Complex Frequency Research Articles

Frequency Domain UWB Channel Characterization and Modeling in Indoor Static Environment

In this article, distance and frequency‐dependent indoor wireless channel characterization and modeling are presented in C, X, and Ku bands at the line of sight (LOS) and nonline of sight (nLOS) scenarios. Complex frequency responses of the indoor radio channel are measured using the frequency‐domain approach. The terminal and measurement setup dependencies of the propagation channel are deconvoluted using closed‐form expressions devised from a two‐port network model approach of the antenna transreceiver system. Subsequently, terminal independent frequency‐domain channel transfer functions (CTFs) and time‐domain channel impulse responses (CIRs) are derived for accurate statistical analysis of large‐ and small‐scale fading parameters of the propagation channel. A distance‐dependent statistical pathloss model is proposed. Successively, a frequency domain channel model using the fifth‐order autoregressive process is proposed. Subsequently, the existence of the multiple clusters in the environment is analyzed by the poles of the transfer functions of the model. Models are validated in both time and frequency domains by comparing the computer‐simulated synthetic model data with the empirical channel responses. The measurement data and the model data are seen to be in good agreement.

CATECHOLASE ACTIVITY AND SUBSTITUENT EFFECT OF NEW HOMOLEPTIC COPPER(II) CHALCONE COMPLEXES

<p>Three new neutral complexes of copper(II) containing chalcone ligands derived from 2'-hydoxyacetophenone and 4-substituted benzaldehydes were synthesized. Complexes were prepared by solution synthesis and characterized by spectroscopy. The catalytic activity of complexes was examined in the reaction of 3,5-di-tertbutylcatehcol (DTBC) oxidation. The kinetics of DTBC catalytic oxidation by copper(II) complexes (1 – 3) was investigated spectrophotometrically under pseudo-first-order conditions. Catalytic parameters, the maximum reaction rate (vmax), Michaelis-Menten constant (KM), catalytic efficiency, catalytic reaction rate constant (kcat), turnover number (TON), and turnover frequencies (TOF) for complexes 1 – 3 in DTBC oxidation were collected. The studied complexes 1 and 2 were found to have moderate catalytic activity, while complex 3 does not show catalytic properties.</p>

Nonlocal dual-phase-lag thermoelastic damping analysis in functionally graded sandwich microbeam resonators utilizing the modified coupled stress theory

Functionally graded (FG) sandwich structures stand as one of the most representative composite structures owing to the thermal resistance and the energy absorption property in non-unfiorm thermal environment. Additionally, accurately estimating thermoelastic damping (TED) is of great importance for the design of high-performance micro/nano-resonators. Nevertheless, the classical TED models fail on the micro/nano-scale structures due to without considering the influences of the spatial size-dependent effects related to heat transfer and elastic deformation. The nonlocal heat conduction model and modified coupled stress theory are responsible for the size-dependent effects. To address this issue, present study aims to conduct the size-dependent TED model of FG sandwich microbeam resonators for TED analysis by incorporating the nonlocal dual-phase-lag (NDPL) heat conduction model and the modified coupled stress theory (MCST). It is assumed that the FG sandwich microbeam resonators consist of a ceramic core and FG surfaces. The energy equation and the transverse motion equation are formulated, and then, the analytical solution is solved by complex frequency method. Exact and closed-form expressions for TED can be obtained through the complex frequency method. The results are validated, and the parameter effects of the nonlocal thermal parameter, the material length-scale parameter, the power-law index and the vibration modes on the TED are analyzed. The results show that the energy dissipation can be reduced by the size-dependent effects resulting in improving the quality factor of microstructures. It is expected that these results may provide a theoretical basis for predicating TED in the design of FG sandwich micro/nano-resonators with high quality factor in extreme heat transfer environment.

