• The Journal of the Acoustical Society of Korea
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• v.32 no.2
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• pp.116-123
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• 2013

This paper represents generation of time-varying underwater acoustic channels by performing scattering simulation with time-varying ocean surface and Kirchhoff approximation. In order to estimate the time-varying ocean surface, 1D Pierson-Moskowitz ocean power spectrum and Gaussian correlation function were used. The computed scattering coefficients are applied to the amplitudes of each impulse of BELLHOP simulation result. The scattering coefficients are then compared with measured doppler spectral density of signal components which were scattered from ocean surface and the correlation time used in the Gaussian correlation function was estimated by the comparison. Finally, bit-error-rate and channel correlation simulations were performed with the generated time-varying channel based on passive time-reversal communication scenario.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.19 no.10
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• pp.1175-1184
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• 2008

Delay-sum back projection(DSBP) algorithm and the time reversal algorithm based on the finite-difference time-domain method are compared. The two algorithms, which operate in the time domain, can process the ultra-wideband (UWB) radar data to generate images that are close to the original location and shape of the target. For the experiment, the UWB radar consists of a network analyzer, a resistive V dipole antenna, a scanner, and a control computer. The radar aperture is synthesized by linearly scanning the antenna. A calibration procedure is applied to the measured data to remove signal distortion and clutter. The two algorithms are applied to the same data on the same platform. It is shown that the DSBP algorithm produces better images but takes longer time to produce the images than the FDTD-TR algorithm.

• Journal of the Institute of Electronics and Information Engineers
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• v.52 no.5
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• pp.66-73
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• 2015

IFFT is one of the key components in OFDM-based communication systems. In this paper, we propose a new memory efficient IFFT design method for OFDM-based communication systems, based on a mapping of three IFFT input signals which consist of modulated data, pilot and null signals. The proposed method focuses on reducing the memory size in the bit-reversal block which requires the largest number of memory cells in IFFT architectures. To reduce the memory size, we propose a selection mapping method based on decimation-in-time (DIT) algorithm. It is shown that the proposed method achieves a memory reduction of about 50% compared to conventional methods.

• Journal of Magnetics
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• v.11 no.2
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• pp.61-65
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• 2006

The physical origin of complex dynamic domain configuration in nonequilibrium magnetic systems with mesoscopic length scales has been studied. An increasing complexity in the spatial feature of the evolution is found to accompany the increasing reversal speed, when a ferromagnetic element is driven by progressively faster switching fields applied antiparallel to the initial magnetization direction. As reversal rates approach the characteristic precession frequencies of spin fluctuations, the thermal energy can boost the magnetization into local configurations which are completely different from those experienced during quasistatic reversal. The sensitive dependence of the spatial pattern on switching speed can be understood in terms of a dynamic exchange interaction of thermally excited spins; the coherent modulation of the spins is strongly dependent on the rise time of switching pulses.

• Journal of Magnetics
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• v.17 no.2
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• pp.124-128
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• 2012

Many different converter topologies have been developed with a view to use the minimum number of switches in order to reduce construction costs. Among this research, the four-switch converter topology with a novel PWM control technique based on the current controlled PWM method is thought to be a good solution. In this paper, a two dimensional time-stepped voltage source finite-element method (FEM) is used to analyze the characteristics of a Flux-Reversal Machine (FRM) with a 4-switch converter. To validate the proposed computational method, a digital signal processor (DSP) installed controller and prototype FRM are built and experiments performed.

• Journal of Magnetics
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• v.19 no.3
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• pp.237-240
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• 2014

For heat-assisted magnetic recording, magnetization reversal probabilities of nano-Pt/MnSb multilayer film with perpendicular magnetic anisotropy under thermal pulse activation were investigated numerically by solving the Landau-Lifshift Gilbert Equation. Magnetic parameters of nano-Pt/MnSb multilayer were used with anisotropy energy of $3{\times}10^5$ erg/cc and saturation magnetization of 2100 G, which offer more than 10 y data stability at room temperature. Scheme of driven magnetic field and thermal pulse on writing mechanism was designed closely to real experiment. This study found that the chosen material is potential to be used as a high density magnetic storage that requires low writing field less than two-hundreds Oersted through definite heating and cooling interval. The possibility of writing data with a zero driven magnetic field also became an important result. Further study is recommended on the thickness of media and thermal pulse design as the essential parameters of the reversal magnetization.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
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• 2004.09a
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• pp.195-198
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• 2004

This study applied EK method to remediate contaminated soil by heavy metal(Pb), tried to increase the efficiency of remediation using zeolite as enhanced EK remediation method to overcome the limit of traditional EK remediation method. Adsorption tests on Pb were conducted to apply the EK extraction and test conditions which contained the electrode reversal, operating time were established. After tests, lead-ions were transported into the specific position of zeolite by EK phenomena, then the immobilization of lead-ions at zeolite was developed. Based on these tests, the efficiency of remediation is different with its test conditions. In addition, the efficiency of remediation was improved dramatically by adding zeolite without electrode reversal and satisfied TCLP regulation of EPA in USA through the whole sample range. Finally, absorption and immobilization capacity of zeolite were proved its excellence and confirmed the possibility of application as enhanced EK remediation method.

• Proceedings of the KIEE Conference
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• summer
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• pp.866-868
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• 2004

The flux-reversal machine(FRM) is a new brushless doubly-salient permanent-magnet machine combining the advantages of the switched-reluctance machine(SRM) and the permanent-magnet machine(PMM) into one machine. FRM has a naturally low inductance, therefore, a low electrical time constant. This feature, combined with its simple construction and low rotor inertia appear to make the FRM attractive as a low-cost high-speed machine. For high-speed applications, two alternative commutation strategies are studied, one using the phase commutation advancing technique and another using the conducting pulse-width control. This paper describes the techniques and reports the corresponding simulated and experimented performance

• Journal of Magnetics
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• v.18 no.1
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• pp.50-56
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• 2013

In this paper, the 6-switch inverter for the Flux-Reversal Motor (FRM) has been presented and compared to the 4-switch inverter for the FRM, which is more popular in cost effective applications. To analyze the FRM, we adopted the two-dimensional time-stepped voltage source finite element method (FEM) that uses the actual pulse width modulation (PWM) voltage waveforms as the input data. As the FRM characteristic analysis of actual pwm voltage input, the torque ripples and iron losses (eddy current and hysteresis loss) of the FRM can be precisely calculated. With the simulated and experimental results, the performance and limitations of the 4-switch FRM which is the cost effective drive compared to the 6-switch FRM drive are provided in more detail.

• The Journal of the Acoustical Society of Korea
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• v.35 no.5
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• pp.375-382
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• 2016

This paper presents the results for the performance improvement of underwater communication in a passive time reversal mirror (PTRM) using ray-based blind deconvolution (RBD). In conventional PTRM, the signal to be recovered is found from matched-filtering the received probe signal. However, the communication performance is degraded because the time-varying impulse response for each data frame is not reflected in the received probe signal. In this study, the time-variant transfer function is estimated from each received data frame using RBD, and the estimated time-variant transfer function is then used to recover the data signal using PTRM. The results from the experimental data show that the suggested method improves the communication performance when comparing with the conventional PTRM.