High Frequency Band Research Articles

Adaptive Non-Uniform Hybrid Beamforming for mmWave Train-to-Ground Communications in High-Speed Railway Scenarios

Higher frequency bands, especially millimeter wave (mmWave), are a key enabler for future high speed railway (HSR) wireless communication systems. Hybrid analog and digital beamforming (HBF) schemes operating at mmWave frequency bands can exploit the sparse nature of the channel by using large scale antenna arrays (LSAs) to shape multiple beams that overcome the severe path loss. However, due to the long and straight geometric characteristic of the rail track, there is a significant variance in the train-to-ground communication rate when the train is near or far from the BS. To address this issue, a novel hybrid beamforming scheme is developed based on a non-uniform beam design. We first model and investigate different beam design schemes in terms of spectral efficiency. Then, an efficient straight route based adaptively analog beamforming scheme with the non-uniform beam is proposed for HSR, which is designed to resist path loss. Finally, an adaptive non-uniform HBF algorithm based on the orthogonal matching pursuit method (OMP) is developed to minimize the rate gap between different locations by selecting the appropriate predefined beams. Numerical results show that the proposed hybrid beamforming scheme can efficiently ensure signal coverage, and reduce the variance of the train-to-ground communication rate along a straight track route.

Beyond Low-Pass Filtering: Graph Convolutional Networks With Automatic Filtering

Graph convolutional networks are becoming indispensable for deep learning from graph-structured data. Most of the existing graph convolutional networks share two big shortcomings. First, they are essentially low-pass filters, thus the potentially useful middle and high frequency band of graph signals are ignored. Second, the bandwidth of existing graph convolutional filters is fixed. Parameters of a graph convolutional filter only transform the graph inputs without changing the curvature of a graph convolutional filter function. In reality, we are uncertain about whether we should retain or cut off the frequency at a certain point unless we have expert domain knowledge. In this paper, we propose Automatic Graph Convolutional Networks (AutoGCN) to capture the full spectrum of graph signals and automatically update the bandwidth of graph convolutional filters. While it is based on graph spectral theory, our AutoGCN is also localized in space and has a spatial form. Experimental results show that AutoGCN achieves significant improvement over baseline methods which only work as low-pass filters.

Intelligent prediction of acoustic performance of landing gear using deep learning

Efficient prediction and evaluation of noise performance are crucial to the design and the optimization of landing gear noise. A systematic method is developed to predict and evaluate landing gear noise in the present study, termed as noise spectrum deep learning model (NSDL). In this algorithm, the encoder and decoder are designed to extract noise features and reconstruct noise data. Specifically, a loss function that takes the identification of both broadband noise and tone noise into account is utilized to guide the training direction of the model, aiming to improve the training efficiency and prediction results of the model. Afterward, the mapping relationship between landing gear experimental parameters and noise features is established by multi-layer perceptron. In this study, the detail of the algorithm is analyzed and discussed based on the results of wind tunnel noise experiment and numerical simulation. The results show that the proposed model can effectively and precisely predict landing gear noise under various conditions, including different flow speeds, angles of attack, number of wheels, and heights of the main strut. For the inflow velocity range of 34–75 m/s, the average error of the overall sound pressure level is restricted below 0.83% (0.6 dB). In case only the angle of attack is changed, the average error is reduced to be less than 0.36% (0.3 dB). The prediction results show that the landing gear noise is mainly broadband noise and tone noise mainly appears in the low frequency and intermediate frequency. With the increase in the inflow speed, the broadband noise increases gradually, and the frequency of tone noise gradually shifts to the high frequency band. Additionally, it is found that, for landing gear with four or six wheels, noise is very sensitive to angles of attack and wheel angles of attack. Consequently, the NSDL method shows significant potential in predicting the sound pressure level of landing gears and is expected to improve the efficiency of evaluation and optimization design for noise reduction of landing gear.

Frequency specificity of aberrant triple networks in major depressive disorder: a resting-state effective connectivity study.

