• The Journal of Information Technology
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• v.6 no.4
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• pp.1-9
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• 2003

This paper is about power line communication(PLC) over the low power voltage grid. The main advantage with power line communication is the use of an existing infrastructure. The PLC channel can be modeled as having multi-path propagation with frequency-selective fading, typical power lines exhibit signal attenuation increasing with length and frequency. OFDM(Orthogonal Frequency Division Multiplexing) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The performance in consideration of the multi-path(echoes) powerline scheme is analyzed and verified by computer simulation.

• Korean Journal of Optics and Photonics
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• v.8 no.6
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• pp.450-455
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• 1997

A pulsed Ti:sapphire laser with a Z-folded cavity, which was pumped by a freqeuncy-doubled Nd:YAG laser, was developed. It shows 120 nm wavelength tuning range from 740 nm to 860 nm with 90 nm FWHM under a pumping energy of 3 mJ with 18% output coupler. Using a birefringent filter, the effective efficiency was improved about 10 times compare to without a birefringent filter. The output energy obtained was $\365mu$Jwith 4 nm FWHM. And the wavelength tuning range was broadened to about 160 nm within an output energy fluctuation of $\pm$10%.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.19 no.5
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• pp.815-821
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• 1994

Short-term fading in a mobile radio channel is mainly caused by multipath reflected wave by local scatterers or natural obstacles surrounding a mobile unit. In this paper, we analyze probability density function(PDF), level crossing rate(LCR) and widths and depths of fades for the measured field strength data in Seoul and Daejeon areas. As a result, the satatistical distribution of signal variations within any small area with the constant mean value becomes the Rayleigh distribution The variation of fade depths is small regardless of the environment such as urban, suburban. etc. However, the variation of fade widths is large with the change of circumstantial structures and environment. the maximum level crossing rate is obtained near the mean value.

• Geophysics and Geophysical Exploration
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• v.3 no.1
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• pp.7-12
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• 2000

The paper discusses a data processing methodology that derives a three dimensional porosity volume information from the 3-D seismic dataset. The methodology consists of preprocessing and inversion procedures. The purpose of the preprocessing is balancing the amplitudes of seismic traces by using reflectivity series derived from sonic and density logs. There are eight sonic logs are available in the study area; therefore, we can compute only 8 balance functions. The balance function for every seismic trace was derived from these 8 balance functions by kriging. In order to derive a wide-band acoustic impedance --similar to the one can be derived from a sonic log- from a band-limited reflection seismogram, we need to recover missing low- and high-frequency information of the seismic trace. For that Purpose we use the autoregressive method.

• Geophysics and Geophysical Exploration
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• v.14 no.1
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• pp.25-41
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• 2011

AVO analysis was conducted on hydrocarbon-bearing structures by applying the crossplot and offset-coordinate amplitude polynomial techniques. To evaluate the applicability of the AVO analysis, it was conducted on synthetic data that were generated with an anticline model, and field data from the hydrocarbon-bearing Colony Sand bed in Canada. Analysis of synthetic data from the anticline model demonstrates that the crossplot method yields zero-offset reflection amplitude and amplitude variation with negative values for the upper interface of the hydrocarbon-bearing layer. The crossplot values are clustered in the third quadrant. The results of AVO analysis based on the coefficients of the amplitude polynomial are similar to those from the crossplots. These well correlated results of AVO analysis on field and synthetic data suggest that both methods successfully investigate the characteristics of the reflections from the upper interface of a hydrocarbon-bearing layer. Analysis based on the incident-angle equation facilitates the application of various interpretation methods. However, it requires the conversion of seismic data to an incident angle gather. By contrast, analysis using coefficients of the amplitude polynomial is cost-effective because it allows examining amplitude variation with offset without involving the conversion process. However, it warrants further investigation into versatile application. The two different techniques can be complement each other effectively as AVO-analysis tools for the detection of hydrocarbon reservoirs.

• Korean Journal of Optics and Photonics
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• v.25 no.1
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• pp.38-43
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• 2014

An external cavity laser supporting two wavelengths is demonstrated by incorporating polymer waveguide Bragg reflectors and a superluminescent light-emitting diode. An oversized rib waveguide structure and Bragg gratings are designed by using the effective-index and transmission-matrix methods. Bragg gratings with different periods are inscribed on a polymer waveguide through double-exposure laser interferometry. In order to tune the cavity loss affected by the reflectivity of Bragg gratings, a Bragg reflectors with varying length is incorporated. Two-wavelength-mode lasing is achieved for the device consisting of 2-mm long, 537-nm period gratings and 2.2-mm long, 540-nm period gratings; the lasing wavelengths are 1554 nm and 1564 nm, with an output power close to 0 dBm, a 20-dB bandwidth of 0.2 nm, and a side-mode suppression ratio of 45 dB.

