• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제9권11호
• /
• pp.4419-4435
• /
• 2015

The use of an efficient packet scheduling scheme for a forward link in satellite communication networks is very important to support fairness for each return channel satellite terminal (RCST) and the service differentiations for user traffics. To support fairness and QoS for each RCST with service-level agreement (SLA), the Adaptive Coding and Modulation (ACM) system in a satellite hub has to process packets with considering modulation and coding (MODCOD) and packet types. Although a DVB-S2 system with ACM scheme has higher transmission efficiency, it cannot offer fairness or quality of service (QoS) to RCSTs. Because the data are transmitted with high MODCOD in regions with clear skies, while data are transmitted using low MODCOD in regions experiencing rain events. In this paper, we propose a two-step scheduling scheme offering fairness and QoS to RCSTs, while minimizing a decrease in throughput. The proposed scheme is carried out performance evaluations using a computer simulation. As results of this simulation, the proposed scheduler was shown to support bandwidth fairness to an individual RCST, and provide a level of QoS differentiation for user traffics.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제10권5호
• /
• pp.2144-2165
• /
• 2016

In a vehicular ad hoc network, the communication links are unsteady due to the rapidly changing topology, high mobility and traffic density in the urban environment. Most of the existing geographical routing protocols rely on the continuous transmission of beacon messages to update the neighbors' presence, leading to network congestion. Source-based approaches have been proven to be inefficient in the inherently unstable network. To this end, we propose an opportunistic beaconless packet forwarding approach based on a modified handshake mechanism for the urban vehicular environment. The protocol acts differently between intersections and at the intersection to find the next forwarder node toward the destination. The modified handshake mechanism contains link quality, forward progress and directional greedy metrics to determine the best relay node in the network. After designing the protocol, we compared its performance with existing routing protocols. The simulation results show the superior performance of the proposed protocol in terms of packet delay and data delivery ratio in realistic wireless channel conditions.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제11권5호
• /
• pp.2416-2432
• /
• 2017

In this paper, we investigate the security performance for cooperative networks over Nakagami-m fading channels. Based on whether the channel state information (CSI) of wiretap link is available or not, optimal relay selection (ORS) and suboptimal relay selection (SRS) schemes are considered. Also, multiple relays combining (MRC) scheme is considered for comparison purpose. The exact and asymptotic closed-form expressions for secrecy outage probability (SOP) are derived and simulations are presented to validate the accuracy of our proposed analytical results. The numerical results illustrate that the ORS is the best scheme and SRS scheme is better than MRC scheme in some special scenarios such as when the destination is far away from the relays. Furthermore, through asymptotic analysis, we obtain the closed-form expressions for the secrecy diversity order and secrecy array gain for the three different selection schemes. The secrecy diversity order is closely related to the number of relays and fading parameter between relay and destination.

• 한국통신학회논문지
• /
• 제28권8C호
• /
• pp.771-778
• /
• 2003

인접 기지국의 송신 신호는 단말기에서 볼 때 간섭 신호로 작용한다. 이러한 간섭 신호로 인하여 순방향 링크 용량이 줄어든다. 인접 기지국의 파일럿 간섭 신호를 제거함으로써 BER 성능을 개선할 수 있으며 순방향 링크 용량을 개선할 수 있다. 본 논문에서는 순방향 링크 용량을 개선할 수 있도록 인접 기지국에서 송신하는 파일럿 간섭 신호를 제거할 수 있는 복조기 구조를 제안하였다. 모의 실험 결과 인접 기지국의 파일럿 간섭 신호를 제거할 때 BER = 10-2를 얻기 위한 Eb/No 값이 간섭신호를 제거하지 않을 때에 비하여 3㏈ 감소하였다.

• Journal of Power Electronics
• /
• 제16권6호
• /
• pp.2243-2257
• /
• 2016

This paper presents a two stage three-phase proton exchange membrane fuel cell (PEMFC) generation system. When the system is connected to a three-phase load, it is very sensitive to the characteristics and type of the load. Especially unbalanced three-phase loads, which result in a pulsating power that is twice the output frequency at the inverter output, and cause the dc-link to generate low frequency ripples. This penetrates to the fuel cell side through the front-end dc-dc converter, which makes the fuel cell work in an unsafe condition and degrades its lifespan. In this paper, the generation and propagation mechanism of low frequency ripple is analyzed and its impact on fuel cells is presented based on the PEMFC output characteristics model. Then a novel method to evaluate low frequency current ripple control capability is investigated. Moreover, a control scheme with bandpass filter inserted into the current feed-forward path, and ripple duty ratio compensation based on current mode control with notch filter is also proposed to achieve low frequency ripple suppression and dynamic characteristics improvement during load transients. Finally, different control methods are verified and compared by simulation and experimental results.

