CMD: A Multi-Channel Coordination Scheme for Emergency Message Dissemination in IEEE 1609.4

In the IEEE 1609.4 legacy standard for multi-channel communications in vehicular ad hoc networks(VANETs), the control channel (CCH) is dedicated to broadcast safety messages while the service channels (SCH's) are dedicated to transmit infotainment service content. However, the SCH can be used as an alternative to transmit high priority safety messages in the event that they are invoked during the service channel interval (SCHI). This implies that there is a need to transmit safety messages across multiple available utilized channels to ensure that all vehicles receive the safety message. Transmission across multiple SCH's using the legacy IEEE 1609.4 requires multiple channel switching and therefore introduces further end-to-end delays. Given that safety messaging is a life critical application, it is important that optimal end-to-end delay performance is derived in multi-channel VANET scenarios to ensure reliable safety message dissemination. To tackle this challenge, three primary contributions are in this article: first, a channel coordinator selection approach based on the least average separation distance (LAD) to the vehicles that expect to tune to other SCH's and operates during the control channel interval (CCHI) is proposed. Second, a model to determine the optimal time intervals in which CMD operates during the CCHI is proposed. Third, a contention back-off mechanism for safety message transmission during the SCHI is proposed. Computer simulations and mathematical analysis show that CMD performs better than the legacy IEEE 1609.4 and a selected state-of-the-art multi-channel message dissemination schemes in terms of end-to-end delay and packet reception ratio.Comment: 15 pages, 10 figures, 7 table

先進ITSのための中継アシスト車車間通信技術の研究

Wireless vehicular communications for advanced Intelligent Transport Systems (ITS) have the potential to support safety driving, enhance the efficiency of transportation and play an important role in the future automated driving system. The vehicle-to-vehicle(V2V), vehicle-to-infrastructure (V2I) and vehicle-to-pedestrian (V2P) communications in the advanced ITS enable safety support applications that can predict potential traffic accidents, warn drivers and, in some cases, directly control vehicles to prevent collisions. Such applications require highly reliable broadcast communications. However, the reliability of wireless communication in vehicular environments suffers from fast fading due to multipath propagation, shadowing , and distance-dependent path loss. In addition, hidden terminal (HT) problem is a great concern in CSMA/CA based wireless networks due to its distributed access nature. The packet delivery rate (PDR) of V2V communications rapidly decreases especially under non-line-of-sight (NLOS) environments such as intersections. A vehicle-roadside-vehicle relay-assisted V2V communication scheme has been proposed to improve the reliability of V2V communications. In the scheme, packets sent from a vehicle can be directly received by other vehicles or relayed by a relay station (RS) to the other vehicles. Then path diversity effect can be obtained that improves PDR of V2V communications. However, when the V2V traffic becomes higher, the number of packets that RS has to retransmit becomes larger. This leads to a large number of packets waiting in the transmit queue of RS and packet congestion happens. If the normal relay scheme is employed, thepackets may be dropped due to the limited queue size. Then the gain obtained by relay-assist may be decreased. A packet payload combining relay (PCRL) scheme is proposed to deal with the congestion issue. In the scheme, multiple V2V packet payloads are combined into a single packet and the resultant packet is rebroadcasted once the channel becomes idle. Analytical and simulated results show that the proposed PCRL scheme can remarkably alleviate the congestion issue and improve the relaying performance.The PCRL scheme, however, still suffers from HT problem. In the intersection environments where LOS propagation between VSs is often unavailable, the packet collision frequently happens due to HTs when RS receives V2V packets. If RS cannot receive V2V packets, the advantage of relay-assist becomes smaller. Therefore an improved PCRL scheme with sectorized receiving RS (SR-V2VC/PCRL) is proposed to mitigate the effect of HT problem as well as alleviating the congestion issue. An analytical model is then developed to analyze the performance of SR-V2VC/PCRL scheme considering a single intersection scenario. Numerical results show that the reliability of V2V communications is significantly improved by the proposed scheme. Furthermore, performance of the SR-V2VC/PCRL scheme is discussed for an urban environment with multiple intersections. In such environment, RSs at intersections should cooperate with each other to obtain the largest diversity gain. After theoretically analyzing the performance of the sectorized receiving scheme under multiple interference sources, large-scale simulations are conducted to evaluate the performance of SR-V2VC/PCRL scheme. It is shown that the SR-V2VC/PCRL remarkably improves the reliability of V2V communications. SR-V2VC/PCRL scheme even performs better when employing higher data rate modulation for V2V and relay transmissions.The aforementioned proposals can remarkably improve the reliability of V2V communications. In order to improve the performance of relay-assist ed scheme when traffic load becomes even higher, a network coding(NC)based PCRL scheme (PCRL-NC) with a payload sorting and selection algorithm is proposed to adapt multiple node environment in an intersection. It is shown that the scheme can benefit from NC in alleviating the congestion issue while effectively mitigating the disadvantage of NC. As a result, the introduction of PCRL-NC to the proposed SR-V2VC/PCRL scheme can remarkably improve the reliability of V2V communications under various traffic environments.　近年，先進的なITS (Intelligent Transport Systems: 高度道路交通システム)のための通信技術への期待が高まっている．これには，車両がその位置や速度などの情報を交換する車車間通信，路側機が車両へ信号状態や道路規制などの情報を提供する路車間通信，車両と歩行者の間で情報の交換を行う歩車間通信などがある．これらにブロードキャスト通信を活用することで，各車両では潜在的な交通事故を予測して運転手に警告し，さらには制動を行うことにより事故を未然に回避できる．さらにこの情報を利用して車両を自動制御することで，交通流を意識した協調型自動走行を実現することが可能になるものと期待されている． 車車間通信を用いて安全運転支援およびより高度な自動走行システムを実現するためには，高信頼，かつ低遅延の無線通信技術が要求される．しかしながら道路上の移動通信では，多重波伝搬によるフェージングや建物によるシャドウイング，さらに自律分散通信システム特有の問題である隠れ端末問題による干渉などの影響で，通信の信頼度が低下する．特に事故発生確率の高い交差点ではその影響が顕著である．　本論文では車車間通信の品質を改善することを目的として，交差点等に中継局(Relay Station; RS)を設置し，車車間通信パケットを転送中継する中継アシスト車車間通信に関する諸技術が提案されている．中継局は交差点付近の信号機などに併設され，高いアンテナ高を有することと，他の車載局に対して見通し内(Line-of-Sight; LOS)伝搬環境にあるため，中継アシストシステムはシャドウイングやフェージングの問題の軽減に有効であることが既に示されている．しかしながら，トラヒックが増加するにつれて中継局での輻輳問題が発生し，中継効果が低下するという課題があった．そこで本論文では中継によるエアトラヒックの増加を抑える方法として，中継送信時に複数のパケットペイロードをまとめて１つのパケットに再構成して送信するペイロード合成中継法を提案する．本提案法により中継局での輻輳問題が解決でき，中継効果が向上することを解析結果から明らかにした．　交差点における中継アシスト車車間通信のもう1つの課題として，中継局受信時に隠れ端末問題の影響で受信成功率が低下することがある．この課題に対しては中継局受信時にセクタアンテナを用いることが有効であることが示されているが，本研究ではペイロード合成中継法にセクタ化受信を組合せたセクタ化受信ペイロード合成中継法を提案し，その効果を理論解析およびシミュレーションにより示した．セクタ化受信によって中継局での受信成功率を改善すると中継すべきパケット数が増加するが，提案法ではペイロード合成によって中継パケットの送信効率を高めることができるので，結果として中継効果を高めて平均パケット伝送成功率を大幅に向上できることを明らかにした．　さらに，複数交差点からなる市街地環境におけるセクタ化受信ペイロード合成中継法の効果を，大規模ネットワークシミュレーションを用いてブロードキャスト配信成功率として総合的に評価した．他の車両および離れた中継局など干渉源が複数存在する市街地環境においても，提案法を用いることによって隠れ端末問題の影響が有効に回避できること，隣接する中継局間で互いに棲分け中継をすることで非常に高い中継効果が得られることを明らかにした．　以上のように提案法は中継アシスト車車間通信の特性を大幅に改善できるが，通信トラヒックがさらに高い環境に対処するため，中継パケットのエアトラヒックをさらに圧縮できる方法として，複数ノード環境に適したネットワークコーディング法を用いたペイロード合成中継法を提案する．本提案法では，車車間ペイロードのソーティングと合成対象パケットの選択アルゴリズムによって複数ノード環境でのネットワークコーディングの弱点を抑えつつ，輻輳問題に有効に対処できることを示した．結果として本提案法をセクタ化受信と組合せることで，幅広い通信トラヒック条件においてブロードキャスト配信成功率が大きく向上することを明らかにした．電気通信大学201

