• Journal of Communications and Networks
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• 제5권2호
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• pp.157-166
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• 2003

This paper deals with the problem of channel identification for Single Input Multiple Output (SIMO) slow fading channels using clustering algorithms. Due to the intrinsic memory of the discrete-time model of the channel, over short observation periods, the received data vectors of the SIMO model are spread in clusters because of the AWGN noise. Each cluster is practically centered around the ideal channel output labels without noise and the noisy received vectors are distributed according to a multivariate Gaussian distribution. Starting from the Markov SIMO channel model, simultaneous maximum ikelihood estimation of the input vector and the channel coefficients reduce to one of obtaining the values of this pair that minimizes the sum of the Euclidean norms between the received and the estimated output vectors. Viterbi algorithm can be used for this purpose provided the trellis diagram of the Markov model can be labeled with the noiseless channel outputs. The problem of identification of the ideal channel outputs, which is the focus of this paper, is then equivalent to designing a Vector Quantizer (VQ) from a training set corresponding to the observed noisy channel outputs. The Linde-Buzo-Gray (LBG)-type clustering algorithms [1] could be used to obtain the noiseless channel output labels from the noisy received vectors. One problem with the use of such algorithms for blind time-varying channel identification is the codebook initialization. This paper looks at two critical issues with regards to the use of VQ for channel identification. The first has to deal with the applicability of this technique in general; we present theoretical results for the conditions under which the technique may be applicable. The second aims at overcoming the codebook initialization problem by proposing a novel approach which attempts to make the first phase of the channel estimation faster than the classical codebook initialization methods. Sample simulation results are provided confirming the effectiveness of the proposed initialization technique.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제11권1호
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• pp.253-271
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• 2017

In this paper, considering that perfect channel state information (CSI) is hard to achieve in practice, the downlink capacity of distributed antenna systems (DAS) with imperfect CSI and multiple receive antennas is investigated over composite Rayleigh fading channel. According to the performance analysis, using the numerical calculation, the probability density function (PDF) of the effective output SNR is derived. With this PDF, accurate closed-form expressions of ergodic capacity and outage probability of DAS with imperfect CSI are, respectively, obtained, and they include the ones under perfect CSI as special cases. Besides, the outage capacity of DAS in the presence of imperfect CSI is also derived, and a Newton's method based practical iterative algorithm is proposed to find the accurate outage capacity. By utilizing the Gaussian distribution approximation, another approximate closed-form expression of outage capacity is also derived, and it may simplify the calculation of accurate outage capacity. These theoretical expressions can provide good performance evaluation for downlink DAS for both perfect and imperfect CSI. Simulation results verify the effectiveness of the theoretical analysis, and the system capacity can be improved by increasing the receive antennas, and decreasing the estimation error or path loss. Moreover, the system can tolerate the estimation error variance up to about 0.01 with a slight degradation in the capacity.

• 한국정보통신학회논문지
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• 제19권7호
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• pp.1531-1538
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• 2015

본 논문에서는 두 개의 송·수신 센서 사이의 위상 오차를 보상하기 위해 터보 등화 기반의 MIMO 수중 통신의 효율적인 복호기 구조를 제시하였다. 시공간 부호화 시스템에서는 다중 송·수신기 사이의 위상 차이는 완벽하다고 가정하였으나, 본 논문에서는 MIMO 수중 통신 환경에서 서로 다른 송·수신기 사이의 위상 오류에 영향을 분석하였다. 그 결과 오류 성능이 심각하게 감소하게 됨을 알 수 있었으며, 오류 성능 영향을 최소화하기 위해 도착 시간 차이를 보상한 DD 위상 추적 방식 기반의 위상 보상 알고리즘을 제안하였다. 본 논문은 MIMO 수중 통신 환경에서의 위상 오류에 영향을 분석하고, 그 영향을 최소화 할 수중 채널 모델에 대한 보상 알고리즘을 제안하였다.

• IEIE Transactions on Smart Processing and Computing
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• 제5권6호
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• pp.383-389
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• 2016

This paper presents a novel automatic white balance (AWB) algorithm for consumer imaging devices. While existing AWB methods require reference white patches to correct color, the proposed method performs the AWB function using only an input image in two steps: i) white point detection, and ii) color constancy gain computation. Based on the dark channel prior assumption, a white point or region can be accurately extracted, because the intensity of a sufficiently bright achromatic region is higher than that of other regions in all color channels. In order to finally correct the color, the proposed method computes color constancy gain values based on the Y component in the XYZ color space. Experimental results show that the proposed method gives better color-corrected images than recent existing methods. Moreover, the proposed method is suitable for real-time implementation, since it does not need a frame memory for iterative optimization. As a result, it can be applied to various consumer imaging devices, including mobile phone cameras, compact digital cameras, and computational cameras with coded color.

• 한국통신학회논문지
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• 제34권9C호
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• pp.906-914
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• 2009

SC-FDMA는 차세대 이동 통신 방식으로 각광받고 있는 3GPP-LTE 시스템의 상향링크에 채택된 통신 방식으로 등화기를 FD-LE로 간단히 구현할 수 있다는 장점을 갖는다. 그러나 FD-LE는 잡음을 증폭하는 단점 때문에, FF 필터는 주파수 영역에서 구현하고 FB 필터는 시간 영역에서 구현하는 Hybrid-DFE보다 성능이 열악하다. 한편, FB 필터의 구현 및 처리 과정이 복잡한 Hybrid-DFE의 단점을 보완하고 FD-LE보다 우수한 성능을 얻기 위한 여러 가지 IB-DFE 알고리즘들이 최근 제안되고 있다. 본 논문에서는 여러 가지 IB-DFE 알고리즘을 3GPP-LTE 상향링크 시스템에 적용하고 BER을 계산하여 성능을 비교한다. 그리고 CAZAC 신호를 사용한 채널추정 오차와 도플러 시프트가 IB-DFE의 성능에 미치는 영향에 대해서 알아본다. 끝으로 IB-DFE 알고리즘의 계산량을 분석하여 설계자가 요구되는 서비스의 특성과 품질에 따라 적절한 알고리즘을 선택하고 반복 횟수를 결정하는 기준을 제시한다.