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  • FACULDADE DE ENGENHARIA DA UNIVERSIDADE DO PORTO

    Successive Interference Cancellation inVehicular Networks to Relieve the

    Negative Impact of the Hidden NodeProblem

    Carlos Miguel Silva Couto Pereira

    Dissertation conducted under theMaster in Electrical and Computers Engineering

    Major Telecommunications

    Supervisor: Ana Aguiar (Ph.D.)

    Co-Supervisor: Jens Mittag (Dipl.-Inform.)

    September, 2011

  • c Carlos Miguel Pereira, 2011

  • Abstract

    In Vehicular Ad-hoc Networks (VANETs), communications are made between vehicles (V2V)and between vehicles and roadside infrastructures (V2I). VANETs are a promising approach to in-crease the safety on roads. However, fast fading characteristics of the received signal due to thehigh mobility of the vehicles and the hidden node problem can lead to unsynchronized packettransmissions, which in turn may result in packet collisions and therefore packet losses. Further-more, safety messages are based on one-hop broadcast, where there are no mechanisms imple-mented for feedback or for reducing collisions due to the hidden node problem, such as Requestto Send/Clear to Send (RTS/CTS).

    It is known that it is possible to, under specific conditions, successfully decode and receiveeven colliding packets. The technique used to achieve this is called Successive Interference Can-cellation (SIC).

    Recently, PhySim-WiFi integrated a physical layer implementation of the Orthogonal Fre-quency Division Multiplexing (OFDM) PHY specification for the 5 GHz band into the NS-3 net-work simulator which permits the study of signal processing techniques, such as SIC, in VANETs.

    In this context, we use NS-3 with PhySim-WiFi to characterize packet collisions in VANETs.Then, we proceed with the analysis of the advantages and disadvantages of implementing SIC inreceivers.

    The feasibility of the receivers is directly related with the user offered load to the channel andcan become infeasible for extreme channel congestion situations due to the high number of packetcollisions that occur in the system. However, when the user offered load to the channel is lowthere are few packet collisions and, therefore, SIC doesnt introduce throughput gains.

    According to our assessment, the probability of packet reception can be increased by 5 to 20 %for packets sent in the entire communication range of the receiver, for scenarios where the useroffered load to the channel is approximately 3.3 Mbps, and achieves the maximum of 40 % ofpacket recovery, for scenarios with approximately 1.1 Mbps.

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  • Resumo

    Em redes veiculares ad-hoc (VANETs), as comunicaes so feitas entre veculos (V2V) e deveculos para infra-estruturas situadas na berma da estrada (V2I). VANETs so uma abordagempromissora para aumentar a segurana nas estradas. No entanto, as caractersticas de fading rpidodo sinal recebido, devido grande mobilidade dos ns, e o problema do n escondido (hiddennode problem) podem levar a transmisses dessincronizadas de pacotes, que podem resultar emcolises de pacotes e, portanto, perdas de pacotes. Alm disso, mensagens destinadas seguranaso baseadas em one-hop broadcast, onde no h mecanismos implementados para feedback oupara reduzir colises devido ao problema do n escondido, tal como o Request to Send/Clear toSend (RTS/CTS).

    conhecido que possvel, mediante condies especficas, descodificar e receber com sucessopacotes que colidam. A tcnica que usada para isto chama-se Successive Interference Cancella-tion (SIC).

    Recentemente, PhySim-WiFi integrou uma implementao da camada fsica da especificaode Orthogonal Frequency Division Multiplexing (OFDM) PHY para a banda de 5GHz no simu-lador de rede NS-3 que permite o estudo de tcnicas de processamento de sinal, como SIC, emVANETs.

    Neste contexto, usamos o NS-3 com PhySim-WiFi para fazermos uma caracterizao detal-hada das colises de pacotes em VANETs. De seguida, procedemos anlise das vantagens edesvantagens de implementar SIC nos receptores.

