• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2009년도 춘계학술대회 논문집
• /
• pp.177-180
• /
• 2009

직경 대 길이비(L/D)의 값이 큰 고체 추진기관에서는 축방향 연소불안정 현상이 발생할 가능성이 높다. 일반적으로 이러한 현상을 억제하기 위해 추진제에 금속입자를 포함시키거나 그레인 설계시 축방향 압력 진동을 억제할 수 있도록 형상을 고안한다. Slotted-Tube형 그레인을 적용한 고체 추진기관은 연소시 Slot의 영향으로 인해 축방향 압력진동이 억제되나 Slot의 길이가 짧을 경우 연소 중반이후부터 실린더부의 영향으로 축방향 압력진동이 증폭될 수 있다. 본 연구에서는 230mm급 고체 추진기관의 연소 시험 결과 중 압력에 대해 스펙트럼 분석 및 음향모드 해석을 수행하여 축방향 압력 진동 현상을 분석하였다.

• 한국경영과학회:학술대회논문집
• /
• 대한산업공학회/한국경영과학회 2002년도 춘계공동학술대회
• /
• pp.418-425
• /
• 2002

Bluetooth is an emerging technology expected to provide users with short range, low cost, pico-cellular wireless connectivity. The access to the medium for Bluetooth is based on a Master driven Time Division Duplexing (TDD) scheme. A slave transmits packets in the reverse slot only after the master polls the slave (or transmits a packet to the slave) in a forward slot. The master transmits packets to a slave in even slots while the slave transmits packets to the master in an odd slot. The way in which the master schedules packets transmission to slaves or polls them determines system performance. In this paper. we propose a traffic adaptive MAC scheduling scheme for Bluetooth. The proposed scheme adopts the ISAR (Intelligent Segmentation and Reassembly) policy, which adjusts the packet size to the traffic patterns, to adapt the polling frequency to the traffic conditions. Also for achieving fairness among master-slave connections our scheme includes a priority policy assinging prioritised service tlimes to each connection. By considering a scenario where a Bluetooth master is used as wireless access point to the Internet, we show that our scheme improve the system throughput and average queue delay with regard to a naive Round Robin (RR) scheme.

• 한국전자통신학회논문지
• /
• 제11권6호
• /
• pp.581-590
• /
• 2016

본 논문은 다수의 RFID 리더가 공간상에서 무선네트워크를 형성하여 리더기마다 소속된 능동형 태그들 관리할 수 있는 효율적인 시스템을 설계 및 구현하고 특히 이 시스템에서 리더기-리더기 간 및 태그-태그 간에 발생하는 충돌을 회피하도록 하는 효율적인 알고리즘을 제안한다. RF 리더기가 무선 네트워크 노드들로서 작동하고 이들이 동기식으로 링크를 형성하여 리더기간 메시지를 주고받을 수 있도록 구성하였다. 능동형 태그 역시 각 리더기와 동기식으로 통신하도록 구성되며 태그 고유 ID를 사용하여 태그 간에 슬롯 점유를 경쟁시키는 알고리즘을 수행한다. 리더기 간 충돌방지를 위하여 각 리더기 노드들은 네트워크 통신 주기에서 서로 다른 시간 구간에서 리더기 기능을 수행하도록 하였다.

• 정보처리학회논문지C
• /
• 제16C권2호
• /
• pp.267-274
• /
• 2009

RFID 리더기가 영역내의 다수의 태그를 인식할 때 태그간의 통신간섭에 의해 태그인식이 방해 받는다. 이를 피하기 위해서는 충돌방지 알고리즘이 필요하게 된다. 충돌방지 알고리즘은 크게 슬롯 알로하 기반 알고리즘과 트리기반 알고리즘으로 나뉜다. 본 논문은 ISO 18000-6 TYPE A에 정의된 알로하 기반의 Framed Slotted ALOHA(FSA) 알고리즘에 태그와 슬롯간의 혼잡도를 이용하여 성능을 개선한 Dynamic Framed Slotted ALOHA-Slot Congestion(DFSA-SC) 알고리즘을 제안한다. 시뮬레이션 결과 최초 태그 수 추정의 정확도를 높여 전체 태그인식 시간이 줄어든 것을 확인 할 수 있었다. 나아가 태그 아이디의 중복성이 클 경우 대표적인 트리기반 알고리즘인 Query Tree 알고리즘보다 제안된 알고리즘이 우수한 것을 확인 할 수 있었다.

• Advances in aircraft and spacecraft science
• /
• 제6권4호
• /
• pp.333-348
• /
• 2019

Low fuel regression rate is the main drawback of hybrid rocket which should be overcome. One of the improvement techniques to this problem is usage of a solid fuel grain with a complicated geometry port, which has been promoted owing to the recent development of additive manufacturing technologies. In the design of a hybrid rocket fuel grain with a complicated geometry port, the understanding of fuel regression behavior is very important. Numerical investigations of fuel regression behavior requires a capturing method of solid fuel surface, i.e. gas-solid interface. In this study, level set method is employed as such a method and the preliminary numerical tool for capturing a hybrid rocket solid fuel surface is developed. At first, to test the adequacy of the numerical modeling, the simulation results for circular port are compared to the experimental results in open literature. The regression rates and oxidizer to fuel ratios show good agreements between the simulations and the experiments, after passing enough time. However, during the early period of combustion, there are the discrepancies between the simulations and the experiments, owing to transient phenomena. Second, the simulations of complicated geometry ports are demonstrated. In this preliminary step, a star shape is employed as complicated geometry of port. The slot number effect in star port is investigated. The regression rate decreases with increasing the slot number, except for the star port with many slots (8 slots) in the latter half of combustion. The oxidizer to fuel ratio increases with increasing the slot number.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제18권2호
• /
• pp.511-527
• /
• 2024

