• Journal of the Institute of Electronics and Information Engineers
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• v.51 no.4
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• pp.3-9
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• 2014

ECMA-392 is the first cognitive radio (CR) specification for personal/portable devices in TV white space. It supports two transmit antennas, for which only open-loop transmit diversity schemes are included. In this paper, we design a simple closed-loop transmit diversity scheme using sub-carrier grouping for ECMA-392 systems. First, sub-carrier grouping types suitable for ECMA-392 systems are described, and then a transmit antenna selection scheme is proposed. In order to reduce feedback information, decision on the transmit antenna selection in the given channel environments is made at the receiver side and the only index of the decided transmit antenna is sent to the transmit side. Through performance comparison to open-loop transmit diversity schemes of ECMA-392, it will be shown that with only a slight feedback overhead, the proposed scheme can improve receiver performance of ECMA-392 systems.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.25 no.4
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• pp.91-100
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• 2017

Movements using aircraft are continuously increasing. This is because options for airlines, including eight national airlines, are becoming more diverse. In particular, FSC and LCC provide different services and prices, so it is not easy to choose, but it is analyzed that safety is the most important factor when selecting airlines in 2013 and 2017(all age groups are the same). There was a difference between 2013 and 2017 for each age group. In particular, the importance of price factors is high in 2017 compared to 2013, and other items in the 40s and 50s are also significantly changed.

• Proceedings of the KIEE Conference
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• 2003.07e
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• pp.79-82
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• 2003

The switching-mode power converter has been widely used because of its features of high efficiency and small weight and size. In the switching-mode power converter, the output voltage is generally controlled by varying the duty ratio of main switch. When a converter operates in steady state, duty ratio of the converter is kept constant. So the power of switching noise is concentrated in specific frequencies. The more white noise is injected, the more conducted EMI is reduced. But output-voltage is not sufficiently regulated. This is the reason why carrier frequency selection topology is proposed. In the case of carrier frequency selection, output-voltage of steady state and transient state is fully regulated. Spectrum analysis is performed on the Phase current and the CM noise voltage. The former is measured with Current Probe and the latter is achieved with LISN, which are connected to the spectrum analyzer respectively.

• Journal of Information Processing Systems
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• v.10 no.2
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• pp.163-175
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• 2014

This paper will focus on improving the performance of orthogonal frequency division multiplexing (OFDM) in Rayleigh fading environments. The proposed technique will use a previously published method that has been shown to improve OFDM performance in independent fading, based on ordered sub-carrier selection. Then, a simple non-iterative method for finding the optimal bit-loading allocation was proposed. It was also based on ordered sub-carrier selection. We compared both of these algorithms to an optimal bit-loading solution to determine their effectiveness in a correlated fading environment. The correlated fading was simulated using the JTC channel models. Our intent was not to create an optimal solution, but to create a low complexity solution that can be used in a wireless environment in which the channel conditions change rapidly and that require a simple algorithm for fast bit loading.

• Informatization Policy
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• v.18 no.3
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• pp.88-107
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• 2011

This paper proposes a prepaid system promotion policy for the 3G(WCDMA) MVNO with regard to the newly included paragraphs 3, 4 and 5 of Article 32 (User Protection) and Article 38 (Provision of Wholesale Telecommunications Services) of the Telecommunication Business Act, which was revised on March 22, 2010. As of June 2011, there are only a few prepaid system subscribers to the mobile communications service due to various limitations, including prepay, interconnection, carrier selection, the MVNO policy, and number portability. However, overseas communications service regulatory agencies and service providers are increasingly presenting policies and strategies for mobile prepaid plans, in order to accommodate the various customer demands that are increasing the use of smart phones and data. This paper advances various proposals concerning promotion of the prepaid system by the 3G MVNO under the current Telecommunication Business Act, including separation of the prepaid data system and the mobile network; introduction of a monthly fixed-rate hybrid prepaid system, a top-up system and USIM system; introduction of mobile network carrier selection; differentiated retail discounts between prepaid and post-paid prices; revision of the retail discount policy for wholesale provision; increase in the number of mandatory service providers; and in-depth consideration of the introduction of a number portability policy for the prepaid and post-paid systems.

