• The Journal of the Acoustical Society of Korea
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• 제13권1E호
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• pp.11-19
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• 1994

상태 지속 밀도를 사용하는 hidden Markov Models(HMM/SD)은 음성 신호의 시간적인 변화를 보다 명확하게 나타낼 수 있다 그러나 상태 지속 밀도가 완만하거나 제한된 상태가 길면 이러한 장점은 감소된다. 이러한 문제점을 해결하기 위하여, 본 논문에서는 상태간의 천이가 특정한 시간 구간에서만 발생하도록 하는 전에 경로 제한 조건을 갖는 HMM/GPC를 제한한다. HMM/GPC는 상태 지속을 제한하고 음성 신호의 시간적 변화를 단순하고 효과적으로 표현할 수 있다. 또한 HMM/SD와 HMM/GPC를 결합한 새로운 형태의 HMM/SD+GPC를 제안하고 성능을 비교하였다. HMM/GPC는 기존 Viterbi 알고리즘을 약간 수정하여 구현될 수 있다. HMM/GPC와 HMM/SD+GPC는 기존 HMM과 HMM/SD에 비하여 우수한 성능을 보일 뿐만아니라 계산량도 매우 작다. 화자도립 단독음 인식 실험에서, HMM/GPC(1.6%)의 최소 오차는 기존 HMM보다 1.1% 낮았고 계산량도 57% 감소하였다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제7권11호
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• pp.2616-2635
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• 2013

In this paper, we propose an iterative transceiver design in a multi-relay multi-user multiple-input multiple-output (MIMO) system. The design criterion is to minimize sum mean squared error (SMSE) under relay sum power constraint (RSPC) where only local channel state information (CSI)s are available at relays. Local CSI at a relay is defined as the CSI of the channel between BS and the relay in the $1^{st}$ hop link, and the CSI of the channel between the relay and all users in the $2^{nd}$ hop link. Exploiting BS transmitter structure which is concatenated with block diagonalization (BD) precoder, each relay's precoder can be determined using local CSI at the relay. The proposed scheme is based on sequential iteration of two stages; stage 1 determines BS transmitter and relay precoders jointly with SMSE duality, and stage 2 determines user receivers. We verify that the proposed scheme outperforms simple amplify-and-forward (SAF), minimum mean squared error (MMSE) relay, and an existing good scheme of [13] in terms of both SMSE and sum-rate performances.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제14권6호
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• pp.2686-2708
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• 2020

This paper investigates the energy efficiency of energy harvesting (EH) bidirectional cooperative sensor networks, in which the considered system model enables the uplink information transmission from the sensor (SN) to access point (AP) and the energy supply for the amplify-and-forward (AF) relay and SN using power-splitting (PS) or time-switching (TS) protocol. Considering the minimum EH activation constraint and quality of service (QoS) requirement, energy efficiency is maximized by jointly optimizing the resource division ratio and transmission power. To cope with the non-convexity of the optimizations, we propose the low complexity iterative algorithm based on fractional programming and alternative search method (FAS). The key idea of the proposed algorithm first transforms the objective function into the parameterized polynomial subtractive form. Then we decompose the optimization into two convex sub-problems, which can be solved by conventional convex programming. Simulation results validate that the proposed schemes have better output performance and the iterative algorithm has a fast convergence rate.

• Journal of Communications and Networks
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• 제13권3호
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• pp.232-239
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• 2011

This paper investigates the user selection problem of successive zero-forcing precoded multiuser multiple-input multiple-output (MU-MIMO) downlink systems, in which the base station and mobile receivers are equipped with multiple antennas. Assuming full knowledge of the channel state information at the transmitter, dirty paper coding (DPC) is an optimal precoding strategy, but practical implementation is difficult because of its excessive complexity. As a suboptimal DPC solution, successive zero-forcing DPC (SZF-DPC) was recently proposed; it employs partial interference cancellation at the transmitter with dirty paper encoding. Because of a dimensionality constraint, the base station may select a subset of users to serve in order to maximize the total throughput. The exhaustive search algorithm is optimal; however, its computational complexity is prohibitive. In this paper, we develop two low-complexity user scheduling algorithms to maximize the sum rate capacity of MU-MIMO systems with SZF-DPC. Both algorithms add one user at a time. The first algorithm selects the user with the maximum product of the maximum column norm and maximum eigenvalue. The second algorithm selects the user with the maximum product of the minimum column norm and minimum eigenvalue. Simulation results demonstrate that the second algorithm achieves a performance similar to that of a previously proposed capacity-based selection algorithm at a high signal-to-noise (SNR), and the first algorithm achieves performance very similar to that of a capacity-based algorithm at a low SNR, but both do so with much lower complexity.

