Structure In Time Domain Research Articles

A Reassessment on Friendly Jamming Efficiency

With the rapid and continuous growth of various types of wireless devices in IoT, securing the communications among heterogeneous devices becomes an emerging issue. A physical layer security scheme, called “friendly jamming”, has drawn great attention recently owing to its ability to protect the confidentiality of the communication as well as to enable message authentication and access control for those already employed, unencrypted, weakly encrypted, or resource constrained devices. We notice that in a large number of cases in which friendly jamming are preferable, the transmitting signals to be protected have varying spectrum utilization at symbol level. In this paper, we rebuild secrecy capacity models and re-evaluate the jamming efficiency by taking this micro time scale non-stationary characteristic into consideration. Our reassessments reveal that jamming efficiency is greatly overestimated in the existing literature. The second part of our work further proposes a waveform design on jamming signal as a means to enhance the jamming efficiency. The basic idea is to consider both time and frequency domain structure of the transmitting signal when designing the jamming signal, making both time and frequency bandwidth largely match to each other. We discuss the implementation details for jamming common QAM and PSK modulated signals. Both simulations and proof-of-concept experiments validate the theoretical correctness of our reassessment and practical effectiveness of our method.

Resource Optimization With Flexible Numerology and Frame Structure for Heterogeneous Services

We explore the potential of optimizing resource allocation with flexible numerology in frequency domain and variable frame structure in time domain, in presence of services with different types of requirements. We analyze the computational complexity and propose a scalable optimization algorithm based on searching in both the primal space and dual space that are complementary to each other. Numerical results show significant advantages of adopting flexibility in both time and frequency domains for capacity enhancement and meeting the requirements of mission critical services.

A fully discrete scheme for the pressure–stress formulation of the time-domain fluid–structure interaction problem

We propose an implicit Newmark method for the time integration of the pressure–stress formulation of a fluid–structure interaction problem. The space Galerkin discretization is based on the Arnold–Falk–Winther mixed finite element method with weak symmetry in the solid and the usual Lagrange finite element method in the acoustic medium. We prove that the resulting fully discrete scheme is well-posed and uniformly stable with respect to the discretization parameters and Poisson ratio, and we provide asymptotic error estimates. Finally, we present numerical tests to confirm the asymptotic error estimates predicted by the theory.

Underwater giant enhancement of broadband diffraction efficiency of surface diffraction gratings fabricated by femtosecond laser

In this study, a simple approach is proposed to improve the diffraction efficiency of surface diffraction gratings on fused silica fabricated by femtosecond laser. Effects of grating period, pulse energy, scanning speed and wavelength on the total diffraction efficiency are also investigated. In addition, the mechanism of the diffraction efficiency's giant enhancement is discussed by finite difference time domain (FDTD) and optical phase shift structure calculation. It is revealed that: (1) Compared with the diffraction efficiency obtained in air, the underwater image via diffraction grating is clearer than that in air and diffraction pattern in water is much brighter; (2) The diffraction efficiency increases with an increase in wavelength and scanning speed, whereas it decreases while the laser pulse energy is high; and (3) This giant enhancement is attributed to the degree of Mie scattering and refraction, which is effectively reduced for the existence of water.

Time-Domain <inline-formula> <tex-math notation="LaTeX">$N$</tex-math> </inline-formula>-Continuous OFDM: System Architecture and Performance Analysis

$N$ -continuous orthogonal frequency-division multiplexing (NC-OFDM) is a promising technique to achieve significant sidelobe suppression for baseband OFDM signals. However, NC-OFDM usually has high implementation complexity that limits its practical applications. Based on conventional NC-OFDM, this paper proposes a new technique, termed time-domain $N$ -continuous OFDM (TD-NC-OFDM), which shifts the processing burden from the frequency domain to the time domain. This is achieved by adding a smooth signal, which is linearly combined by rectangularly pulsed OFDM basis signals consisting of a novel basis set. We prove that TD-NC-OFDM with the basis set is essentially equivalent to conventional NC-OFDM. Furthermore, using the time-domain structure, an asymptotic spectrum analysis of $N$ -continuous OFDM signals is obtained by developing a closed-form expression related to sidelobe decaying. This paper also examines the impact of TD-NC-OFDM on the received signal-to-interference-plus-noise ratio (SINR) and derives closed-form expressions.

