• Journal of Communications and Networks
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• v.5 no.4
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• pp.354-364
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• 2003

A general method for the evaluation of the symbol error probability (SER) of ultra wideband (UWB) systems with various kind of modulation schemes (N-PAM, M-PPM, Bi-Orthogonal), in presence of multipath channel, multiuser and strong narrowband interference, is presented. This method is shown to be able to include all the principal multiaccess techniques proposed so far for UWB, time hopping (TH), direct sequence (DS) and optical orthogonal codes (OOC). A comparison between the performance of these multiple access and modulation techniques is given, for both ideal Rake receiver and minimum mean square error (MMSE) equalizer. It is shown that for all the analyzed multiple access schemes, a Rake receiver exhibits a high error floor in presence of narrowband interference (NBI) and that the value of the error floor is in-fluenced by the spectral characteristics of the spreading code. As expected, an MMSE receiver offers better performance, representing a promising candidate for UWB systems. When the multiuser interference is dominant, all multiple access techniques exhibit similar performance under high-load conditions. If the number of users is significantly lower than the spreading factor, then DS outperforms both TH and OOC. Finally 2PPM is shown to offer better performance than the other modulation schemes in presence of multiuser interference; increasing the spreading factor is proposed as a more effective strategy for SER reduction than the use of time diversity.

• Journal of Advanced Navigation Technology
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• v.5 no.1
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• pp.11-18
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• 2001

In this paper, we evaluated the performance of Orthogonal MC/FH-CDMA system considering the hit of frequency hopping patterns, and obtained the degree of the performance improvement by adopting TCM scheme under frequency selective Rayleigh fading channel. From the performance analysis, it is known that the error performance is improved, since delay time relative to symbol time is reduced as the number of carrier is increased. On the other hand, it is found that BER performance is improved about $5{\times}10^{-2}$ degree by adopting TCM scheme, and higher S/N is required if the QoS of voice (BER = $10^{-3}$) must be achieved.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.66 no.1
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• pp.179-185
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• 2017

Communication technologies hold a significant place in the swarm flight of drones for surveillance, inspection of disasters and calamities, entertainment performances, and drone collaborations. A GCS(ground control station) for the control of drone swarms needs its devoted communication method to control a large number of drones at the same time. General drone controllers control drones by connecting transmitters and drones in 1:1. When such an old communication method is employed to control many drones simultaneously, problems can emerge with the control of many transmitter modules connected to a GCS and frequency interference among them. This study implemented a transmitter controller to control many drones simultaneously with a communication chip of 2.4GHz ISM band and a Cortex M4-based board. It also designed a GCS to control many transmitter controllers via a network. The hierarchical method made it possible to control many more drones. In addition, the problem with frequency interference was resolved by implementing a time- and frequency-sharing method, controlling many drones simultaneously, and adding the frequency hopping feature. If PPM and S.BUS protocol features are added to it, it will be compatible with more diverse transmitters and drones.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.29 no.1
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• pp.225-234
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• 2019

Bitcoin is a cryptocurrency with characteristics such as de-centralization and distributed ledger, and these features are maintained through a mining system called "proof of work". In the mining system, mining difficulty is adjusted to keep the block generation time constant. However, Bitcoin's current method to update mining difficulty does not reflect the future hash power, so the block generation time can not be kept constant and the error occurs between designed time and real time. This increases the inconsistency between block generation and real world and causes problems such as not meeting deadlines of transaction and exposing the vulnerability to coin-hopping attack. Previous studies to keep the block generation time constant still have the error. In this paper, we propose a machine-learning based method to reduce the error. By training with the previous hash power, we predict the future hash power and adjust the mining difficulty. Our experimental result shows that the error rate can be reduced by about 36% compared with the current method.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.7 no.6
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• pp.1153-1160
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• 2003

In this paper, the performance of the UWB communication systems is analyzed in the presence of the Narrow Band Interference(NBI). UWB communication systems are modeled as using the Pulse Position Modulation(PPM). In this system, a Gaussian monocycle is used as the received pulses. The NBI is considered as a zero-mean random process with a constant spectral power density over its whole bandwidth. We obtain the mathematical expressions for describing the effect of the NBI on the UWB system. And it can be shown that the suppression of the effect of NBI on the UWB systems is available by adjusting the PPM related parameter.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.55 no.12
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• pp.526-533
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• 2006

