• Steel and Composite Structures
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• 제50권4호
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• pp.429-441
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• 2024

This paper studies the free vibration behavior of trapezoidal shaped coupled double-layered graphene sheets (DLGS) system using first-order shear deformation theory (FSDT) and incorporating nonlocal elasticity theory. Two nanoplates are assumed to be bonded by an interlayer van der walls force and surrounded by an external kelvin-voight viscoelastic medium. The governing equations together with related boundary condition are discretized using a mapping-differential quadrature method (DQM) in the spatial domain. Then the natural frequency of the system is obtained by solving the eigen value matrix equation. The validity of the current study is evaluated by comparing its numerical results with those available in the literature and then a parametric study is thoroughly performed, concentrating on the series effects of angles and aspect ratio of GS, viscoelastic medium, and nonlocal parameter. The model is used to study the vibration of DLGS for two typical deformation modes, the in-phase and out-of-phase vibrations, which are investigated. Numerical results indicate that due to Increasing the damping parameter of the viscoelastic medium has reduced the frequency of both modes and this medium has been able to overdamped the oscillations and by increasing stiffness parameters both in-phase and out-of-phase vibration frequencies increased.

• Journal of Communications and Networks
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• 제16권3호
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• pp.280-292
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• 2014

This article presents a novel cooperative spectrum sharing (CSS) scheme. The primary transmitter transmits a complex Quadrature amplitude modulation (QAM) signal in the first phase, and CSS occurs in the second phase. The secondary transmitter with the largest forwarding channel gain among the nodes that successfully decode the primary signal in the first phase is selected for CSS. This selected node employs a pulse-amplitude modulation (PAM) signal for primary information message (IM) instead of the QAM signal, and it employs a modified PAM signal for the secondary IM. The proposed modified PAM signal depends on the amplitude of the primary PAM signal. This method results in no mutual interference and negligible primary interference constraint and allows a higher degree of exploitation of spatial diversity, thus enabling increase in secondary power to improve primary transmission. The outage performance is enhanced in both the primary and secondary systems. The critical region, in which the primary outage performance is enhanced with the proposed CSS scheme, can be adjusted and widened by varying either the modulation cooperation sharing factor or the number of secondary transmitters.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.322-326
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• 2008

In this paper, we have studied the images which have been reconstructed by using combination of images acquired by the variation of operating frequency. When inner images have been reconstructed, they have been superposed by the surface state effect. In this case, the images of the phase object can be enhanced by the contrast of inner images. There is a kind of specimen, one is a reference block having 1/4T, 1/2T, 3/4T side drilled holes as main run piping material of the steam generator in nuclear power plants. It has been shown that the two results of defect shapes have better than before in this processing and phase contrast grow about twice. And we have constructed the acoustic microscope by using a quadrature detector that enables to acquire the amplitude and phase of the reflected signal simultaneously. Further more we have studied the reconstruction method of the amplitude and phase images, the enhancement method of the defect images' contrast.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집B
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• pp.176-180
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• 2001

In this paper, we have studies the images have been reconstructed by using combination of images which has been acquired by the variation of operating frequency. When inner images has been reconstructed, inner image has been superposition by surface state effect. In this case, image enhancement the phase object and enhance the contrast of inner image. In the result of the specimen for the round defect with 2mm diameter, for the types of the depth are 1.5mm, 2mm, 2.5mm, and 3mm, it has been show that the shape of defect has better than before this processing and phase contrast grow large twice. And we have constructed the acoustic microscope by using quadrature detector that is able simultaneously to acquired the amplitude and phase of the reflected signal. Father more we have studied the reconstruction method of the amplitude and phase images and the enhancement method of the defect images' contrast.

• 한국정보통신학회논문지
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• 제20권7호
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• pp.1229-1236
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• 2016

수중 음향 다중경로 채널에서 수중음향 통신 시스템의 성능은 채널의 시변적 경계면 상태 변동에 의해 영향을 받는다. 이러한 채널에서 시공간적 변동에 의해 송신 신호와 수신 신호의 위상과 주파수가 일치하지 않아 반송 주파수 옵셋이 발생되고 위상편이키잉 방식의 수중음향통신시스템의 성능을 저하시킨다. 본 논문에서는 수중 음향 통신 채널의 시 공간적 변동 채널에서 위상 코드를 적용한 반송 주파수 옵셋 보상 추정 기법의 성능을 평가하였다. 위상 코드는 주파수 옵셋을 추정하고 보상하기 위한 코드로 CAZAC를 적용하였으며, 실내 수조에서 성능을 평가하였다. QPSK 시스템에 위상코드를 적용한 결과는 적용하지 않은 경우보다 약 4-6배 비트오류율이 개선되었다.

