• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.4
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• pp.424-432
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• 2016

World's leading countries are developing next generation digital broadcasting system specifications to support UHDTV (ultra-high definition television) contents and other various services. In order to maximize the transmission capacity by using the bandwidth efficiently, most broadcasting systems adopt MIMO-OFDM. In distributed-MIMO systems, multiple transmit antennas are spatially separated and therefore result in multiple timing offsets. To overcome this problem, this paper proposes a technique using a null symbol to detect each individual signal from distributed transmit antennas. By inserting null symbols before preambles, the receiver can distinguish the signals between each transmit antennas and perform frame synchronization. When the reception time difference is shorter than 500 samples, the proposed method outperforms the conventional method.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.281-286
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• 2005

This paper presents sensitivity analysis of generalized rapid relaxation inversion (GRRI) algorithm for inverting controlled-source audio-frequency magnetotelluric (CSAMT) data. The algorithm was originally developed by modifying the RRI algorithm to recover a two-dimensional (2-D) conductivity structure of the Earth from MT data, but can be extended to include CSAMT data if it is combined with 2.5-D forward modeling. These GRRI approximate sensitivities are validated by comparison with exact 1-D and 2.5-D sensitivities. The comparison shows that the GRRI sensitivity is a good approximation to the exact sensitivity and has about half magnitude of the RRI sensitivity. Although the magnitude of the GRRI sensitivity is still slightly larger than that of the 2.5-D sensitivity, both sensitivities are broadly similar in shape when source-receiver offsets are greater than one skin depth on the Earth.

• Korean Journal of Materials Research
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• v.23 no.8
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• pp.430-434
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• 2013

Quantum dots(QDs) with their tunable luminescence properties are uniquely suited for use as lumophores in light emitting device. We investigate the microstructural effect on the electroluminescence(EL). Here we report the use of inorganic semiconductors as robust charge transport layers, and demonstrate devices with light emission. We chose mechanically smooth and compositionally amorphous films to prevent electrical shorts. We grew semiconducting oxide films with low free-carrier concentrations to minimize quenching of the QD EL. The hole transport layer(HTL) and electron transport layer(ETL) were chosen to have carrier concentrations and energy-band offsets similar to the QDs so that electron and hole injection into the QD layer was balanced. For the ETL and the HTL, we selected a 40-nm-thick $ZnSnO_x$ with a resistivity of $10{\Omega}{\cdot}cm$, which show bright and uniform emission at a 10 V applied bias. Light emitting uniformity was improved by reducing the rpm of QD spin coating.At a QD concentration of 15.0 mg/mL, we observed bright and uniform electroluminescence at a 12 V applied bias. The significant decrease in QD luminescence can be attributed to the non-uniform QD layers. This suggests that we should control the interface between QD layers and charge transport layers to improve the electroluminescence.

• Journal of Positioning, Navigation, and Timing
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• v.6 no.2
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• pp.79-86
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• 2017

Precise Point Positioning-Real Time Kinematic (PPP-RTK) refers to a technology that combines PPP with network-RTK in which a user does not directly receive observed data from a reference station but receives State-Space Representation (SSR) messages corrected for error components from a central processing station through Networked Transport of RTCM via Internet Protocol (NTRIP) or Digital Multimedia Broadcasting (DMB) for purposes of positioning. SSR messages, which refer to corrections used in PPP-RTK, are generated by a central processing station using real-time observed data collected from reference stations and account for corrections needed due to the ionosphere, troposphere, satellite orbital errors, satellite time offsets, and satellite biases. This study used a type of SSR message provided in South Korea, known as Korea-SSR (K-SSR), to implement a PPP-RTK algorithm based on code-pseudorange measurements and validated its accuracy within the reference station network. In order to validate the accuracy of the implemented algorithm outside of the network, the K-SSR was extrapolated and applied to positioning in reference stations in Changchun, China (CHAN) and Japan (AIRA). This also entailed a quantitative evaluation that measured improvements in accuracy in comparison with point positioning. The results of the study showed that positioning applied with extrapolated K-SSR correction data was more accurate in both AIRA and CHAN than point positioning with improvements of approximately 20~50%.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.92.1-92.1
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• 2014

