• Journal of the Institute of Electronics Engineers of Korea TC
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• v.45 no.4
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• pp.31-37
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• 2008

Conventional cooperative communications can attain gain of spatial diversity and path loss reduction because destination node independently received same signal from source node and relay node located between source node and destination node. However, these techniques bring about decreased spectral efficiency with relay node and increased complexity of receiver by using maximal ratio combining (MRC). This paper has proposed cooperative ARQ protocol that can improve the above problems and can get the better performance. This method can increase the spectral efficiency than conventional cooperative communication because if the received signal from source node is satisfied by the destination preferentially, the destination transmits ACK message to both relay node and source node and then recovers the received signal. In addition, if ARQ message indicates NACK relay node operates selective retransmission and we can increase reliability of system compared with that of general ARQ protocol in which source node retransmits data. In the proposed protocol, the selective retransmission and ARQ message are to be determined by comparing log-likelihood ratio (LLR) computation of received signal from source node with predetermined threshold values. Therefore, this protocol don't waste redundant bandwidth with CRC code and can reduce complexity of receiver without MRC. We verified spectral efficiency and BER performance for the proposed protocol through Monte-Carlo simulation over Rayleigh fading plus AWGN.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39A no.9
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• pp.535-547
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• 2014

We investigate BER (Bit Error Ratio) performance according to the gain of spatial and frequency diversities in uplink SC-FDMA of SIMO (Single Input Multiple Output) systems. The main results of the analysis in this paper are as follows. First, we prove that performance of integrated system for considering spatial and frequency diversity combining in parallel is equivalent with the performance of sequential system for performing diversity combining in sequence. By signal modeling, it is demonstrated that the performances of both systems are the same when the frequency diversity combining technique of the sequential system is equal to diversity combining technique of the integrated system, and spatial diversity combining technique of the sequential system is performed as MRC in advance of frequency diversity combining. Secondly, it is found that effect on the BER performance is different according to the gain of spatial and frequency diversities, respectively. The frequency diversity gain increases by increasing the number of subcarrier. It might affect the performance improvement of high SNR(Signal to Noise Ratio) while it maintains gap between performances of ZF(Zero Forcing) and MMSE(Minimum Mean Square Error) in frequency diversity combining schemes. Also, spatial diversity gain increases as the number of receiving antennas increases. It means that it can reduce performance gap between ZF and MMSE in frequency diversity combining schemes by increasing the number of receiving antennas. In addition, it might affect the performance improvement of the whole SNR. Finally, through the analysis of performance according to the spatial diversity gain, the performance of ZF in frequency diversity combining is equal to the MMSE if the number of receiving antennas is 6 or more.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.8 no.6
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• pp.63-69
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• 2008

In this paper, our goal is to find a power-effective protocol that improves the accuracy of transmission in sensor networks. Therefore we propose a cooperative communication protocol based on MRC(Maximal Ratio Combining) and cyclic code. In our proposal, one sensor node assists two others to communicate with a clusterhead that can get diversity effect and MRC can improve diversity effect also. The proposed protocol with cyclic code can correct error up to 3-bit and reduce decoding complexity compared with convolutional code. Simulation results reveal proposed protocol can save the network energy up to 6dB over single-hop protocol at BER(Bit Error Rate) of $10^{-2}$.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.9 no.11
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• pp.4623-4643
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• 2015

This paper considers physical-layer security protocols in multicast cognitive radio (CR) networks. In particular, we propose dual-hop cooperative decode-and-forward (DF) and randomize-and-forward (RF) schemes using partial relay selection method to enhance secrecy performance for secondary networks. In the DF protocol, the secondary relay would use same codebook with the secondary source to forward the source's signals to the secondary destination. Hence, the secondary eavesdropper can employ either maximal-ratio combining (MRC) or selection combining (SC) to combine signals received from the source and the selected relay. In RF protocol, different codebooks are used by the source and the relay to forward the source message secretly. For each scheme, we derive exact and asymptotic closed-form expressions of secrecy outage probability (SOP), non-zero secrecy capacity probability (NzSCP) in both independent and identically distributed (i.i.d.) and independent but non-identically distributed (i.n.i.d.) networks. Moreover, we also give a unified formula in an integral form for average secrecy capacity (ASC). Finally, our derivations are then validated by Monte-Carlo simulations.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.6 no.9
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• pp.2037-2051
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• 2012

Cognitive radio (CR) is envisioned as a promising paradigm of exploiting intelligence for enhancing efficiency of underutilized spectrum bands. In CR, the main concern is to reliably sense the presence of primary users (PUs) to attain protection against harmful interference caused by potential spectrum access of secondary users (SUs). In this paper, evolutionary algorithms, namely, particle swarm optimization (PSO) and genetic algorithm (GA) are proposed to minimize the total sensing decision error at the common soft data fusion (SDF) centre of a structurally-centralized cognitive radio network (CRN). Using these techniques, evolutionary operations are invoked to optimize the weighting coefficients applied on the sensing measurement components received from multiple cooperative SUs. The proposed methods are compared with each other as well as with other conventional deterministic algorithms such as maximal ratio combining (MRC) and equal gain combining (EGC). Computer simulations confirm the superiority of the PSO-based scheme over the GA-based and other conventional MRC and EGC schemes in terms of detection performance. In addition, the PSO-based scheme also shows promising convergence performance as compared to the GA-based scheme. This makes PSO an adequate solution to meet real-time requirements.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.41 no.12
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• pp.69-75
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• 2004

