• 한국경영과학회지
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• 제18권3호
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• pp.35-49
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• 1993

The node coloring problem is a problem to color the nodes of a graph using the minimum number of colors possible so that any two adjacent nodes are colored differently. This problem, along with the edge coloring problem, has a variety of practical applications particularly in item loading, resource allocation, exam timetabling, and channel assignment. The node coloring problem is an NP-hard problem, and thus many researchers develop a number of heuristic algorithms. In this paper, we survey and classify those heuristics with the emphasis on how an algorithm orders the nodes and colors the nodes using a determined ordering.

• 한국컴퓨터정보학회논문지
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• 제18권2호
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• pp.69-76
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• 2013

RFID 시스템에서 리더간 간섭은 일정한 서비스 영역에서 제한된 주파수를 사용하기 때문에 발생하며 수동형 태그의 가독율을 떨어뜨리는 주요 원인이 된다. 그러므로 제한된 주파수 자원 환경에서 가독율을 최대화하려면 리더간 주파수 간섭을 최소화시켜야 한다. 본 논문에서는 RFID 리더간 주파수 간섭 최소화 문제를 FDM/TDM 혼합방식의 제약만족문제로 모델링하고 기존의 백트래킹 탐색 알고리즘을 적용하여 각각의 리더에게 최적의 채널을 할당한다. 제약 만족 문제의 해를 구하기 위해서 백트래킹을 이용한 깊이우선탐색을 실행하는데 이 때 탐색되는 노드의 순서를 효과적으로 배열하는 변수 순서화 방법이 중요하다. 본 논문의 실험에서 적용된 변수 순서화 알고리즘들은 그래프 채색에 효과적인 것으로 알려져 있다. 제안한 제약만족문제 모델의 성능을 입증하기 위하여 수동형 UHF RFID 시스템 환경에서 시뮬레이션하여 간섭조건을 만족하면서 각각의 리더에게 최적의 채널을 할당한다.

• 한국정보통신학회논문지
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• 제15권7호
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• pp.1465-1470
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• 2011

본 논문에서는 무선 애드혹 네트워크에서 복수개 채널을 이용한 효율적인 매체접속제어 프로토콜로써 DM-MMAC (Doze Mode Multi-Chanel MAC)을 제안한다. 기존의 IEEE 802.11 프로토콜의 경우 물리계층에서 복수개의 가용 채널이 주어지나 MAC 계층에서는 오직 고정된 단일채널을 통한 접속만이 가능하다. 이를 보완하기 위한 프로토콜로써 여러 가지 멀티채널 MAC 프로토콜이 제안되었으나 대부분의 경우 채널수가 적고 호스트의 수가 많아지면 전송 처리율(throughput)이 급격히 저하되는 문제점이 있다. 본 연구에서는 기존 MMAC 프로토콜의 채널 할당 및 채널점유 과정을 단순화하고 호스트 간 충돌가능성을 줄임으로써 절전효과와 함께 전송률 향상을 도모하였다.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2002년도 ITC-CSCC -2
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• pp.967-970
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• 2002

Multihop wireless networks consist of mobile terminals with personal communication devices. Each terminal can receive a message from a terminal and send it to the other terminal. In this paper, we discuss edge coloring problems related to multihop wireless networks. We show some relations about the problems.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권8호
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• pp.2797-2820
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• 2015

In order to accommodate huge number of antennas in a limited antenna size, a large scale antenna array is expected to have a three dimensional (3D) array structure. By using the Active Antenna Systems (AAS), the weights of the antenna elements arranged vertically could be configured adaptively. Then, a degree of freedom (DOF) in the vertical plane is provided for system design. So the three-dimension MIMO (3D MIMO) could be realized to solve the actual implementation problem of the massive MIMO. However, in 3D massive MIMO systems, the pilot contamination problem studied in 2D massive MIMO systems and the inter-cell interference as well as inter-vertical sector interference in 3D MIMO systems with vertical sectorization exist simultaneously, when the number of antenna is not large enough. This paper investigates the interference management towards the above challenges in 3D massive MIMO systems. Here, vertical sectorization based on vertical beamforming is included in the concerned systems. Firstly, a cooperative joint vertical beams adjustment and pilot assignment scheme is developed to improve the channel estimation precision of the uplink with pilots being reused across the vertical sectors. Secondly, a downlink interference coordination scheme by jointly controlling weight vectors and power of vertical beams is proposed, where the estimated channel state information is used in the optimization modelling, and the performance loss induced by pilot contamination could be compensated in some degree. Simulation results show that the proposed joint optimization algorithm with controllable vertical beams' weight vectors outperforms the method combining downtilts adjustment and power allocation.