Signatures of transient electron localisation in high-order harmonic generation

ABSTRACTWe identify and explain signatures of ongoing single-electron nonadiabatic dynamics in the high-order harmonic generation (HHG) spectrum of H+2. We establish the necessary laser parameter criteria for the modulation of the highest energy section of the HHG spectrum by suppressed ionisation due to transient electron localisation. This minimum is also shown to enable the emergence of an unusually complex interference structure at the highest energies of the spectrum caused by the breaking of inversion symmetry during sequential recombination events of the electron wavepacket. By inspecting the time-domain structure of the emitted radiation, we identify the role of transient electron localisation in modulating the amplitudes of individual peaks within the attosecond pulse train. Finally, an analysis of the phase mismatch accumulated within this highly structured region of HHG is performed, showing qualitative differences in coherence lengths present in different regions of the spectrum.

PAPR Reduction in OFDM Signals: An Adaptive-Network-Based Fuzzy Inference Approach

In this paper, an adaptive-network-based fuzzy inference system (ANFIS) based scheme is analyzed and proposed for reducing the peak-to-average power ratio (PAPR) in multicarrier signals under additive white Gaussian noise and multipath fading (Raleigh) channel environment. This scheme involves training of ANFIS structure in time domain using Orthogonal Frequency Division Multiplexing signals with low PAPR, such as those obtained by approximate gradient project---null subcarrier switching (AGP---NCS) method. Once the ANFIS module is trained, the proposed scheme approximately offers similar reduction in PAPR as compared to AGP---NCS method, with significantly less convergence time and computational complexity. he results show that proposed approach is not only less complex but also maintains the data rate and bit error rate performance compared with other conventional schemes.

Time—window Complexity and its Application in the Fault Diagnosis of Bearing

Different from conventional spectral method, complexity analysis treaded with the signals’ time domain structure feature. Before, due to the no stationary and the unevenness of state space of mechanical signals, the state information is likely to get a loss. Now, the time-window complexity is proposed to overcome certain limitations of complexity itself in some extent. It will help to extract the state features of mechanical systems in different states. The concept and algorithm of time-window complexity is introduced detailed. The way of applying the time-window complexity for fault diagnosis is discussed. The mechanical signals of ball bearing with slight flaw and that of four kinds of impact-rubbing states of a typical rotor are then studied. The results show that time-window complexity can reflect the early fault of ball bearing, and differentiate the four kinds of rotor impact-rubbing states properly, which provides another effective way for fault diagnosis of mechanical systems.

Aerodynamic loads and aeroelastic responses of large wind turbine tower-blade coupled structure in yaw condition

An effective method to calculate aerodynamic loads and aeroelastic responses of large wind turbine tower-blade coupled structures in yaw condition is proposed. By a case study on a 5 MW large wind turbine, the finite element model of the wind turbine tower-blade coupled structure is established to obtain the modal information. The harmonic superposition method and modified blade-element momentum theory are used to calculate aerodynamic loads in yaw condition, in which the wind shear, tower shadow, tower-blade modal and aerodynamic interactions, and rotational effects are fully taken into account. The mode superposition method is used to calculate kinetic equation of wind turbine tower-blade coupled structure in time domain. The induced velocity and dynamic loads are updated through iterative loop, and the aeroelastic responses of large wind turbine tower-blade coupled system are then obtained. For completeness, the yaw effect and aeroelastic effect on aerodynamic loads and wind-induced responses are discussed in detail based on the calculating results.

Damage Detection in Bridge Structure Using Vibration Data under Random Travelling Vehicle Loads

Due to the random nature of the road excitation and the inherent uncertainties in bridge-vehicle system, damage identification of bridge structure through continuous monitoring under operating situation become a challenge problem. Methods for system identification and damage detection of a continuous two-span concrete bridge structure in time domain is presented using interaction forces from random moving vehicles as excitation. The signals recorded in different locations of the instrumented bridge are mixed with signals from different internal and external (road roughness) vibration sources. The damage structure is also modelled as the stiffness reduction in one of the beam element. For the purpose of system identification and damage detection three different output-only modal analysis techniques are proposed: The covariance-driven stochastic subspace identification (SSI-COV), the blind source separation algorithms (called Second Order Blind Identification) and the multivariate AR model. The advantages and disadvantages of the three algorithms are discussed. Finally, the null-space damage index, subspace damage indices and mode shape slope change are used to detect and locate the damage. The proposed approaches has been tested in simulation and proved to be effective for structural health monitoring.

