• ETRI Journal
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• 제34권3호
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• pp.330-340
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• 2012

In many applications of wireless sensor actor networks (WSANs) that often run in harsh environments, the reduction of completion times of tasks is highly desired. We present a new time-aware, energy-aware, and starvation-free algorithm called Scate for assigning tasks to actors while satisfying the scalability and distribution requirements of WSANs with semi-automated architecture. The proposed algorithm allows concurrent executions of any mix of small and large tasks and yet prevents probable starvation of tasks. To achieve this, it estimates the completion times of tasks on each available actor and then takes the remaining energies and the current workloads of these actors into account during task assignment to actors. The results of our experiments with a prototyped implementation of Scate show longer network lifetime, shorter makespan of resulting schedules, and more balanced loads on actors compared to when one of the three well-known task-scheduling algorithms, namely, the max-min, min-min, and opportunistic load balancing algorithms, is used.

• Journal of Communications and Networks
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• 제18권6호
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• pp.892-901
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• 2016

In the multi-user multiple-input multiple-output (MIMO) system, orthogonal random beamforming (ORBF) scheme is proposed to serve multiple users simultaneously in order to achieve the multi-user diversity gain. The opportunistic space-division multiple access system (OSDMA-S) scheme performs multiple weighting matrices during the training phase and chooses the best weighting matrix to be used to broadcast data during the transmitting phase. The OSDMA-S scheme works better than the original ORBF by decreasing the inter-user interference during the transmitting phase. To save more time in the training phase, a partly random multiple weighting matrices selection scheme is proposed in this paper. In our proposed scheme, the Base Station does not need to use several unitary matrices to broadcast pilot symbol. Actually, only one broadcasting operation is needed. Each subscriber generates several virtual equivalent channels with a set of pre-saved unitary matrices and the channel status information gained from the broadcasting operation. The signal-to-interference and noise ratio (SINR) of each beam in each virtual equivalent channel is calculated and fed back to the base station for the weighting matrix selection and multi-user scheduling. According to the theoretical analysis, the proposed scheme relatively expands the transmitting phase and reduces the interactive complexity between the Base Station and subscribers. The asymptotic analysis and the simulation results show that the proposed scheme improves the throughput performance of the multi-user MIMO system.

• 대한전자공학회논문지TC
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• 제46권2호
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• pp.18-26
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• 2009

송신단에서 채널 상태 정보를 알 수 있는 경우, 적응 알고리즘을 통한 전송 및 다중사용자 스케줄링을 통해 시스템 전송률을 향상시킬 수 있다. 본 논문에서는 비트 인터리버와 결합한 부호화된 직교 주파수 다중 분할 (BIC-OFDM; Bit-Interleaved Coded Orthogonal frequency Division Multiplexing) 기법을 기반으로 하는 다중안테나 (MIMO; Multiple-Input Multiple Output) 시스템을 고려한다. 먼저 Levin-Campello 알고리즘을 개선한 비트 로딩 (bit-loading) 기법을 제안하고, 이를 다중안테나 시스템으로 확장하여 한정된 개수의 신호 성상을 사용하는 데 따르는 잔여 파워 문제를 극복하는 알고리즘을 제시한다. 실험 결과는 제안하는 기법이 시스템 성능을 개선시키며 특히 높은 신호 대 잡음비 (SNR; Signal-to-Noise Ratio) 영역에서 기존의 기법에 비하여 큰 성능 이득을 제공함을 보여준다.

• Journal of Positioning, Navigation, and Timing
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• 제7권2호
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• pp.53-59
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• 2018

This paper proposes a pseudo-random beamforming technique for time-synchronized mobile base stations (BSs) for multi-cell downlink networks which have mobility. The base stations equipped with multi-antennas and mobile stations (MSs) are time-synchronized based on global positioning system (GPS) signals and generate a number of transmit beamforming matrix candidates according to the predetermined pseudo-random pattern. In addition, MSs generate receive beamforming vectors that correspond to the beam index number based on the minimum mean square error (MMSE) using transmit beamforming vectors that make up a number of transmit beamforming matrices and wireless channel matrices from BSs estimated via the reference signals (RS). Afterward, values of received signal-to-interference-plus-noise ratio (SINR) with regard to all transmit beamforming vectors are calculated, and the resulting values are then feedbacked to the BS of the same cells along with the beam index number. Each of the BSs calculates each of the sum-rates of the transmit beamforming matrix candidates based on the feedback information and then transmits the calculated results to the BS coordinator. After this, optimum transmit beamforming matrices, which can maximize a sum-rate of the entire cells, are selected at the BS coordinator and informed to the BSs. Finally, data signals are transmitted using them. The simulation results verified that a sum-rate of the entire cells was improved as the number of transmit beamforming matrix candidates increased. It was also found that if the received SINR values and beam index numbers are feedbacked opportunistically from each of the MSs to the BSs, not only nearly the same performance in sum-rate with that of applying existing feedback techniques could be achieved but also an amount of feedback was significantly reduced.

• 한국통신학회논문지
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• 제33권1A호
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• pp.50-58
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• 2008

다중사용자 무선 채널 환경에서 공간분할 다중접속 기법을 사용함으로서 전체 시스템의 전송률을 대폭 증가시킬 수 있다. 송신단에서 정확한 채널정보를 알고 있는 경우, 전송률을 적절하게 조절함으로서 시스템 성능을 보다 향상시킬 수 있다. 뿐만 아니라, 주파수 선택적 페이딩 특성을 갖는 광대역 무선 채널에서는 채널 부호화를 통해 채널 다이버시티를 활용하여 신뢰성이 높은 데이타 전송이 가능하도록 BIC-OFDM 기술을 사용한다. 본 논문에서는 제한된 궤환정보를 이용한 기회적 스케줄링 기법과 결합된 적응 변조 시스템을 제안하고자 한다. 제안된 기법은 주파수 다이버시티와 다중 사용자 다이버티시를 이용하여 링크 성능을 항샹시킬 수 있다. 또한, 적은양의 궤환정보를 위해 모든 부채널 대신 하나의 OFDM심볼을 기준으로 적응 변조 기법을 사용한다. 모의 실험에서는 본 논문에서 제안한 기법이 적절한 계산복잡도를 가지면서도 상당한 링크 성능 향상이 있음을 보여주고자 한다.