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Abstract of Doctoral Dissertation

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Graduate School of Global Information and Telecommunication Studies, Waseda University

Abstract of Doctoral Dissertation

Studies on Cooperative Communication in Wireless LAN

無線

LAN

における協調的通信に 関する研究

RABARIJAONA Verotiana Hanitriniala

Global Information and Telecommunication Studies (Wireless Communication and Satellite Communication II)

Date: February 2012

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The last few decades have witnessed the exponential development of telecommunications worldwide. In particular, the demand for communication ubiquity drew the researchers’ focus on wireless communications. Being able to communicate anytime and anywhere was the main goal advertized by wireless communication technologies.

Similarly to people speaking to each other at the same time, wireless communication devices share the same medium to exchange information. Collisions between two messages may thus occur. How to control the access of devices to the medium so as to avoid collisions is one of the fundamental problems of wireless communications. The most popular protocol developed to address this issue is the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol, implemented in the well-known IEEE 802.11 standard. CSMA/CA implements a Distributed Coordination Function (DCF) to administer the medium access.

Nowadays, the increasing presence on the market of electronic devices with wireless connection capabilities is driven by decreasing unit cost and enhancing portability and services. The consequences are a higher user density and mobility in wireless networks. This trend offers a new challenge compelling for improving scalability and adaptability for all layers of the wireless systems. As a result, new paradigms in system design have been sought for by the research community, notably leading to the notion of cooperation between network nodes.

Cooperation has been a topic of high interest for the last decade and has first been proposed to alleviate the wireless medium impairments. Indeed, the random nature of the wireless medium, also called wireless channel, causes random attenuations of the signal strength, called fading, which impairs the reliability of transmitting from one network node to another. This work focuses on the implementation of cooperation at the MAC layer of an 802.11 wireless network.

While cooperation was first explored as a new spatial diversity technique, we address the issue of wasteful duplicate transmissions and investigate on-demand cooperative transmission. The objective is to provide cooperative relaying only when the original transmission fails. In contrast with the space diversity techniques where several copies of the same data are transmitted from different locations with users experiencing different levels of fading, we propose a protocol that leverages the diversity benefits for error recovery.

An overview of 802.11 MAC protocol is presented in Chapter 1 with the basic frame sequence of the CSMA/CA protocol and its behavior in case of error. The historical background of cooperation, its motivations and its basic principles are introduced. The fundamental cooperation algorithms are described and the aspects of cooperation at the MAC layer contributing to the diversity, the error recovery or the data rate improvement are investigated.

Finally, we define the scope of this research and describe the constraints and the requirements to consider in a cooperative protocol.

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Chapter 2 explores the potential contribution of cooperation at the MAC layer on a theoretical basis. A cooperative MAC protocol is proposed to reduce the overall latency of the network and to enable packet recovery after an error. The protocol introduces a new interframe space called Cooperation Interframe Space (CIFS) and a Request To Cooperate (RTC) control packet to perform cooperation. Exploring the performances in an ideal channel and a saturated network sets the baseline regarding the expectations from a cooperative MAC protocol in more realistic scenarios. The theoretical analyses are carried out while assuming a fixed partnership, i.e. a station’s partner never changes; and the partners are assumed to be assigned randomly.

Consequently a more adequate partnership selection can yield higher performances. Once the contribution of cooperation is confirmed theoretically with a random partnership, a low complexity SNR-based dynamic partnership is investigated and the cooperative MAC protocol is refined. It is shown that the use of cooperation can expand the coverage area of a terminal while maintaining a certain data rate.

