• KSII Transactions on Internet and Information Systems (TIIS)
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• 제6권2호
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• pp.593-609
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• 2012

We propose a scheme to reduce the overhead associated with channel state information (CSI) feedback required for opportunistic scheduling in wireless access networks. We study the case where CSI is partially overheard by mobiles and thus one can suppress transmitting CSI reports for time varying channels of inferior quality. We model the mechanism of feedback suppression as a Bayesian network, and show that the problem of minimizing the average feedback overhead is NP-hard. To deal with hardness of the problem we identify a class of feedback suppression structures which allow efficient computation of the cost. Leveraging such structures we propose an algorithm which not only captures the essence of seemingly complex overhearing relations among mobiles, but also provides a simple estimate of the cost incurred by a suppression structure. Simulation results are provided to demonstrate the improvements offered by the proposed scheme, e.g., a savings of 63-83% depending on the network size.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권3호
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• pp.1311-1324
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• 2019

In the traditional cellular network communication, the cellular user and the base station exchange information through the uplink channel and downlink channel. Meanwhile, device-to-device (D2D) users access the cellular network by reusing the channel resources of the cellular users. However, when cellular user channel conditions are poor, not only D2D user cannot reuse its channel resources to access the network, but also cellular user's communication needs cannot be met. To solve this problem, we introduced a novelty D2D communication mechanism in the downlink, which D2D transmitter users as half-duplex (HD) relays to assist the downlink transmission of cellular users with reusing corresponding spectrum. The optimization goal of the system is to make the cellular users in the bad channel state meet the minimum transmission rate requirement and at the same time maximize the throughput of the D2D users. In addition, i for the purpose of improving the efficiency of relay transmission, we use two-antenna architecture of D2D relay to enable receive and transmit signals at the same time. Then we optimized power of base station and D2D relay separately with consideration of backhaul interference caused by two-antenna architectures. The simulation results show that the proposed HD relay strategyis superior to existing HD and full-duplex (FD) models in the aspects of system throughput and power efficiency.

• 한국통신학회논문지
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• 제41권7호
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• pp.723-731
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• 2016

5G 이동무선통신에서 요구되는 증가된 데이터 속도를 제공하기 위한 핵심적인 기술 축은 대규모 어레이를 활용하는 MIMO 전송을 통한 주파수 효율의 향상이다. MIMO 전송을 위해서는 CSI-RS (channel state information-reference signaling)를 이용하는 채널 추정 및 추정 결과에 적합한 빔포밍이 필요하며, 따라서 빔포밍 벡터를 정의하는 코드북의 설계는 매우 중요한 이슈이다. 본 논문에서는 공간채널모델을 이용하여 생성된 채널의 통계적인 특성을 활용하여 DFT (discrete Fourier transform) 행렬 기반 다중 랭크 코드북의 설계 방안을 제안하였다. 제안 방식은 인접한 안테나 원소 간 위상차의 분포와 전송 레이어 별 선택되는 코드벡터의 특징을 고려하여 PMI (precoding matrix indicator)의 구조 변경을 하였으며, LTE (long-term evolution) 시스템에서 사용 중인 3GPP 표준 코드북과의 성능 비교를 통하여 제안 방식의 이득을 산출하고 검증하였다.

• 한국통신학회논문지
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• 제37B권10호
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• pp.938-946
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• 2012

간섭정렬(Interference Alignment : IA)은 높은 신호 대 잡음 비(Signal to Noise Ratio : SNR)에서 다중 사용자 간섭 채널에서 도달 가능한 최대 자유도를 얻을 수 있는 데이터 전송 기법이다. 그러나 대부분의 간섭정렬에 대한 연구는 네트워크 내의 모든 채널의 정보를 송, 수신단에서 완벽하게 알아야 하는 비현실적인 가정에 기반을 둔다. 본 논문에서는 K 사용자 다중안테나 간섭채널에서 간섭정렬을 위한 채널 상태 정보 (Channel-State Information : CSI)의 궤환 량을 효율적으로 감소시킬 수 있는 CSI 궤환 구조를 제안한다. 기존의 IA기법은 K의 2차함수의 형태로 궤환 오버헤드가 증가하는 반면, 제안된 궤환 구조는 송, 수신 단 사이에서 계산된 IA 송신기의 순차적인 교환을 통하여 K에 선형적으로 증가하는 오버헤드를 갖는다. 또한, 주어진 궤환 구조 하에서 제한된 궤환 채널에 의한 잔여 간섭의 세기를 분석하였으며, 이 때 발생하는 잔여 간섭을 최소화 할 수 있는 IA 조건, 즉, 주어진 채널환경에 따라 각 수신단에서 잔여 간섭을 최소화할 수 있는 최적의 IA 쌍을 순차적으로 결정하는 알고리즘을 제안한다.

