• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.2C
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• pp.105-111
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• 2011

In this paper, we propose a novel Alamouti space-time transmission scheme for orthogonal frequency division multiplexing (OFDM) based asynchronous cooperative communication systems with low relay complexity. The conventional scheme requires an additional operation likes time-reversal at the relay nodes besides the simple multiplications at the relay nodes, which result in increasing the complexity of relay nodes. Unlike the conventional scheme, exploiting the simple combination of the symbols at the source node and the simple multiplications at the relay nodes, the proposed scheme achieves the second order diversity gain by obtaining the Alamouti code structure at the destination node. Simulation results show that the proposed scheme achieves the second order diversity gain and has the same bit error rate performance as the conventional scheme.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37B no.10
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• pp.859-876
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• 2012

Since the existing protocols separated mobility management part and routing protocol part in their design and used a flooding, they suffer from the high control overhead, thereby limiting performance. In this paper, we use a tree-based mobility management method and present a simple and efficient routing protocol that exploits the topology information which is built additionally through mobility management. Thus, the mobility management and the routing protocol closely cooperate to optimize control overhead. Furthermore, we use a progressive path discovery method to alleviate traffic congestion around IG and a unicast-based broadcast method to increase the reliability of message delivery and to judge link validity promptly. The proposed protocol reduces control overhead greatly and works in a stable manner even with the large number of nodes and high mobility. This was proven by comparing with the AODV protocol that employs the hybrid mobility management protocol.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.2
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• pp.275-280
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• 2016

In this paper, we propose a novel relay selection technique which utilizes a priori decoding information at relays for buffer-aided successive relaying networks. In the conventional relaying schemes, a single relay pair is selected for receiving data from the source and transmitting data to the destination. In the proposed technique, however, all relays except the relay selected for transmitting data to the destination try to decode the received signal from the source, and they store the data if they succeed decoding. The proposed technique selects the relay such that it can succeed its own transmission and it maximizes the number of relays successfully decoding the data from the source at the same time. It is shown that the proposed relaying technique significantly outperforms the conventional buffer-aided relaying schemes in terms of outage probability through extensive computer simulations.

• Journal of Satellite, Information and Communications
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• v.10 no.1
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• pp.49-55
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• 2015

In cognitive radio network (CRN), spectrum sensing is an elementary level of technology for non-interfering to licensed user. Required sample number for spectrum sensing is directly related to the throughput of secondary user and makes the tradeoff between the throughput of secondary user and interference to primary user. Required spectrum sensing sample is derived from required false alarm, detection probability and minimum required SNR of primary user (PU). If we make clustering and minimize the required transmission boundary of secondary user (SU), we can relax the required PU SNR for spectrum sensing because the required SNR for PU signal sensing is related to transmission range of SU. Therefore we can achieve efficient throughput of CRN by minimizing spectrum sensing sample. For this, we design the tradeoff between gain and loss could be obtained from clustering, according to the size of cluster members through game theory and simulation results confirm the effectiveness of the proposed method.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.7A
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• pp.640-648
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• 2011

We propose relay selection schemes using STBC (Space Time Block Coding) technique in OFDM (Orthogonal Frequency Division Multiplexing)-based wireless systems. The proposed schemes select the optimum relay having the maximum instantaneous equivalent channel gain among multiple candidate relays. Also, in order to reduce the system overhead, a symbol grouping method which groups some amount of symbols before selecting the optimum relay is proposed. The simulation results show that the proposed relay selection schemes can obtain more selection diversity gain as the number of selectable relay candidates increases. Furthermore, the proposed scheme with the symbol grouping can reduce system overhead without any degradation of the performance in fading channels with low frequency selectivity.

• Electronics and Telecommunications Trends
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• v.28 no.6
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• pp.86-98
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• 2013

최근 다양한 형태의 스마트 기기 출현과 대중적 보급으로 고속 데이터 전송에 대한 수요가 나날이 증가하고 있어 소형셀 기술에 대한 이동통신 사업자들의 관심이 높다. 이에 데이터 요구량이 많은 위치에 소형 셀을 다층으로 밀집 배치하고 매크로 기지국 및 소형 셀 기지국의 밀접한 협력을 통해 무선 네트워크의 용량을 증가시키기 위한 기술들에 대한 요구가 높아지고 있다. 매크로 셀과 소형 셀이 다층으로 배치된 구조에서 고려해 볼 수 있는 요소기술들은 단말의 다중 노드 연결, 효율적인 이동성 보장, 효율적인 셀/이동단말 발견, 이종 듀플렉스캐리어 집성, 셀 간 간섭 관리 등이며, 본 논문에서는 이들을 기반으로 최근 3GPP에서 활발히 논의되고 있는 LTE 소형셀 향상 표준화 동향에 대해 기술한다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38C no.4
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• pp.378-386
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• 2013