Major depressive disorder (MDD) has been associated with aberrant effective connectivity (EC) among the default mode network (DMN), salience network (SN), and central executive network (CEN)-collectively referred to as triple networks. However, prior research has predominantly concentrated on broad frequency bands (0.01-0.08 Hz or 0.01-0.15 Hz), ignoring the influence of distinct rhythms on triple network causal dynamics. In the present study, we aim to investigate EC alterations within the triple networks across various frequency bands in patients with MDD. Utilizing a data-driven frequency decomposition approach and a multivariate Granger causality analysis, we characterized frequency-specific EC patterns of triple networks in 49 MDD patients and 54 healthy controls. A support vector machine classifier was subsequently employed to assess the discriminative capacity of the frequency-specific EC features. Our findings revealed that, compared to controls, patients exhibited not only enhanced mean EC within the CEN in the conventional frequency band (0.01-0.08 Hz), but also decreased mean EC from the SN to the DMN in a higher frequency band (0.12-0.18 Hz), and increased mean EC from the CEN to the SN in a sub-frequency band (0.04-0.08 Hz); the latter was significantly correlated with disease severity. Moreover, optimal classification performance for distinguishing patients from controls was attained by combining EC features across all three frequency bands, with the area under the curve (AUC) value of 0.8831 and the corresponding accuracy, sensitivity, and specificity of 89.97%, 92.63%, and 87.32%, respectively. These insights into EC changes within the triple networks across multiple frequency bands offer valuable perspectives on the neurobiological basis of MDD and could aid in developing frequency-specific EC features as potential biomarkers for disease diagnosis.

A stacked dielectric magnetoelectric dipole antenna with embedded ceramics

Abstract A stacked dielectric magnetoelectric antenna with embedded ceramics (SDMEA-EC) is proposed. The antenna consists of a stacked dielectric resonator antenna with embedded ceramics and a leaf-shaped electric dipole (LSE-Dipole). To improve the performance of the magnetoelectric dipole antenna, a novel verification method is proposed by analyzing equivalent magnetic currents to meet the Huygens element principle. The stacked structure with embedded ceramics is helpful for expanding the impedance bandwidth. The electric dipole not only participates in the radiation but also is a part of the feed network, which simplifies the overall structure. The prototype of the proposed stacked dielectric resonator magnetoelectric antenna achieves an impedance bandwidth of 20.2 % in the high frequency band (5.35 GHz–6.55 GHz) and 13.4 % in the low frequency band (4.54 GHz–5.19 GHz), an over 6.7 dBi gain, and a cross-polarization discrimination (XPD) of greater than 30 dB.

Numerical simulation and reduction of balance valve noise based on considering quadrupole and dipole in different frequency bands

The natural gas balance valve is an important part to control the flow of natural gas and use it safely. When the pressure before the valve is close to the upper limit, there will be a strong noise at this time. Aiming at the problem that the aerodynamic noise of the balance valve with large pressure drop is difficult to predict accurately, a new numerical simulation method of aerodynamic noise considering quadrupole and dipole in different frequency bands is proposed. The LES numerical method is used to obtain the pressure fluctuation and velocity fluctuation information. Taking the pulsation information as the source term, the dipole and quadrupole noises are solved respectively. The results show that the sound pressure level in the low-frequency band is mainly dominated by both quadrupole noise and dipole noise, while in the higher-frequency band, it is mainly dominated by dipole noise. The total sound pressure level is obtained by considering the quadrupole and dipole noises in different frequency bands, and it does not exceed 5% with the experimental error. Aiming at the problem that the total sound pressure level exceeds 85 dB(A) when the pressure drop is 0.30 MPa, the noise numerical simulation of the balance valve equipped with the noise reduction sleeve is carried out, and the hole diameter and hole distance of the sleeve are optimized. The total sound pressure level of the balance valve with optimized noise reduction sleeve is lower than 85 dB(A).

Dual-band filtering antenna with independent predetermined frequencies and low cross-polarization levels

In this paper, a dual-band filtering antenna with independent predetermined frequencies and low cross-polarization levels is proposed. In order to realize the independent control of frequencies of two radiation passbands, multiple additional structures such as rectangular slots, rectangular parasitic patches, and U-shaped slots are integrated on the traditional rectangular patch antenna to introduce new resonant modes and radiation nulls to the antenna, simultaneously. Since the generation structures of each frequency point are different, they do not affect each other. Besides, to obtain low cross-polarization levels, each additional structure is carefully adjusted such that the current vectors of each resonant frequency along the cross-polarized direction are quite weak. The proposed antenna operates in the 5G sub-6G (4050–4294 MHz) and the Wi-Fi 5.8 GHz band (5700–5920 MHz), and achieves cross-polarization levels of −37.8 and −36.8 dB in the lower and higher frequency bands, respectively. Compared to other multi-band filtering antennas, the proposed one has the advantages of independent predetermined frequencies and lower cross-polarization levels.

