• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.46.2-46.2
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• 2017

Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high frequency Very Long Baseline Interferometry (VLBI) observations. In previous works it has been shown that non-dispersive (e.g. tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT, as well as the instrumental effects. In this work, we implement a further phase transfer between two FPT residuals (i.e. so-called FPT2) to calibrate the ionospheric effects based on their frequency dependence. We show that after FPT2, the coherence time at 3 mm can be further extended beyond 8 hours, and the residual phase errors can be sufficiently canceled by applying the calibration of another source, which can have a large angular separation from the target (> $20{\circ}$). Calibrations for all-sky distributed sources with a few calibrators are also possible after FPT2. One of the strengths and uniqueness of this calibration strategy is the suitability for high frequency all-sky survey observations including very weak sources. We discuss the introduction of a pulse calibration system in the future to calibrate the remaining instrumental effects and allowing the possibility of imaging the source structure at high frequencies with FPT2, where all phases are fully calibrated without involving any sources other than the target itself.

• Journal of Advanced Navigation Technology
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• v.26 no.6
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• pp.496-503
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• 2022

In the reality that the fishing population is decreasing and the single-man fishing vessels is increasing, mandatory equipment for navigation and radio equipments for the safety of fishing boats has continued to be added. Therefore, many equipment such as navigation, communication and fishing are installed in the narrow steering room, so it is very confusing and a number of monitors are placed in the front, which is a factor that degrades the function of maritime observation. To solve this problem, we studied an interface that integrates and operates to major radio facilities such as very high frequency-digital selective calling equipment (VHF-DSC), automatic identification system (AIS) and fishing boat location transmission device (V-pass) into one multi function display (MFD) based on LAN. In addition, IEC61162-450 UDP packets and IEC61162 sentence were applied to exchange data through link between MFD and radio equipments, and additional messages needed for each equipment and function were defined. The integrated MFD monitor is easily operated by the menu method, and the performance of the interface was evaluated by checking the distress and emergency communication functions related to maritime safety and the message transmission status by equipment.

• ETRI Journal
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• v.36 no.1
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• pp.42-50
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• 2014

Orthogonal frequency division multiplexing (OFDM) is a modulation technique that allows the transmission of high data rates over wideband radio channels subject to frequency selective fading by dividing the data into several narrowband and flat fading channels. OFDM has high spectral efficiency and channel robustness. However, a major drawback of OFDM is that the peak-to-average power ratio (PAPR) of the transmitted signals is high, which causes nonlinear distortion in the received data and reduces the efficiency of the high power amplifier in the transmitter. The most straightforward method to solve this problem is to use a nonlinear mapping algorithm to transform the signal into a new signal that has a smaller PAPR. One of the latest nonlinear methods proposed to reduce the PAPR is the $L_2$-by-3 algorithm, which is based on the discrete sliding norm transform. In this paper, a new algorithm based on the $L_2$-by-3 method is proposed. The proposed method is very simple and has a low complexity analysis. Simulation results show that the proposed method performs better, has better power spectral density, and is less sensitive to the modulation type and number of subcarriers than $L_2$-by-3.

• ETRI Journal
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• v.36 no.4
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• pp.554-563
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• 2014

The burgeoning growth of real-time applications, such as interactive video and VoIP, places a heavy demand for a high data rate and guarantee of QoS from a network. This is being addressed by fourth generation networks such as Long-Term Evolution (LTE). But, the mobility of user equipment that needs to be handed over to a new evolved node base-station (eNB) while maintaining connectivity with high data rates poses a significant challenge that needs to be addressed. Handover (HO) normally takes place at cell borders, which normally suffers high interference. This inter-cell interference (ICI) can affect HO procedures, as well as reduce throughput. In this paper, soft frequency reuse (SFR) and multiple preparations (MP), so-called SFRAMP, are proposed to provide a seamless and fast handover with high throughput by keeping the ICI low. Simulation results using LTE-Sim show that the outage probability and delay are reduced by 24.4% and 11.9%, respectively, over the hard handover method - quite a significant result.

• 전자공학회논문지 IE
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• v.48 no.4
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• pp.30-39
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• 2011

This paper proposed a new concept of information space called Smart Floor. Smart Floor is an intelligent space where a mobile robot can read and write specific information through Radio Frequency IDentification (RFID) tags which are mounted on Smart Floor to drive its goal position. RFID tag packaging technology is described for building Smart Floor. Also a mobile robot equipped passive RFID System with ultra high frequency (UHF) bandwidth has developed. The information that consists of an absolute position in the Smart Floor and desired direction saved on RFID tags is a simulated Q-value based on Q-learning algorithm. Proposed Smart Floor will be a proper method to communicate between space and robot.

