• The Transactions of the Korean Institute of Electrical Engineers B
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• v.54 no.2
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• pp.101-109
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• 2005

The newly developed speed sensorless control scheme is proposed to estimate both motor speed and rotor resistance simultaneously using variable rotor flux. The rotor flux is given as sinusoidal waveform with an amplitude and a frequency without affecting precise torque control. Especially the proposed method makes the simultaneous estimation of rotor resistance and speed with high precision even though at the low speed area including a few rpm. Moreover, on-line identification of rotor resistance can be performed simply without calculating troublesome trigonometric functions and complicated integral computation. Therefore, the proposed system can be accomplished by using very cheap microprocessors for several applications. The results of the numerical simulations and experiments demonstrate that this method is effective to estimate the speed and on-line identification of rotor resistance for sensorless induction motors.

• ETRI Journal
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• v.45 no.2
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• pp.226-239
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• 2023

Challenges in cognitive radio and tactical communications include recognizing anonymously received signals and estimating parameters in a blind or semi-blind manner. In this paper, we examine this issue for orthogonal frequency division multiplexing (OFDM) signaling. There are several parameters in OFDM signaling, and the blind receiver must extract and consider the synchronization issue. We assume that the blind receiver is aware of modulation type, OFDM, and not aware of chip duration and the length of cyclic prefix. First, we present new criteria based on kurtosis to estimate these parameters and compare their performance at different levels of additive white Gaussian noise with methods based on correlation, kurtosis, maximum likelihood, and matched filter. Then, we perform synchronization and estimate the start time based on these criteria and several new criteria in two steps: fine and coarse synchronization. Finally, in a more practical setup, we present the idea of jointly estimating the mentioned parameters and the signal start time as coarse synchronization. We compare different criteria and show that one of the proposed criteria has the highest efficiency.

• Structural Engineering and Mechanics
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• v.73 no.6
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• pp.651-666
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• 2020

In this study, the spatial variation mechanisms of large far-field earthquakes at engineering scales are first investigated with data from the 2008 Ms 8.0 Wenchuan earthquake. And a novel 'coherency cut-off frequency' is proposed to distinguish the spatial variations in ground motions in the low-frequency and high-frequency ranges. Then, a practical piecewise coherency model is developed to estimate and characterize the spatial variation in earthquake ground motions, including the effects of source-to-site distances, site conditions and neighboring topography on these variations. Four particular earthquake records from dense seismograph arrays are used to investigate values of the coherency cut-off frequency for different source-to-site distances. On the basis of this analysis, the model is established to simulate the spatial variations, whose parameters are suitable for both near- and far-field earthquake conditions. Simulations are conducted to validate the proposed model and method. The results show that compared to the existing models, the proposed model provides an effective method for simulating the spatial correlations of ground motions at local sites with known source-to-site distances.

• Journal of Nutrition and Health
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• v.31 no.2
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• pp.220-230
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• 1998

The purpose of this study was to develop and validate the Food frequency questionnaire (FFQ) for dietary studies of Koreans. One hundred and five food items for the Food frequency questionnaire were selected based on information of frequently consumed foods from National Nutrition Survey Reports and on raw data from a dietary survey on diabetic patients. Frequency of consumption was determined through nine categories ranging from more than three times a day to almost never to indicate how often the specified amount of each food item was consumed during the past month. Three portion sizes were given for each food item(small, medium or large) with respect to a stated medium portion. Seventy-three healthy women served for the validation study. They completed both the FFQ and a 3-day diet record. The FFQ estimate of mean nutrient intake was higher by 10-20% than that of the 3-days diet record and the Spearman correlation coefficients between the two methods ranged from 0.26 to 0.59 . The degree of agreement was from 36% to 38% when nutrients intake assessed by the FFQ and 3day diet record were classified within the same quintile. On the whole , the result of this study seemed to be in good agreement with other studies. Therefore the FFQ developed in this study is considered to be a reliable tool in assessing the dietary habits of Korean.

