Spreading sequences for fast switching process in spin-valve nanopillars

Gold functions are well-known for designing sequences with good correlation properties. This paper reveals a fascinating relationship between Gold functions and Golay-Davis-Jedwab (GDJ) Boolean functions to design sequences with low correlation and low PAPR, which is the first of its kind. The bent and semi-bent properties of the Gold functions are utilized to derive the correlation of the resultant binary spreading sequence sets, while the complementary properties of the GDJ Boolean functions are used to derive the low PAPR. Based on this idea we propose three new sets of binary spreading sequences with low PAPR and low correlation. The proposed sequence sets have the potential to be used as spreading sequences in non-orthogonal multiple access (NOMA). They have a very large set size, which in turn results in a high overloading factor as compared to the existing sequence sets with the same length and the same correlation, which are generated through systematic constructions.

This paper presents new binary spreading sequences, obtained by tabu search. These sequences exhibit odd correlation properties concerning the minimax criterion better than those of the classical sequence families, such as the well known Kasami and Gold sequences. In particular, the design process takes into account both odd and even correlation properties which means that global performance is compromised. These new sequences also have same mean-square correlation performance as classical ones.

It is well known that jointly optimal multiuser detection for code division multiplex access (CDMA) systems has complexity which grows exponentially with the number of users. Recently, several authors [S. Ulukus and R. D. Yates, April 1998], [C. Sankaran and A. Ephremides, Sept. 1998], [C. Schlegel and A. Grant, 2000] have reported that, for certain special sets of spreading sequences, optimal multiuser detection in synchronous CDMA systems can be performed with computational complexity which is polynomial in the number of users. In this paper, we show that the existing polynomial complexity (PC) algorithms of [S. Ulukus and R. D. Yates, April 1998], [C. Sankaran and A. Ephremides, Sept. 1998], [C. Schlegel and A. Grant, 2000] lead to efficient algorithms for a wider class of spreading sequences than initially proposed. We identify these sequences and prove the existence of optimal polynomial-complexity algorithms for detecting synchronous CDMA signals using these sequences. We also give constructions of sets of binary antipodal spreading sequences for which optimal polynomial-complexity algorithms exist and show that for any sequence length N, we can construct at least N such sequences.

The effects of a noise-like weak signal in the resonant switching of the magnetization driven by spin transfer torque have been systematically studied by means of micromagnetic simulations in both in-plane spin valves and out-of-plane magnetic tunnel junctions. The use of a noise-like resonant excitation allows the overpassing of some critical points compared to the traditional microwave assisted switching. In particular, the application of binary spreading sequences (BSS) does not require the precise knowledge of the resonant frequency and of the optimum relative phase, allowing the use of the same resonant excitation for all the devices in a matrix of memory cells, even if small variations of the physical and geometrical parameters are present. In addition, we also found that in spin valves the switching probabilities close to 100% can be achieved with a smaller current pulse, and in magnetic tunnel junctions a significant energy saving can be obtained.

This paper is devoted to the generation and evaluation of the Chaos-based M-ary spreading sequences on communications systems. Sequences obtained by repeating a truncated and multi-ary quantized chaotic series are compared with classical m-sequences by means of the autocorrelation and cross-correlation properties and power-spectral features. Anti-noise performance of binary sequences and chaotic-based M-ary spreading sequences has been compared in the case of the same single-frequency interferences. Studies have shown that spectral features and anti-noise performance of chaotic-based M-ary spreading sequences which have great researching value are better than binary sequences. DOI: http://dx.doi.org/10.11591/telkomnika.v10i8.1654 Full Text: PDF

Very recently the tuned liquid column damper (TLCD) is receiving an increasing interest from researchers concerned with vibration control, to be considered an alternative device with respect to the tuned mass damper (TMD), since the former has low cost, easy adjustment, flexible installation. However, in recent studies the authors [1] have pointed out that for TMD the analytical formulation provides results that are in good agreement with the experimental ones, while for TLCD it has been deducted that the analytical formulation needs further investigation. In fact using the classical formulation of the problem, numerical results are very different from the experimental results obtained by the authors using the facilities at the experimental dynamic laboratory of University of Palermo. In particular it has been shown that the total liquid length should be corrected in an effective one, but in a different way from what has been done in literature, where only the variation of section of the vessel has been taken into account. On the other hand, from experimental investigations it is seen that the liquid moves more in the central area of the tube and less in the area in contact with the side walls. This aspect plays a fundamental role for capturing the real performance of TLCD. In fact, being the TLCD a special type of auxiliary damping device which relies on the inertia of liquid column in a U-tube to counteract the forces acting on the structure, then it is necessary to identify the effective moving liquid mass. To aim at this, in this paper the authors differentiate the total liquid mass into a liquid dead mass and a liquid dynamic mass, then introducing these values into a properly modified mathematical formulation numerical results match the experimental ones for all tests.

