• The Journal of the Acoustical Society of Korea
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• v.41 no.2
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• pp.159-165
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• 2022

In this paper, a frame synchronization structure for the Binary Phase Shift Keying (BPSK) modulation method in underwater acoustic communication was proposed. The proposed frame synchronization structure is largely divided into two. First, the approximate position and frequency offset of the frame are obtained by non-coherent correlation and sliding Fast Fourier Transform (FFT) method. Second, after compensating for the frequency error to the received signal, the exact position of the frame is obtained by coherent correlation method. Maritime experiments were conducted to confirm the performance of the 2-STEP frame synchronization structure. It was showed that the limitations of the non-coherent correlation and sliding FFT method can be verified when the power of the received signal was greatly reduced due to the channel characteristics. As a result, stable frame synchronization could be obtained by compensating for the frequency error and then using the coherent correlation method.

• Journal of the Optical Society of Korea
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• v.11 no.3
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• pp.138-141
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• 2007

We suggest that supercontinuum can be used for absorption spectroscopy to observe the exciton levels of a semiconductor nano-structure. Exciton absorption spectrum of a GaAs/AlGaAs quantum well was observed using supercontinuum generated by a microstructrured fiber pumped by a femtosecond (fs) pulsed laser. Significantly narrower peaks were observed in the absorption spectrum from 11 K up to room temperature than photoluminescence (PL) spectrum peaks. Because supercontinuum is coherent light and can readily provide high enough intensity, this method can provide a coherent ultra-broad band light source to identify exciton levels in semiconductors, and be applicable to coherent nonlinear spectroscopy such as electromagnetically induced transparency (EIT), lasing without inversion (LWI) and coherent photon control in semiconductor quantum structures.

• Journal of Advanced Marine Engineering and Technology
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• v.18 no.1
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• pp.60-72
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• 1994

The effect of free stream tubulence on the coherent structure in the near wake of a circular cylinder was investigated by a conditional sampling technique. The measurements were made from C.T.A. with hot wire I-probe and a Split-film sensor. Contours of phase-averaged velocity and vorticities were presented and discussed. It was found that the value of the vortex strength increased with increasing free stream turbulence which can enhance the roll-up of the shear layer.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.10A
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• pp.1006-1012
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• 2008

This paper presents a Duvall-structure-based adaptive beamforming method which efficiently cancels coherent and incoherent interferences. The proposed method exploits several correlation vectors to increase the dimension of the weight vector, compared to the existing method which uses a single correlation vector only. The increased dimension of the weight vector leads to an improvement in the signal-to-interference plus noise ratio (SINR) performance. Moreover, the proposed method can suppress more interferences than the existing one. Simulation shows that the former is superior to the latter in terms of the steady-state and transient responses.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2454-2462
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a heated circular cylinder were examined in a wind tunnel. Turbulent intensities, rms values of temperature and turbulent convective heat flux distributions in the heated cylinder wake with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. A phase averaging method was also used to estimated coherent motion in the near wake. It is found that the vertical turbulent motion in the stably stratified flow case dissipates faster than that of the neutral case, i.e., vertical growth of vortical structure is suppressed under the strongly stratified condition. The coherent motion of temperature makes a large contribution like velocity coherent motion. However, the coherent motions of temperature fluctuation become very different with the change of experimental conditions, though the velocity coherent motions are quite similar in all experimental conditions.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1863-1868
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• 2003

Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong internal waves, or separated, fluctuating wake and persistent late wakes. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex the late wake far downstream of the object. Unlike in homogeneous fluid, the flow field downstream self-develops coherent vortex even after diminishing of the near wake, no matter how small the stratification is. This paper present a computational approach to simulate the generation of the coherent vortex structure in late wake of a moving sphere submerged in weakly stratified fluid. The results are in consistent with several experimental observations and the vortex stretching mechanism is employed to explain the process of coherence.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.6B
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• pp.625-637
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• 2002

This study extends the modified linear feedback equalizer (MLFE), normally used for differentially coherent DPSK systems, to the equalization of double differentially coherent PSK (DDPSK) signals. By feeding back into the feedback part after modifying the equalizer output using decision feedback-based demodulation, the proposed equalizer can operate like an equalizer with a decision feedback structure. Simulation results showthat performances of the decision feedback demodulation-based feedback equalizer (DFD-FE) approach those of the DFE/coherent.

• Journal of the Korean Institute of Telematics and Electronics B
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• v.32B no.11
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• pp.1481-1488
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• 1995

Due to the low loss, broadband and accurate short time delay properties of optical fiber, it has attracted as a delay medium for high speed and broad-band signal processing. In this paper, we consider the coherent optical fiber filter of IIR lattice structure, which uses coherent light sources and consists of directional couplers whose coupling coefficients are restricted between 0 and 1. Considering restrictions of directional coupler, the design formulae and condition for realibility of optical fiber filter of IIR lattice structure which makes the optimal use of optical signal energy are derived.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.6
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• pp.1661-1668
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• 1990

In a turbulent premixed flame stabilized by the streamline step, and dominated by a coherent eddy, a flame micro-structure was investigated with analyzing the signals of temperature, the ion current, and schieren phtographs simultaneously. Generally the contours of large scale coherent eddies of schlieren photographs was considered as the flame front, however, the main reaction can be occurred within the eddy as a structure of fine flamelets scale. The surrounding burned gas of flamelets could not propagate to a unburned mixture, obstructing flamelets from propagating to a unburned mixture. Consequently, it could restrain flashback. The main reaction region was found to be located at higher temperature of the burned gas rather than at maximum rms of fluctuating temperature. The peak probability of higher temperature was 6 times greater than that of lower temperature. As it was difficult to infer a flame structure from PDF distribution of the fluctuating temperature in form of bimodal shape, it should be taken into consideration with other informations related to the sensitive flame front, for instance, ion current.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.3
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• pp.1028-1039
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• 1996

This study was focused on the examination of the flame structure and the combustion characteristics of diffusion flame which was formed the turbulent shear flow of a double coaxial air jet system. The shear flow was formed by the difference velocity of surrounding air jet(U$\_$s/) and center air jet (U$\_$c/). So experimental condition was divided S-type flame (.lambda. > 1) and C-type flame (.lambda. < 1) by velocity ratio .lambda. (=U$\_$s//U$\_$c/). For examination of the flame structure and the combustion characteristics in diffusion flame, coherent structure was observed in flame by schlieren photograph method. We measured fluctuating temperature and ion current simultaneously and accomplished the statistical analysis of its. According to schlieren photograph, the flame was stabilized in the rim of the direction of lower velocity air jet, coherent eddy was produced and developed by higher velocity air jet. The statistical data of fluctuating temperature and ion current was indicated that reaction was dominated by higher velocity air jet. The mixing state of burnt gas and non-burnt gas was distributed the wide area at Z = 100 mm of C-type flame.