• Journal of Communications and Networks
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• 제16권1호
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• pp.18-25
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• 2014

In this paper, we propose a codebook-based interference alignment (IA) scheme in the constant multiple-input multiple-output (MIMO) interference channel especially for the uplink scenario. In our proposed scheme, we assume cooperation among base stations (BSs) through reliable backhaul links so that global channel knowledge is available for all BSs, which enables BS to compute he transmit precoder and inform its quantized index to the associated user via limited rate feedback link. We present an upper bound on the rate loss of the proposed scheme and derive the scaling law of the feedback load to maintain a constant rate loss relative to IA with perfect channel knowledge. Considering the impact of overhead due to training, cooperation, and feedback, we address the effective degrees of freedom (DOF) of the proposed scheme and derive the maximization of the effective DOF. From simulation results, we verify our analysis on the scaling law to preserve the multiplexing gain and confirm that the proposed scheme is more effective than the conventional IA scheme in terms of the effective DOF.

• Journal of information and communication convergence engineering
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• 제18권1호
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• pp.1-7
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• 2020

This paper proposes a fast-processing and low-cost hardware multiple input multiple output (MIMO) channel emulator. The channel emulator is an important component of hardware-based simulation systems. The novelty of this work is the use of sharing and pipelining functions to reduce hardware resource utilization while maintaining a high sample rate. In our proposed emulator, the samples are created sequentially and interpolated to ensure the sample rate is equal to the base band rate. The proposed 4 × 4 MIMO requires low-cost hardware resource so that it can be implemented on a single field-programmable gate array (FPGA) chip. An implementation on Xilinx Virtex-7 VX980T was found to occupy 10.47% of the available configurable slice registers and 12.58% of the FPGA's slice lookup tables. The maximum frequency of the proposed emulator is 758.064 MHz, so up to 560 different paths can be processed simultaneously to generate 560 × 758 million × 2 × 32 bit complex-valued fading samples per second.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권4호
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• pp.1535-1554
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• 2016

This paper proposes a novel channel assignment scheme called multi-cluster based dynamic channel assignment (MC-DCA) to improve system performance for the downlink of dense femtocell networks (DFNs) based on orthogonal frequency division multiple access (OFDMA) and frequency division duplexing (FDD). In order to dynamically assign channels for femtocell access points (FAPs), the MC-DCA scheme uses a heuristic method that consists of two steps: one is a multiple cluster assignment step to group FAPs using graph coloring algorithm with some extensions, while the other is a dynamic subchannel assignment step to allocate subchannels for maximizing the system capacity. Through simulations, we first find optimum parameters of the multiple FAP clustering to maximize the system capacity and then evaluate system performance in terms of the mean FAP capacity, unsatisfied femtocell user equipment (FUE) probability, and mean FAP power consumption for data transmission based on a given FUE traffic load. As a result, the MC-DCA scheme outperforms other schemes in two different DFN environments for commercial and office buildings.

• 한국산업정보학회논문지
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• 제20권6호
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• pp.1-10
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• 2015

본 논문에서는 음영 지역을 위한 다중 중계기 기반의 주파수 효율적인 OFDMA(orthogonal frequency division multiple access) 전송 시스템을 제안한다. 본 논문에서는 다중 중계기를 통해 확보된 다중 경로에 주파수 공간 블록 부호를 적용하여 음영 지역의 시스템 성능을 향상시킨다. 또한, 주기적 프리픽스 부족으로 인해 발생하는 채널 주기성 손실을 복원하는 과정이 포함된 주기적 프리픽스를 사용하지 않는 OFDMA 시스템을 제안한다. 본 논문에서 제안된 주기적 프리픽스를 사용하지 않는 OFDMA 시스템의 성능이 충분한 주기적 프리픽스를 사용하는 기존 OFDMA 시스템의 성능과 거의 동일하여, 기존 시스템 대비 주파수 효율이 향상되는 것을 실험을 통해서 확인할 수 있다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권2호
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• pp.635-656
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• 2019

In this paper, we investigate the degrees of freedom (DoF) of a multi-cell multi-user multiple-input multiple-output (MIMO) interference broadcast channel (IBC) with non-cooperation distributed base stations (BS), where each BS serves users of its corresponding cell. When all BSs simultaneously transmit their own signals over the same frequency band in the MIMO IBC, the edge users in each cell will suffer the inter-cell interference (ICI) and inter-user interference (IUI) signals. In order to eliminate the ICI and IUI signals, a distributed space time interference alignment (DSTIA) approach is proposed where each BS has only limited access to distributed moderately-delay channel state information at the transmitter (CSIT). It is shown that the DSTIA scheme can obtain the appreciate DoF gains. In addition, the DoF upper bound is asymptotically achievable as the number of antenna at each BS increases. It is shown that the DSTIA method can get DoF gains over other interference alignment schemes with delayed CSIT in literature. Moreover, the DSTIA method can attain higher DoFs than the IA schemes with global CSIT for certain antenna configurations.

