• Bulletin of the Korean Chemical Society
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• v.30 no.11
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• pp.2701-2706
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• 2009

The polymer blends containing polypyrrole (PPy) and the sulphonic acids such as β-naphthalene sulfonic acid (NSA), camphor sulfonic acid (CSA), and dodecylbenzenesulfonic acid (DBSA) were synthesized by in situ deposition technique in an aqueous media using ammonium per sulfate (APS) as an initiator. The obtained films were characterized by scanning electron microscopy (SEM), and the thermal behavior of these polymer blends was analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The temperature-dependent (DC) conductivity of the obtained films shows a semiconducting behavior with a negative temperature coefficient of resistivity (TCR). The conductivity data were also analyzed through Mott’s equation, which provides the variable range hopping model in three dimensions. The parameters such as density of states at the Fermi energy, hopping energy, and hopping distance were calculated for PPy, PPy-NSA, PPy-CSA, and PPy-DBSA films, and the data were compared.

• Journal of the Korean Ceramic Society
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• v.40 no.9
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• pp.849-854
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• 2003

Temperature dependent thermoelectric power (TEP) of La$_{2.1}$ Sr$_{1.9}$ Mn$_3$O$_{10}$ system has been studied in the temperature range 80-373 K. In the low temperature ferromagnetic regime, TEP (S) follows an expression of formS＝S$_{0}$ ＋S$_{1.5}$ T$^{1.5}$ ＋S$_4$T$^4$ over the wide range of temperature. The broad peak below the ferromagnetic transition and complicated temperature dependence of S may be understood on the basis of electron-magnon scattering as predicted for an itinerant ferromagnet. High temperature TEP data can be well fitted with Mott's small polaron hopping model.

• Korean Journal of Materials Research
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• v.20 no.3
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• pp.161-166
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• 2010

The ac, dc conductivity and dielectric properties of $DyCoO_3$ were reported in the temperature range of 77 - 300K and in the frequency range of 20 Hz - 100 kHz. It was observed that at low temperature, ac conductivity is much higher than dc conductivity and the hopping carrier between localized states near the Fermi level was the dominant loss mechanism. A comparison of the measured ac conductivity $\sigma(\omega)$ was made with some of the models of hopping conductivity of the proposed earlier in the literature. It was observed that in $DyCoO_3$ the measured ac conductivity, over the entire frequency and temperature region, can be explained reasonably well by assuming two contributions $\sigma_1(\omega)$ and $\sigma_2(\omega)$ to the measured $\sigma(\omega)$. The first, $\sigma_1(\omega)$, which dominates at low temperature, may be due to impurity conduction in a small polaron; the second, $\sigma_2(\omega)$, which dominates at higher temperatures, depending on the frequency of measurements, may be due to the hopping of a small polaron and is reasonable for the dielectric relaxation peak.

• Journal of Advanced Navigation Technology
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• v.6 no.4
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• pp.318-325
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• 2002

In this paper, the effects of the interference environments on the performance of the time hopping(TH) binary PPM impulse radio(IR) system are presented. Based on the monocycle pulse available within the frequency of 3.1~10.6 GHz permitted for application by FCC, a PPM-modulated TH IR system simulator was designed and followed by the analysis of the monocycle pulse characteristics as well as the system performance. Particularly for the evaluation of the system performance, the multiple access interference and the narrowband system interference signals were considered as the interference signals. Since the narrowband system interference signal has very narrow bandwidth and very large amplitude compared with those of IR system, the analysis of the IR system performance was implemented by considering the interference power and band fraction ratio of the narrowband interference signal.