Discriminating rapid eye movement sleep from wakefulness by analyzing high frequencies from single-channel EEG recordings in mice

Rapid eye movement sleep (REMS) is characterized by the appearance of fast, desynchronized rhythms in the cortical electroencephalogram (EEG), similar to wakefulness. The low electromyogram (EMG) amplitude during REMS distinguishes it from wakefulness; therefore, recording EMG signal seems to be imperative for discriminating between the two states. The present study evaluated the high frequency components of the EEG signal from mice (80–500 Hz) to support REMS detection during sleep scoring without an EMG signal and found a strong positive correlation between waking and the average power of 80–120 Hz, 120–200 Hz, 200–350 Hz and 350–500 Hz. A highly negative correlation was observed with REMS. Furthermore, our machine learning approach demonstrated that simple EEG time-series features are enough to discriminate REMS from wakefulness with sensitivity of roughly 98 percent and specificity of around 92 percent. Interestingly, assessing only the higher frequency bands (200–350 Hz as well as 350–500 Hz) gives significantly greater predictive power than assessing only the lower end of the EEG frequency spectrum. This paper proposes an approach that can detect subtle changes in REMS reliably, and future unsupervised sleep-scoring approaches could greatly benefit from it.

Sound absorption behaviors of composite functionally graded acoustic structure under hydrostatic pressure

In order to overcome the drawback that the sound absorption band of traditional cavity resonance acoustic structure is narrow, the functionally graded acoustic structure (FGAS) is designed in this paper according to the characteristics of functionally graded materials (FGMs), and considering the nonlinear term of the structural strain, the graded finite element method (G-FEM) is employed to establish the acoustic calculation model for FGAS under hydrostatic pressure. The correctness of the established acoustic calculation model is verified. Three types of composites functionally graded acoustic structures (CFGAS) are designed based on geometric gradient and material property gradient. The underwater sound performance of CFGAS is investigated, and the effects of hydrostatic pressure on the sound absorption performance of CFGAS are also studied. Numerical results indicate that, the convergence and accuracy of the underwater acoustic calculation model based on the G-FEM are superior than that based on the traditional FEM. In addition, the CFGAS can improve the sound absorption performance of traditional cavity acoustic structure in the middle and high frequency bands, and its acoustic performance is insensitive to pressure, which makes FGMs have potential applications in deep water environment.

Damping vibration in three-dimensional helically tapered rod with power-law thickness

Tapered structure with power-law thickness has been regarded as an efficient approach for passive vibration control. In this paper, a three-dimensional (3D) helically tapered structure with power-law feature is proposed to reduce the vibration in rod structure. Compared with the traditional one-dimensional tapered rod, this novel helix type can compress the length of tapered region by the coiling configuration and transform the flexural wave modes into various complex modes. Drawing on its folded characteristic, the helix structure can generate highly efficient space utilization and achieve a superior vibration attenuation effect in the low frequency band. Numerical investigations on the dynamics and reflection coefficient analyses of different rod types are conducted and the reduction mechanism of 3D helically tapered rod is revealed. It is found that the larger helical radius can contribute to the reduction in low-frequency vibration, but it deteriorates the vibration performance in broadband. To overcome this “sudden collapse” phenomenon in the damping performance caused by the low stiffness, the novel step helically tapered rod is presented. According to the introduced multimodal and multidirectional mode-coupling effects, this step 3D helix structure not only makes up for the shortcoming of vibration control in low frequency range, but also further improves the innate advantage of reduction effect in the high frequency band. Meanwhile, parametric studies of step 3D helically tapered rod are performed and the effect of different constituent materials is discussed to expand the utility. To validate these analyses, experimental tests are also conducted. The results demonstrate its remarkable vibration reduction in broadband frequency.