In this paper, we propose a multi-way relaying system, in which N communicating nodes interchange their information one another by the help of a multiple-antenna non-regenerative relay station (RS). While the conventional multi-way relaying requires 2N transmission phases to complete the data exchange, the proposed system completes the mission with N phases composed of one multiple access phase and N-1 broadcast phases. For effective broadcast transmission, the proposed system pairs users for signal transmission with a new RS beamforming matrix not to interfere with the nodes of different pairs and a self-interference canceler at each node. The performance evaluation in terms of the average sum rate shows that the proposed system outperforms the conventional one with more significant gain when the number of RS antennas is comparable to the number of communicating nodes. The proposed schemes can be applicable to marine communications where the ships need to share their information with extended coverage.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.8C
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• pp.645-654
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• 2012

In this paper, we consider a two-way relay (TWR) system, where two user nodes exchange their information within two transmission phases, by the help of a relay node adopting physical layer network coding. In the system, two users transmit their binary phase shift keying symbols simultaneously in the first phase, and the relay node decodes the XORed version of two user data and broadcasts it back to two users in the second phase. The performance of the system is analyzed in terms of the average end-to-end symbol error probability in Nakagami-m fading channels, for which a tight upper bound is derived in a closed form to provide an accurate and handy estimate on the performance. The results show that our upper bounds are almost indistinguishable from simulation results for various channel and system configurations. In addition, the optimal relay location and power allocation for various conditions can be obtained quickly with our analysis.

• Proceedings of the Korea Water Resources Association Conference
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• 2018.05a
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• pp.62-62
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• 2018

무선영상취득시스템(WIA 시스템, Wireless Image Acquisition System)은 라즈베리 파이에 전용 카메라와 WiFi 모듈을 장착하여, 하천의 영상을 실시간으로 촬영하여 무선으로 서버로 전송하는 시스템이다. 이 시스템이 갖는 가장 큰 이점은 시스템을 구성하는 비용이 매우 저렴하다는 점이다. 라즈베리 본체와 카메라 모듈, WiFi 모듈 모두 매우 저렴하고, 또 사용하는 전력이 작아서 상용 전원이 아닌 태양광 발전이나 배터리 등을 이용할 수 있다. 따라서 비용과 장소에 구애받지 않고 손쉽게 어디든지 설치하여 하천의 상시 감시나 계측에 활용할 수 있다. 또한, 상용 전원을 이용하지 않아도 되기 때문에, 산간벽지나 오지 등의 소하천 관리에도 적합하다. 본 연구에서는 이 WIA 시스템을 경상남도 김해시의 대청천에 적용하여 홍수 시 하천의 수표면을 촬영하고, 촬영된 동영상을 분석하여 수위와 유속을 동시에 계측하여 유량을 산정하였다. 라즈베리 파이에 $640{\times}480$ 화소의 카메라를 장착하여 10분 간격으로 10초간의 동영상을 촬영하고, 이를 WiFi 모듈을 이용하여 무선으로 서버로 전송한다. 전송된 동영상을 분석하기 전에 설치 지점의 3차원 좌표 변환 자료와 횡단면 좌표를 입력하여 대상 지점의 측정 매개변수를 설정한다. 즉, 이들 자료에서 영상 내의 표정점과 측정선을 설정해 둔다. 그 다음, 전송된 동영상을 시공간 영상으로 만들어 수위를 분석한다. 비슷한 방법으로 동영상에서 유속을 분석하고, 분석된 수위와 유속, 그리고 미리 설정된 횡단면 좌표를 이용하여 유량을 산정해 낸다. 설치된 WIA 시스템을 실제로 운용하여, 2017년 9월 11일의 06:10~19:00의 호우 사상 전체를 분석하였다. 10분 간격으로 촬영된 10초간 동영상 중에서 적절한 분석이 가능한 영상 77개에서 수위와 유속을 분석한 결과, 최대 수위는 0.746 m(간이수위표 기준), 최대 유속은 0.962 m/s, 최대 유량은 $12.977m^3/s$에 이르렀다. 지점 특성상 다른 유속계를 이용한 검증은 사실상 불가능하였다. 또, 하폭이 넓어서 일출 전과 일몰 후의 촬영 자료는 분석이 어려운 점이 있다. 이러한 기술적 문제들을 보완하면, WIA 시스템을 이용한 소하천의 수위와 유속 측정 시스템은 경제성이고 효율이 높은 관측시스템으로 유망할 것으로 기대된다.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.19 no.7
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• pp.1546-1553
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• 2015

In cognitive radio technology, the overall capacity of communications systems can be improved without allocating additional bands by allowing secondary system to utilize the band when a primary system who has right to use the band does not utilize it. Therefore, spectrum sensing to determine the existence of primary system is utmost important in the cognitive radio technology. In this work, we consider a novel cooperative spectrum sensing in cognitive cellular systems. Especially, we consider the case in which heterogeneous primary systems coexist, i.e., heterogenous transmission power and sensing requirement of primary system, such that only portion of users in cognitive cellular systems are able to detect the primary system. In this case, we propose new cooperative spectrum sensing with multiple sensing stages to properly detect the existence of primary systems in this kind of situations. Moreover, we analyze the performance of conventional cooperative spectrum sensing schemes such as OR-rule, AND-rule and MAJORITY-rule under the existence of heterogeneous primary systems. Finally, we investigate the performance of the proposed scheme through computer based simulations and show that the existence of primary systems can be determined accurately by using our proposed scheme.