• Proceedings of the Korean Information Science Society Conference
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• 2000.04a
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• pp.665-667
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• 2000

본 논문에서는 방향 네트웍에서 안전한 통신 프로토콜을 연구한다. 본 논문은 Dolev 등의 방법[6]을 따라 일반적인 네트웍을 한 쌍의 전송자와 수신자가 여러개의 채널을 통해 연결되어 있는 모델로 단순화한다. Dolov 등은 채널이 모두 양방향이거나 또는 방향이 ？ 전송자에서 수신자인 경우만 다루었으나 여기서는 양방향과 단방향 채널이 동시에 존재하는 경우를 다룬다. 단방향 채널에는 전송자에서 수신자로의 방향 뿐만아니라 그 역방향으로만 통신이 가능한 채널도 포함된다. 본 논문은 정보이론적으로 안전한 통신이 가능하기 위한 필요충분조건을 구하고 효율적인 알고리즘을 제시한다. 이 알고리즘은 단순화한 모델에서의 채널을 정점분리 경로로 대응시켜 일반적인 방향 네트웍에서 적용될 수 있다.

• Information and Communications Magazine
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• v.12 no.5
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• pp.85-93
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• 1995

이동통신 서비스에 대한 수요가 크게 증가하므로 무선접속방식에 있어서 채널용량의 분석이 매우 중요한 요소로 부각되고 있다. 그러나 이동통신채널은 환경에 따라 매우 다르므로 용량을 정확하게 분석하기 어렵다. 이 논문에서는 PCS 환경에서 채널용량에 관계되는 요소들을 정량적으로 분석하고 이들을 사용하여 전형적인 CDMA와 TDMA 방식에 대한 채널용량을 분석하였다. 제시한 채널모델은 가능한 모델중에 하나가 될 수 있고, 제안된 채널 용량 분석방법중 요구되는 $E_b/N_a$ 값의 계산방식은 이동통신 시스팀에 있어 각 요소가 성능에 미치는 영향을 정량적으로 분석하는데 도움이 된다.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.21 no.7
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• pp.1335-1341
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• 2017

In this paper, a new architecture of multiple conference servers which use extended control channel framework in SIP(session initiation protocol) session has been presented. For this purpose, in this study, a new extended control channel framework architecture which can distribute total conference system loads to multiple servers effectively has been presented. In the implementation, extended control channels have been connected by using SIP sessions that was established between each conference servers, and extended control channel messages which can be transferred through control channels have been designed in this study. These extended control channel messages can distribute system load effectively between multiple conference servers, and conference information data format that can represent extended control channel framework has also been designed. Furthermore, exchange procedures of extended control channel messages have also been presented. The performance of the proposed system has been analysed by simulation. The analysis results show that average SIP messages delay time and average media stream delay time have improved.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.4A
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• pp.205-213
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• 2012

In this paper, we propose a new channel hopping scheme based on IEEE 802.11h as a good countermeasure against jamming attacks in IEEE 802.11 wireless networks. 802.11h Dynamic Frequency Selection (DFS) is a mechanism which enables hopping to a best channel with full channel measurement, not a randomly chosen channel, when the current link quality degradation occurs due to interferers such as military radars. However, under jammer attacks, this needs a time for full channel measurement before a new channel hopping and due to link disconnection during the time network performance degradation is inevitable. In contrast, our proposed schemes make an immediate response right after a jammer detection since every device is aware of next hopping channel in advance. To do this, a next hopping channel is announced by Beacon frames and the channel is selected by full channel measurement within Beacon intervals. Simulation results show that proposed scheme minimizes throughput degradation and keeps the advantages of DFS.

• Journal of Digital Convergence
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• v.12 no.12
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• pp.329-335
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• 2014

In this paper, to improve the performance of PB/MC-CDMA system, we have researched about the decision feedback channel estimation method using the pilot symbol with data symbol. The PB/MC-CDMA system is able to obtain the improved BER by frequency diversity gain and frequency domain equalizer in the frequency selective fading channel. However, when it is not the estimating of channel exactly, it is degrading the performance of BER in the system for occurred the interference among users. To improve the performane system in the multi-user environment the proposed system is using the decision feedback to estimate channel using the channel estimated value of the first stage and second stage. The proposed system is evaluated by computer simulation. The proposed system is not only able to improve the performance of BER by decreasing the interference to each user, but also the proposed system is possible to reduce number of pilot symbol to estimate the channel. Therefore, it confirmed the proposed system improves the performance than the conventional system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.10A
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• pp.992-1003
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• 2007