• Journal of the Korea Institute of Military Science and Technology
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• v.20 no.3
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• pp.413-420
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• 2017

We investigate the performance of a true time delay(TTD) phase shifter to reduce the beam squint caused by frequency changes of a phased array antenna in wideband communication systems. To design a high gain phased array antenna, we need a long TTD, which causes high RF loss, low resolution and large dimension of TTD phase shifters. To overcome the problems, we propose a schematic of dual TTD phase shifters, which consists of short time delay(STD) in radio frequency(RF) part and long time delay(LTD) in intermediate frequency(IF) part. Our analysis results show that the proposed scheme reduces the required bits and delay time in RF band of the TTD compared to the conventional single TTD scheme.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.17 no.10
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• pp.2883-2902
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• 2023

Since the existing anti-jamming methods, including intelligent methods, have difficulty against high-speed reactive jamming, we studied a new methodology for jamming utilization instead of avoiding jamming. Different from the existing jamming utilization techniques that harvest energy from the jamming signal as a power supply, our proposed method can take the jamming signal as a favorable factor for frequency detection. Specifically, we design an intelligent differential frequency hopping communication framework (IDFH), which contains two stages of training and communication. We first adopt supervised learning to get the jamming rule during the training stage when the synchronizing sequence is sent. And then, we utilize the jamming rule to improve the frequency detection during the communication stage when the real payload is sent. Simulation results show that the proposed method successfully combated high-speed reactive jamming with different parameters. And the communication performance increases as the power of the jamming signal increase, hence the jamming signal can help users communicate in a low signal-to-noise ratio (SNR) environment.

• Journal of Positioning, Navigation, and Timing
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• v.10 no.4
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• pp.315-333
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• 2021

Due to the Global Navigation Satellite System (GNSS) modernization, recently launched GNSS satellites transmit signals at various frequency bands such as L1, L2 and L5. Considering the Korean Positioning System (KPS) signal and other GNSS augmentation signals in the future, there is a high probability of applying more complex communication techniques to the new GNSS signals. For the reason, GNSS receivers based on flexible Software Defined Radio (SDR) concept needs to be developed to evaluate various experimental communication techniques by accessing each signal processing module in detail. This paper proposes a novel SDR-based A-GNSS receiver capable of processing multi-GNSS/RNSS signals at multi-frequency bands. Due to the modular structure, the proposed receiver has high flexibility and expandability. For real-time implementation, A-GNSS server software is designed to provide immediate delivery of satellite ephemeris data on demand. Due to the sampling bandwidth limitation of RF front-ends, multiple SDRs are considered to process the multi-GNSS/RNSS multi-frequency signals simultaneously. To avoid the overflow problem of sampled RF data, an efficient memory buffer management strategy was considered. To collect and process the multi-GNSS/RNSS multi-frequency signals in real-time, the proposed SDR A-GNSS receiver utilizes multiple threads implemented on a CPU and multiple NVIDIA CUDA GPGPUs for parallel processing. To evaluate the performance of the proposed SDR A-GNSS receiver, several experiments were performed with field collected data. By the experiments, it was shown that A-GNSS requirements can be satisfied sufficiently utilizing only milliseconds samples. The continuous signal tracking performance was also confirmed with the hundreds of milliseconds data for multi-GNSS/RNSS multi-frequency signals and with the ten-seconds data for multi-GNSS/RNSS single-frequency signals.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.23 no.11
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• pp.1228-1238
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• 2012

In this paper, we design, fabricate and measure two kinds of high-frequency packages for K-band CMOS FMCW radar chips using RF via structures. The packages are fabricated with the conventional PCB process and LTCC process. The design centering of the packages is performed at 24 GHz and impedance variation caused by the wire bonding and RF via structure is fully evaluated using 3D electromagnetic simulation. The RF via structure with characteristic impedance of $50{\Omega}$ is used to reduce impedance mismatch loss. Two kinds of test packages with back-to-back connected RF paths are fabricated and measured for the design verification of the PCB-based package and LTCC package. Their measured results show an insertion loss of less than 0.4 dB at 24 GHz and less than 0.5 dB for 20~29 GHz. The measured return loss is less than -13 dB for the PCB-based package and less than -15 dB for the LTCC package in the frequency band, but the return loss of the package itself is predicted to be better than that of the test package by about 5 dB, because the ripples of the back-to-back connection typically degrade the return loss by 5 dB or more.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.57.2-57.2
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• 2017

KVN is a millimeter VLBI array composed of three 21m-diameter radio telescopes at Seoul, Ulsan and Jeju island in Korea. KVN has unique simultaneous multi-frequency receiving systems, which enable us to correct phase fluctuation of troposphere by transferring phase solution of low frequency data to higher frequency data. Although KVN can achieve very high performance up to 130 GHz through multi-frequency technique, imaging capability is highly limited because of lack of the number of baselines. In order to enhance imaging capability and maximizing multi-frequency capability, we plan to extend KVN baselines from 3 to 10 (or more) by constructing new KVN stations. This talk introduce expected performances, science cases, required budgets and periods of the Extended-KVN.