Investigation of Antiviral Activities of Nickel and Copper Complexes with Macrocyclic Ligands against Crimean-Congo Hemorrhagic Fever by In Silico Calculations

For the first time, electronic characteristics of potential drug candidates and their inhibitory activities have been linked thanks to this work. Synthesized copper and nickel complexes with trans-N1,N8-bis(2-cyanoethyl)-2,4,4,9,11,11-hexamethyl-1,5,8,12-tetraazacyclotetradecane (tet-bx) ligand, as well as the proposed hypothetical complexes, were properly examined by the appropriate calculation method in atomic and molecular dimensions. The appropriate calculation level was achieved by using the IR spectroscopic data of the tet-bx ligand. The experimental and calculated bond stretching frequencies were compared for synthesized complexes [Ni(tet-bx)](ClO4)2 (1), [Cu(tet-bx)](ClO4)2 (2), [Ni(tet-bx)(NCS)2] (3), and [Ni(tet-bx)(ClO4)Cl] (5). Some bond stretching frequencies of hypothetical complexes [Cu(tet-bx)(NCS)2] (4) and [Cu(tet-bx)(ClO4)Cl] (6) have also been proposed and their molecular structure were determined. To analyze the electronic behavior of the examined complexes at the atomic level, Fukui function indices (nucleophilic f+ and electrophilic f- populations) were determined. Furthermore, antibacterial and antiviral inhibition efficiency of the complexes against Crimean-Congo hemorrhagic fever has been investigated by docking studies

The new Subspace-based poly-reference Complex Frequency (S-pCF) for robust frequency-domain modal parameter estimation

Subspace identification has been widely used in modal parameter estimation. The formulation of a parametric subspace-driven modal identification technique is achieved by using the Singular Value Decomposition (SVD) to remove most the uncertainties present in the null space of the system matrices used in the identification process, which leads to a more robust estimation of the modal properties. This idea has been used in the formulation of time domain modal identification techniques like the (time-domain) Eigensystem Realization Algorithm and the Stochastic Subspace identification techniques. In this paper, a subspace implementation of the new poly-reference Complex Frequency (pCF) formulated in Modal Model is proposed. The idea is to combine SVD with the new pCF to derive a new system identification technique that operates in the frequency domain by using either the Frequency Response Function or the Half Spectrum as primary data. Similarly to the ERA, the idea behind the subspace implementation of the pCF is to use different subspaces of the observability and frequency-domain controllability matrices, and carry out eigen-system realizations to estimate the modal properties corresponding to each chosen subspace. In order to illustrate its robustness, the subspace-driven pCF is applied to a simulated and two practical application examples.

A multi-layered corrugated resonator acoustic metamaterial with excellent low-frequency broadband sound absorption performance

How to design acoustic metamaterials with excellent low frequency broadband sound absorption and mechanical properties is a concern and bottleneck in the field of structural vibration and noise control. Based on the principle of Helmholtz resonator and lightweight multi-functional sandwich structure, we propose a multi-layered corrugated resonator acoustic metamaterial (MLCRAM) and fabricate a sample using 3D printing technology. Through theoretical and simulated studies of the MLCRAM model, it is presumed that the MLCRAM has excellent sound absorption performance in the frequency range of 500 to 1000 Hz, which is mainly attributed to the parallel combination of resonant cavities. Then the experimental test of the sample is also carried out to verify the reliability of both theoretical and simulated methods. By analyzing the complex frequency plane, the influence of coupling effects between resonant cavities on the sound absorption properties of the MLCRAM. Subsequently, the overall sound absorption mechanism of the MLCRAM is further analyzed in terms of sound velocity, sound pressure amplitude, and power dissipation density. It has been demonstrated that MLCRAM exhibits excellent low-frequency broadband sound absorption performance, providing a reference for the design of novel acoustic metamaterials.

