• 한국통신학회논문지
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• 제33권6A호
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• pp.635-642
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• 2008

본 논문에서는 time-varying Rayleigh fading channel에서 분산 안테나 시스템의 최적 전력 분배 기법을 제안한다. 기존 관련 연구에서는 각 기지국으로부터 수신되는 전력의 차이를 기지국간 전력 분배에 고려하지 않았으나, 최근 들어 빠르게 변하는 small scale fading까지 Channel State Information (CSI)로하여 전력 분배를 하는 경우에 대한 연구가 진행되었다. 하지만, 실제 무선통신 환경에서 송수신단간의 거리 차이 때문에 피드백이 반영된 신호가 수신될 때의 페이딩 채널은 피드백을 하는 시점과는 차이가 있고, 이로 인한 성능저하가 나타난다. 본 논문에서는 이러한 성능저하를 극복하고 CSI 피드백의 부담을 경감시키기 위하여 small scale fading을 제외하고, 느리게 변하는 경로손실과, 섀도잉에 의한 large scale fading값만을 피드백하는 분산 안테나 시스템을 제안하고 Bit Error Rate (BER)을 최소화하기 위한 최적 전력 분배 공식을 유도했다. 빠른 페이딩 환경에서, 제안된 최적 전력 분배 기법이 small scale fading까지 고려한 전력 분배 기법보다 월등히 좋은 성능을 보이며, 피드백 지연이 없는 환경에서도 1dB 이내의 성능을 보이는 것을 확인했다. 또한, 본 논문에서 Rayleigh fading channel을 가정하여 유도한 전력 분배 기법이 Line-of-Sight (LOS) 성분의 크기가 작은 Ricean fading channel에서도 최소의 BER과 거의 비슷한 값을 달성함을 보였다.

• The Journal of the Acoustical Society of Korea
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• 제17권3E호
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• pp.47-52
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• 1998

This paper prolposes a VFM (Variable Fading Memory)Kalman filtering and applies it to the parameter estimation for time-varying signals. By adaptively calculating the fading memory, the proposed algorithm does not require any predetermined fading memory when estimating the time-varying signal parameter. Moreover, the proposed algorithm has faster convergence speed than fixed fading memory one in case the signal contains an impulsive outlier. The performance of parameter estimation for time-varying signal is evaluated by computer simulation for two cases, one of which is the chirp signal whose frequency varies linearly with time and the other is the chip signal with an impulsive outlier. The experimental results show that the VFM Kalman filtering estimates the parameter of the chirp signal more rapidly than the fixed fading memory one in the region of an outlier.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2006년도 하계종합학술대회
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• pp.17-18
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• 2006

In this paper, we propose a robust detection scheme of OSTBCs with channel estimation errors over time-selective fading channels. Channel estimation errors are inevitable over time-selective fading channels and even small channel estimation errors dramatically degrade the performance of space-time block coding schemes. Therefore, it is desired to investigate the effect of channel estimation errors on the performance of the proposed detection scheme compared with the existing detection scheme. The proposed detection scheme minimizes noise enhancement and impact of channel estimation errors which occur in an existing detection scheme. It is shown by simulations that the proposed detection scheme performs better than the existing detection scheme over time-selective fading channels.

• Journal of Communications and Networks
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• 제11권5호
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• pp.473-479
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• 2009

Conventional frequency domain equalization (FDE) schemes were originally devised for quasi-static channels. Thus, such equalization schemes could suffer from significant performance degradation in fast-fading channels. This paper proposes a frequency domain equalization algorithm to mitigate the effect of fast time-varying fading. First, a mathematical expression is derived to quantify the total interference resulting from the time variation of the channel. Then, the proposed approach attempts to eliminate the effect of time-variations of the channel. This cancellation allows efficient use of the classical FDE structures in fast time-varying fading environments, although they are built upon the quasi-static channel model. Simulation results of bit-error-rate performance are provided to demonstrate the effectiveness of the proposed algorithm.

• 한국전자파학회:학술대회논문집
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• 한국전자파학회 2003년도 종합학술발표회 논문집 Vol.13 No.1
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• pp.239-242
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• 2003

Transmit diversity using STBC(Space Time Block Code) provides the same diversity gain as MRRC(Maximal Ratio Receiver Combining), when the fading channel is constant across two consecutive symbols. But, when the channel condition is changed for the two consecutive symbols, the transmit diversity using STBC does not offer good performance due to the large doppler shift. In this paper, we have proposed a interference cancellation scheme for performance enhancement in transmit diversity using STBC over time selective fading channel. Simulation results for various doppler shift rates are presented for the transmit diversity using the proposed scheme.