Front–back localization in simulated rectangular rooms

The front–back sound localization performance of human subjects was investigated in 3 virtual rooms with different acoustic characteristics, and in an anechoic chamber. The three chosen rectangular rooms had the same width–length–height ratio. Their size and reverberation time were systematically varied in order to disentangle the respective effects on the localization cues, perceived during listening tests. The sound absorption was distributed equally over all surfaces. The head related transfer function (HRTF) used in simulation of the receiver was based on measurements on an artificial head. Four stimuli, with different spectra and time domain structure, were presented to the listeners: broadband noise, orchestra legato sound, orchestra staccato sound, and noise containing two one-third octave band components around 0.5 and 3.15kHz. Significant differences in localization performance were found between sounds presented in the smallest room and the large rooms, and between the anechoic room and the two large rooms. People’s localization performance was significantly different between staccato and legato sound stimuli, and it was significantly worse for the noise containing two one-third octave band components compared to the other stimuli. Also learning effects were observed.

Epilogue

The front–back sound localization performance of human subjects was investigated in 3 virtual rooms with different acoustic characteristics, and in an anechoic chamber. The three chosen rectangular rooms had the same width–length–height ratio. Their size and reverberation time were systematically varied in order to disentangle the respective effects on the localization cues, perceived during listening tests. The sound absorption was distributed equally over all surfaces. The head related transfer function (HRTF) used in simulation of the receiver was based on measurements on an artificial head. Four stimuli, with different spectra and time domain structure, were presented to the listeners: broadband noise, orchestra legato sound, orchestra staccato sound, and noise containing two one-third octave band components around 0.5 and 3.15 kHz. Significant differences in localization performance were found between sounds presented in the smallest room and the large rooms, and between the anechoic room and the two large rooms. People’s localization performance was significantly different between staccato and legato sound stimuli, and it was significantly worse for the noise containing two one-third octave band components compared to the other stimuli. Also learning effects were observed.

Terabit Nyquist PDM-32QAM signal transmission with training sequence based time domain channel estimation.

We propose a time domain structure of channel estimation for coherent optical communication systems, which employs training sequence based equalizer and is transparent to arbitrary quadrature amplitude modulation (QAM) formats. Enabled with this methodology, 1.02 Tb/s polarization division multiplexed 32 QAM Nyquist pulse shaping signal with a net spectral efficiency of 7.46 b/s/Hz is transmitted over standard single-mode fiber link with Erbium-doped fiber amplifier only amplification. After 1190 km transmission, the average bit-error rate is lower than the 20% hard-decision forward error correction threshold of 1.5 × 10(-2). The transmission distance can be extended to 1428 km by employing intra-subchannel nonlinear compensation with the digital back-propagation method.

ESPRIT-Based Carrier Frequency Offset Estimation for OFDM Direct-Conversion Receivers

Over the last years, there has been growing interest in making user terminals more efficient in terms of cost, size, and power consumption. A step forward in this direction is represented by direct-conversion receiver (DCR) architectures, which directly convert the received waveform from the radio frequency (RF) to baseband, thereby avoiding any intermediate frequency (IF) stage. One major issue of DCR devices is the presence of RF impairments, which greatly complicate fundamental receiver functions, including the synchronization task. In this paper we propose a carrier frequency offset (CFO) recovery scheme for OFDM DCRs plagued by frequency-selective In-phase/Quadrature (I/Q) imbalances. Our approach is based on the ESPRIT algorithm and relies on the transmission of a typical OFDM training preamble having a repetitive structure in the time domain. Numerical simulations indicate that the proposed scheme outperforms existing state-of-the-art alternatives at the price of a certain increase of the processing load.

Comparison of linear spring and nonlinear FEM methods in dynamic coupled analysis of floating structure and mooring system

This paper compares the dynamic coupled behavior of floating structure and mooring system in time domain using two numerical methods for the mooring lines such as the linear spring method and the nonlinear FEM (Finite Element Method). In the linear spring method, hydrodynamic coefficients and forces on the floating body are calculated using BEM (Boundary Element Method) and the time domain equation is derived using convolution. The coupled solution is obtained by simply adding the pre-determined spring constants of the mooring lines into the floating body equation. In FEM, the minimum energy principle is applied to formulate the nonlinear dynamic equation of the mooring system with a discrete numerical model. The ground contact model and Morison formula for drag forces are also included in the formulation. The coupled solution is obtained by iteratively solving the floating body equation and the FEM equation of the mooring system. Two example structures such as weathervane ship and semi-submersible structure are analyzed using linear spring and nonlinear FEM methods and the difference of those two methods are presented. By analyzing the cases with or without surge-pitch or sway-roll coupling stiffness of mooring lines in the linear spring method, the effect of coupling stiffness of the mooring system is also discussed.

