• 대한전자공학회논문지TC
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• 제49권4호
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• pp.34-40
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• 2012

무선통신에서 페이딩은 피할 수 없는 문제이다. 그러므로 전송신호에 있어서는 BER 형태의 오류개념이 도입된다. 다른 페이딩 채널들 상에서 이러한 오류들의 동작을 인식하는 것이 필요하다. CoherentMPSK의 평균 BER에 대한 수학적인 해법을 얻기 위해서 몇 가지 기법들을 제안한다. 본 논문에서, 느리고 평탄한 라이시안 페이딩 채널 상에 diversity의 영향도 또한 분석되어진다. 여기서, 변조 지표 값 M은 변화하고 이 변화의 효과들 또한 묘사되어진다. 다양한 diversity값과 페이딩 파라미터에 따른 성능 곡선들은 믿을 수 있는 통신 시스템을 위하여 무선 채널을 설계하고 평가하는데 유용하다.

• 한국통신학회논문지
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• 제36권4C호
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• pp.232-240
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• 2011

Water-filling 알고리즘은 적응 MIMO 시스템의 ergodic 용량을 최대화하는 효율적인 전력 할당 방식이지만 이산 변조 지쉬discrete modulation index)를 사용하는 실제의 시스템의 경우 과도한 잉여 전력(residual power)으로 인해 스펙트럼 효율이 감소하는 단점이 있다. 본 논문에서는 water-filling 알고리즘의 잉여 전력을 효율적으로 재할당 함으로써 적응 MIMO 시스템의 스펙트럼 효율을 향상시키는 새로운 전력 할당 알고리즘을 제안한다. 알고리즘의 성능을 검증하기 위해 터보 코드가 적용된 적응 MIMO 시스템을 구성하고 다양한 환경에서 시뮬레이션을 수행한다 시뮬레이션 결과, 레일리이 페이딩 환경에서 SNR이 20dB일 때 제안된 알고리즘의 스펙트럼 효율이 기존의 water-filling 알고리즘에 비해 대략 8.9% 향상됨을 알 수 있었다.

• Journal of Power Electronics
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• 제19권4호
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• pp.968-979
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• 2019

Recently, there has been increased interest in the study of multiphase machines due to their higher fault-tolerant capability when compared to their conventional three-phase counterparts. For six-phase machines, stator windings configured with a single isolated neutral (1N) provide significantly more post-fault torque/power than two isolated neutrals (2N). Hence, this configuration is preferred in applications where post-fault performance is critical. It is well known that min-max injection has been commonly used for three-phase and multiphase machines in healthy condition to maximize the modulation limit. However, there is a lack of discussion on min-max injection for post-fault condition. Furthermore, the effects in terms of the common-mode voltage (CMV) in modulating signals has not been discussed. This paper investigates the effect of min-max injection in post fault-tolerant control on the voltage and speed limit of a symmetrical six-phase induction machine with single isolated neutral. It is shown that the min-max injection can minimize the amplitude of reference voltage, which maximizes the modulation index and post-fault speed of the machine. This in turn results in a higher post-fault power.

• Journal of Astronomy and Space Sciences
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• 제37권4호
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• pp.219-228
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• 2020

A CubeSat platform has become a popular choice due to inexpensive commercial off-the-shelf (COTS) components and low launch cost. However, it requires more power-efficient and higher-data rate downlink capability for space applications related to remote sensing. In addition, the platform is limited by the size, weight and power (SWaP) constraints as well as the regulatory issue of licensing the radio frequency (RF) spectrum. The requirements and limitations have put optical communications on promising alternatives to RF communications for a CubeSat platform, owing to the power efficiency and high data rate as well as the license free spectrum. In this study, we analyzed the performance of optical downlink communications compatible with CubeSat platforms in terms of data rate, bit error rate (BER) and outage probability. Mathematical models of BER and outage probability were derived based on not only the log-normal model of atmospheric turbulence but also a transmitter with a finite extinction ratio. Given the fixed slot width, the optimal guard time and modulation orders were chosen to achieve the target data rate. And the two performance metrics, BER and outage data rate, were analyzed and discussed with respect to beam divergence angle, scintillation index and zenith angle.