• IEIE Transactions on Smart Processing and Computing
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• v.6 no.1
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• pp.39-46
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• 2017

In this paper, we propose an integer frequency offset estimation and compensation method based on PLC preamble correlation in DOCSIS 3.1 Downstream system. The proposed method determines the PLC preamble subcarrier location recovered from PLC data and the one obtained from PLC preamble correlation. We showed the performance of PLC preamble detection in the received signal through the maximum value detection of PLC preamble correlation. Thus we can estimate and compensate for the integer frequency offset by computing the difference of PLC subcarrier locations.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.11B
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• pp.1556-1564
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• 2001

Reed-Solomon code, block interleaving and SI (side information) are frequently used in SFH (slow frequency hopping) system. Erasing those symbols in the hit frequency slot greatly increases the error connection capacity. Packet error rate has been the major performance measure for SFH system. The analysis of BER has been limited to the case of perfect Sl, in which neither miss nor false alarm exists. BER with imperfect Sl has been obtained only by Monte Carlo simulation. In this paper, we present a unified solution to estimate BER with imperfect Sl. It is shown that previous formulae for packet error rate or BER with perfect Sl are special cases in the proposed solution. The computed BER with false alarm and miss of frequency hit is verified by comparing with the simulation result.

• ETRI Journal
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• v.42 no.3
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• pp.324-332
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• 2020

In this paper, a blind symbol timing offset (STO) estimation method is proposed for offset quadrature phase-shift keying (OQPSK) modulated signals, which also works for other linearly modulated signals (LMS) such as binary-PSK, QPSK, 𝜋/4-QPSK, and minimum-shift keying. There are various methods available for blind STO estimation of LMS; however, none work in the case of OQPSK modulated signals. The popular cyclic correlation method fails to estimate STO for OQPSK signals, as the offset present between the in-phase (I) and quadrature (Q) components causes the cyclic peak to disappear at the symbol rate frequency. In the proposed method, a set of close and approximate offsets is used to compensate the offset between the I and Q components of the received OQPSK signal. The STO in the time domain is represented as a phase in the cyclic frequency domain. The STO is therefore calculated by obtaining the phase of the cyclic peak at the symbol rate frequency. The method is validated through extensive theoretical study, simulation, and testbed implementation. The proposed estimation method exhibits robust performance in the presence of unknown carrier phase offset and frequency offset.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.7
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• pp.676-684
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• 2012

Numerical analysis was performed to investigate the heavy-weight impact noise of apartment houses. The FEM is practical method for prediction of low-frequency indoor noise. The results of numerical analysis, the shape of the acoustic modes in room-2 are similar to that of acoustic pressure field at the fundamental frequency of acoustic modes. And the acoustic pressure was amplified at the natural frequency of the acoustic modes and structural modes. The numerical analysis result of sound pressure level at 63 Hz and 125 Hz octave-band center frequency are similar to the test results, but at 250 Hz and 500 Hz have some errors. Considering most of bang-machine force spectrum exists below 100 Hz, the noise at 250 Hz and 500 Hz are not important for heavy-weight impact noise. Thus, the FEM numerical analysis method for heavy-weight impact noise can apply to estimate heavy-weight impact noise for various building systems.

• Journal of the Korean Society of Safety
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• v.31 no.1
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• pp.1-6
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• 2016

Severity and frequency of industrial accidents are typically used to assess the "absolute" risk associated with the industrial machines and devices ("items") which are subject to safety certification or self-declaration of conformity. However, the "relative" risk associated with a particular item can further be assessed based on unit severity and unit frequency where the total number of item in use is taken into account. This study first attempts to estimate the total number of each item in use which was recently selected for safety certification or self-declaration of conformity. The appropriateness of such selection is recapitulated based on the relative risk involved. Analysis results indicate that depending on items, the relative risk is differentiated from the absolute risk. Recent selection of items for safety certification or self-declaration of conformity is then revisited for its validity. The relative risk based on unit severity and unit frequency of industrial accidents, together with the absolute risk, may be used to further categorize items for safety certification or self-declaration of conformity in the future.

• Smart Structures and Systems
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• v.21 no.5
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• pp.583-590
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• 2018

Nowadays, many tunnels have been commissioned for several decades, which require effective inspection methods to assess their health conditions. The ambient vibration test has been widely adopted for the damage identification of concrete structures. In this study, the vibration characters of tunnel lining shells built with forepoling method was analyzed based on the analytical solutions of the Donnell-Mushtari shell theory. The broken rock, foreploing, rock-concrete contacts between rock mass and concrete lining, was represented by elastic boundaries with normal and shear stiffness. The stiffness of weak contacts has significant effects on the natural frequency of tunnel lining. Numerical simulations were also carried out to compare with the results of the analytical methods, showing that even though the low nature frequency is difficult to distinguish, the presented approach is convenient, effective and accurate to estimate the natural frequency of tunnel linings. Influences of the void, the lining thickness and the concrete type on natural frequencies were evaluated.