Distributed multiple-input multiple-output (MIMO) system is a new architecture for next generation wireless communication systems with distributed multiple antennas. In this paper, an uplink model for multiuser distributed MIMO systems with wideband code-division multiple-access (CDMA) technology is presented. We focus on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. The spectral efficiency of the uplink system with single-user detectors at the base stations under multipath fading channels is investigated carefully. Two cases with different assumptions of interference knowledge are studied in detail. The spreading sequences of the CDMA systems are assumed to employ loosely synchronized (LS) spreading sequences and random spreading sequences respectively. LS spreading sequences exhibit a so-called interference free window (IFW), where both the auto-correlation and cross-correlation of the codes become zero. Therefore LS sequences have the promise of mitigating the effects of inter symbol interference (ISI) and multiple access interference (MAI) in multipath fading channels. It is shown by numerical results that the uplink spectral efficiency of the distributed MIMO-CDMA system using LS sequences can significantly outperform that of the system using random spreading sequences. Copyright © 2005 John Wiley & Sons, Ltd.

Biomass fast pyrolysis in screw reactors: Prediction of spent coffee grounds bio-oil production through a monodimensional model

The flow structuration of silicon oil (Prandtl number of 10.3) in a open cylindrical pool heated from the center of the surface is investigated numerically. Our purpose is to perform the numerical simulation of experimental results obtained by Favre [Phys. Fluids 9, 1473 (1997)] who observed transitions between steady and axisymmetric flows at sufficiently low values of the Marangoni number (Ma) and various types of instability depending on the height of the fluid. The hydrothermal wave regime has been obtained at critical values of Ma which depend on the Bond number and on the aspect ratio. The numerical results are in good agreement with the experimental ones.

The authors consider the radiation problem of an aperture on a dielectric-coated perfectly conducting cylinder. The aperture is modeled by an infinite line source. The modeling follows the approach of R. Paknys (1985). In the experimental study, a longitudinal X-band slot (8-12 GHz) was set on the conducting surface of the cylinder. Theoretical results are in good agreement with the experimental ones except for the oscillations present in the numerical results. >

A few studies have been reported on the transient vibration of clamped beams impulsed by a spherical impactor. The governing equation of the transient vibration becomes a nonlinear equation due to the contact force between the beam and impactor, and due to the axial force caused by bending of the beam. Although effects of contact force and axial force on the transient vibration of the beam have been studied independently in some reports, one can find no reports discussing both effects. In the present study, it is assumed that the contact force can be estimated by the Hertzian force-deformation relationship. A numerical approach has been proposed for taking into account effects of both contact force and axial force. With the comparison of numerical results and experimental ones, it follows that one can estimate accurately the deflection of the beam and the axial force caused by bending using the proposed approach. From numerical results, furthermore, it has been made clear that one cannot neglect the effect of the axial force caused by bending on the deflection in the case of the clamped thin beam.

This research work aims to investigate the effect of fabrication techniques on the physico‐mechanical properties of pistachio shell (PS) filled glass‐epoxy composites and to compare the experimental findings with numerical simulation results using finite element analysis (FEM). These hybrid composites are fabricated by vacuum‐assisted resin transfer molding (VARTM) and hand layup (HL) techniques with fixed glass fiber fraction (20 wt. %) but different PS powder loadings of 0–30 wt. %. The composites are then characterized for physical, compositional, micro‐structural and mechanical properties. The micro‐structural analysis of the filler and composites is performed using electron microscopy and stereo‐microscopy. To identify the phases present in both the raw filler and the composite, an x‐ray diffraction test is performed. The presence of functional groups is identified with the help of Fourier transform infrared spectroscopy (FTIR). It is observed that there is a reasonable improvement in the mechanical properties of the composites with increase in PS powder content, irrespective of the fabrication process used. The density and void fraction are also greatly affected by the amount of filler particles present in the composites and also on the composite fabrication technique. The FEM analysis is also carried out using ANSYS 22.0 workbench to determine the characteristic properties numerically. The comparison of experimental and numerical results yields that the properties obtained by using VARTM results are close to experimental ones with errors lying in the range of 1%–4%. These composites can possibly find potential applications in light duty structures and wear resistant applications.Highlights Development of hybrid composites using HL and VARTM techniques. Physical and mechanical properties alter with filler loading. Evaluation of mechanical properties using finite element analysis. Validating the model by comparing experimental and numerical results.

Transient analysis of the DSIFs and dynamic T-stress for particulate composite materials—Numerical vs. experimental results

The spontaneous ignition of coal stockpile causes serious safety and economic problems. Such spontaneous ignition occurs in coal stockpile when the rate of heat released by the oxidation of coal is greater than the rate of heat lost to the surroundings. In this study, a two-dimensional unsteady model is adopted for studying spontaneous ignition and the numerical results are compared with experimental results. The numerical results are in a good agreement with the experimental ones. Depending on the porosity, the internal maximum temperature, pressure, and oxygen mass fraction during spontaneous ignition are investigated. On the basis of the numerical results, the transient temperature variations for several shapes of coal stockpiles are analyzed. Further, the physical mechanisms of hot-spot formation and spontaneous ignition are analyzed.

The authors consider a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with M-ary phase-shift keyed (MPSK) signaling. Users in the system use complex signature sequences. A bounding technique is used to obtain arbitrarily tight upper and lower bounds on the probability of bit error. Numerical results are presented for Frank-Zadoff-Chu (FZC) and near-optimal four-phase complex sequences, as well as binary Gold sequences, for binary phase shift keying, (BPSK), 4-PSK, and 8-PSK. Results show that complex sequences can yield better performance than binary sequences in MPSK signaling. Using complex sequences, the number of signature sequences with good correlation properties is greatly enlarged.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>