• 한국정보통신학회논문지
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• 제20권7호
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• pp.1277-1282
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• 2016

차세대 무선 통신 시스템에서는 RF 에너지 하베스팅 기술을 이용하여 센서의 전원 부족 문제를 해결하고자 한다. 본 논문에서는 채널 추정 오차가 존재하는 에너지 하베스팅 네트워크에서 무선 정보 및 전력 동시 전송을 위한 효율적인 알고리즘을 제안하고자 한다. 먼저, 1차원의 완전 검색을 통해 최적의 채널 추정 주기를 찾은 후, MMSE 채널 추정기를 이용하여 채널을 추정한다. 추정된 채널 값을 기반으로, 최소 필요 획득 에너지 조건을 만족시켜주면서 데이터 전송률을 최대화할 수 있는 파워 할당 및 분할 방안을 제안하였다. 시뮬레이션을 통해 제안 방안은 완벽한 채널 추정을 가정한 최적 방안에 비해 10% 미만의 성능 저하가 있었지만, 기존 방안과 비교할 시에는 데이터 전송률을 20% 이상 향상시킴을 확인 하였다.

• 한국통신학회논문지
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• 제41권5호
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• pp.539-545
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• 2016

이 논문에서는 단말 간 직접 통신(Device-to-Device: D2D) 링크가 있는 셀룰러 망 안에서 증폭 후 전달 (Amplify-and- Forward: AF) 방식의 조력국을 사용하여 D2D 전송률을 높이고 D2D 단말기가 셀룰러 망에 미치는 간섭을 제어하는 방법을 제안한다. 전체 시스템의 quality-of-service (QoS)를 높이기 위해 두 가지 방법으로 조력국을 설계한다. 첫 번째는 D2D 단말기가 셀룰러 망에 미치는 간섭을 제한하면서 최저 D2D 전송률을 최대화하는 문제이고, 두 번째는 이것의 dual 문제인 각 D2D 단말기의 최소 전송률을 보장하면서 D2D 단말기가 셀룰러망에 미치는 간섭을 최소화하는 문제이다. 또한, 이 문제들은 semi-definite programming (SDP) 문제로 바뀔 수 있음을 보인다. 실험 결과에서는 AF 조력국이 시스템의 성능을 획기적으로 향상시킬 수 있음을 보인다.

• 한국태양에너지학회 논문집
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• 제34권1호
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• pp.81-90
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• 2014

The Jeju-Korea power system is a small-sized network with a system demand ranging from a autumn minimum of 350MW to a summer peak of 716MW. Because Jeju island is well exposed to north-east winds with high speed, applications to connect to Jeju power system are flooded. Considering physical/environmental constraints, Jeju Self-governing Province has also target for the wind power capacity of 1,350MW by 2020. It amounts to two or three times of Jeju average-demand power and wind power limit capacity announced by Korea Power Exchange (KPX) company. Wind farm connection agreements will be signed to maximize utilization of wind resource. In spite of submarine cable HVDC connected to Korea mainland, Jeju power system is independently operated by frequency and reserve control. This study reevaluates wind power limit based on the KPX criteria from 2016 to 2020. First of all wind power generation limit are affected by off-peak demand in Jeju power system. Also the possibility capacity rate of charging wind power output is evaluated by using energy storage system (ESS). As a result, in case of using 110MWh ESS, wind power limit increases 33~55MW(30~50% of ESS), wind power constraint energy decreases from 68,539MWh to 50,301MWh and wind farm capacity factor increases from 25.9 to 26.1% in 2020.

