timing estimate variance, the acceptable noise level, and the dead-zone thresholds. It allows us to achieve a compromise between, on the one hand, the ability to track and compensate for frequency drift and for changes in the transmission media, and on the other hand, immunity against unnecessary phase corrections. The performance of the postulated timing function is examined by means of simulations.

Frequency Domain Echo Canceller for DMT-Based Systems

This letter addresses echo cancellation for digital subscriber line (DSL) communications with discrete multitone (DMT) modulation. The described adaptive echo canceller operates entirely in the frequency domain. The method allows estimation and subtraction of an echo signal impaired by intercarrier and intersymbol interference present when the impulse response of the echo channel is longer than the cyclic prefix. Implementation aspects for symmetric and asymmetric DMT-based systems are described together with ways to lower the computational complexity. Finally, computer simulation results are presented that validates the proposed method.

An adaptive gain control with a variable step size for use in high-speed data communication systems

A high-precision adaptive gain control is an important function in high-speed data communication systems. The nonsymmetric statistics of the received signal results in a biased value of gain, particularly if 2B1Q line code is used, which contains a periodically repeated synchronization word, i.e., a Barker sequence. A bias-free gain is obtained using a step size that depends on the statistics of the received signal. The additional computations may be successfully approximated by shift-add operations.

A wavelet-based expert system for digital subscriber line topology identification

This work proposes a new method for automatically identifying topologies of lines with one or more sections in a telephone network. The method is based on the examination of both impulse response and time-domain reflectometry trace of a line under test. They are analyzed using a method based on the wavelet transform that identifies and extracts features that contain information about the line topology. Those features are interpreted by an expert system composed of three sequential modules that estimate, respectively, the type of line makeup serial or bridge tap, the lengths of the line sections, and the corresponding cable type, which are the parameters that completely identify the topology according to the assumed model. A thorough comparison with two state-of-the-art methods is also presented using several twisted-pair copper cables. The results show that the proposed method provides good accuracy with respect to topology identification at low computational cost. Copyright

Detection and Localization of Load Coils from One-Port Measurements

This paper deals with detection and localization of load coils in the copper access network. The load coil is a legacy device that must be removed before deploying broadband services on twisted-pair transmission lines. The proposed methods do not require dedicated hardware but utilize the already installed transceivers for one-port measurements of the line input impedance. The number of load coils is detected by counting the number of resonance peaks in the line impedance spectrum. An approximative model of the symmetric loaded line is presented, from which a low-complexity load coil locator is derived. For nonsymmetric lines, a load coil locator using model-based optimization is proposed. The two methods are compared and evaluated by computer simulations and by laboratory measurements on real transmission lines. The estimation results indicate that up to five load coils can be robustly detected and located with the proposed methods.

Backscattering in Twisted-Pair Nonhomogeneous Transmission Lines

The development of the communication networks tends gradually toward exploiting higher frequencies, sometimes even reaching the lowest microwave band (P-band). As the signal bandwidth used for transmission over twisted-pairs increases, as recommended by G.fast and other broadband systems, new phenomenon was observed, namely, backscattering. Motivated by the measurements of copper cables in frequency band up to 400 MHz, we propose a novel backscattering model. It may be productively applied to the problem of loop diagnostics. The methods to accurately and reliable determine the relevant transmission-line parameters are sine qua non condition to appropriately exploit the potential of short-to-medium range access lines. In this paper, a recursive formulation of the frequency-domain response of the backscattering is used for a space–time characterization. To confirm the practical use of the finding, we evaluate the properties of a loop using wideband, high-frequency

Binder Identification Using Pattern Recognition on Phantom Measurements

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