In this paper, we presented the synchronization module between RF shower system and RFID Reader to extend interrogating range on Mobile RFID system, Costas Loop and FPLL(Frequency/phase Lock Loop) were used. We achieved compromised range of 3MHz locking frequency, 1ms locking time and figured out remarkable Hopping frequency of the Reader. The prototype of the new designed RFID system has been tested with ISO18000-6 type-B Tag. The read range between designed RFID Reader and Tag has been measured, it increased triple times by adjusting the Shower system output level.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.97-97
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• 2010

A new electric application method was developed to prepare epoxy/organoclay nanocomposite for the electrical insulation in the AC electric fields and it could be also used in the field of various viscous polymer/organoclay systems. The applied AC electric field condition was as follows; (1) inter-electrode distance: 40 mm, (2) application voltage: 3-11 kV, (3) frequency: 60~1,000 Hz, and (4) application time: 0~60 min. To characterize the epoxy/clay nanocomposite, WAXS and TEM analyses were confirmed. In order to explain how the organic modifier affects the exfoliation phenomena, a mechanism of the oscillating collision of the quaternary ammonium head was proposed and the effects of the AC voltage and frequency and the organoclay content were studied.

• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.172-172
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• 1999

Perovskite Manganese Oxide has been intensively investigated since the discovery of the colossal magnetoresistive(CMR) effect. In this paper, we studied the effect of temperature dependence and various doping dependence of rare earth site ions of La0.7-xPrxCa0.3MnO3 series using Ultraviolet Photoelectron spectroscopy(UPS). They show unusual temperature dependent features and the doped rare earth ions seem to affect the electron-phonon coupling strongly. We found clear evidence of metal-insulator transition from the spectral density at the Fermi level. but the transition temperature is lower than that deduced from transport measurements. Also we found that the spectral features change as time goes on implying that the surface of these materials is somewhat unstable in the vacuum. We can conclude from these results that the surface oxygen atoms correlated to the hopping electrons can escape from the material into the vacuum and that the surface state of these material is different from the bulk state.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2000.10a
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• pp.15-20
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• 2000

In this paper. Thermally Stimulated Current spectra have been measured in the Polypy - romellitimide (polyimide). As a result of the investigation. three peaks have been observed: $A(50^{\circ}C)$, $B(120^{\circ}C)$, $C(200^{\circ}C)$. Among three peaks, C peak has been observed in detail. The C peak is shown to be created due to the polarization of ionic space charge from the experimental results which were measured as a function of forming time, forming temperature. forming voltage and collecting voltage. As a result, we have measured the TSC in ion ($Na^+$) implanted - Polyimide, also found the peak at $215^{\circ}C$. Therefore, TSC characteristics in the C peak is probably caused by residual impurity. And The activation energy caculated by means of initial rising method 1.8eV, hopping distance. $90{\AA}$. and the mobility. $2.88{\times}10^{-11}cm^2/v.s$

• Journal of Korea Multimedia Society
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• v.16 no.5
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• pp.601-609
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• 2013

One of the most serious factors constraining the next generation cellular mobile consumer communication systems will be the severe co-channel interference experienced at the cell edge. Such a capacity-degrading impairment combined with the limited available spectrum resource makes it essential to develop more spectrally efficient solutions to enhance the system performance and enrich the mobile user's application services. This paper proposes a unique hybrid method of frequency hopping (FH) and subcarrier-reuse-partitioning that can maximize the system capacity by efficiently utilizing the available spectrum while at the same time reduce the co-channel interference effect. The main feature of the proposed method is that it applies an optimal combination of different frequency reuse factors (FRF) and FH-subcarrier allocation patterns into the partitioned cell regions. From the simulation results, it is shown that the proposed method can achieve the optimum number of subcarrier subsets according to the frequency-reuse distance and results in better performance than the fixed FRF methods, for a given partitioning arrangement. The results are presented in the context of both blocking probability and BER performances. It will also be shown how the proposed scheme is well suited to FH-OFDMA based cellular systems aiming at low co-channel interference performance and optimized number of subcarriers.