• 설비공학논문집
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• 제10권6호
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• pp.743-752
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• 1998

Experiments were performed to characterize single-phase heat transfer behavior of submerged liquid jet with multiple nozzle normally impinging on the smooth and extended surfaces. Arrays of 9 and 36 nozzles were used, with diameters of 0.5 to 2.0mm providing nozzle area ratio (AR) from 0.05 to 0.2. The square pin fin arrays were chosen as extended surfaces and the effects of geometrical parameters such as fin height, the ratio of fin width to channel width on heat transfer enhancement were examined. Single nozzle characteristics were also evaluated for comparison. The results clearly showed that heat transfer enhancement could be realized by using multiple nozzles at the constant volume flow rate. The average Nusselt number of multiple nozzle impingement on the smooth surface was correlated by the following equation : Nu/$Pr\frac{1}{3}=0.94 Re^{0.56}N^{-0.12}AR^{0.50}$The average heat transfer coefficients of multiple nozzle impingement on the extended surfaces decreased with increasing fin height and the ratio of fin width to channel width. The effectiveness of ex-tended surfaces ranged from 1.5 to 3.5 depending on the fin height, the ratio of fin width to channel width of pin fin arrays, nozzle number and nozzle area ratio.

• Journal of Communications and Networks
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• 제8권3호
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• pp.290-296
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• 2006

In this paper, a generalized blind adaptive algorithm is introduced for multi-user detection of direct sequence code division multiple access (OS-COMA) wireless communication systems. The main property of the proposed algorithm is its ability to resolve the multipath fading channel resulting in inter symbol interference (ISI) as well as multiple access interference (MAI). Other remarkable properties are its low complexity and mitigation to the near-far problem as well as its insensitivity to asynchronous transmission. The proposed system is based on the minimization of the output energy and convergence to the minimum mean square error (MMSE) detector. It is blind in the sense that it needs no knowledge of the other users' signatures, only the intended user signature and timing are required. Furthermore, the convergence of the minimum output energy (MOE) detector to the MMSE detector is analytically proven in case of M-ary PSK. Depicted results show that the performance of the generalized system dominates those previously considered. Further improvements are obtained when multiple input multiple output (MIMO) technique is employed.

• 한국정보통신학회논문지
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• 제20권5호
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• pp.979-985
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• 2016

본 논문에서는 확장 미디어 제어 채널을 사용하여 시스템의 부하를 효율적으로 분산 처리 할 수 있는 다중 서버구조의 분산 인스턴트 메시지 시스템을 제안하였다. 미디어 제어 채널은 실시간 전송 분야에서 SIP 서버와 클라이언트 사이의 신뢰성 있는 제어 채널 설정 및 유지 기능을 제공한다. 본 연구에서는 이 미디어 제어 채널의 기능을 확장하여 멀티미디어 데이터를 포함한 다량의 인스턴트 메시지를 효율적으로 다중 서버에 분산 처리 할 수 있는 새로운 구조의 인스턴트 메시지 시스템을 제안하였다. 이를 위하여 본 연구에서는 사용자 수의 증가에 따라 복수 개의 서버에게 동적으로 부하를 분산 처리하기 위한 미디어 제어 채널 메시지가 설계되었고 서버들 사이에서 미디어 제어 채널 메시지의 교환 절차가 제시되었다. 제안된 시스템의 성능은 시뮬레이션을 통하여 분석하였다.

• ETRI Journal
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• 제27권6호
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• pp.768-776
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• 2005

We compare the achievable throughput of time division multiple access (TDMA) multiple-input multiple-output (MIMO) schemes illustrated in the 3rd Generation Partnership Project (3GPP) MIMO technical report, versus the sum-rate capacity of space-time multiple access (STMA). These schemes have been proposed to improve the 3GPP high speed downlink packet access (HSDPA) channel by employing multiple antennas at both the base station and mobile stations. Our comparisons are performed in multi-user environments and are conducted using TDMA such as Qualcomm's High Data Rate and HSDPA, which is a simpler technique than STMA. Furthermore, we present the unified optimal power allocation strategy for HSDPA MIMO schemes by exploiting the similarity of multiple antenna systems and multi-user channel problems.

• 한국통신학회논문지
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• 제35권1C호
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• pp.45-54
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• 2010

이 논문에서는 다중사용자 (multi-user) 다중안테나 (multiple-in multiple-out : MIMO) 시스템을 위한 전송기법을 제안한다. Costa가 증명한 더티페이퍼코딩 (dirty-paper coding: DPC)이론에 따르면 송신기가 수신기의 가우시안 분포를 갖는 간섭 신호를 알고 있는 경우 간섭 신호에 상관 없이 채널 캐패시티를 얻을 수 있음이 알려져 있다. 단독사용자 채널의 경우 간단하며 효율적인 DPC기법들이 알려져 있으나 다중사용자 환경에는 실제 구현하는데 있어 복잡도와 관련해 많은 문제를 가지고 있다. 이 논문에서 우리는 다중사용자 환경에서 송신기에 알려진 갑선신호를 효율적으로 제어할 수 있는 네트워크 채널 행렬 삼각화 (network channel matrix triangulation: NMT)기법을 제안하고자 한다. 제안하는 NMT 알고리즘은 다중사용자 MIMO 채널을 서로 독립된 병렬의 Single-input Single-output (SISO) 채널들로 변환하여 단독사용자 환경을 위해 제안되어 있는 기존 DPC기법들을 다중사용자 환경에서도 사용 가능케 한다. 시뮬레이션 결과를 통해 제안된 알고리즘이 다중사용자 환경의 채널 캐패시티에 거의 접근함을 보일 것이다.