• 한국통신학회논문지
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• 제34권7A호
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• pp.562-567
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• 2009

본 논문에서는 여러 송수신 안테나를 갖는 송신기(source node), 중계기(relay node), 수신기(destination node)로 구성된 두 홉(two-hop) 빔포밍(beamforming) 중계 시스템을 고려한다. 송수신 심벌간 평균제곱오차(mean square error: MSE)를 최소화하는 송수신기 빔포밍 벡터와 중계기 가중치 행렬을 설계한다. 이때, 송신기 또는 중계기에 송신 전력을 줄이기 위하여, 국소(local)부동식전력제약(inequality power constraint)을 사용한다. 제약식이 있는 평균제곱오차 최소화 문제는 라그랑즈(Lagrange) 방법을 써 제약식이 없는 최적화 문제로 바꿀 수 있고, Karush-Kuhn-Tucker (KKT) 조건으로부터 그 해를 얻을 수 있다. 제안한 중계 시스템에 송신기와 중계기 송신전력을 각각 국소부동식으로 제약하여, 그 결과 두 홉에 채널 상태가 다를 경우, 최대 신호대잡음비(signal-to-noise ratio: SNR)를 얻는 기존 방식과 대등한 성능을 내며, 동시에 송신기 또는 중계기 송신 전력을 줄일 수 있다. 이를 모의실험을 통해 확인하였다.

• International Journal of Aeronautical and Space Sciences
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• 제8권1호
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• pp.10-20
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• 2007

For spacecraft attitude control, reaction wheel (RW) steering laws with more than three wheels for three-axis attitude control can be derived by using a control allocation (CA) approach.1-2 The CA technique deals with a problem of distributing a given control demand to available sets of actuators.3-4 There are many references for CA with applications to aerospace systems. For spacecraft, the control torque command for three body-fixed reference frames can be constructed by a combination of multiple wheels, usually four-wheel pyramid sets. Multi-wheel configurations can be exploited to satisfy a body-axis control torque requirement while satisfying objectives such as minimum control energy.1-2 In general, the reaction wheel steering laws determine required torque command for each wheel in the form of matrix pseudo-inverse. In general, the attitude control command is generated in the form of a feedback control. The spacecraft body angular rate measured by gyros is used to estimate angular displacement also.⁵ Combination of the body angular rate and attitude parameters such as quaternion and MRPs(Modified Rodrigues Parameters) is typically used in synthesizing the control command which should be produced by RWs.¹ The attitude sensor signals are usually corrupted by noise; gyros tend to contain errors such as drift and random noise. The attitude determination system can estimate such errors, and provide best true signals for feedback control.⁶ Even if the attitude determination system, for instance, sophisticated algorithm such as the EKF(Extended Kalman Filter) algorithm⁶, can eliminate the errors efficiently, it is quite probable that the control command still contains noise sources. The noise and/or other high frequency components in the control command would cause the wheel speed to change in an undesirable manner. The closed-loop system, governed by the feedback control law, is also directly affected by the noise due to imperfect sensor characteristics. The noise components in the sensor signal should be mitigated so that the control command is isolated from the noise effect. This can be done by adding a filter to the sensor output or preventing rapid change in the control command. Dynamic control allocation(DCA), recently studied by Härkegård, is to distribute the control command in the sense of dynamics⁴: the allocation is made over a certain time interval, not a fixed time instant. The dynamic behavior of the control command is taken into account in the course of distributing the control command. Not only the control command requirement, but also variation of the control command over a sampling interval is included in the performance criterion to be optimized. The result is a control command in the form of a finite difference equation over the given time interval.⁴ It results in a filter dynamics by taking the previous control command into account for the synthesis of current control command. Stability of the proposed dynamic control allocation (CA) approach was proved to ensure the control command is bounded at the steady-state. In this study, we extended the results presented in Ref. 4 by adding a two-step dynamic CA term in deriving the control allocation law. Also, the strict equality constraint, between the virtual and actual control inputs, is relaxed in order to construct control command with a smooth profile. The proposed DCA technique is applied to a spacecraft attitude control problem. The sensor noise and/or irregular signals, which are existent in most of spacecraft attitude sensors, can be handled effectively by the proposed approach.