    A viabilidade dos receptores est directamente relacionada com a carga do utilizador ofere-cida ao canal e pode tornar-se intolervel em situaes de congestionamento extremo de canal porcausa do elevado nmero de colises que ocorrem no sistema. No entanto, quando a carga doutilizador oferecida ao canal baixa, existem poucas colises de pacotes e, por isso, SIC no trazganhos de throughput.

    De acordo com a nossa avaliao, a probabilidade de recepo de pacotes pode ser melho-rada entre 5 a 20 % para pacotes enviados dentro da distncia de comunicao do receptor, paracenrios em que a carga do utilizador oferecida ao canal aproximadamente 3.3 Mbps, e atinge omximo de 40 % de recuperao de pacotes, para cenrios em que a carga com aproximadamente1.1 Mbps.

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  • Acknowledgments

    I would like to thank Jens Mittag for accepting me in his group to develop this work and forall the help he gave me during these last months concerning the work and my stay in Karlsruhe. Ialso want to express my gratitude to Professor Ana Aguiar, who supported and helped me so thatI could fulfill this work.

    To my friends for all the laughts we share.To my close family for their unconditional support.To Raquel for being the most important person in my life.

    Carlos Pereira

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  • I do not fear computers. I fear the lack of them.

    Isaac Asimov

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  • Contents

    1 Introduction 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    2 State of the Art 52.1 Vehicular Ad-Hoc Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    2.2.1 IEEE 802.11.p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3 Challenges of VANETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    2.3.1 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.3 MAC operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    2.4 Simulation of VANETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4.1 The NS-3 Network Simulator . . . . . . . . . . . . . . . . . . . . . . . 152.4.2 PhySim-WiFi Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.5 Interference cancellation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.5.1 Successive Interference Cancellation . . . . . . . . . . . . . . . . . . . . 23

    2.6 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    3 Characterization of Packet Collisions 273.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.2 Evaluation Parameters and Metrics . . . . . . . . . . . . . . . . . . . . . . . . . 313.3 Scenario Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.4 Modifications to PhySim-WiFi 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . 36

    4 Evaluation 374.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    4.1.1 Packet Reception and Packet Drop Probabilities . . . . . . . . . . . . . . 374.1.2 Characterization of packet drops . . . . . . . . . . . . . . . . . . . . . . 434.1.3 Characterization of Packet Collisions . . . . . . . . . . . . . . . . . . . 484.1.4 Probability of Recovery through SIC . . . . . . . . . . . . . . . . . . . . 54

    5 Conclusions and Future Work 575.1 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

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  • x CONTENTS

    References 59

  • List of Figures

    2.1 Spectrum allocation for DSRC. . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 802.11p Operating Channels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 IEEE 802.11p WAVE Protocol Stack. . . . . . . . . . . . . . . . . . . . . . . . 72.4 IEEE 802.11p frame with OFDM preamble and PLCP header. . . . . . . . . . . 82.5 Block diagram of the transmitter for the OFDM PHY. . . . . . . . . . . . . . . . 92.6 Block diagram of the receiver for the OFDM PHY. . . . . . . . . . . . . . . . . 92.7 General MAC frame format with explicit frame Control field. . . . . . . . . . . . 102.8 Example of IEEE 802.11 DCF operation. . . . . . . . . . . . . . . . . . . . . . 112.9 Interframe Space in IEEE 802.11. . . . . . . . . . . . . . . . . . . . . . . . . . 112.10 Hidden Node. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.11 Four-way handshake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.12 Architecture of the physical layer emulation within NS-3. . . . . . . . . . . . . . 172.13 The PhySim-WiFi state machine. . . . . . . . . . . . . . . . . . . . . . . . . . . 182.14 Encoding of the payload. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.15 Channel modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.16 Frame Reception. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.17 StartReceive() event. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.18 EndPreamble() event. . . . . . . . . . . .

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