A least mean squares blind equalization techniques for OFDM systems

Classical multicarrier systems based on the discrete Fourier transform(dft) make use of aguard interval (gi) in order to enable a low complexity equalization scheme. Thisguard interval consists of a redundant prefix cyclically appended to each block of modulated symbols so as to exploit the cyclic convolution property of thedft. Therefore, besides decreasing the useful transmitted symbol rate, this technique is very specific todft-basedofdm systems. In order to implement a digital modulator, an oversampled version of the continuous signal that would be produced by the all-analog ideal modulator is often computed. This amounts to appending null symbols to the block of symbols to be modulated. This work shows that forcing the presence of these null symbols at the appropriate places on the receiver side is sufficient to equalize the channel. Here, a linear equalizer is adapted by minimizing a quadratic criterion based on the energy of the subband signals that should be zero. Since no knowledge about theuseful data is required, this method performs blind equalization. Moreover, it requires neither a guard interval nor any reference symbol. As a result, for a given channel bitrate budget, the data rate is increased     

Subspace methods for the blind identification of multichannel FIRfilters

This paper addresses a problem arising in a context of digital communications. A digital source is transmitted through a continuous channel (the propagation medium), and several measurements are performed at the receiver, either by means of several sensors, or by oversampling the received signal compared to the emission rate. Given only these observations, the baseband equivalents of the corresponding channels have to be recovered. An orthogonality property between “signal” and “noise” subspaces is exploited to build some quadratic form whose minimization yields the desired estimates up to a scale factor. This is in the same spirit as recent works by Tong et al. (see Proc. 25th Asilomar Conf., p.856-860, 1991) but requires fewer computations. Numerical simulations demonstrate the performance of the proposed methods in a channel identification context     

Sulfur Localization in NiPtAl/Superalloy Systems After High Temperature Isothermal Oxidation

Sulfur distribution and its evolution after isothermal oxidation have been studied using secondary ion mass spectrometry. In this work, the systems consist of a Ni-based single-crystal superalloy with a (Ni,Pt)Al bond coat. Two superalloys are compared: AM1 and MCNG. The latter contains Hf as a reactive element. Isothermal oxidations are performed at 1,100 °C under synthetic air for 1, 10, 50, 500 and 1,000 h. After oxidation, sulfur is detected both in the internal and external zones of the bond coat, at the bond coat/oxide interface and inside the oxide scale. An increase of the sulfur signal with oxidation time is registered at the bond coat/oxide interface for the MCNG-based systems whereas the opposite trend is observed for the AM1-based ones.     

A Necessary Condition on the Location of Pilot Tones for Maximizing the Correction Capacity in OFDM Systems

This paper presents a new view of the Bose-Chaudhuri-Hocquengem (BCH) code through the addition of some flexibility to the syndromes distribution in the transmitted sequence. In order to get this flexibility, we derive a necessary condition (NC) on the syndromes distribution for decoding BCH codes, which includes the already known Hartmann-Tzeng proposition. This NC is essentially deduced from the decoding process of BCH code, and is related to the locator polynomial and the requested constraints to guarantee a maximal error-correction capacity. The obtained results have the advantage to be applicable for any considered field (finite or not). Furthermore, we prove that when the correction capacity is equal to 2 or 3, the obtained NC becomes also sufficient. This result is very useful in some practical transmission systems such as orthogonal frequency-division multiplexing systems. Once the pilot tones considered in such systems verify the necessary and sufficient condition, it becomes possible to both reduce the peak-to-average-power rate and correct the impulse noise, present in such multicarrier systems. The usefulness of the presented analysis and the exploitation of the derived condition on the pilot tones distribution is illustrated by simulation results in the case of the Hiperlan2 system     

The effect of nitrogen on the passivation mechanisms and electronic properties of chromium oxide layers

In this work, several Physical Vapour Deposition (PVD) films of chromium and chromium nitride were grown by adjusting the nitrogen flow rate. It was found that films containing lower nitrogen content exhibited better corrosion behaviour in relation to their microstructure. Moreover, the resistivities of the samples were found to increase with N content due to a decrease in the number of metallic bonds in the coatings and a loss of oxide layer conductivity. The latter is linked to a decrease of charge carrier density by the incorporation of species into the passive layers, which decreases cation vacancy density.     

Influence of fluxing in the preparation of bulk Fe-based glassy alloys

Fe-based master ingots, with composition [(Fe₅₀Co₅₀)₇₅B₂₀Si₅]₉₆Nb₄, have been prepared by arc-melting and subsequently purified using the fluxing technique. Rods, with diameters from 1 to 2.5 mm, ribbons and wires with a maximal diameter of 200 μm have been produced from these purified master ingots. The amorphous structure, thermal stability, hardness and magnetic properties of the specimens were studied. In particular, we show by high-energy synchrotron X-ray diffraction that the rods are fully amorphous up to a maximal diameter of 2 mm. Moreover, the [(Fe₅₀Co₅₀)₇₅B₂₀Si₅]₉₆Nb₄ glassy alloy exhibits high hardness (HV = 1050) combined with good soft-magnetic properties (H_c = 63 A/m).