• 한국통신학회논문지
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• 제32권9A호
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• pp.888-897
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• 2007

본 논문에서는 802.16e OFDMA/TDD 셀룰러 시스템에서 Sector Throughput과 Fairness를 향상시키기 위한 상향링크 동적자원할당 알고리즘을 제안한다. 제안된 알고리즘을 설명하기에 앞서 상하향링크 채널 정보의 차이를 언급하고, Round-robin 알고리즘에 사용자의 위치에 따라 할당 받을 수 있는 부채널 수를 제한하는 기존의 FLR 알고리즘과 전송전력 최소화 또는 전송률 최대화 Inner Closed-loop Power Control 알고리즘을 적용함으로서 10% 정도의 Throughput과 공평성 측면에서 높은 성능 향상을 볼 수 있다. 또한 상향링크 채널 사운딩을 통한 상향링크 채널 정보를 이용하여 FASA 알고리즘을 수행함으로써 FLR 을 적용한 Round-robin에 비하여 31.8%의 Sector Throughput이 향상되었다. User Selection, 부채널 할당 알고리즘, 전력할당 알고리즘과 Simulation Methodology 등은 Part I 논문에서 이미 언급한 바와 같다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권8호
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• pp.3498-3511
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• 2016

We investigate the channel state information (CSI) in multi-input multi-output (MIMO) cooperative networks that employ the amplify-and-forward transmission scheme. Least squares and expectation conditional maximization have been proposed in the system. However, neither of these two approaches takes advantage of channel sparsity, and they cause estimation performance loss. Unlike linear channel estimation methods, several compressed channel estimation methods are proposed in this study to exploit the sparsity of the MIMO cooperative channels based on the theory of compressed sensing. First, the channel estimation problem is formulated as a compressed sensing problem by using sparse decomposition theory. Second, the lower bound is derived for the estimation, and the MIMO relay channel is reconstructed via compressive sampling matching pursuit algorithms. Finally, based on this model, we propose a novel algorithm so called sparsity adaptive expectation maximization (SAEM) by using Kalman filter and expectation maximization algorithm so that it can exploit channel sparsity alternatively and also track the true support set of time-varying channel. Kalman filter is used to provide soft information of transmitted signals to the EM-based algorithm. Various numerical simulation results indicate that the proposed sparse channel estimation technique outperforms the previous estimation schemes.

• Journal of information and communication convergence engineering
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• 제5권1호
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• pp.35-41
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• 2007

In this paper, a modified Fuzzy C-Means (MFCM) algorithm is presented for nonlinear blind channel equalization. The proposed MFCM searches the optimal channel output states of a nonlinear channel, based on the Bayesian likelihood fitness function instead of a conventional Euclidean distance measure. In its searching procedure, all of the possible desired channel states are constructed with the elements of estimated channel output states. The desired state with the maximum Bayesian fitness is selected and placed at the center of a Radial Basis Function (RBF) equalizer to reconstruct transmitted symbols. In the simulations, binary signals are generated at random with Gaussian noise. The performance of the proposed method is compared with that of a hybrid genetic algorithm (GA merged with simulated annealing (SA): GASA), and the relatively high accuracy and fast searching speed are achieved.

• Journal of Communications and Networks
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• 제14권2호
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• pp.199-205
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• 2012

This work investigates the progressive linear precoder design for packet retransmissions in multi-input multi-output (MIMO) systems with multiple codewords and automatic repeat request (ARQ) bundling feedback. Assuming perfect channel state information, a novel progressive linear ARQ precoder is proposed in the perspective of minimizing the packet error rate. We devise the ARQ precoder by combining power loading and sub channel pairing between current retransmission and previous transmissions. Furthermore, we extend the design to the case that the channel estimation error exists. Numerical results show that the proposed scheme can improve the performance of MIMO ARQ systems significantly regardless of the channel estimation error.

• 유통과학연구
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• 제16권1호
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• pp.47-58
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• 2018

Purpose - China has become the world's largest market of Korean culture goods. However the distribution and information channel from which Chinese consumers can access to Korean contents remains under-explored. This study aimed to investigate Chinese consumers' information accessing behavior. Research design, data, and methodology - We collected the sample of 800 customers who are exposed to Korean Wave in China. Regression analysis is used to confirm the relationships in the research model. Results - The results showed that official channels such as state owned sites still play a significant role, Chinese consumers prefer to use unofficial channels and the private SNS is the most popular one. It is also found that users with low interests in Korean contents prefer offline/indirect channels while people who are highly immersed into Korean culture and spend longer time on it tend to favor online/unofficial channels. The results confirms that there are some users' characteristics such as gender, age, income and experience of visiting Korea are relevant to explain their propensity to select distribution and information channels for Korean contents. Conclusions - The study enables managers to develop the more effective distribution channel strategies according to the different target consumer groups.

• 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.