• KSII Transactions on Internet and Information Systems (TIIS)
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• v.15 no.6
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• pp.2204-2224
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• 2021

This paper studies secure wireless transmission from a multi-antenna transmitter to a single-antenna intended receiver overheard by multiple eavesdroppers with considering the imperfect channel state information (CSI) of wiretap channel. To enhance security of communication link, the artificial noise (AN) is generated at transmitter. We first design the robust joint optimal beamforming of secret signal and AN to minimize transmit power with constraints of security quality of service (QoS), i.e., minimum allowable signal-to-interference-and-noise ratio (SINR) at receiver and maximum tolerable SINR at eavesdroppers. The formulated design problem is shown to be nonconvex and we transfer it into linear matrix inequalities (LMIs). The semidefinite relaxation (SDR) technique is used and the approximated method is proved to solve the original problem exactly. To verify the robustness and tightness of proposed beamforming, we also provide a method to calculate the worst-case SINR at eavesdroppers for a designed transmit scheme using semidefinite programming (SDP). Additionally, the secrecy rate maximization is explored for fixed total transmit power. To tackle the nonconvexity of original formulation, we develop an iterative approach employing sequential parametric convex approximation (SPCA). The simulation results illustrate that the proposed robust transmit schemes can effectively improve the transmit performance.

• Journal of Biosystems Engineering
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• v.36 no.2
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• pp.116-121
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• 2011

This study was conducted to develop a robust navigator which could be in positioning for precision farming through developing a plural GPS receiver with 4 sets of GPS antenna. In order to improve positioning accuracy by integrating GPS signals received simultaneously, the algorithm for processing plural GPS signal effectively was designed. Performance of the algorithm was tested using a simulation program and a fixed point on WGS 84 coordinates. Results of this study are aummarized as followings. 1. 4 sets of lower grade GPS receiver and signals were integrated by kalman filter algorithm and geometric algorithm to increase positioning accuracy of the data. 2. Prototype was composed of 4 sets of GPS receiver and INS components. All Star which manufactured by CMC, gyro compass made by KVH, ground speed sensor and integration S/W based on RTOS(Real Time Operating System)were used. 3. Integration algorithm was simulated by developed program which could generate random position error less then 10 m and tested with the prototype at a fixed position. 4. When navigation data was integrated by geometrical correction and kalman filter algorithm, estimated positioning erros were less then 0.6 m and 1.0 m respectively in simulation and fixed position tests.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38A no.1
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• pp.10-18
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• 2013

Recently, a fixed complexity LR(fcLR) technique was proposed. Also QR-LRL signal detection method was proposed in which all constellation symbols are tried as the symbol corresponding to the least reliable layer (LRL), thereby achieving high error performance. In this paper, we combine these two efficient methods to propose a novel detection method. When the LRL is disregarded in the process of LR, the worst case complexity of LR is significantly reduced. Also, the proposed method is shown to be superior to the conventional fcLR-based detection method from the perspective of error performance. Simulations are performed to demonstrate the efficacy of the proposed method.

• Korean Journal of Optics and Photonics
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• v.15 no.4
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• pp.385-390
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• 2004

In this paper, we designed a 4-bit optical true time-delay(TTD) for phased array antennas(PAAs), which is composed of a wavelength-fixed optical source, 2 ${\times}$ 2 optical MEMS switches, and fiber-optic delay lines. A 4-bit TTD with a unit time delay difference of 6 ps for 10-GHz PAAs has been implemented. Measurement results on time delay show an error of -0.4 ps at maximum, corresponding to a radiation angle error of less than 1.63$^{\circ}$. Thus, the TTD implemented in this research performs in excellent agreement with theory. Each TTD line, composed of MEMS switches and fiber-optic delay lines, connected to the corresponding antenna element has insertion loss in between 1.36 ㏈ and 2.40 ㏈ depending upon the setup of the switches. On the other hand, the insertion loss difference between TTD lines was 0.32 ㏈ at maximum for a fixed radiation angle. The TTD structure proposed in this paper might be more reliable and economical than those previously proposed using tunable wavelength sources if proper power equalization either with gain control of RF amplifiers or variable attenuators is achieved.