Performance and Reliability Evaluation for DSRC Vehicular Safety Communication

<p>Inter-Vehicle Communication (IVC) is a vital part of Intelligent Transportation System (ITS), which has been extensively researched in recent years. Dedicated Short Range Communication (DSRC) is being seriously considered by automotive industry and government agencies as a promising wireless technology for enhancing transportation safety and efficiency of road utilization. In the DSRC based vehicular ad hoc networks (VANETs), the transportation safety is one of the most crucial features that needs to be addressed. Safety applications usually demand direct vehicle-to-vehicle ad hoc communication due to a highly dynamic network topology and strict delay requirements. Such direct safety communication will involve a broadcast service because safety information can be beneficial to all vehicles around a sender. Broadcasting safety messages is one of the fundamental services in DSRC. In order to provide satisfactory quality of services (QoS) for various safety applications, safety messages need to be delivered both timely and reliably. To support the stringent delay and reliability requirements of broadcasting safety messages, researchers have been seeking to test proposed DSRC protocols and suggesting improvements. A major hurdle in the development of VANET for safety-critical services is the lack of methods that enable one to determine the effectiveness of VANET design mechanism for predictable QoS and allow one to evaluate the tradeoff between network parameters. Computer simulations are extensively used for this purpose. A few analytic models and experiments have been developed to study the performance and reliability of IEEE 802.11p for safety-related applications. In this thesis, we propose to develop detailed analytic models to capture various safety message dissemination features such as channel contention, backoff behavior, concurrent transmissions, hidden terminal problems, channel fading with path loss, multi-channel operations, multi-hop dissemination in 1-Dimentional or 2-Dimentional traffic scenarios. MAC-level and application-level performance metrics are derived to evaluate the performance and reliability of message broadcasting, which provide insights on network parameter settings. Extensive simulations in either Matlab or NS2 are conducted to validate the accuracy of our proposed models.</p>Dissertatio