• Journal of Power Electronics
• /
• 제16권3호
• /
• pp.1176-1189
• /
• 2016

In this paper, a novel quasi-direct power control (Q-DPC) scheme based on a resonant frequency adaptive proportional-resonant (PR) current controller with a variable frequency resonant phase locked loop (RPLL) is proposed, which can achieve a fast power response with a unity power factor. It can also adapt to variations of the generator frequency in T-type Three-level shaft synchronous generator (SSG) converters. The PR controller under the static α-β frame is designed to track ac signals and to avert the strong cross coupling under the rotating d-q frame. The fundamental frequency can be precisely acquired by a RPLL from the generator terminal voltage which is distorted by harmonics. Thus, the resonant frequency of the PR controller can be confirmed exactly with optimized performance. Based on an instantaneous power balance, the load power feed-forward is added to the power command to improve the anti-disturbance performance of the dc-link. Simulations based on MATLAB/Simulink and experimental results obtained from a 75kW prototype validate the correctness and effectiveness of the proposed control scheme.

• 한국통신학회논문지
• /
• 제28권3C호
• /
• pp.271-278
• /
• 2003

본 논문에서는 CDMA 하향링크에서 최적화가 용이한 새로운 다층 신경망을 제안하고 이를 적용한 신경망 수신기를 레이크 수신기와 비교하여 CDMA 하향링크에서 간섭이 제거되어 성능이 개선되었음을 입증하였다. 새로운 다층신경망은 기존의 다층신경망에 비해 시스템 복잡도가 개선되고 최적화가 용이하면서 기존의 다층신경망과 동일한 간섭제거에 의한 성능 향상 효과를 얻을 수 있어 실제 시스템에 적용하기에 적합하다. CDMA 하향링크에서 요구되는 고속의 데이터 전송을 위해 BLAST를 시작으로 STS, STTC, STC 등 다양한 STD 기술들이 제시되었으나 타사용자의 정보부재로 다중사용자 검파가 불가하고 충분한 수의 안테나를 장착할 수 없는 하향링크의 한계로 실질적으로 채널효율증대 효과가 미흡한 실정이다. ［10］-［15］이러한 문제점을 해결하기 위하여 복잡한 채널환경에 대해 적응능력이 뛰어나고, 고속의 병렬처리의 장점을 갖으며, 실시간 구현이 용이한 최적화된 단계층 신경망을 이용해서 다중사용자 간섭을 제거하였다. 여기에 STC 기술들을 접목시킨다면 CDMA 단말기의 획기적인 채널효율증대 가능성을 기대할 수 있을 것이다.

• Journal of Positioning, Navigation, and Timing
• /
• 제7권2호
• /
• pp.61-71
• /
• 2018

Existing radio positioning systems have a drawback that the attitude of user's tag is difficult to be determined. Although forward link angle of arrival (FLAOA) technology that uses measurements of array antenna arranged in a tag among the angle of arrival (AOA) technologies can estimate attitude and positioning of tags, it cannot extend the estimated results into three-dimensional (3D) results due to complex non-linear model displayed because of the effects of 3D positioning and attitude in tags. This paper proposed a radio navigation technique that determines 3D attitude and positioning via FLAOA / time of arrival (TOA) integration. According to the order of determining attitude and positioning, two integration techniques were proposed. To analyze the performance of the proposed technique, MATLAB-based simulations were used to verify the performance. The simulation results showed that the first proposed method, TOA-FLAOA integrated technique, showed about 0.15 m of positioning error, and $2-3^{\circ}$ of attitude error performances regardless of the positioning space size whereas the second method, differenced FLAOA-TOA integrated technique, revealed a problem that a positioning error became larger as the size of the positioning space became larger.

• 한국시뮬레이션학회논문지
• /
• 제22권1호
• /
• pp.41-51
• /
• 2013

이 논문에서는 EV-DO나 브로드캐스트 멀티캐스트 서비스와 같은 CDMA 셀룰러 표준을 목표로 하는 시스템 수준 시뮬레이터의 설계 방법론을 소개한다. 셀 레이아웃, 모바일 분포, 채널 모델링, 수신 전력 계산, 스케줄링, 패킷 에러 예측, 트래픽 생성 등의 절차를 따라 가면서, 시스템 수준 시뮬레이터의 기본 구조와 시뮬레이션 흐름을 기술하였다. 시뮬레이터를 모델링하기 위해, CDMA 시스템과 EV-DO 방송 시스템의 순방향 링크에서의 패킷 데이터 전송을 고려하였다. 셀룰러 표준을 위한 시스템 수준 시뮬레이터는 상위 수준 언어로 모델링 및 개발 되었고, 용량과 커버리지를 포함한 에어 접속부 성능 지표들을 계산 및 예측하는 데에 이용되었다.

• 한국정보통신학회논문지
• /
• 제9권8호
• /
• pp.1649-1655
• /
• 2005

동위상(I) 파일럿 채널에 할당된 셀룰러 셀 CCSSC와 직교위상(Q) 파일럿 채널에 할당된 핫 스팟 셀 HSCSSC가 공존하는 광대역 OFCDM 시스템에 환경하에서 분리된 I/Q채널 CCSC를 이용한 탐색 알고리즘을 제안하였다. 제안된 알고리즘은 이동 기지국에서 무선 인터넷을 사용하고자 할 때 셀룰러 셀 CCSSC의 영향으로 감소하는 최상의 핫 스팟 셀 HSCSSC을 빠르게 추적하는데 적합하다. 시뮬레이션 결과 제안된 셀 추적 알고리즘이 기존의 셀 추적 알고리즘과 비교하여 훨씬 빠른 결과를 수행할 수 있음을 보였다.