The Performance of Hollow Pyramidal Microwave Absorber Using Different Slot Size

Microwave absorber is commonly used in defense, electromagnetic compatibility (EMC)/electromagnetic interference (EMI) reduction and anechoic chamber application. Conventional electromagnetic (EM) absorbers have some constraints in practical handling due to its heavy weight. In this paper, the research focuses on the development of high performance and lightweight microwave absorber. A lightweight and simple design configuration of different rectangular slot size array implemented on hollow pyramidal microwave absorber are carried out. There are two different designs of different slot size array calculated based on 3GHz, 6GHz and 9GHz frequency slot size. The two designs have an opposite slots size array arrangement namely different slot size 3GHz, 6GHz, 9GHz order design array and different slot size 9GHz, 6GHz, 3GHz order design array. The absorption is measured using the Naval Research Laboratory (NRL) arch free space method in the frequency range of 1GHz to 12GHz covering L, S, C and X band. In the measurement result, the maximum absorption performance is obtained by the different slot size 9GHz, 6GHz, 3GHz order design array which is up to -44.23dB at X-band. The measurement results for both designs array show good absorption performance which exhibit below than -20dB especially at high frequency band. The proposed designs have been identified as the new approach to increase absorption improvement over a broad frequency range application.

Broadband vibration analysis of the periodically stitched sandwich structure

This work presents a numerical study on the broadband vibration analysis of periodically stitched sandwich structures. Firstly, a finite element model is established for the unit cell, which is capable of taking into account different layers of the sandwich as well as the stitches. The wave propagation properties in different frequency bands are analyzed by applying the Floquet-Bloch theorem to the boundaries of the unit cell. Then, the influences of structural design parameters on broadband dynamic responses are discussed. Simulation results indicate that the dynamic response of the investigated stitched sandwich structure is mainly determined by various far-field propagating waves. Structural dynamic response in the medium and high frequency ranges is significantly suppressed by introducing stitches into the sandwich structure. A more intensive stitch density, larger stitch diameter, and thicker core layer could make the resonance peaks of structural displacement response shift towards lower frequency, which is more obvious in the medium and high frequency bands.

Recovery of autonomic dysfunction assessed by heart rate variability after acute stroke: an observational study

Background/Aims Early mobilisation after acute stroke is recommended worldwide, and autonomic dysfunction is a common complication. However, little is known about autonomic adaptation to postural changes during early mobilisation. The aim of the present study was to clarify the recovery process of autonomic dysfunction after acute stroke regarding adaptation to postural changes. Methods A total of 23 patients (mean age 60 ± 11 years) were included in this observational study. Those with a history of arrhythmia, heart failure and diabetes mellitus with diabetic neuropathy were excluded. Autonomic functions were assessed using heart rate variability. Electrocardiograms were recorded when the patients were lying down, sitting up and then lying back down again for 5 minutes in each position. The indices of heart rate variability, total power, high frequency band and low frequency/high frequency ratio, which correspond to total, parasympathetic and sympathetic activity respectively, were calculated to evaluate participants' autonomic function during the 7 days after the onset of stroke. Results From days 3 to 7 after stroke onset, the low frequency/high frequency ratio was significantly higher, and the total power and high frequency were reciprocally lower in the sitting position than in the supine position. On day 7, the total power and high frequency were significantly higher while the low frequency/high frequency ratio was lower than the total power and high frequency values that were recorded on the second and third days after stroke. Blood pressure levels and heart rates also changed, corresponding to changes in heart rate variability. Conclusions The patients' autonomic function, as well as heart rate and blood pressure levels, changed gradually during the first 7 days after onset of stroke. The results could lead to optimised clinical practice in safe early stroke rehabilitation.

Triple-Mode Resonant Backfire Feed Antenna With Nearly Equal E-/H-Plane and Inverse Taper Patterns

A design approach to triple-mode resonant feed antenna is advanced for parabolic reflector antenna system. The antenna is composed of a slit-loaded 1.5-wavelength sectorial dipole, a primary corner reflector, and an auxiliary circular reflector. The corner angle of the primary reflector can be employed to properly narrow down the H-plane half-power beam width (HPBW), while with the E-plane one rarely affected, such that equal E-/H-plane characteristic can be yielded accordingly. Unlike traditional backfire antennas, high-order resonant modes are employed to generate inverse taper radiation pattern at the high frequency band. As is numerically demonstrated and experimentally validated, the corner angle of the primary reflector can be eventually determined as 45∘. The antenna exhibits an impedance bandwidth of up to about 79.7%, and a nearly equal E-/H-plane bandwidth of up to about 29.4%. Average HPBW of about 65∘, gain up to about 8.9 dBi with fluctuation of less than 1 dB can be achieved within the equal E-/H-plane bandwidth. The inverse taper radiation pattern at the high frequency band can provide effective edge illumination and uniform aperture field distribution in parabolic reflector antenna systems.