Optimization of hybrid microperforated panel and nonuniform space-coiling channels for broadband low-frequency acoustic absorption

Broadband noise cancellation at low frequencies (<1000 Hz) with thin absorbers has been a long-standing demand in acoustic engineering. Here, a design method is proposed for a subwavelength hybrid absorber, consisting of a microperforated panel (MPP) backed by nonuniform space-coiling channels. The low-frequency absorption performance is analyzed by impedance theory and complex frequency method. The sensitivity of the absorption band to the parameters of the MPP and the coiled channels is investigated, based on which a genetic algorithm is employed to costume quasi-perfect absorption at the desired frequency. Furthermore, a parallel integration of multiple inhomogeneous units is developed to broaden the absorption spectrum. An optimization strategy is proposed to effectively mitigate the performance degradation caused by the impedance coupling effect. The influence of the number of combined units and the objective function is analyzed for the optimization results. Numerical simulations, experiments, and theoretical predictions verify the effectiveness of the design method. Furthermore, the as-designed combination absorber achieves continuous absorption from 305 Hz to 1088 Hz. The proposed design approach may offer a promising solution for practical applications in broadband noise reduction.

Dynamic behavior of axially functionally graded pipe conveying gas–liquid two-phase flow

The dynamic behavior of an axially functionally graded pipeline conveying gas–liquid two-phase flow is studied using the Euler–Bernoulli beam model and the generalized integral transform method (GITT). The influence of changes in functionally graded material parameters, boundary conditions, and gas volumetric fraction of two-phase flow on the linear stability of functionally graded pipes is investigated. The results indicate that the gradient of elastic modulus has a significant influence on the complex frequency, critical velocity for instability, and coupling flutter of the functionally graded pipeline. Additionally, the gradient of density has a significant effect on the complex frequency of the functionally graded pipe. The boundary conditions of the pipe have a significant influence on its dynamic response. The pipe exhibits higher stability when both ends are fixed, followed by the combination of fixed support and simple support. Conversely, the condition of simply supported at both ends is the most susceptible to instability. The gas volumetric fraction also plays a significant role in the dynamic behavior of functionally graded pipes conveying gas–liquid two-phase fluids.

Deep Cascade-Learning Model via Recurrent Attention for Immunofixation Electrophoresis Image Analysis.

Immunofixation Electrophoresis (IFE) analysis has been an indispensable prerequisite for the diagnosis of M-protein, which is an important criterion to recognize diversified plasma cell diseases. Existing intelligent methods of IFE diagnosis commonly employ a single unified classifier to directly classify whether M-protein exists and which isotype of M-protein is. However, this unified classification is not optimal because the two tasks have different characteristics and require different feature extraction techniques. Classifying the M-protein existence depends on the presence or absence of dense bands in IFE data, while classifying the M-protein isotype depends on the location of dense bands. Consequently, a cascading two-classifier framework suitable to the two tasks respectively may achieve better performance. In this paper, we propose a novel deep cascade-learning model, which sequentially integrates a positive-negative classifier based on deep collocative learning and an isotype classifier based on recurrent attention model to address these two tasks respectively. Specifically, the attention mechanism can mimic the visual perception of clinicians, where only the most informative local regions are extracted through sequential partial observations. This not only avoids the interference of redundant regions but also saves computational power. Further, domain knowledge about SP lane and heavy-light-chain lanes is also introduced to assist our attention location. Extensive numerical experiments show that our deep cascade-learning outperforms state-of-the-art methods on recognized evaluation metrics and can effectively capture the co-location of dense bands in different lanes.

Approaches for improvement of reliability of the Prony’s method computation

Parametric spectral analysis methods such as the Prony’s method can estimate the frequencies and amplitudes of a signal, conforming to their model, with great precision. At the same time, the addition of noise to the signal can lead to a complete model breakdown which leads to erroneous parameter values. This is especially true for the impulse noise. The article explores several possible algorithms which can be applied to the Prony’s method in order to refine the results and make them more noise resistant. Such algorithms include signal segmentation methods where the results of each segment processing influence the final estimate as well as the conceptually related method of point skipping. An approach based on the use of non-Euclidean norms as a measure of the linear algebraic equation system’s solution quality is developed and tested. Initially, the methods are applied to model digital signals, comprised of harmonic components with varying complex frequencies and amplitudes. Additive white Gaussian and impulse noise is added to the model signals. The results are then applied to the noisy results of a real-life synthetic aperture synthesis experiment obtained in the intermediate zone of radiation.