• Publications of The Korean Astronomical Society
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• v.28 no.3
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• pp.95-100
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• 2013

A fiber-optic reference signal transmission system, which transmits the 1.4 GHz reference signal from H-maser to receiver cabin in radio telescopes, was adopted for compensating the phase changes due to temperature variation and antenna movement. At the first experiment, the remote signal's phase changed more than 15 degrees at 1.4 GHz. We found unstable components in sub-system experiments and replaced them. The main cause of unstable phase stability was the unaligned polarization axis between Laser Diode and Mach-Zehnder Modulator (MZM). The improved system stability showed $1{\times}10^{-16}$ allan standard deviation at 1,000 sec integration time with the antenna fixed. When the antenna moves in the azimuth axis, the 1.4 GHz remote signal showed the phase change smaller than 0.2 degrees.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.21 no.4
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• pp.718-723
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• 2017

We propose a pilot hopping scheme that improves the limited system capacity due to pilot contamination in multi-cell environment with large-scale antenna arrays at a base station, assuming the infinite number of antennas. In the conventional fixed pilot scheme, each user obtains the same signal-to-interference ratio (SIR) over a long period of time. Therefore, a user with strong interference has continuously low SIR which degrades its service quality. In the proposed pilot hopping scheme, different pilot signals are used for each time slot, and different amounts of interference are received every time. When such a pilot hopping technique is applied, the SIR fluctuates at every time slot. When the Hybrid Automatic Repeat & reQuest (HARQ) technique is applied in such a channel, the outage probability and transmission rate are improved. We show that there is the performance gain of the proposed scheme over the conventional scheme through computer simulations.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.14 no.7
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• pp.721-726
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• 2003

This paper present a waveguide slot 16${\times}$16 assay antenna with diaphragms for 38 GHz frequency band. Diaphragms are used to control the impedance and to minimize the return loss of the structure. A 20 dB Chebyshev array of 16 waveguide slots has been designed by optimizing the positions and sizes of the diaphragms. A serial feeding system with $\pi$- and T-junction power dividers has also designed to get a 20 dB Chebyshev power distribution. A 16${\times}$16 slot array was designed for a Fixed Wireless Access System at 38 GHz frequency band, manufactured using a computer controlled milling technique, and measured for the return -19 dB and the frequency band of 740 MHz.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.12
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• pp.1903-1913
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• 2016

In this paper, we propose 2-stage beamformer with linear receiver in wireless backhaul downlink system where macro base station has large antenna array with sub-array structure. Also, to compare the system capacity, we apply 3-stage beamformer with zero-forcing precoder and calculate the achievable sum rate of received small cell base stations. Considering scattering and path-loss property of wireless backhaul channel, we combine precoding technique for spatial multiplexing and beamforming technique to overcome path-loss. Therefore, we design DFT-based fixed beam patterns for the first stage. The simulation results show that considering spatial multiplexing, proposed 2-stage beamformer with linear receiver can increase the achievable sum rate as well as reduce the feedback information.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.20 no.12
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• pp.1279-1286
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• 2009

This paper addresses the analysis of the interference produced between the LEO(Low Earth Orbit) satellite constellation and FS(Fixed Service) system operating in the same frequency and area. At first, we calculates the interference of FS system from the LEO satellite constellation depending on the number of LEO satellite antenna beams. Simulation results show that the amount of interference that was calculated from each region. This result can be used to define the carrier level for protecting FS system from total interference by LEO satellite constellation. In the second scenario, we calculates the interference of LEO satellite system earth station by the FS link depending on radius of protection area. The presented results can be used to design FS systems minimizing interference to earth station.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.25 no.1
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• pp.99-107
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• 2014

Abruptly occurred obstacles on highway threaten driving safety. Radar draws the attention to the collision avoidance system because it can be fully operational in all weather, and day and night condition. This paper presents the design, implementation and performance test results of pulsed Doppler radar system for detection and warning of road obstacles. The system is designed to consider highway environment and detection capability about various fixed and moving obstacles. The system consists of 4 subsystems, which include antenna unit, transmitter and receiver unit, radar signal & data processing unit, and controller & display unit. The core technologies include clutter map based change detection for fixed obstacles detection, Doppler estimation for velocity detection of moving targets, and azimuth angle estimation method using monopulse for lane estimation and tracking. The design performance of the developed radar system is verified through experiments using a fixed reference target and moving vehicles in test highway.