• Journal of Astronomy and Space Sciences
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• 제30권2호
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• pp.79-82
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• 2013

We present the Hilbert-Huang transform (HHT) analysis on the quasi-periodic modulation of SMC X-1. SMC X-1, consisting of a neutron star and a massive companion, exhibits superorbital modulation with a period varying between ~40 d and ~65 d. We applied the HHT on the light curve observed by the All-Sky Monitor onboard Rossi X-ray Timing Explorer (RXTE) to obtain the instantaneous frequency of the superorbital modulation of SMC X-1. The resultant Hilbert spectrum is consistent with the dynamic power spectrum while it shows more detailed information in both the time and frequency domains. According to the instantaneous frequency, we found a correlation between the superorbital period and the modulation amplitude. Combining the spectral observation made by the Proportional Counter Array onboard RXTE and the superorbital phase derived in the HHT, we performed a superorbital phase-resolved spectral analysis of SMC X-1. An analysis of the spectral parameters versus the orbital phase for different superorbital states revealed that the diversity of $n_H$ has an orbital dependence. Furthermore, we obtained the variation in the eclipse profiles by folding the All Sky Monitor light curve with orbital period for different superorbital states. A dip feature, similar to the pre-eclipse dip of Her X-1, can be observed only in the superorbital ascending and descending states, while the width is anti-correlated with the X-ray flux.

• 전자공학회논문지
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• 제50권8호
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• pp.15-20
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• 2013

본 논문에서는 UHF (ultra high frequency) 대역에서 협대역 통신 시스템을 설계할 경우 필연적으로 겪게 되는 주파수 옵셋 (offset)의 영향을 감쇄시킬 수 있는 dual-band FSK (frequency shift keying) 변조 방식을 제안한다. 협대역 통신 시스템의 경우 광대역 통신 시스템에 비하여 주파수 옵셋에 의한 성능 열화가 상대적으로 큰 단점이 있으며 이로 인하여 버스트 모드 (burst mode)로 동작하는 협대역 통신 시스템의 경우 신호 검출 및 초기 동기 획득이 용이치 않은 문제가 있다. 본 논문에서 제안하는 dual-band FSK 변복조 방식은 버스트 모드 협대역 통신 시스템에서 초기 주파수 옵셋이 큰 경우에도 안정적으로 동작할 뿐만 아니라 주파수 다이버시티 (frequency diversity)를 얻을 수 있어 통신 시스템의 높은 신뢰도가 요구되는 긴급 통신망 설계 등에 활용될 수 있다.

• ETRI Journal
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• 제26권6호
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• pp.565-574
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• 2004

Differential unitary space-time modulation (DUSTM) has emerged as a promising technique to obtain spatial diversity without intractable channel estimation. This paper presents a study of the application of DUSTM on multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with frequency-selective fading channels. From the view of a correlation analysis between subcarriers of OFDM, we obtain the maximum achievable diversity of DUSTM on MIMO-OFDM systems. Moreover, an efficient implementation strategy based on subcarrier reconstruction is proposed, which transmits all the signals of one signal matrix in one OFDM transmission and performs differential processing between two adjacent OFDM blocks. The proposed method is capable of obtaining both spatial and multipath diversity while reducing the effect of time variation of channels to a minimum. The performance improvement is confirmed by simulation results.

• 한국정보통신학회논문지
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• 제5권1호
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• pp.52-60
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• 2001

본 논문에서는 나카가미 페이딩 환경에서 다중 반송파 MFSK-DPSK/DS-CDMA 합성 변조기법을 제안하였다. 또한, 다중경로 페이딩에 강한 특성과 주파수 이용 효율이 높은 다중반송파 DS-CDMA 합성변소 방식을 같이 사용하여 분석하였다. 그러한 변조방식은 비동기식 MFSK와 MFSK에 기반한 DPSK를 이용한 결합 주파수 위상 합성변조 방식이다. 결과로부터 전력효율과 대역폭 효율이 일반적인 통신 시스템보다 다중캐리어 합성 변조시스템이 효율을 개선시킬 수 있음을 알았다.