2758. Clinical Significance and Antifungal Susceptibility Profile of 103 Clinical Scedosporium Species Complex and Lomentospora prolificans Isolated from NIH Hospitalized Patients

Abstract Background Members of the fungal genera Scedosporium species complex and Lomentospora have been increasingly recognized as emerging opportunists affecting immunocompromised patients. Reduced susceptibility to systemic antifungals is common, and optimal treatments are incompletely described. Methods 103 clinical isolates from NIH, Bethesda, MD, were investigated. The identity of each isolate was confirmed at the species level via PCR-sequencing of internal transcribed spacer (ITS) of the ribosomal DNA (rDNA) region and the calmodulin gene. Antifungal susceptibility testing was conducted in accordance with the CLSI M38-A3 guidelines. Patient data were collected retrospectively. Results The frequency of Scedosporium species complex and Lomentospora prolificans isolates are detailed in Figure 1. In vitro susceptibility results of eight antifungals against all isolates are listed in Table 1. The novel antifungal olorofim showed the lowest MICs against all Scedosporium and L. prolificans, followed by micafungin. Among triazoles, voriconazole (VRC) showed lower MIC values against clinical members of Scedosporium. Amphotericin and posaconazole (POS) demonstrated species-specific and inter-species variable activity. Itraconazole, isavuconazole, and terbinafine had high MICs against Scedosporium and L. prolificans. Clinical data were available for 90 isolates. 9 patients (28 isolates) had disease or infection (Table 2). All but one case occurred in immunocompromised hosts, and all patients were treated with a regimen that included VRC or POS. Five patients died. Three patients with chronic granulomatous disease were cured following hematopoietic cell transplant. 24 patients (62 isolates) had colonization, of which 58 isolates reflected respiratory colonization in patients with bronchiectasis. Conclusion Our data support that species-specific and inter-species differences exist in the distribution of antifungal susceptibility patterns among Scedosporium and L. prolificans. Our in vitro data suggest that olorofim may be a promising therapy for Scedosporium and L. prolificans. Our clinical data suggest that host status, in conjunction with effective antifungal therapy, is an important determinant in treatment outcomes. Disclosures All Authors: No reported disclosures

Elastic Lateral Torsional Buckling of Beams Under External Moments: A New Type of Nonconservative Problem

The important character of the applied moment with a fixed line of direction as implied in deriving the governing differential equations of LTB for elastic beams, however, is not represented in stating the boundary conditions based on conventional energy approaches. This paper sets out to consider the boundary conditions derived by conventional potentials in order to identify the ignored second-order moments that need to be added for maintaining the initial direction of the applied moment during LTB. By taking into account the non-zero work done by these ignored moments for modifying original potentials, the nonconservative nature of the elastic LTB of beam under external moments is clarified in two aspects — by discovering the path-dependent property of the non-zero work and by demonstrating that only trivial solution of LTB mode is expected from the analytical solution of the governing differential equations. Using the concept of dynamic instability, an analysis procedure combining the numerical solution of an eigen-value problem and the bisection method is proposed to identify the first appearance of imaginary or conjugate complex frequencies and to determine the value of the critical moment. Demonstrative examples are considered for illuminating the elastic LTB behavior using conventional energy methods and revised virtual work equation. It has been found that the critical moments given by conventional potentials are generally not consistent with each other unless the work done by the ignored second-order moments vanishes. However, consistent critical moment, under which the free vibrating amplitude of elastic beam is increased unboundedly with time, can be obtained by the kinetic approach based on any revised virtual work equation, indicating the necessity of considering the nonconservative nature of non-follower applied moment.