A Novel Wave-transparent and Decoupling Antenna in Broadband Dual-Polarized Dual-Band Shared-Aperture Array

This letter proposes a novel wave-transparent and decoupling antenna in broadband dual-Polarized dual-Band shared-aperture array. It is implemented by kneading the double-arrow-stripe-shaped decoupling structure into the frequency-selective metasurface structure of the lower-frequency-band (LFB) antenna arms. The LFB antenna radiator can be considered as frequency-selective and array-decoupling metasurface for the higher-frequency-band (HFB) antennas placed below. The antenna system consists of 2 × 2 HFB (3.3-3.6 GHz)arrays and an LFB (1.65-2.58 GHz) antenna with wave-transparent and decoupled structure (WTDS). Then, without adding additional structure, the in-band isolations of co- and cross-polarized between HFB antennas are promoted to >25.6 dB and >25 dB only by relying on the antenna form of the LFB antenna itself.

Foreground separation and constraints on primordial gravitational waves with the PICO space mission

PICO is a concept for a NASA probe-scale mission aiming to detect or constrain the tensor to scalar ratio r, a parameter that quantifies the amplitude of inflationary gravity waves.We carry out map-based component separation on simulations with five foreground models and input r values r in = 0 and r in = 0.003. We forecast r determinations using a Gaussian likelihood assuming either no delensing or a residual lensing factor A lens = 27%. By implementing the first full-sky, post component-separation, map-domain delensing, we show that PICO should be able to achieve A lens = 22% – 24%. For four of the five foreground models we find that PICO would be able to set the constraints r < 1.3 × 10-4 to r < 2.7 × 10-4 (95%) if r in = 0, the strongest constraints of any foreseeable instrument. For these models, r = 0.003 is recovered with confidence levels between 18σ and 27σ. We find weaker, and in some cases significantly biased, upper limits when removing few low or high frequency bands. The fifth model gives a 3σ detection when r in = 0 and a 3σ bias with r in = 0.003. However, by correlating r determinations from many small 2.5% sky areas with the mission's 555 GHz data we identify and mitigate the bias. This analysis underscores the importance of large sky coverage. We show that when only low multipoles ℓ ≤ 12 are used, the non-Gaussian shape of the true likelihood gives uncertainties that are on average 30% larger than a Gaussian approximation.

An Equivalent Differential Method for Active Damping of LCL Type Grid-Connected Inverters With Grid-Side Inductor Voltage Feedback

The differential feedback of grid-side inductor voltage can effectively increase the damping of grid-connected inverters. Compared with proportional feedback of filter capacitor current, it can increase the output impedance of the grid-connected inverters, which is more conducive to reducing the impact of grid background harmonics on the grid-in current. However, there is a differential link in its damping branch, and existing differential implementation methods were pursuing to be ideal differential in wide frequency band, as a consequence, high frequency components of the grid-side inductor voltage are amplified, which affects the quality of grid-in current. For this reason, this paper proposes a band-pass filter in series with a lead compensation unit to replace the ideal differential. The major advantage different from the conventional methods is that the proposed strategy is only equivalent to the ideal differential around the resonance frequency band. Specifically, it features lower gain at low and high frequency bands. Finally, theoretical analysis and experiment results show that the proposed strategy can reduce the influence of the added damping path on the fundamental component of grid-in current and features better suppression effect on the harmonics.