In astronomical spectroscopy, stable auto-guiding and accurate target centering capabilities are critical to increase the achievement of high observation efficiency and sensitivity. We developed an instrument control software for the Immersion GRating INfrared Spectrograph (IGRINS), a high spectral resolution near-infrared slit spectrograph with (R=40,000). IGRINS is currently installed on the McDonald 2.7 m telescope in Texas, USA. We had successful commissioning observations in March, May, and July of 2014. The role of the IGRINS slit-viewing camera (SVC) is to move the target onto the slit, and to provide feedback about the tracking offsets for the auto-guiding. For a point source, we guide the telescope with the target on the slit. While for an extended source, we use another a guide star in the field offset from the slit. Since the slit blocks the center of the point spread function, it is challenging to fit the Gaussian function to guide and center the target on slit. We developed several center finding algorithms, e.g., 2D-Gaussian Fitting, 1D-Gaussian Fitting, and Center Balancing methods. In this presentation, we show the results of auto-guiding performances with these algorithms.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.24 no.9B
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• pp.1795-1805
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• 1999

In an OFDM (Orthogonal Frequency-Division Multiplexing) system, the sampling frequency offset between the transmitter and receiver is known to cause the interchannel interference (ICI), resulting in performance degradation. In this paper, we propose two time-domain techniques to estimate the sampling frequency offset, especially for a high data-rate OFDM system. The first technique estimates the sampling frequency offset by using the phase difference between two received samples with a fixed amount of time interval, corresponding to the transmitted training symbol, under the assumption of perfect symbol and carrier offset synchronization. The second technique estimates the sampling frequency offset and carrier frequency offset jointly, when the two offsets exist together, by using two training symbols with different frequency components and using a sample algebraic calculation. The proposed estimation techniques for sampling frequency offset cause no time delay due to all time-domain processing, and have a good performance due to no ICI effect. The performances of the proposed techniques are demonstrated by various simulations.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.16 no.8
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• pp.1631-1641
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• 2012

A TDMA system in satellite communication has been utilized. Especially DVB-S2 was standardized and now operated in satellite broadcasting system. In this paper, we propose a TDMA frame structure appropriate for special purpose which has the good reliablilty in a poor RF environment even if frequency efficiency is decreased. TDMA frame duration is 12 seconds which is long duration in comparison with general TDMA system with several ms. Designing the frame structure, time and frequency shift in single frame duration are considered. Simulation results show that the proposed frame structure and synchronization method has robust synchronization performance when the terminal is even in low SNR as well as 25 kHz frequency offsets.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.1B
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• pp.66-78
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• 2002

In this paper, we propose a new symbol offset estimation technique for an orthogonal frequency-division multiplexing (OFDM)-based wireless LAN. When both inter-symbol interference (ISI) and inter-channel interference (ICI) do not exist in an OFDM symbol, symbol offsets cause circular shifts in the estimated channel impulse response (CIR) by the amount of symbol offset. Also, the power delay profile of a typical multipath wireless channel can be modeled by exponentially decaying function, and most energy of multipath channel is concentrated at the beginning part of the CIR. Based on these properties, the proposed symbol offset estimation technique estimates the CIR, which is circularly shifted by the amount of symbol offset, and then calculates the partial mean power from the estimated impulse response by using a moving window with a finite length. And, symbol offset can be estimated from the index of a moving window having the maximal partial mean power. The proposed technique can reduce noise effect in the process of the CIR estimation, and remove ISI and ICI using repetitive training symbol structure in time-domain for minimum training overhead. The performances of the proposed symbol offset estimation technique in typical indoor channels are demonstrated by computer simulation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39A no.7
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• pp.372-379
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• 2014

When a hand-held device with limited battery transmits and receives data using short-range wireless communication systems, such as WLAN(Wireless Local Area Network) or high speed WPAN(Wireless Personal Area Network), instead of mobile communication systems, the device is able to reduce the power consumption due to the reduced transmission distance. However, if there are many WLAN or high speed WPAN systems around the device, non-negligible power may be consumed by receiving and decoding the packets which have nothing to do with the device. In this paper, we propose a scheme reducing the power consumption by including physical-layer ID in WLAN or WPAN packets and avoiding unnecessary packet receptions. Also, we describe a method to determine the optimum number of physical-layer IDs.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.7A
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• pp.702-709
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• 2008

This paper presents a frequency offset estimation scheme which can be used for packet based OFDM communication systems. The proposed scheme detects the failure of performing coarse frequency offset estimation and compensates the error of estimated coarse frequency offset. The preamble structure considered in this paper is based on the preamble specified in IEEE802.11a and IEEE802.11p standards. We performed simulation to compare the performance according to the different number of reference packets used to detect the failure of performing coarse frequency offset estimation. The simulation results show that the proposed scheme has better performance than the conventional scheme in the low SNR(below 2dB) environment.