• Journal of Korea Society of Industrial Information Systems
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• v.25 no.5
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• pp.13-21
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• 2020

This paper proposes a sign reversal channel switching method in space-frequency block code for orthogonal frequency division multiplexing system. In case of sending source data on other antenna, it is necessary for the receiver to change combining method according to the channel variation. If one does not know the predefined channel switching sequence, it is not possible to decode the received data precisely. In transmit data symbols' exchanges for a channel switching, data symbols are exchanged according to a format of space-frequency block code. In this paper, we proposes a simple sign reversal method except exchanging data symbols between transmit antennas. It is shown that this method occurs another combining method for a simple encryption in the receiver.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.8A
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• pp.1298-1310
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• 2001

이 논문에서는 채널 상태가 천천히 변할 때, 직교 주파수 분할 다중화-부호분할 다중접속 시스템에 적합한 채널 추정 방법을 제안하고 성능을 분석한다. 제안한 채널 추정 방법은 시간과 주파수 영역 모두에 파일럿 심볼을 삽입하고, 이산 퓨리에 변환 후 영을 채워 넣는 방법을 이용한 내삽법으로 모든 부채널의 전달 함수를 얻는다. 또한 변환 영역에서 저대역 여파를 통해 받은 파일럿 신호에 존재하는 가산성 백색 정규 잡음의 영향을 상당히 줄일 수 있다. 이 때 저대역 여파기의 차단 주파수는 채널의 다중경로 수에 따라 정해진다. 같은 방법을 이용한 내삽법을 시간축으로 적용하여 시간에 따라 변하는 부채널의 채널 응답을 얻을 수 있다. 이 논문에서는 여러 가지 내삽법에 대한 평균 제곱 오차 성능을 수식적으로 제시한다. 제안한 저대역 여파를 결합한 내삽법을 쓰면 채널의 통계적 특성에 관한 정보 없이, 그리고 훨씬 적은 계산량으로 선형 최소 평균 제곱 오차 추정기와 비슷한 성능을 얻는다. 레일리 감쇄 채널에 대한 모의 실험을 통해 같은 비트 오류율에 대한 신호 대 잡음 비의 이득이 있음을 보인다.

• Proceedings of the Korean Institute of Communication Sciences Conference
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• 1987.10a
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• pp.165-172
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• 1987

• Journal of the Korea Institute of Information and Communication Engineering
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• v.15 no.10
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• pp.2100-2112
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• 2011

An orthogonal code hopping multiplexing (OCHM) technique has been proposed for accommodating a large number of users with low channel activities than the number of orthogonal codewords through statistical multiplexing in downlink cellular systems. In this paper, a multiple input multiple output (MIMO) antenna based OCHM system is proposed to improve the performance. Each modulated symbol is repeated N times and the N repeated symbols are transmitted simultaneously using N transmit antennas. Through repetitions, the effect of perforations that the OCHM system experiences is decentralized among the repeated symbols and the full perforation probability is significantly reduced. Each receiver detect the transmitted signal using its pre-assigned code hopping pattern. Simulation results show that the proposed scheme saves the required energy for a given frame error rate (FER).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38A no.8
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• pp.697-703
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• 2013

We propose an efficient frequency-time domain resource allocation scheme in multi-carrier (MC) direct-sequence code-division multiple-access (DS/CDMA) communications. We consider, as a power allocation strategy in the frequency domain, transmitting each user's DS waveforms over the user's sub-band with the largest channel gain. We then consider rate adaptation in the time domain, where the data rate is adapted such that a desired transmission quality is maintained. We analyze the achievable average data rate of the proposed scheme with fixed average transmission power, and compare the performance to single carrier DS/CDMA systems with power and rate adaptations.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39C no.2
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• pp.199-208
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• 2014