• 한국통신학회논문지
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• 제27권5C호
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• pp.422-434
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• 2002

본 논문에서는 대용량 트래픽의 효율적 전송을 위해 B-DCS(Broadband-Digital Cross-connect System)망, 간선링, 지선링의 3계위 구조 광섬유 기간전송망을 제안하며 각 계위에서 경제적으로 망의 생존도를 보장하는 설계방법을 연구한다. 링 구축비용을 혼합정수계획(MIP) 모델을 이용해 수리적으로 최적화할 경우 비다항식 완전 문제(NP-complete problem)가 되어 실시간 내에 해를 구하기가 어려워지므로 본 논문에서는 B-DCS망을 위한 운용회선 및 예비회선의 최적할당 문제, 간선링을 위한 다중링 부하최적화 문제, 지선링을 위한 중첩링 부하최적화 문제에 대한 휴리스틱 알고리즘을 각각 개발하여 사례분석을 통해 그 효용성을 보인다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제11권2호
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• pp.611-627
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• 2017

For enhancing the coverage of wireless networks and increasing the spectrum efficiency, small cell networks (SCNs) are considered to be one of the most prospective schemes. Most of the existing literature on resource allocation among non-cooperative small cell base stations (SBSs) has widely drawn close attention and there are only a small number of the cooperative ideas in SCNs. Based on the motivation, we further investigate the cooperative approach, which is formulated as a coalition formation game with power control algorithm (CFG-PC). First, we formulate the downlink sub-channel resource allocation problem in an SCN as a coalition formation game. Pareto order and utilitarian order are applied to form coalitions respectively. Second, to achieve more availability and efficiency power assignment, we expand and solve the power control using particle swarm optimization (PSO). Finally, with our proposed algorithm, each SBS can cooperatively work and eventually converge to a stable SBS partition. As far as the transmit rate of per SBS and the system rate are concerned respectively, simulation results indicate that our proposed CFG-PC has a significant advantage, relative to a classical coalition formation algorithm and the non-cooperative case.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제8권8호
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• pp.2626-2646
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• 2014

This study presents a power and Physical Resource Blocks (PRBs) joint allocation algorithm to coordinate uplink (UL) interference in the device-to-device (D2D) underlaying Long Term Evolution-Advanced (LTE-A) networks. The objective is to find a mechanism to mitigate the UL interference between the two subsystems and maximize the weighted sum throughput as well. This optimization problem is formulated as a mixed integer nonlinear programming (MINLP) which is further decomposed into PRBs assignment and transmission power allocation. Specifically, the scenario of applying imperfect channel state information (CSI) is also taken into account in our study. Analysis reveals that the proposed PRBs allocation strategy is energy efficient and it suppresses the interference not only suffered by the LTE-A system but also to the D2D users. In another side, a low-complexity technique is proposed to obtain the optimal power allocation which resides in one of at most three feasible power vectors. Simulations show that the optimal power allocation combined with the proposed PRBs assignment achieves a higher weighted sum throughput as compared to traditional algorithms even when imperfect CSI is utilized.

• Journal of Communications and Networks
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• 제18권1호
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• pp.50-64
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• 2016

Nowadays, smart meter (SM) technology is widely effectively used. In addition, power allocation (PA) and channel selection (CS) are considered problems with many proposed approaches. In this paper, we will suggest a specific scenario for an SM configuration system and show how to solve the optimization problem for transmission between SMs and the data concentrator unit (DCU), the center that collects the data from several SMs, via simulation. An efficient CS with PA scheme is proposed in the TV white space system, which uses the TV band spectrum. On the basic of the optimal configuration requirements, SMs can have a transmission schedule and channel selection to obtain the optimal efficiency of using spectrum resources when transmitting data to the DCU. The optimal goals discussed in this paper are the maximum capacity or maximum channel efficiency and the maximum allowable power of the SMs used to satisfy the quality of service without harm to another wireless system. In addition, minimization of the interference to the digital television system and other SMs is also important and needs to be considered when the solving coexistence scenario. Further, we propose a process that performs an interference analysis scheme by using the spectrum engineering advanced Monte Carlo analysis tool (SEAMCAT), which is an integrated software tool based on a Monte-Carlo simulation method. Briefly, the process is as follows: The optimization process implemented by genetic evolution optimization engines, i.e., a genetic algorithm, will calculate the best configuration for the SM system on the basis of the interference limitation for each SM by SEAMCAT in a specific configuration, which reaches the solution with the best defined optimal goal satisfaction.

• Journal of Communications and Networks
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• 제15권1호
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• pp.102-110
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• 2013

This paper considers a cooperative orthogonal frequency division multiple access (OFDMA)-based cognitive radio network where the primary system leases some of its subchannels to the secondary system for a fraction of time in exchange for the secondary users (SUs) assisting the transmission of primary users (PUs) as relays. Our aim is to determine the cooperation strategies among the primary and secondary systems so as to maximize the sum-rate of SUs while maintaining quality-of-service (QoS) requirements of PUs. We formulate a joint optimization problem of PU transmission mode selection, SU (or relay) selection, subcarrier assignment, power control, and time allocation. By applying dual method, this mixed integer programming problem is decomposed into parallel per-subcarrier subproblems, with each determining the cooperation strategy between one PU and one SU. We show that, on each leased subcarrier, the optimal strategy is to let a SU exclusively act as a relay or transmit for itself. This result is fundamentally different from the conventional spectrum leasing in single-channel systems where a SU must transmit a fraction of time for itself if it helps the PU's transmission. We then propose a subgradient-based algorithm to find the asymptotically optimal solution to the primal problem in polynomial time. Simulation results demonstrate that the proposed algorithm can significantly enhance the network performance.