• 한국기후변화학회지
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• 제34권14호
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• pp.5621-5634
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• 2021

The life cycle of El Niño-Southern Oscillation (ENSO) typically follows a seasonal march, with onset in spring, developing during summer, maturing in boreal winter, and decaying over the following spring. This feature is referred to as ENSO phase locking. Recent studies have noted that seasonal modulation of the ENSO growth rate is essential for this process. This study investigates the fundamental effect of a seasonally varying growth rate on ENSO phase locking using a modified seasonally dependent recharge oscillator model. There are two phase locking regimes associated with the strength of the seasonal modulation of growth rate: 1) a weak regime in which only a single peak occurs and 2) a strong regime in which two types of events occur either with a single peak or with a double peak. Notably, there is a seasonal gap in the strong regime, during which the ENSO peak cannot occur because of large-scale ocean-atmosphere coupled processes. We also retrieve a simple analytical solution of the seasonal variance of ENSO, revealing that the variance is governed by the time integral of seasonally varying growth rate. Based on this formulation, we propose a seasonal energy index (SEI) that explains the seasonal gap and provides an intuitive explanation for ENSO phase locking, potentially applicable to global climate model ENSO diagnostics.

• Journal of Power Electronics
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• 제14권1호
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• pp.61-73
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• 2014

Multiphase machines are characterized by high power density, enhanced fault-tolerant capacity, and low torque pulsation. For a voltage source inverter supplied multiphase machine, the probability of load imbalances becomes greater and unwanted low-order stator voltage harmonics occur. This paper deals with the PWM control of multiphase inverters under unbalanced load conditions and it proposes a novel near-five-vector SVPWM algorithm based on the five-phase six-leg inverter. The proposed algorithm can output symmetrical phase voltages under unbalanced load conditions, which is not possible for the conventional SVPWM algorithms based on the five-phase five-leg inverters. The cause of extra harmonics in the phase voltages is analyzed, and an xy coordinate system orthogonal to the ${\alpha}{\beta}z$ coordinate system is introduced to eliminate low-order harmonics in the output phase voltages. Moreover, the digital implementation of the near-five-vector SVPWM algorithm is discussed, and the optimal approach with reduced complexity and low execution time is elaborated. A comparison of the proposed algorithm and other existing PWM algorithms is provided, and the pros and cons of the proposed algorithm are concluded. Simulation and experimental results are also given. It is shown that the proposed algorithm works well under unbalanced load conditions. However, its maximum modulation index is reduced by 5.15% in the linear modulation region, and its algorithm complexity and memory requirement increase. The basic principle in this paper can be easily extended to other inverters with different phase numbers.

• 한국광학회지
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• 제9권3호
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• pp.168-174
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• 1998

본 연구에서는 광학매질의 비선형 특징을 측정하기 위해, 자체집속현상을 이용하는 기존의 측정법과는 다르게, 매질을 통과한 광원의 위상섭동으로 인하여 far-field에 발생한 공간적 광장도 분포를 이용하는 far-field 위상 변조량 측정법을 사용하였다. 미선형 매질을 통과한 조사과의 위상 변화가 far-field 영역에서 나타나는 겅간적 위상 변조량을 실험적으로 측정아였고, 이를 전산시\ulcorner을 통하여 비교분석하였다. 파장 616nm, 펄스폭 106ps의 분포궤한 섹소레이저(DFDL)와 파장 308nm인 DFDL의 제2고조파를 조사광으로 사용하였고, 매질 $CS_2$와 $BaF_2$의 비선형 특성을 실험적으로 측정한후 이 값을 전산시늉값과 비교하였다. 이 실험에서 파장 616nm에 대해 $CS_2$와 $BaF_2$의 비선형 굴절률 $1.2{\times}10^{-11}$ esu와 $1.0{\times}10^{-13}$ esu를 각각 얻었고, 파장 308nm에 대해 $BaF_2$의 비선형 흡수계수 $5.0{\times}10^{-11}$cm/W를 얻었다. 이들 값은 지금까지 발표된 값들과 잘 일치한다.

