• 한국통신학회논문지
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• 제29권7A호
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• pp.844-851
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• 2004

무선통신에서의 전송 매체인 air interface는 주위의 여러 환경적인 요인에 의해서 다양한 에러가 존재할 수 있다. 이러한 조건에서는 채널에 대한 신뢰도가 유선일 때보다 크게 떨어지며 더욱이 ad hoc 환경인 IEEE 802.15.3 High-rate WPAN에서는 이러한 에러에 대한 의존도가 더욱 클 수 있다. 이미 WLAN에서 이와 관련된 연구가 있었으며 그 중 하나가 각 무선 노드들의 큐 상태에 따라서 큐의 크기가 크다면 채널 상태가 나쁘다고 판단하고 그 무선 노드에게 할당할 리소스를 줄이는 방법이나. 이는 중앙의 controller가 항상 각 노드들을 모니터링하고 있어야 하는 단점이 있다. 그래서 이러한 단점을 보완하고자 본 논문에서는 무선 이동노드의 위치에 의해 달라지는 링크의 에러 상태에 따라 controller가 노드들에게 할당하는 채널 시간의 크기를 동적으로 변화시켜서 piconet 전체의 throughput을 향상시킬 수 있는 MAC을 제안한다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권3호
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• pp.950-975
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• 2016

Device-to-Device (D2D) communication underlaying cellular networks is a promising add-on component for future radio communication systems. It provides more access opportunities for local device pairs and enhances system throughput (ST), especially when mobile relays (MR) are further enabled to facilitate D2D links when the channel condition of their desired links is unfavorable. However, mutual interference is inevitable due to spectral reuse, and moreover, selecting a suitable transmission mode to benefit the correlated resource allocation (RA) is another difficult problem. We aim to optimize ST of the hybrid system via joint consideration of mode selection (MS) and RA, which includes admission control (AC), power control (PC), channel assignment (CA) and relay selection (RS). However, the original problem is generally NP-hard; therefore, we decompose it into two parts where a hierarchical structure exists: (i) PC is mode-dependent, but its optimality can be perfectly addressed for any given mode with additional AC design to achieve individual quality-of-service requirements. (ii) Based on that optimality, the joint design of MS, CA and RS can be viewed from the graph perspective and transferred into the maximum weighted independent set problem, which is then approximated by our greedy algorithm in polynomial-time. Thanks to the numerical results, we elucidate the efficacy of our mechanism and observe a resulting gain in MR-aided D2D communication.

• 한국경영과학회:학술대회논문집
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• 한국경영과학회 2006년도 추계학술대회
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• pp.674-677
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• 2006

Orthogonal frequency division multiple access (OFDMA) has emerged as one of the core multiple access schemes in 4G to mitigate the problems of frequency selectivity and inter-symbol interferences. In OFDMA, cooperative transmission can be adopted in transmission between each user and base station. Cooperative transmission can provide better channel environments and additional transmitter diversity. In this paper, we formulate a user and subcarrier assignment problem in OFDMA system with user cooperation and propose an efficient scheme to solve it.

• 한국정보통신설비학회:학술대회논문집
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• 한국정보통신설비학회 2004년도 하계학술대회
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• pp.302-307
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• 2004

이 논문은 제한된 대역폭을 가진 고정 채널 할당 문제를 유전알고리즘을 이용하여 최적화하는 방법을 제시한다. 또한 이 논문에서는 유전알고리즘의 미세조정 능력 향상을 위해 지역 최적화 방법을 고안하였다. 고정채널 할당 문제를 행렬로 표현하여 지역 최적화 알고리즘에서는 채널을 수평 또는 수직으로 움직임으로써 유전 알고리즘의 성능 향상을 꾀한다. 제안된 방법은 기존의 방법보다 뛰어난 성능 향상을 나타냄을 실험결과에서 보일 것이다.

• Journal of Communications and Networks
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• 제18권6호
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• pp.913-925
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• 2016

In-band wireless full-duplex is a promising technology that enables a wireless node to transmit and receive at the same time on the same frequency band. Due to the complexity of self-interference cancellation techniques, only base stations (BSs) are expected to be full-duplex capable while user terminals remain as legacy half-duplex nodes in the near future. In this case, two different nodes share a single subchannel, one for uplink and the other for downlink, which causes inter-node interference between them. In this paper, we investigate the joint problem of subchannel assignment and power allocation in a single-cell full-duplex orthogonal frequency division multiple access (OFDMA) network considering the inter-node interference. Specifically, we consider two different scenarios: i) The BS knows full channel state information (CSI), and ii) the BS obtains limited CSI through channel feedbacks from nodes. In the full CSI scenario, we design sequential resource allocation algorithms which assign subchannels first to uplink nodes and then to downlink nodes or vice versa. In the limited CSI scenario, we identify the overhead for channel measurement and feedback in full-duplex networks. Then we propose a novel resource allocation scheme where downlink nodes estimate inter-node interference with low complexity. Through simulation, we evaluate our approaches for full and limited CSIs under various scenarios and identify full-duplex gains in various practical scenarios.

