• KSII Transactions on Internet and Information Systems (TIIS)
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• v.13 no.5
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• pp.2400-2413
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• 2019

Doubly-selective (DS) fading channel is often occurred in many orthogonal frequency division multiplexing (OFDM) communication systems, such as high-speed rail communication systems and underwater acoustic (UWA) wireless networks. It is challenging to provide an accurate and fast estimation over the doubly-selective channel, due to the strong Doppler shift. This paper addresses the doubly selective channel estimation problem based on complex exponential basis expansion model (CE-BEM) in OFDM systems from the perspective of distributed compressive sensing (DCS). We propose a novel DCS-based improved sparsity adaptive matching pursuit (DCS-IMSAMP) algorithm. The advantage of the proposed algorithm is that it can exploit the joint channel sparsity information using dynamic threshold, variable step size and tailoring mechanism. Simulation results show that the proposed algorithm achieves 5dB performance gain with faster operation speed, in comparison with traditional DCS-based sparsity adaptive matching pursuit (DCS-SAMP) algorithm.

• Journal of Internet Computing and Services
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• v.16 no.1
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• pp.101-108
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• 2015

This study paid attention to the notable increase of the use of diet/exercise apps on smartphones. Based on channel expansion theory and technology acceptance model (TAM), this study investigated how three predictors-Internet information use efficacy, Internet information credibility, smartphone use efficacy-affected one's attitude toward and intention to use diet/exercise apps. Results from a path analysis indicated that Internet information use efficacy and smartphone use efficacy positively predict the perceived ease of use of diet/exercise apps. Internet information credibility positively predicts the perceived usefulness of such apps. Moreover, there were gender differences in the effects of both Internet information use efficacy and Internet information credibility on the perceived usefulness of diet/exercise apps.

• Proceedings of the KIEE Conference
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• 2004.11c
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• pp.240-242
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• 2004

A channel allocation algorithm of multi-user OFDM(orthogonal frequency division multiplexing) system is presented. The proposed algorithm is to reduce the complexity of the system, using the GNN(gradual neural network) with gradual expansion scheme and the algorithm attempts to allocate channel with good channel gain to each user. The method has lower computational complexity and less iteration than other algorithms.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.6
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• pp.1582-1589
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• 1994

The characteristics Prandt1-Meyer expansion of supersonic flow with condensation through a wavy wall in a channel are investigated by experiment and numerical direct marching method of characteristics. In the present study, for the case of moist air flow in the type of indraft supersonic wind tunnel, the dependency of location of formation and reflection of the oblique shock wave generated by the wavy wall and the distribution of flow properties, on the specific humidity and temperature at the entrance of wavy wall and the attack angle of the wavy wall to the main stream is clarified by schlieren photograph, distribution of static pressure and Mach number, and plots of numerical results. Also, we confirm that the wavy wall plays an important key role in the formation of oblique shock wave, and that the effect of condensation on the flow field appears apparently.

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.4
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• pp.468-474
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• 2011

The flow field around baffle located sudden expansion and contraction channel was measured by PIV method and the effect according to height change of baffle built in the flow field was evaluated. The inlet flow velocity and the baffle height influence mutually to the size and flow pattern of the recycle flow of the back of the baffle and the size of the area of the water power jump passing the upper part of the baffle. In case of Reynolds number $Re=4{\times}10^3$, the critical value of baffle height is estimated around h/H=1.6 and there was a decreasing tendency as the inlet flow velocity was increased.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.19 no.1
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• pp.27-35
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• 2023

Pressure tubes are the main components of PHWR core and serve as the pressure boundary of the primary heat transport system. However, because pressure tubes have changed their geometrical dimensions under the severe operating conditions of high temperature, high pressure and neutron irradiation according to the increase of operation time, all dimensional changes should be predicted to ensure that dimensions remain within the allowable design ranges during the operation. Among the deformations, the diameter expansion due to creep leads to the increase of bypass flow which may not contribute to the fuel cooling, the decrease of critical channel power and finally the deration of the power to maintain the operational safety margin. This study is focused on the modeling of the expansion of the pressure tube diameter based on the operating conditions and measured diameter data. The pressure tube diameter expansion was modeled using the neutron flux and temperature distributions of each fuel channel and each fuel bundle as well as the measured diameter data. Although the basic concept of the current modeling approach is simple, the diameter prediction results using the developed methodology showed very good agreement with the real data, compared to the existing methodology.

• ETRI Journal
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• v.25 no.4
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• pp.266-269
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• 2003

Polymeric multi-channel optical interconnection devices that are usually fabricated by transfer molding are indispensable for parallel interconnection in high speed, high capacity optical communication systems. This paper proposes a design technique considering the thermal behavior of materials, such as shrinkage and expansion during the molding process, to satisfy geometrical requirements that have less than 1 ${\mu}m$ tolerance. We also designed molds considering the thermal effects of the materials and fabricated multi-channel optical fiber connectors that have less than 1 ${\mu}m$ tolerance.

• Journal of electromagnetic engineering and science
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• v.7 no.4
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• pp.195-200
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• 2007

Doubly-selective channel estimator for orthogonal frequency division multiplexing(OFDM) systems is proposed in this paper. Based on the generalized complex exponential basis expansion model(GCE-BEM), we describe the time-variant channel with time-invariant coefficients over multiple OFDM blocks. The time variation of the channel destroys the orthogonality between subcarriers, and the resulting channel matrix in the frequency domain is no longer diagonal, but the main interference comes from the near subcarriers. Based on this, we propose a channel estimator with low pilot ratio. We first develop a least-square(LS) estimator under the assumption that only the maximum Doppler frequency and the channel order are known at the receiver, and then verify that the correlation matrix of inter-channel interference(ICI) is a scaled identity matrix based on which we derive an optimal pilot insertion scheme for the LS estimator in the sense of minimum mean square error. The proposed estimator has the advantages of low pilot ratio and robustness against inter-carrier interference.

• 한국연소학회:학술대회논문집
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• 2012.04a
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• pp.79-81
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• 2012

Flame stabilization conditions and combustion noise characteristics induced by premixed flames in sudden expansion channels were experimentally investigated. Nozzle size and channel scale were varied continuously, and variation of flame behaviors was examined. Combustion noise was observed at specific configurational conditions, and their mechanism was investigated. This study will help understand premixed flame instability at the burner surface.

• Journal of electromagnetic engineering and science
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• v.6 no.4
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• pp.203-208
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• 2006

Channel estimation and data detection for OFDM systems over time- and frequency-selective channels are investigated. Relying on the complex exponential basis expansion channel model, a pilot-embedded channel estimation scheme with low computational complexity and spectral efficiency is proposed. A periodic pilot sequence is superimposed at a low power on information bearing sequence at the transmitter before modulation and transmission. The channel state information(CSI) can be estimated using the first-order statistics of the received data. In order to enhance the performance of channel estimation, we recover the transmitted data which can be exploited to estimate CSI iteratively. Simulation results show that the proposed method is suitable for doubly-selective channel estimation for the OFDM systems and the performance of the proposed method can be better than that of the Wiener filter method under some conditions. Through simulations, we also analyze the factors which can affect the system performances.