• 대한전자공학회논문지
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• 제27권5호
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• pp.784-792
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• 1990

In this paper, attempts have been made to compare the subjective performance of sub-band coding (SBC) techniques incorporating four representative modulation transfer functions (MTF's) of th human visual system(HVS): Sakrison, Nill, Ngan, and Rao. In SBC, the frequency band of input signal is split into 16 equal sub-bands. In addition, 28 sub-band splitting schemd which splits more sub-bands in low spatial frequency is considered to compare the 4 MTF's effectively. In encoding of each sub-band, the weight of each sub-band obtained from the MTF of HVS is applied to bit allocation process which minimize the weighted mean square error (WMSE). The differential pulse code modulation(DPCM) coder is used to encode the lowest sub-band and the pulse code modulation(PCM) coder is used for the rest of sub-bands. It is found that the images incorporating the MTF of Rao yields the best results in subjective criteria, followed by Ngan, Nill, Sakrison, and the images not employing the HVS.

• 한국인터넷방송통신학회논문지
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• 제10권2호
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• pp.159-165
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• 2010

다중 부호 변조는 무선 환경에서 고속의 데이터 전송을 위해 개발되었지만 직교 부호(OC)의 제한된 자원과 높은 평균 전력 대 최대 전력 비(PAPR)와 같은 치명적인 두 가지 문제점을 가지고 있다. 본 논문에서는 위와 같은 문제점을 해결하고 사용자의 요구된 서비스 질(QoS)에 따라 네 가지 변수들로 조절 할 수 있는 가변성 고속 비트율을 얻기 위하여 AOCG(Advanced Orthogonal Code Group)-OFDM(Orthogonal Frequency Division Multiplexing) 이라 부르는 새로운 변조 기술을 제안한다.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1990년도 한국자동제어학술회의논문집(국제학술편); KOEX, Seoul; 26-27 Oct. 1990
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• pp.1021-1026
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• 1990

Accuracy improvement of a self-mixing semiconductor laser range finder is predicted by simulation, in which the laser modulation current is reshaped to give an ideal triangular waveform of the optical frequency change. The maximum range measurement error of less than 0.1% in a wide range of O.1m to 1m is expected by the reshaping of the modulation current. Experimental verification of the effect of current reshaping on the linearization of the derivative of the optical frequency change curve is given.

• Journal of Power Electronics
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• 제16권3호
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• pp.994-1003
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• 2016

Two major obstacles in the utilization of electrical vehicles are their price and range. The collaboration of direct torque control (DTC) with induction motor (IM) is preferred for its low cost, easy implementation, and parameter independency. However, in terms of edges, the method has drawbacks, such as variable switching frequency and undesired current harmonic distortion. These drawbacks result in acoustic noise, reduced efficiency, and electromagnetic interference. A feed-forward approach for stator-flux-oriented DTC with space vector pulse-width modulation is presented in in this paper. The outcome of the proposed method is low current harmonic distortion with fixed switching frequency while preserving the torque performance and simple application feature of basic DTC. The method is applicable to existing and forthcoming IM drive systems via software adaptation. The validity of the proposed method is confirmed by simulation and experimental results.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2005년도 춘계학술대회 논문집 전기기기 및 에너지변환시스템부문
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• pp.197-199
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• 2005

The Cold cathode fluorescent lamp (CCFL) are widely used to illuminate the liquid crystal display(LCD). Ballasts are required for CCFL because the lamp need high starting voltage and behave negative dynamic resistant characteristics in the desired region of operation. Dimming methods of CCFL are used to pulse frequency modulation(PFM) or pulse width modulation(PWM). In this paper, CCFL driving and control circuit is designed by half-bridge type series and parallel resonant inverter that variable frequency modulation method to control the output voltage current. The validity of this study is confirmed from the simulation and experimental results.

• International Journal of Precision Engineering and Manufacturing
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• 제8권2호
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• pp.80-84
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• 2007

We propose a new displacement measurement method using a phase modulation homodyne interferometer and a tunable laser diode as a light source to determine an arbitrary length with a resolution in the order of 10 pm. In the proposed instrument, the displacement of a movable mirror in the interferometer can be converted to a frequency shift of the tunable laser diode. We discuss the principles of the proposed method, the instrumentation, and the experimental results, and compare the proposed method with two commercial displacement sensors. The commercial sensors used are a heterodyne interferometer, the interpolation error of which is also measured, and a capacitive sensor.