OFDM-MIMO radar assisted dual-function radar communication system using index modulation

Dual-function radar communication (DFRC) system exploits a hardware platform to achieve both radar and communication functions. Compared to utilizing separate radar and communication systems, DFRC has better performance in spectrum utilization. In this paper, assisted by orthogonal frequency division multiplexing-multiple input multiple output (OFDM-MIMO) radar, we propose a DFRC system architecture combining generalized space shift keying (GSSK) modulation and subcarrier index modulation (SIM) techniques. In this scheme, linear frequency modulation (LFM) signal is explored as the transmission waveform of OFDM-MIMO radar to complete the radar function. Furthermore, the proposed scheme combines GSSK and SIM to carry the communication information to the antenna index and subcarrier index to complete the communication function. Based on this architecture, a novel low complexity frequency detection communication demodulation algorithm is proposed, and the achievable rate communication performance of the system is analyzed. Numerical and simulation results verify the feasibility and effectiveness of the proposed system.

Study on the acoustic performance of bending metasurface with ultra-thin broadband quasi-perfect sound absorption

In order to achieve low-frequency broadband noise control under subwavelength thickness, this paper established a theoretical model for calculating the sound absorption coefficient of curled acoustic metasurface (CAM) composed of embedded hole, equal width curled channel, and gradient width curled channel, which is based on the thermal viscosity equivalent model and impedance equivalent model. Considering the over resistance effect caused by multi-unit composite structure, a multi-parameter control strategy was used to design bending acoustic metasurface (BAM) units for perfect sound absorption at four discrete frequencies of 546 Hz, 563 Hz, 585 Hz, and 601 Hz, which is based on the complex frequency plane method. and the thicknesses of these unites are only (1.2 cm). The broadband and efficient sound absorption effect of the four parallel composite BAM units was theoretically studied. The simulation and experiment verified that the composite BAM not only achieved perfect sound absorption at 573 Hz with a subwavelength thickness of 12 mm, but also had an efficient sound absorption(α > 0.8) frequency band of 512Hz-621Hz, with a bandwidth up to 109 Hz. The research in this paper provides a certain theoretical basis for the design of low-frequency broadband compact acoustic structures, and has certain reference value for low-frequency broadband noise control.

Dynamic continualization of masonry-like structured materials

Block-lattice materials featuring periodic planar running-bond tessellation of regular rigid blocks and linear elastic homogeneous isotropic interfaces are considered. The governing equations of the discrete masonry-like Lagrangian model are properly manipulated via the novel enhanced continualization scheme, in such a way as to obtain equivalent integral type non-local continua, whose band structure turns out to be coincident with that of the corresponding discrete models. The formal Taylor series expansion of the integral kernels allows deriving homogeneous generalized micropolar higher-order continuum models, characterized by non-local constitutive and inertial terms. The enhanced continualization exhibits thermodynamic consistency in the definition of the overall non-local constitutive tensors, as well as qualitative agreement and quantitative convergent matching of the complex frequency band structure in the regime of both homogeneous and non-homogeneous Bloch waves. The theoretical findings are effectively validated by studying the dispersion relations and the spatial attenuation properties, as referred to realistic representative cases of masonry-like block-lattice micro-structures.

Three-Dimensional Plate Dynamics in the Framework of Space-Fractional Generalized Thermoelasticity: Theory and Validation

This work aims to study the dynamics of 3D plates under uniform and nonuniform temperature distributions in the framework of the space-fractional generalized thermoelasticity (S-FGT) approach. The quadratic eigenvalue problem is obtained, which means that the thermoelastic damping plays a meaningful role due to the plate’s thermal energy absorption. The plate’s complex frequency spectrum and mode shapes (free ends) under two different temperature distributions are considered for different values of the fractional continua order α and the length scale parameter l. For the first four frequencies, the fractional modes closest to the experimental results and the classical modes are presented with the absolute differences between them. For the nonuniform temperature distribution case, the mode shape analysis is performed assuming that modulus of elasticity, thermal expansion, and specific heat parameters are functions of the temperature. The primary outcomes of the paper can be stated as follows: 1) the S-FGT approach analysis gives more reliable results than the classical (local) theory; 2) the peak point of the out-of-plane mode amplitude is shifted toward the warmed zone; 3) a mode shifting is observed for the uniform temperature distribution in contrast to the nonuniform temperature distribution; 4) the fractional order derivative and length scale parameter depend on temperature, similar to other material properties such as elastic modulus, specific heat, and coefficients of thermal expansion; 5) a decrease in the fractional order is observed, while temperature increases for the fixed length scale parameter. These novelties indicate that the S-FGT approach establishes a new model for analyzing materials under heating, and the results may be beneficial for designing thermal structures.