Measurement of Currents Induced on Overhead Line From Very Close Natural and Triggered Lightning

Induced currents on an overhead line due to a natural lightning with a strike distance of 20 m and a triggered lightning with a strike distance of 8 m were analyzed in time domain, frequency domain, and time–frequency domain, respectively. The results indicated that the energy peak of most currents lagged far behind its time-domain peak. The arrival time of different frequency components of the induced currents varies nonmonotonically with respect to frequency, especially that of the currents caused by the natural lightning. The energy of the highest frequency component is not necessarily the strongest. When it occurs earlier than the time-domain current peak, the protection measures designed by referring the time-domain threshold voltage may not start as expected, and accidental damage that may be incurred owing to that energy in the higher frequency band is more easily coupled to the terminal. Compared with the natural lightning, the smaller time delay between current peak and its energy maxima is yielded in the triggered lightning, as well as the shorter duration time of both time-domain current profile and energy distribution. Additionally, the consistency among currents induced by the triggered lightning is better than that excited by the natural lightning.

Heart rate variability is associated with global cognition and cognitive impairment: Results from the einstein aging study

AbstractBackgroundThe role of autonomic function in cognitive impairment is unclear. Heart rate variability (HRV) provides a feasible, non‐invasive measure of cardiac autonomic control. We examined whether HRV is associated with global cognition and cognitive impairment using 7‐day ambulatory ECG assessments in the Einstein Aging Study.MethodAnalyses included 84 participants free of dementia (mean age 78.1 (± 5.2) years; 82% female; 39% non‐Hispanic White, 44% non‐Hispanic Black, mean education 14.7 (± 3.3 years)). Participants wore a small, single lead ECG device continuously over 7 days. Power spectral analyses were applied to determine HRV over 5‐minute epochs in the high (0.15‐0.40 Hz, hfHRV) and low (0.04‐0.15 Hz, lfHRV) frequency bands, and values were log transformed. Demographics, comorbidities, and cognition were assessed during clinic visits. Two tests per domain evaluated memory, executive function, language, visuo‐spatial and attention with impairment defined as performance ≥ 1.5 SD below the age, sex, education standardized norm on at least one test. Global cognition was assessed using the MoCA, and Mild Cognitive Impairment (MCI) was defined using Jak‐Bondi criteria. Associations of HRV with cognition were examined using logistic (MCI, domain specific impairment) and linear (MoCA) regression adjusting for age, sex, race/ethnicity and education, diabetes, and hypertension.ResultMean MoCA score was 24 (SD 3.5), 21 (25%) had MCI and impairment was identified in 25% (memory), 28.6% (executive function), 21.4% (attention), 17.9% (language) and 27.4% (visuospatial). Both hfHRV and lfHRV were inversely associated with MCI (OR per 1 SD increase in log HRV: 0.47, p = 0.02 for hfHRV and 0.42, p = 0.01 for lfHRV) and with memory impairment (OR per 1 SD increase in log HRV: 0.52, p = 0.03 for hfHRV and 0.43, p = 0.01 for lfHRV). Higher hfHRV and lfHRV were each associated with better MoCA score (β for 1 SD increase in log HRV: 0.65, p = 0.046 for hfHRV; 0.80, p = 0.02).ConclusionWhether HRV is associated with cognitive impairment remains unclear, particularly the role of hfHRV, indicative of parasympathetic control. Findings suggest that hfHRV is related to cognitive performance and impairment. Results for lfHRV also suggest joint effects of the sympathetic and parasympathetic systems.

Efficient reconfigurable architecture to enhance medical image security

Medical images are one of the most critical and sensitive types of data in information systems. For the secure storage and transfer of medical images, confidentiality is the most important aspect. This paper presents efficient embedding technique to enhance medical image security. The Gaussian filters are used as preprocessing to remove high frequency components and then applied to cumulative distribution function (CDF) 5/3 wavelet to obtain LL band features. Similarly, the LL band features of cover image are obtained. The alpha bending technique combines both the LL band features of cover and secret image to form first level of encryption and now with other high frequency bands of LH, HL and HH applied to Inverse CDF 5/3 obtains an encrypted image which is then transferred along with key obtained through other bands of LH, HL and HH. The key generated acts as additional level of security and similarly, at the receiver the opposite operations are performed to obtain the original secrete image. The performance is measured in terms of Peak signal-to-noise ratio (PSNR) and is compared with existing techniques to validate the results. Further, the entire architecture is synthesized on spartan 6 field-programmable gate array (FPGA) board to compare the hardware utilizations.