This paper proposes a block-based self-organizing time-division multiple access (BSO-TDMA) protocol for very high frequency (VHF) data exchange system (VDES) in shipborne ad-hoc networks (SANETs). The BSO-TDMA reduces the collisions caused by the simultaneous transmission of automatic identification system (AIS) messages by uniformly allocating channel resources using a block-wise frame. For this purpose, the BSO-TDMA includes two functional operations: (1) frame configuration and (2) slot allocation. The first operation consists of block division and block selection. A frame is divided into multiple blocks, each consisting of fixed-size subblocks, by using the reporting interval (RI) of the ship. Then, the ship selects one of the subblocks within a block by considering the number of occupied slots for each subblock. The second operation allocates the slots within the selected subblock for transmitting AIS messages. First, one of the unoccupied slots within the selected subblock is allocated for the periodic transmission of position reports. Next, to transmit various types of AIS messages, an unoccupied slot is randomly selected from candidate slots located around the previously allocated slot. Experimental simulations are conducted to evaluate the performance of BSO-TDMA. The results show that BSO-TDMA has better performance than that of the existing SOTDMA.

• 한국통신학회:학술대회논문집
• /
• 한국통신학회 1991년도 추계종합학술발표회논문집
• /
• pp.137-140
• /
• 1991

The TDX-10 Time Switch architecture has modularity, high reliability and considerable large switch fabric having separated and both-way 1K time slot interchange switching circuit elements. In this paper, we present key functions, architecture, features and traffic characteristic of TDX-10 Time Switch. And we also describe some basic implementation technologies such as Frame Base Read-Write Separation Method, Multi-Write Method and Read-Write Separation Technique with Dual-port Memory.

• 한국항행학회논문지
• /
• 제12권6호
• /
• pp.554-558
• /
• 2008

본 논문에서는 현재에 항공감시 통신링크로 사용되는 다중접속방식인 STDMA(Self -organized Time Division Multiple Access) 통신 프로토콜이 ICAO 규격에 따라 설계하였다. 다중접속방식 통신프로토콜은 감시데이터링크 시스템의 MAC 영역에서 사용된다. 통신시뮬레이션 프로그램을 이용하여 다중접속방식 프로토콜을 ICAO 규격서 제시된 순서에 맞게 프로그램을 구현하였고, 시뮬레이션 결과를 통해 확인하였다. 데이터 전송시 보고율에 따라서 슬롯할당과 예약이 발생되고, 슬롯 번지수도 계산되었다.

• 한국정보컨버전스학회:학술대회논문집
• /
• 한국정보컨버전스학회 2008년도 International conference on information convergence
• /
• pp.199-204
• /
• 2008

Sensor nodes have various energy and computational constraints because of their inexpensive nature and ad-hoc method of deployment. Considerable research has been focused at overcoming these deficiencies through faster media accessing, more energy efficient routing, localization algorithms and system design. Our research attempts to provide a method of improvement MAC performance in these issues. We show that traditional carrier-sense multiple access(CSMA) protocols like IEEE 802.11 do not handle the first constraint adequately, and do not take advantage of the second property, leading to degraded latency and throughput as the network scales in size, We present more efficient method of a medium access for real-time wireless sensor networks. Proposed MAC protocol is a randomized CSMA protocol, but unlike previous legacy protocols, does not use a time-varying contention window from which a node randomly picks a transmission slot. To reduce the latency for the delivery of event reports, it carefully decides a fixed-size contention window, non-uniform probability distribution of transmitting in each slot within the window. We show that it can offer up to several times latency reduction compared to legacy of IEEE 802.11 as the size of the sensor network scales up to 256 nodes using widely used simulator ns-2. We, finally show that proposed MAC scheme comes close to meeting bounds on the best latency achievable by a decentralized CSMA-based MAC protocol for real-time wireless sensor networks which is sensitive to latency.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제13권9호
• /
• pp.4390-4407
• /
• 2019

The paucity of pilot signals in Massive MIMO system is a vital issue. To accommodate substantial number of users, pilot signals are reused. This leads to interference, resulting in pilot contamination and degrades channel estimation at the Base Station (BS). Hence, mitigation of pilot contamination is exigency in Massive MIMO system. The proposed Time Shifted Pilot Signal Transmission with Pilot signal Hopping (TSPTPH), addresses the pilot contamination issue by transmitting pilot signals in non-overlapping time interval with hopping of pilot signals in each transmission slot. Hopping is carried by switching user to new a pilot signal in each transmission slot, resulting in random change of interfering users. This contributes to the change in channel coefficient, which leads to improved channel estimation at the BS and therefore enhances the efficiency of Massive MIMO system. In this system, Uplink Signal Power to Interference plus Noise Power Ratio (SINR) and data-rate are calculated for pilot signal reuse factor 1 and 3, by estimating the channel with Least Square estimation. The proposed system also reduces the uplink Signal power for data transmission of each User Equipment for normalized spectral efficiency with rising number of antennas at the BS and thus improves battery life.