Code division multiple access (CDMA) is one of the promising medium access control (MAC) schemes for underwater acoustic sensor networks because of its robustness against frequency-selective fading and high frequency-reuse efficiency. As a way of performance evaluation, sea or lake experiment has been employed along with computer simulation.. In this study, we design the underwater CDMA forward-link transceiver and evaluate the feasibility aginst harsh underwater acoustic channel in water-tank first. Then, based on the water-tank experiment results, we improved the transceiver and showed the improvements in a lake experiment. A pseudo random noise code acquisition process is added for phase error correction before decoding the user data by means of a Walsh code in the receiver. Interleaving and convolutional channel coding scheme are also used for performance improvement. Experimental results show that the multiplexed data is recovered by means of demultiplexing at receivers with error-free in case of two users while with less than 15% bit error rate in case of three and four users.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2008.11a
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• pp.247-250
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• 2008

본 논문에서는 다중안테나-직교 주파수 분할 다중화(multiple input multiple output orthogonal frequency division multiplexing, MIMO-OFDM)시스템 중 공간 다중화와 전송 다이버시티 이득을 동시에 얻기 위해 제안된 double space-time block code(D-STBC) OFDM 시스템에 대해 기존 수신 기법인 그룹단위 간섭 제거 기법의 문제점을 보완하는 새로운 간섭 제거 기법을 제안한다. 또한 STBC의 다이버시티 이득을 극대화 할 수 있는 컴포넌트 인터리버 구조를 송수신단에 추가함으로써 수신 성능을 크게 향상시킨다. 이동방송 수신을 고려한 빠른 시변 채널환경에서의 전산 모의 실험을 통해 STBC의 부호직교성 상실로 인한 성능열화 현상이 개선됨을 보이고 다비어시티 이득에 의해 수신 성능 향상을 확인한다.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2003.11a
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• pp.47-50
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• 2003

본 논문에서는 동영상을 효율적으로 부호화하기 위한 새로운 다중해상도 움직임 보상 방법과 잉여 양자화 방법을 제안한다. 본 논문에서 제안하는 동영상 부호화기는 다단계 이산웨이브렛 분해 움직임 예측 및 움직임 보상 블록 Tree 의 구성 및 블록 분할. 적응적 산술 부호화기로 구성된다 제안된 동영상 부호화기는 단순하면서도 낮은 연산량을 필요로 하며, 임베디드 특성과 SNR 계위 부호화 특성과 같은 좋은 기능을 제공한다. 또한 기존에 제안되었던 이산웨이브렛변환을 이용하는 동영상 부호화 방법과 비교하여 우수한 성능을 제공한다.

• The Journal of the Acoustical Society of Korea
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• v.36 no.1
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• pp.39-48
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• 2017

CDMA (Code Division Multiple Access) is promising medium access control schemes for underwater acoustic sensor networks because of its robustness against frequency-selective fading and high frequency-reuse efficiency. In this paper, we design diversity schemes of underwater CDMA transceiver for the forward and reverse links. User data are multiplexed by Walsh code and a pseudo random noise code acquisition process is added for phase error correction before decoding the user data at the receiver. Then, the diversity reception using equal gain combining and maximal ratio combining is performed in order to minimize performance degradation caused by rich multipath fading of underwater acoustic channel. We evaluated the performance of diversity transceiver through lake experiment, which was performed at Lake Kyungcheon, Mungyeong city using two transmitters and two receivers placed 460 m apart at an average depth of 40 m. The lake experiment results show that user data are recovered with error-free in both of the forward and reverse links.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.46 no.7
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• pp.13-23
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• 2009

In this paper, the network performance of a turbo coded optical code division multiple access (CDMA) system with cross-layer, which is between physical and network layers, concept is analyzed and simulated. We consider physical and MAC layers in a cross-layer concept. An intensity-modulated/direct-detection (IM/DD) optical system employing pulse position modulation (PPM) is considered. In order to increase the system performance, turbo codes composed of parallel concatenated convolutional codes (PCCCs) is utilized. The network performance is evaluated in terms of bit error probability (BEP). From the simulation results, it is demonstrated that turbo coding offers considerable coding gain with reasonable encoding and decoding complexity. Also, it is confirmed that the performance of such an optical CDMA network can be substantially improved by increasing e interleaver length and e number of iterations in e decoding process. The results of this paper can be applied to implement the indoor optical wireless LANs.