• Proceedings of the Optical Society of Korea Conference
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• 2000.08a
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• pp.62-63
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• 2000

다양한 분야의 요구에 따라 반사율이 99.995% 이상인 고반사 저손실 반사경의 제작이 가능해 지면서 반사경의 산란, 투과, 흡수 및 손실 등을 측정하는 여러 측정 방법들도 더불어 연구되어 왔다. 적용 분야에 따라 사용될 반사경의 특성이 결정되는데 그 중 중력파 측정 장치$^{(1)}$ , 광자 감쇠 분광기$^{(2)}$ 등의 분야에서는 투과, 산란 및 흡수가 모두 작은 저손실 반사경을 요구한다. 한편 링 레이저 자이로스코프의 경우 반사경의 산란이 곧 lock-in을 결정하여 성능을 제한하는 중요한 변수가 된다. 따라서 이 응용의 경우 반사경의 산란을 정확히 측정하는 일은 중요하며 위치에 따른 산란분포 정보를 알면 링 레이저 자이로스코프의 성능 예측과 개선이 더욱 쉬워진다. 산란을 측정하는 직접적인 방법에는 ARS(angle resolved scattering)과 TIS(total integrated scattering)이 있는데, 본 연구에서는 TIS 측정 장비를 반사경의 위치에 따른 산란분포까지 측정할 수 있도록 mapping 기능을 첨가하고 이온빔 스퍼터링에 의해 제작된 저산란 반사경을 측정하기 위해 높은 분해능을 갖도록 구성하였다. 반사경의 응용분야의 필요에 의해 45$^{\circ}$ 산란을 측정하였다. (중략)

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.9
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• pp.1113-1118
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• 2011

In order to evaluate the characteristics of solid-solid contact interfaces, reflection or transmission techniques involving normal-incidence longitudinal waves are generally used. However, these normal-incidence techniques are of limited use in field applications such as in the inspection of welded parts. The oblique-incidence ultrasonic technique may be an alternative for overcoming these problems. However, in this technique, the mode conversion at the contact interfaces should be taken into account along with the normal and tangential interface stiffness. In this study, we have suggested a theoretical model for obliqueincidence ultrasonic waves at the contact interfaces and analyzed their reflection and transmission characteristics. Experimental results showed that the measured reflection coefficient and second harmonic wave agreed well with the suggested theoretical model. Consequently, the oblique-incidence technique can be a promising method for evaluating the characteristics of the contact interfaces.

• Journal of Advanced Navigation Technology
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• v.15 no.3
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• pp.349-354
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• 2011

In this paper, H-polarized scattering problems by a resistive strip grating with zero resistivity at strip-edges over a grounded dielectric plane according to the strip width and grating period, the relative permittivity and thickness of a dielectric layer, and incident angles of a transverse electric (TE) plane wave are analyzed by applying the Fourier-Galerkin Moment Method (FGMM). The tapered resistivity of resistive strips has zero resistivity at strip edges, then the induced surface current density on the resistive strip is expanded in a series of Chebyshev polynomials of the second kind as a orthogonal ploynomials. The sharp variations of the reflected power are due to resonance effects were previously called wood's anomallies, the numerical results for the reflected power are compared with those of uniform resistivity in the existing papers.

• The Journal of the Acoustical Society of Korea
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• v.38 no.1
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• pp.114-119
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• 2019

The transmitted acoustic signals are severely influenced by multiply reflected signals from boundaries, such as sea surface and bottom in the shallow water. Very large reflection signals from boundaries cause inter-symbol interference so that the performance of the underwater acoustic communication is degraded. Usually, the waveform shaping filters are used to prevent the reflected signals under this kind of acoustic channel. Especially, the raised cosine filter is widely used, which can also be used to restrict the bandwidth of the transmitted signal. In this study, we evaluate the raised cosine filter for image data transmission in the shallow water, and propose a new modified raised cosine filter. The QPSK (Quadrature Phase Shift Keying) system is used for the underwater acoustic communication simulations with different distances and symbol rates. As a result, the bit error rate was reduced from the minimum 1.0 % to the maximum 32 %.