• 대한음성학회지:말소리
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• 제66호
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• pp.73-86
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• 2008

Beamforming is one of the spatial filtering techniques which extract only desired signals from noisy environments using microphone arrays. Fixed beamforming is a simple concept and easy to implement. However, it does not show good performance in real noisy conditions. As an adaptive beamforming, Frost algorithm can be a good candidate. It uses the concept of the linearly constrained minimum variance (LCMV) algorithm. The difference between the Frost and the LCMV algorithm is the error correction scheme which is very effective feature in the aspect of performance. In this paper, as quadrature mirror filtering (QMF)-based filterbank is utilized as the pre-processing of the Frost beamformning, the filter length and the learning rate of each band is optimized to improve the performance. The performance is measured by the signal-to-noise ratio (SNR) and the Bark's scale spectral distortion (BSD).

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2003년도 The Fifth Asian Computational Fluid Dynamics Conference
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• pp.61-62
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• 2003

In this paper, the new higher order wall boundary conditions are proposed for solving the incompressible flows. The square driven cavity flows are simulated by using the variable order method of lines with the present wall boundary conditions. The variable order method of lines is constructed by the spatial discretization, i.e., the variable order proper convective scheme for convective terms and the modified differential quadrature method for diffusive terms, and time integration. The 2nd, 4th, and 6th order solutions are presented and these results show this higher order boundary conditions are very promising for the incompressible flow simulations.

• 한국군사과학기술학회지
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• 제11권3호
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• pp.19-29
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• 2008

Underwater acoustic(UWA) communication is one of the most difficult field in terms of severe channel environments such as multipath propagation, high temporal and spatial variability of channel conditions. Therefore, it is important to model and analyze the characteristics of underwater acoustic channel such as multipath propagation, transmission loss, reverberation, and ambient noise. In this paper, UWA communication channel is modeled with a ray tracing method and applied to image transmission. Quadrature phase shift keying(QPSK) and multichannel decision feedback equalizer(DFE) are utilized as phase-coherent modulation method and equalization technique, respectively. The objective is to improve the performance of the image transmission using vertical sensor array instead of single sensor in the viewpoint of bit error rate(BER), constellation diagram, and received image quality.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2004년도 ICEIC The International Conference on Electronics Informations and Communications
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• pp.110-113
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• 2004

Underwater acoustic(UWA) communication is one of the most difficult field because of several factors such as multipath propagation, high temporal and spatial variability of channel conditions. Therefore, it is important to model and analyze the characteristics of underwater acoustic channel such as multipath propagation, transmission loss, reverberation, and ambient noise. In this paper, UWA communication channel is modeled with a ray tracing method and applied to image transmission. Quadrature phase shift keying(QPSK) and multichannel decision feedback equalizer(DFE) are utilized as phase-coherent modulation method and equalization technique, respectively. The objective is to improve the performance of vertical sensor array than that of single sensor in the viewpoint of bit error rate(BER), constellation output, and received image quality.

• The Journal of the Acoustical Society of Korea
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• 제29권1E호
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• pp.11-27
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• 2010

Nowadays, ultrasound Doppler imaging is widely used in assessing cardiovascular functions in the human body. However, a major drawback of ultrasonic Doppler methods is that they can provide information on blood flow velocity along the ultrasound beam propagation direction only. Thus, the blood flow velocity is estimated differently depending on the angle between the ultrasound beam and the flow direction. In order to overcome this limitation, there have been many researches devoted to estimating both axial and lateral velocities. The purpose of this article is to survey various two-dimensional velocity estimation methods in the context of Doppler imaging. Some velocity vector estimation methods can also be applied to determine tissue motion as required in elastography. The discussion is mainly concerned with the case of estimating a two-dimensional in-plane velocity vector involving the axial and lateral directions.