Chapter 3 investigates a fuzzy logic based cooperative protocol, FuzzyCoop. Researchers have studied the contribution of cooperation at the physical layer, especially focusing on outage probability and capacity regions; however the selection of an optimal partner to cooperate with remains a fundamental issue for efficient cooperation. Besides the inherent wireless channel impairments, the number of users and the portability of the devices leaves crucial points to be addressed, namely the network density and the terminals’ mobility. Therefore, the implementation of a dynamic and adaptive partner selection becomes even more important. The proposed partner selection scheme involves parameters related to the channel instantaneous conditions, the transmissions history and the mobility of the terminals. The partner will be chosen based on a fuzzy logic algorithm. To ease the comprehension of the fuzzy logic-based partnership scheme, a brief introduction to fuzzy logic is first given. Since an optimal partner should share a good channel with both the source and the destination, a source-destination consensus based selection is introduced. Cooperation implies transmitting data for another terminal, in other words, a station uses its own resources while not directly benefiting from the transmission. To compensate for the resources utilization of a cooperative communication, a contention-free channel access is offered to the cooperating terminal as a reward. In addition to the low latency and low overhead packet retransmission, this reward comes as another incentive to cooperate. FuzzyCoop is compared to the low complexity protocol introduced in Chapter 2 and to the DCF function implemented in CSMA/CA. A fuzzy logic-based partner selection shows a higher successful cooperation ratio compared to a selection relying only on the SNR.

FuzzyCoop contributes to the network’s performances when we vary the packet size, the packet generation rate, the maximum diameter of the area around the access point and the number of terminals.

Chapter 4 introduces the integration of cooperation in rate adaptive networks. There is an

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increasing demand for high data rate due to the spreading of multimedia (voice, audio, video) enabled device. In order to maximize the data rate in accordance with the wireless channel conditions, rate adaptation comes as a potential solution. Nonetheless, the duration of transmissions with low data rate occupies the channel to the deficit of the terminals with high data rate. Considering this issue and the improvements gained from cooperation in the previous chapters, the merging of a cooperative MAC protocol with rate adaptive techniques comes as the next logical step of this research. The proposed cooperative rate adaptation (CRA) protocol aims for an adequate partner selection, reactive transmission rate selection, low overhead protocol, and fairness. A rate selection based on the channel conditions is used and the cooperating partner selects the retransmission rate reactively according the received RTC. CRA eliminates the overhead added by FuzzyCoop for the consensus-based partner selection in Chapter 3. It is evaluated in comparison with a non-cooperative and a cooperative rate adaptation schemes, namely Receiver-Based Auto Rate (RBAR) and Cooperative Receiver-Based Auto Rate (CRBAR). The results show that CRA outperforms both protocols and results in a tradeoff between the packet delivery ratio and the throughput. In particular, CRA helps to cope with the network’s density and the users’ mobility by maintaining good performances when the number of terminals or the stations’ velocity increase.

Chapter 5 sums up this thesis and concludes this work on cooperative MAC protocols for 802.11 networks.

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List of academic achievements

Category (Subheadings)

Articles in refereed journals

○ Verotiana H. Rabarijaona, Akeo Masuda and Shigeru

Shimamoto,“DCF-based Cooperative MAC Protocol Employing Fuzzy Logic Partner Selection Scheme”, IEICE Transactions on Communication, Vol. E94B, No.9, pp.2610-2619, 2011.

○ Verotiana H. Rabarijaona, Akeo Masuda, Dinh Chi Hieu,

Thomas Bourgeois and Shigeru Shimamoto, “Cooperative Rate Adaption Scheme for Wireless Networks with Improved Fairness, Delay and Transmission Reliability”, International Journal of Computer Networks & Communications, Vol. 3, No. 1, pp. 13-26, 2012

Presentations at international

conferences

○ Verotiana H. Rabarijaona and Shigeru Shimamato,

“Partnership-based Cooperative MAC Protocol”, Proc. IEEE CCNC’10, Las Vegas, Nevada, 2010.

○ Verotiana H. Rabarijaona and Shigeru Shimamoto, “Coverage

Area Extention Through a Cooperative MAC Protocol”, IEEE ICWITS, Honolulu, Hawaii, 2010.

Other papers

Dinh Chi Hieu, Verotiana H. Rabarijaona, Akeo Masuda and Shigeru Shimamoto, “Intelligent Local Avoided Collision (iLAC) MAC protocol for Very high speed wireless network”, to appear in IEICE Transactions on Communication, Vol.E95-B,No.02, Feb. 2012.

Others (Presentations)

Verotiana Rabarijaona and Shigeru Shimamoto,

“Partnership-based Cooperative MAC Protocol”, Exchange

seminar between Hanyang University and Waseda University,

Hanyang University, Seoul, Japan, Oct. 2009.

参照

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