• Korean Journal of Materials Research
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• v.25 no.9
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• pp.487-491
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• 2015

In photovoltaic power generation where minority carrier generation via light absorption is competing against minority carrier recombination, the substrate thickness and material quality are interdependent, and appropriate combination of the two variables is important in obtaining the maximum output power generation. Medici, a two-dimensional semiconductor device simulation tool, is used to investigate the interdependency in relation to the maximum power output in front-lit Si solar cells. Qualitatively, the results indicate that a high quality substrate must be thick and that a low quality substrate must be thin in order to achieve the maximum power generation in the respective materials. The dividing point is $70{\mu}m/5{\times}10^{-6}sec$. That is, for materials with a minority carrier recombination lifetime longer than $5{\times}10^{-6}sec$, the substrate must be thicker than $70{\mu}m$, while for materials with a lifetime shorter than $5{\times}10^{-6}sec$, the substrate must be thinner than $70{\mu}m$. In substrate fabrication, the thinner the wafer, the lower the cost of material, but the higher the cost of wafer fabrication. Thus, the optimum thickness/lifetime combinations are defined in this study along with the substrate cost considerations as part of the factors to be considered in material selection.

• Journal of Communications and Networks
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• v.12 no.6
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• pp.529-532
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• 2010

Multi-carrier demultiplexer/demodulator (MCDD) in an on-board processing (OBP) satellite used for digital multimedia services has two typical architectures according to the channel demultiplexing procedure: Multistage multi-carrier demultiplexer (M-MCD) or poly-phase fast Fourier transform (PPF). During the channel demultiplexing, phase and quantization errors influence the performance of MCDD; those errors affect the bit error rate (BER) performance of M-MCD and PPF differently. In this paper, we derive the phase error variances that satisfy the condition that M-MCD and PPF have the same signal to noise ratio according to quantization bits, and then, with these results, analyze the BER performances of M-MCD and PPF. The results provided here may be a useful reference for the selection of M-MCD or PPF in designing the MCDD in an OBP satellite communications system.

• ETRI Journal
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• v.34 no.5
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• pp.759-762
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• 2012

Carrier aggregation is an essential feature in the Long Term Evolution-Advanced (LTE-A) system, which allows the scalable expansion of the effective bandwidth to be delivered to user equipment (UE) through the concurrent use of radio resources across multiple component carriers (CCs). This system's optimal radio-resource use has received much attention under simultaneous access (SA) scenarios for multiple CCs (m-CCs). This letter establishes how many CCs a UE should simultaneously connect to maintain maximum uplink capacity. Under the m-CC LTE-A system, the spectral efficiency of the m-CC SA scheme ($m{\geq}2$) is compared with that of CC selection (CCS). Numerical results reveal that the 2-CC SA scheme outperforms CCS and performs almost equally to the m-CC SA scheme ($m{\geq}3$).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.4A
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• pp.291-301
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• 2009

In this paper, the effect of ISI(Inter-Symbol Interference) and ICI(Inter-Carrier Interference) due to time difference of arrival on OFDMA-based mobile multi-hop relay (MMR) systems is analyzed. Analyses are performed for the ISI caused by the previous OFDMA symbol transmitted from neighboring macro or relay cell as well as the ISI caused by the next OFDMA symbol transmitted from neighboring macro or relay cell. Then, an effective SINR(Signal to Interference plus Noise Ratio) estimation method and a path selection method considering time difference of arrival are proposed to minimize the effect of ISI and ICI. It is shown by simulation that the performance degradation caused by time difference of arrival can be significantly mitigated when the proposed path selection method is applied to the uplink of OFDMA-based MMR systems.

• Journal of Hydrogen and New Energy
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• v.27 no.4
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• pp.404-411
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• 2016

To select appropriate oxygen carrier candidates for chemical looping combustion, reduction characteristics of seven oxygen carriers were measured and discussed using three different reduction gases, such as $H_2$, CO, and $CH_4$. Moreover, attrition losses of those oxygen carriers also measured and compared. Among seven oxygen carrier particles, OCN703-1100 and NiO/bentonite particles showed higher oxygen transfer capacity than other particles, but these particles showed more attrition loss than other particles. C14 and C28 particles which used as cheap oxygen carriers in European country showed lower oxygen transfer capacity and less attrition loss. Based on the experimental results, we could select OCN717-R1SU, NC001, and N002 particles as candidates for future works because these oxygen carriers showed enough oxygen transfer capacity and good attrition resistance.