• Journal of the Institute of Electronics Engineers of Korea CI
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• v.45 no.4
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• pp.53-63
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• 2008

Reader collision may occur when neighboring RFID readers use the same channel at the same time. Especially when the readers are operated in dense mode, even though many channels are available, because of frequent reader collisions we can not guarantee the performance of RFID readers. Conventional solutions such as FH(Frequency Hopping) or LBT(Listen Before Talk) are not effective in this situation because they can not schedule RFID readers effectively when RFID readers are operated in multi-channel, dense reader mode, In this paper, we propose a new RFID reader anti-collision algorithm which employs LBT, random backoff before channel access, and probabilistic channel hopping at the same time. While LBT and Random backoff before channel access reduces collisions between competing readers, probabilistic channel hopping increases channel utilization by adaptively changing the hopping probability by reflecting the reader density and utilization. Simulation results shows that our algorithm outperforms conventional methods.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.7B
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• pp.1021-1033
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• 2001

본 논문에서는 초광대역 시간 도약 임펄스 무선 전송 (ultra wideband time hopping impulse radio) 다원 접속 시스템을 위한 잡음 부호 기반의 새로운 시간 도약 시퀀스를 제안한다. 임펄스 무선 전송 시스템에 대한 지금까지의 연구에서는 대부분 이상적인 랜덤 시간 도약 시퀀스를 사용하여 여러 사용자가 다원 접속한다고 가정하였다. 본 논문에서는 동일 차수의 다수 m-시퀀스와 십진화기를 이용하여 구현 가능한 시간 도약 시퀀스를 제안하고, 여러 사용자의 동기식 및 비동기식 접속 상황에 대한 모의 실험을 통해 이상적인 랜덤 시퀀스와 유사한 우수한 비트오율 성능을 얻을 수 있음을 확인하였다. 또한 제안된 시간 도약 시퀀스를 사용하는 경우 좀 더 일반적인 상황인 비동기식 다원 접속이 동기식 다원 접속에 비해 우수한 성능을 얻을 수 있음을 분석한다.

• ETRI Journal
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• v.35 no.5
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• pp.748-756
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• 2013

This paper addresses the problem of online hop timing detection and frequency estimation of multiple frequency-hopping (FH) signals with antenna arrays. The problem is deemed as a dynamic one, as no information about the hop timing, pattern, or rate is known in advance, and the hop rate may change during the observation time. The technique of particle filtering is introduced to solve this dynamic problem, and real-time frequency and direction of arrival estimates of the FH signals can be obtained directly, while the hop timing is detected online according to the temporal autoregressive moving average process. The problem of network sorting is also addressed in this paper. Numerical examples are carried out to show the performance of the proposed method.

• Proceedings of the IEEK Conference
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• 2004.06a
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• pp.75-78
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• 2004

In this paper, we analyze the performance of transmitter and receiver of W-CDMA system using the channel coding and space time code in downlink over ITU-R realistic channel model. We can improve the data rate and the reliability of communications by a channel code, and we can get a diversity gain by using the space time code over fading channels. We also apply the RAKE receiver to improve a performance in multi-path fading channel environment.

• Proceedings of the IEEK Conference
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• 2005.11a
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• pp.845-848
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• 2005

This paper describes a 4.224GHz CMOS charge pump PLL for Mode 1 MB-OFDM UWB hopping carrier generation. It includes a qudrature VCO of which the frequency range is from 3.98GHz to 4.47GHz(@ 0.4 to 1.5 V), a divider, a PFD, a loop filter, a charge pump, and a lock detector. Designed in a 0.18um CMOS technology, the PLL draws 6.6mA from a 1.8V supply. The phase noise of the designed VCO is -133dBc/Hz@3MHz.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.14 no.2
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• pp.139-147
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• 2003

In this paper we proposed system parameter values of M-ary PPM/TH(Pulse Position Modulation/Time Hopping) ultra-wideband systems having data rate of 100 Mbps for the frequency band, which is allocated by Federal Communications Commission(FCC). We also analyzed performance of the proposed system in the AWGN channels. According to the results of performance analysis, It is observed that available pulse duration is very limitative for this system. In case of constant data rate, if M and the pulse repetition numbers are increased, then the system performance is improved and the user number is decreased.