• ETRI Journal
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• 제26권6호
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• pp.622-634
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• 2004

In this paper, we discuss the design problem and the robustness of space-frequency trellis codes (SFTCs) for multiple input multiple output, orthogonal frequency division multiplexing (MIMO-OFDM) systems. We find that the channel constructed by the consecutive subcarriers of an OFDM block is a correlated fading channel with the regular correlation function of the number and time delay of the multipaths. By introducing the first-order auto-regressive model, we decompose the correlated fading channel into two independent components: a slow fading channel and a fast fading channel. Therefore, the design problem of SFTCs is converted into the joint design in both slow fading and fast fading channels. We present an improved design criterion for SFTCs. We also show that the SFTCs designed according to our criterion are robust against the multipath time delays. Simulation results are provided to confirm our theoretic analysis.

• 한국통신학회논문지
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• 제28권5C호
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• pp.506-513
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• 2003

Most existing space-time coding (STC) schemes have been developed for flat fading channels. To obtain antenna diversity gain, they rely on channel state information (CSI) required at the receiver through channel estimation techniques. This paper proposes a new decision feedback decoding scheme for Alamouti-based space-time block coding (STBC) transmission over time-selective fading channels. In wireless channels, time-selective fading effects arise mainly due to Doppler shift and carrier frequency offset, Modelling the time-selective fading channels as the first-order Gauss-Markov processes, we use recursive algorithms such as Kalman filtering, LMS and RLS algorithms for channel tracking. The proposed scheme consists of the symbol decoding stage and channel tracking algorithms. Computer simulations confirm that the proposed scheme shows the better performance and robustness to time-selectivity.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제6권10호
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• pp.2493-2508
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• 2012

The performance of a MIMO system with adaptive modulation (AM) and space-time coding over Ricean fading channels for perfect and imperfect channel state information (CSI) is presented. The fading gain value is partitioned into a number of regions by which the modulation is adapted according to the region the fading gain falls in. Under a target bit error rate (BER) constraint, the switching thresholds for AM are given. Based on these results, we derive the calculation formulae of the theoretical spectrum efficiency (SE) and average BER. As a result, closed-form SE expression and accurate BER expression are respectively obtained. Besides, using the approximation of complementary error function, a tightly closed-form approximation of average BER is also derived to simplify the calculation of accurate theoretical BER. Computer simulation shows that the theoretical SE and BER are in good agreement with the corresponding simulation, and the approximate BER is also close to the accurate one. The results show that the AM scheme in Ricean fading channel provides better SE than that in Rayleigh fading channel due to the direct-path propagation, and has performance degradation in SE and BER for imperfect CSI.

• 한국주조공학회지
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• 제24권2호
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• pp.85-93
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• 2004

Mechanical property of hypereutectic Al-Si alloy is changed according to size and distribution of primary Si. Consequently, the study on the refinement for primary Si is progressed for a long time. But such effect of refinement comes out fading phenomena with the lapse of time. Therefore, this study investigated the optimum condition of primary Si refinement for hypereutectic Al-Si alloy. And we observed various primary Si size with P's fading phenomena. The experiment results were as follows. For experiment of primary Si refinement, we made hypereutectic Al-Si alloy with various amounts of P addition. As a result of experiment, we obtained the fine microstructure at 0.01wt.%P. And the optimum condition of P addition, for preventing from growth of primary Si by P fading, is estimated 0.1wt.%P.

• Journal of Communications and Networks
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• 제6권1호
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• pp.1-8
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• 2004

In this paper, we investigate the properties and performance of space-time bit-interleaved coded modulation (STBICM) systems in fast Rayleigh fading channels. We first show that ST-BICM with QPSK signaling in fast fading channels possesses the uniform distance property, which makes performance analysis tractable. We also derive the probability distribution of the squared Euclidean distance between space-time symbols assuming uniform bit-interleaving. Based on the distribution, we show that the diversity order for each codeword pair becomes maximized as the frame length becomes sufficiently long. This maximum diversity order property implies that the bit-interleaver transforms an ST-BICM system over transmit diversity channels into an equivalent coded BPSK system over independent fading channels. We analyze the performance of ST-BICM in fast fading channels by deriving an FER upper bound. The derived bound turns out very accurate, requiring only the distance spectrum of the binary channel codes of ST-BICM. Numerical results demonstrate that the bound is tight enough to render an accurate estimate of performance of ST-BICM systems.