In this paper, a channel estimation technique based on pilot tones, which does not degrade channel estimation performance even with the existence of symbol timing offset (STO) in OFDM systems, is proposed. The proposed technique performs channel estimation by interpolating channels with respect to amplitude and phase with a minimum computational complexity, differently from the conventional interpolation techniques. The proposed technique requires neither the estimation of fine STO in advance nor trigonometric operation for phase interpolation, signifying a significant reduction in computational complexity. Since the performance of the proposed technique does not depend on the STO present in OFDM systems. It can be directly applied to the following areas in OFDM-based communication system: elimination of fine STO estimation step in the synchronization procedure, elimination of STO estimation step in multiuser uplink, and channel estimation in multi-hop relay system. It is verified by computer simulation that the proposed technique can improve the performance of channel estimation significantly in the presence of STOs, compared with previous channel estimation techniques based on pilot tones.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.7A
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• pp.1152-1161
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• 2001

본 논문에서는 이동국 수신 신호의 SINR을 최대화하는 빔 성형 알고리듬을 이용하여, 폐쇄 루프 송신 빔 성형기술을 적용한 순방향 링크 CDMA 시스템에서의 안테나 수, 귀환 채널의 구조, 귀환 지연 등과 시스템 성능과의 상관관계를 연구하였다. 폐쇄 루프 전송 빔 성형에서는 이동국이 각 안테나가 겪는 채널을 추정하여 수신 SINR을 최대화시키는 가중치 벡터를 계산하고, 귀환채널을 통해 가중치 벡터의 양자화된 진폭과 위상정보를 전송한다. 컴퓨터 모의 실험 결과는 송신 안테나가 2개, 3개, 4개로 늘어남에 따라 빔 성형 이득은 단일 송신 안테나와 비교해 $10^{-5}$ BER 근방에서 4.2dB, 5.8dB, 7dB로 증가하지만 양자화 오류에 의한 성능 저하 또한 0.1dB, 0.6dB, 1.3dB로 커지는 것을 보여준다. 또한 순방향 채널의 최대 도플러 주파수가 100Hz일 때에는 귀환 채널을 통한 소신 가중치 벡터를 보다 빠르게 갱신하는 것이 가중치 벡터의 양자화 레벨의 수를 늘여주는 것보다 $10^{-5}$ BER 근방에서 0.6dB 더 좋은 성능을 보이며, 최대 도플러 주파수가 10Hz일 때에는 가중치 벡터의 갱신 속도를 늘이기 보다 양자화 레벨의 수를 늘여주어 정확한 가중치 벡터를 전송하는 편이 0.9dB의 성능 향상을 보인다. 두 전력제어 그룹 길이의 귀환지연으로 인한 성능저하는 채널의 최대 도플러 주파수가 50Hz인 경우가 채널의 최대 도플러 주파수가 100Hz인 경우보다 $10^{-5}$ BER 근방에서 0.3dB 정도 더 작다. 또한 AOS가 3$^{\circ}$인 경우가 AOS가 $10^{\circ}$인 경우보다 $10^{-5}$ BER 근방에서 1.9dB 정도, 주파수 선택적 페이딩 채널이 주파수 비선택적 페이딩 채널보다 $10^{-5}$ BER 근방에서 1dB 정도 귀환 지연으로 인한 성능의 저하가 작다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.11
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• pp.1119-1126
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• 2011

In conventional microchannel heat exchangers, only one kind of fluid (hot or cold) flows in each plate. The channels contain different kinds of fluid depending on the vertical position, but they have the same kind of fluid at all horizontal positions. Therefore, there is a slower heat transfer rate in the horizontal direction than in the vertical direction. We propose a microchannel heat exchanger in which hot and cold fluid flows alternately in each plate to improve the thermal performance. This novel channel array requires a special design for the inlet and outlet. The proposed channel array has a faster heat transfer rate than a conventional channel array. The thermal performance of the novel channel array increases with increasing Reynolds number and Prandtl number, but it decreases as the ratio of solid to fluid thermal conductivity increases.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.18 no.10
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• pp.2389-2396
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• 2014

Spectrum handoff is the process of exchanging progressing data transmission from the current channel to another idle channel. The essential goal of spectrum handoff in CR(Cognitive Radio) networks is to perform consistent data transmission while sustaining performance of ongoing transmission of secondary users. This handoff procedure can cause additional latency that eventually affects on the performance of CR transmission. Channel prediction method is expected to avoid the disruption to primary users and to reduce the handoff latency. In this paper, adaptive channel prediction is proposed to cope with time-varying channel and an adaptive channel prediction based proactive handoff procedure is designed to enhance data transmission performance.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.5 no.3
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• pp.435-441
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• 2001

In this paper, we proposed adaptive MMSE receiver, which use channel equalizer to eliminate the interference due to multi-path fading and adaptive filter to eliminate the multiple access interference. The unique features of proposed receiver schemes are as following. We use pilot channel to estimate the channel coefficients exactly and guard symbols which are inserted periodically to estimate channel coefficients exactly without interference from user signals. The length of channel equalizer also can be reduced with the help of guard symbols. Especially utilizing adaptive code-matched filter(AMMSE) when the user population is high and SNR is not low we accepts excellent performance improvement.