Spectral-envelope–group-delay models for transients

Transient signals such as plosives in speech or Castanets in audio do not have a specific modulation or periodic structure in time domain. However, in the spectral domain they exhibit a prominent modulation structure, which is a direct consequence of their narrow time localization. Based on this observation, a spectral-domain AM-FM model for transients is proposed. The spectral AM-FM model is built starting from real spectral zero-crossings. The AM and FM correspond to the spectral envelope (SE) and group delay (GD), respectively. Taking into account the modulation structure and spectral continuity, a local polynomial regression technique is proposed to estimate the GD function from the real spectral zeros. The SE is estimated based on the phase function computed from the estimated GD. Since the GD estimation is parametric, the degree of smoothness can be controlled directly. Simulation results based on synthetic transient signals generated using a beta density function are presented to analyze the noise-robustness of the SEGD model. Three specific applications are considered: (1) SEGD based modeling of Castanet sounds; (2) appropriateness of the model for transient compression; and (3) determining glottal closure instants in speech using a short-time SEGD model of the linear prediction residue.

Optical cavities and waveguides in hyperuniform disordered photonic solids

Using finite difference time domain and band structure computer simulations, we show that it is possible to construct optical cavities and waveguide architectures in hyperuniform disordered photonic solids that are unattainable in photonic crystals. The cavity modes can be classified according to the symmetry (monopole, dipole, quadrupole,etc.) of the confined electromagnetic wave pattern. Owing to the isotropy of the band gaps characteristic of hyperuniform disordered solids, high-quality waveguides with freeform geometries (e.g., arbitrary bending angles) can be constructed that have no analogue in periodic or quasiperiodic solids. These capabilities have implications for many photonic applications.

Влияние возраста после зачатия на характеристики интерференционной электромиограммы у новорожденных детей

The purpose of the study was to investigate neuromuscular status in the preterm infants using surface electromyography (EMG). 10 preterm (gestational age 31/32 weeks) and 10 term infants (gestational age 38/39 weeks) participated in the study on the 2nd, 4th and 6th postnatal week. Linear and novel non-linear parameters of were used to quantitize interference EMG (iEMG) in 4 muscles (mm. bic. br. dext., tric. br. dext, tib. ant. sin., gastr. sin. In the preterm infants aged 33-37 weeks both linear and non-linear sEMG parameters were significantly lower in comparison with the term infants. Thus, sEMG of the preterm infants was characterized by a more "primitive" time-domain structure, lower amplitude and spectrum frequency. Also, unlike to term newborns, in the preterm infants the lifetime dynamics ofsEMG parameters was retarded. Thus, the motor system of the preterm infant is likely less prepared to the postnatal life due to shorter existence in utero. Nonetheless, sEMG of the preterm infants may be characterized as a more "complex" in comparison with the term newborns of the same post-conceptual age. It may be attributed to new postnatal sensory stimuli.

Linear water waves: the horizontal motion of a structure in the time domain

AbstractThe framework of the linearized theory of water waves in the time domain is used to examine the horizontal motion of an unrestrained floating structure. One of the principal assumptions of the theory is that an infinitesimal disturbance of the rest state will lead to an infinitesimal motion of the fluid and structure. It has been known for some time that for some initial conditions the theory predicts an unbounded horizontal motion of the structure that violates this assumption, but the possibility does not appear to have been examined in detail. Here some circumstances that lead to predictions of large motions are identified and, in addition, it is shown that not all non-trivial initial conditions lead to violations of the assumptions. In particular, it is shown that the horizontal motion of a floating structure remains bounded when it is initiated by the start up of a separate wave maker. The general discussion is supported by specific calculations for a vertical circular cylinder.

Time-domain analysis of nonlinear circuits performed on the basis of functional Volterra series

A method is proposed for determination of Volterra kernels from known images. It is shown that frequency-domain kernels should be considered at the points where their analyticity is violated. The timedomain structure of Volterra kernels is determined. A nonlinear microwave amplifier is simulated in the presence of a wideband input signal.