• Journal of applied mathematics & informatics
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• 제24권1_2호
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• pp.421-426
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• 2007

The problem of vertex labeling with a condition at distance two in a graph, is a variation of Hale's channel assignment problem, which was first explored by Griggs and Yeh. For positive integer $p{\geq}q$, the ${\lambda}_{p,q}$-number of graph G, denoted ${\lambda}(G;p,q)$, is the smallest span among all integer labellings of V(G) such that vertices at distance two receive labels which differ by at least q and adjacent vertices receive labels which differ by at least p. Van den Heuvel and McGuinness have proved that ${\lambda}(G;p,q){\leq}(4q-2){\Delta}+10p+38q-24$ for any planar graph G with maximum degree ${\Delta}$. In this paper, we studied the upper bound of ${\lambda}_{p,q}$-number of some planar graphs. It is proved that ${\lambda}(G;p,q){\leq}(2q-1){\Delta}+2(2p-1)$ if G is an outerplanar graph and ${\lambda}(G;p,q){\leq}(2q-1){\Delta}+6p-4q-1$ if G is a Halin graph.

• 한국경영과학회지
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• 제23권3호
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• pp.135-151
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• 1998

The demand of large capacity in coming cellular systems makes inevitable the deployment of small cells, rendering more frequent handoff occurrences of calls than in the conventional system. The key issue is then how effectively to reduce the chance of unsuccessful handoffs, since the handoff failure is less desirable than that of a new call attempt. In this study, we consider the control policies which give priority to handoff calls by limiting channel assignment for the originating new calls, and allow queueing the new calls which are rejected at their first attempts. On this system. we propose the problem of finding an optimal call control strategy which optimizes the objective function value, while satisfying the requirements on the handoff/new call blocking probabilities and the new call delay. The objective function takes the most general form to include such well-known performance measures as the weighted average carried traffic and the handoff call blocking probability. The problem is formulated into two different linear programming (LP) models. One is based on the direct employment of steady state equations, and the other uses the theory of semi-Markov decision process. Two LP formulations are competitive each other, having its own strength in the numbers of variables and constraints. Extensive experiments are also conducted to show which call control strategy is optimal under various system environments having different objective functions and traffic patterns.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제4권5호
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• pp.691-708
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• 2010

An important issue of supporting multi-users with diverse quality-of-service (QoS) requirements over wireless networks is how to optimize the systematic scheduling by intelligently utilizing the available network resource while, at the same time, to meet each communication service QoS requirement. In this work, we study the problem of a variety of communication services over multi-media heterogeneous cognitive OFDM system. We first divide the communication services into two parts. Multimedia applications such as broadband voice transmission and real-time video streaming are very delay-sensitive (DS) and need guaranteed throughput. On the other side, services like file transmission and email service are relatively delay tolerant (DT) so varying-rate transmission is acceptable. Then, we formulate the scheduling as a convex optimization problem, and propose low complexity distributed solutions by jointly considering channel assignment, bit allocation, and power allocation. Unlike prior works that do not care computational complexity. Furthermore, we propose the FAASA (Fairness Assured Adaptive Sub-carrier Allocation) algorithm for both DS and DT users, which is a dynamic sub-carrier allocation algorithm in order to maximize throughput while taking into account fairness. We provide extensive simulation results which demonstrate the effectiveness of our proposed schemes.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권11호
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• pp.5253-5270
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• 2016

With device-to-device (D2D) communications, an inactive user terminal can be utilized as a relay node to support multi-hop communication so that connective experience of the cell-edge user as well as the capacity of the whole system can be significantly improved. In this paper, we investigate the spectrum sharing for a cooperative relay assisted D2D communication underlying a cellular network. We formulate a joint relay selection and channel assignment problem to maximize the throughput of the system while guaranteeing the quality of service (QoS) requirements of cellular users (CUs) and D2D users (DUs). By exploiting coalition formation game theory, we propose two algorithms to solve the problem. The first algorithm is designed based on merge and split rules while the second one is developed based on single user's movement. Both of them are proved to be stable and convergent. Simulation results are presented to show the effectiveness of the proposed algorithms.

• 한국통신학회논문지
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• 제36권1A호
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• pp.73-81
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• 2011

IEEE 802.11 물리 (Physical) 계층과 MAC (Medium Access Control) 계층은 다수의 채널과 데이터 레이트를 제공한다. IEEE 802.11 멀티 라디오 애드혹 네트워크의 성능을 향상 시키기 위해서는 가용한 채널과 데이터 레이트를 효율적으로 할당해야 한다. 하지만 IEEE 802.11 멀티 레이트 네트워크에서는 낮은 레이트 링크가 높은 레이트 링크에 영향을 미쳐서 네트워크 성능을 심각히 저하시키는 문제인, RA (Rate Anomaly)가 발생한다. 따라서, 본 논문에서는 다른 데이터 레이트를 사용하는 링크들을 멀티 채널로 분할시킴으로써 이러한 RA문제를 완화하는 모델 기반 레이트 분할 (Model-based Rate Separation, MRS) 알고리즘을 제안한다. MRS 알고리즘은 IEEE 802.11 싱글 홉 (single-hop) 네트워크의 성능을 측정하는 기존의 성능 모델을 활용하여 낮은 레이트 링크와 높은 레이트 링크를 분산시킨다. 시뮬레이션을 통해 멀티 라디오 애드혹 네트워크에서 MRS 알고리즘이 기존 알고리즘들에 비해 향상된 네트워크 성능을 나타냄을 확인하였다.