Thin metamaterial using acoustic black hole profiles for broadband sound absorption

The multi-pancake absorber is a thin acoustic metamaterial composed of periodically arranged thin annular cavities separated by the plates and connected by a main pore, enabling low frequency sound absorption. However, with a constant main pore radius, this structure exhibits tonal absorption behavior characterized by narrow peaks with varying amplitudes due to an imbalance in losses between the modes. This work investigates a geometric variation of the main pore, focusing on the designs with a gradually decreasing radius known as acoustic black hole profiles, to achieve broadband absorption. An analytical model based on the transfer matrix approach and lumped elements is proposed to predict the acoustic properties. Validation is conducted through thermo-visco-acoustic finite element simulations and impedance tube measurements. Multiple main pore profiles are investigated and the complex frequency plane representation is used to improve the balance of losses. An understanding of the influence of the main pore radius at the entrance and backing of the sample, allows design of a decreasing main pore radius profile that results in a high absorption coefficient value across a broad frequency range. Non-linear decreasing main pore profiles can be used, if necessary, to accentuate losses and achieve absorption peaks of similar amplitude.

Compact deep neural networks for real-time speech enhancement on resource-limited devices

In real-time applications, the aim of speech enhancement (SE) is to achieve optimal performance while ensuring computational efficiency and near-instant outputs. Many deep neural models have achieved optimal performance in terms of speech quality and intelligibility. However, formulating efficient and compact deep neural models for real-time processing on resource-limited devices remains a challenge. This study presents a compact neural model designed in a complex frequency domain for speech enhancement, optimized for resource-limited devices. The proposed model combines convolutional encoder–decoder and recurrent architectures to effectively learn complex mappings from noisy speech for real-time speech enhancement, enabling low-latency causal processing. Recurrent architectures such as Long-Short Term Memory (LSTM), Gated Recurrent Unit (GRU), and Simple Recurrent Unit (SRU), are incorporated as bottlenecks to capture temporal dependencies and improve the performance of SE. By representing the speech in the complex frequency domain, the proposed model processes both magnitude and phase information. Further, this study extends the proposed models and incorporates attention-gate-based skip connections, enabling the models to focus on relevant information and dynamically weigh the important features. The results show that the proposed models outperform the recent benchmark models and obtain better speech quality and intelligibility. The proposed models show less computational load and deliver better results. This study uses the WSJ0 database where clean sentences from WSJ0 are mixed with different background noises to create noisy mixtures. The results show that STOI and PESQ are improved by 21.1% and 1.25 (41.5%) on the WSJ0 database whereas, on the VoiceBank+DEMAND database, STOI and PESQ are improved by 4.1% and 1.24 (38.6%) respectively. The extension of the models shows further improvement in STOI and PESQ in seen and unseen noisy conditions.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion.

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

Robust H-Infinity Tracking Control for a Valve-Controlled Hydraulic Motor System with Uncertain Parameters in the Complex Load Environment.

A valve-controlled hydraulic motor system operating in a complex environment is subject to complex load changes. In extreme cases, the load can be regarded as a disturbance signal with complex frequency and strong amplitude fluctuations, which greatly affects the speed stability of the hydraulic motor and reduces the operating efficiency. In this paper, the structure of valve-controlled hydraulic motor systems is analyzed, and a valve-controlled hydraulic motor system model with uncertain parameters is established after considering the actual target parameter error and model linearization error. Different from the common H-infinity control, which regards the load disturbance as external disturbance, this paper presents a robust H-infinity tracking control strategy, which considers uncertain parameters and the load torque of the valve-controlled hydraulic motor system as internal disturbances. The simulation results show that the proposed control scheme has better control characteristics and robustness than the traditional PID control.