• 디지털융복합연구
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• 제20권1호
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• pp.185-190
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• 2022

Bluetooth는 다양한 소비자 전자 장치 간의 연결 매체로 널리 사용되는 공통 무선 기술이다. Bluetooth 수신기는 주로 신호 대 잡음비 성능 개선을 위해 일반적으로 Viterbi 알고리즘을 채택하지만 전송단에서의 irrational modulation index(무리수 변조 지수)로 수신단의 지속적인 탐색 및 추정이 필요하며 이를 위한 복잡한 하드웨어와 계산을 요구한다. 본 논문에서는 이러한 복잡도 문제를 해결하기 위해 비동기 최대 우도 추정(MLE, Maximum-likelihood Estimation) 기반 8-state Viterbi FSM을 제안한다. 본 논문에서 제안한 최적의 Viterbi FSM은 전송단에서의 변조 지수에 대한 사전 정보 및 추정이 필요 없으며 GFSK(Gaussian Frequency Shift Keying) 심볼 검출이 가능하다. 제안한 알고리즘에 대한 성능은 HV1/HV2 패킷을 사용하여 평가하였으며, 시뮬레이션 결과는 DD(Decision Direct)와 같은 이상적인 접근 방식과 비교하여 10^-3 BER에서 약 2dB 성능 향상을 보여주었다.

• 대한통합의학회지
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• 제11권4호
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• pp.27-39
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• 2023

Purpose : The aim of this study was to identify the influence of transient isokinetic exercise on cardiac autonomic modulation and muscle properties in healthy male subjects. Methods : Twenty-eight healthy males underwent isokinetic exercise of both knee joints using a Biodex systems 3 isokinetic dynamometer with an angular velocity of 60 °/sec. The changes in activity of the autonomic nervous system, as determined by heart rate variability (HRV), and in muscle properties were evaluated at three times: pre-exercise, immediately post-exercise, and 10 min post-exercise. Results : The time domain analysis of HRV revealed significant changes in the beat count and mean and minimal heart rate (HR) measured at pre-exercise, immediately post-exercise, and 10 min post-exercise (p<.001). The beat count and mean HR were markedly increased immediately post-exercise compared to pre-exercise, but then significantly decreased at 10 min post-exercise (p<.001). All parameters of the frequency domain were significantly altered by isokinetic exercise (p<.01). The low frequency/high frequency (LF/HF) ratio, as an index for the sympathovagal balance, was elevated by exercise and remained at a similarly high level at 10 min post-exercise (p<.01). The muscle properties of rectus femoris were changed as follows: Muscle tone and stiffness were significantly increased between pre-exercise and immediately post-exercise (p<.001), and between pre-exercise and at 10 min post-exercise (p<.001). Whereas, the elasticity showed no significant change. Conclusion : These results demonstrated that transient isokinetic exercise could induce changes in cardiac autonomic control and muscle properties. In particular, up-regulation of LF/HF ratio after exercise signifies thus enhanced sympathetic modulation by isokinetic exercise. Therefore, it is needed to understand the cardiovascular risks that may arise during isokinetic exercise for providing the basic evidence to establish appropriate isokinetic exercise protocols as effective rehabilitation exercises.

• 한국정보통신학회논문지
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• 제5권4호
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• pp.699-705
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• 2001

본 논문에서는 DPSK변조 방식을 고려하여 나카가미-m 페이딩채널로 다중경로 페이딩을 모델화 하였으며, 각 브랜치에서의 나카가미 페이딩 지수(m)는 전파 환경에 따라 일정하지 않을 것이며 각 MRC 다이버시티의 브랜치에서 나카가미 페이딩 지수가 낮은 값을 가지는 브랜치를 제거할 경우 성능이 좋아질 수 있을 것이다. 따라서 본 논문에서는 나카가미 페이딩 지수를 임의의 값으로 주었을 경우 광대역 DS/ /CDMA 시스템에서 각각의 경로로 수신된 신호의 평균 SNR(signal to Noise Ratio)이 시스템의 BER에 미치는 영향을 컴퓨터 시뮬레이션을 통해 분석하고 나카가미 페이딩 지수(m)값이 낮은 브랜치를 제거한 경우의 광대역 DS/CDMA 셀룰라 시스템에서 각각의 경로로 수신된 신호의 평균 SNR(signal to Noise Ratio)이 시스템의 BER에 미치는 영향을 컴퓨터 시뮬레이션을 통해 분석하였다.