In the paper, system is combined multiband system with DS-UWB techniques with properties including low peak-to-average power ratio, robustness to multiuser interference and excellent security. Because each sub-band is not satisfied with coherence bandwidth, rake receiver in each sub-band is applied to the Proposed system receiver. Output of rake receiver is combined by using Maximal Ratio combining technology. In this paper we mathematically analyse the BER of the DS-UWB system with singleband and multiband systems in the narrow interference channel condition and multi user interference channel condition, the simulation results show that proposed scheme is getting robuster with increasing of the number of subbands.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.45 no.7
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• pp.45-52
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• 2008

In typical cellular systems using frequency reuse scheme, the terminal suffers a performance degradation due to the intercell interference signals from adjacent cells as the terminal moves toward the cell boundary. In this paper, a signal transmission and reception scheme which achieve spatial multiplexing and beamforming gain from a distributed MIMO (multiple-input multiple-output) channel using multiple-antenna terminal is proposed for the spectral efficiency enhancement in a multi-cell downlink environment, when geographically separated base stations cooperatively transmit signals. In particular, we analyze the effective signal-to-interference ratio and spectral efficiency of the proposed scheme for different frequency reuse patterns and for varying numbers of receive antennas, and compare with the performance of the MRC (maximal ratio combining) reception scheme in typical cellular systems. We evaluate the amount of transmission efficiency of the scheme by comparing the performance near the cell boundary where the strong intercell interference is experienced.

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.65 no.4
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• pp.316-320
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• 2016

This paper proposes an alternate time-switched multiplexed space-frequency block coding technique for single-carrier modulation with frequency domain equalization. The traditional multiplexed space-frequency block coding technique for single-carrier modulation uses multiple groups of two transmitters and suppresses the interference signals of other SFBC groups at the receiver. In this paper, we reconfigure transmit signals to adapt them for alternate time-switched multiplexed SFBC for single-carrier modulation with frequency domain equalization and receiver structures and propose a structure for transmitter and receiver, show that its performance is better than the traditional algorithm by simulations.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2010.07a
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• pp.12-14
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• 2010

오늘날 무선통신시스템은 제한된 주파수 자원을 효율적으로 사용하기 위한 다양한 기술이 적용되어 있다. 적응변조기법은 현재 채널 상황에 적응하여 적절한 변조방식을 선택하는 방법으로 통신시스템의 수율(throughput)을 높일 수 있어 대부분의 무선통신 시스템에서 사용되고 있다. 변조 선택의 기준으로는 보통 현 채널의 신호대간섭비(SNR)가 사용되며, 채널의 통계적 특성을 이용하면 변조 선택을 위한 SNR 경계 결정을 최적화시킬 수 있다. 본 논문에서는 송신단에서 안테나 선택을 하고 (transmit antenna selection) 수신단에서 MRC(maximal-ratio combining)를 사용하는 TAS/MRC 시스템에서 적응변조기법을 적용했을 때, 변조 선택을 위한 SNR 경계 결정법을 살펴보고 그 성능을 분석한다.

• Proceedings of the Korea Multimedia Society Conference
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• 2001.06a
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• pp.256-260
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• 2001

본 논문은 광대역 무선통신환경에서 3GPP(3rd Generation Partnership Project)에 기반한 비동기 IMT-2000 시스템의 송수신 시스템을 구성하여 기지국의 수신성능을 분석하였다. 이동국과 기지국간의 Uplink 채널 환경을 고려하였으며, 기지국의 수신신호는 다중경로 페이딩(Multi-path Fading)과 다중접속간섭(Mult Access Interference : MAI)의 영향으로 성능일 열화된다. 이에 대한 대책으로 Array 안테나를 채용하였을 경우와 MRC-Diversity(Maximal Combining Diversity)를 채용하였을 경우의 수신성능을 동일한 환경에서 비교 분석하였으며, 각각의 시스템에 간섭제거기를 직렬로 비교분석하였다. 본 논문에서 적용한 Array 안테나는 수신 신호의 방향에 따라 적응적으로 추적하여 수신 SNR(Singnal-to-Noise power Ratio)을 최대로 형성하여 시스템의 성능을 개선하는 기법이며, 안테나의 방향성(Directivity :D=2.67)을 이용하는 3-element Array 안테나를 채용하였다. 최대비 합성 다이버시티 기법은 다이버시티 기법 중 수신효율이 가장 좋으며 가지수(L=2)를 함수를 사용하였다. 성능분석 결과 다중경로 페이딩과 다중접속간섭 환경에서 최대비 합성 다이버시티 기법이 Array 안테나보다 수신효율이 우수하였다.