Research On MAC Protocol In Vehicular Ad Hoc Networks Based On WAVE

Vehicle Ad Hoc networks (VANETs) have received increasing attention fromacademia and industry since the boom of Intelligent Transportation Systems (ITS).Based on the rapid development of wireless communication technology,VANETs plays an important role in many aspects of traffic safety, such asemergency warning, traffic congestion management and mobile entertainment et al.Some characteristics of VANETs, like dynamic network topology, changeable Nodedensity and Complex network environment, bring Challenges to the design of MACprotocol.The thesis focused on MAC protocol of VANETs to improve the reliability ofMAC layer broadcast. The characteristic of VANETs was first introduced and specialrequirements it brought to MAC protocol was then proposed. A survey of VANETsMAC showed that IEEE802.11p protocol has better applicability then other VANETsMAC protocols. However,802.11p protocol has a serious drawback that it suffersfrom high packet loss rate for broadcast, especially in the case of high node density.There are two reasons for high packet loss rate. On one hand, Vehicles predictablybroadcast beacon frame in the CCH channel, resulting in the congestion of the CCHchannel. On the other hand, in VANETs a lot of applications rely on broadcast service,but the BEB backoff algorithm dose not work for broadcast. This thesis proposed twocorresponding solutions to improve802.11p protocol.A new VANETs MAC protocol H-MAC (Hybrid-MAC) was proposed tosolve the congestion problem caused by the beacon frame. H-MAC protocol divided aframe cycle into two parts. The first part is reservation period which is composed of anumber of slots with same structure, and the number of the slots is dynamicallychanged according to the number of nodes in the neighbor table. The second part iscompetition based period. Each node in the network had its own slot in the reservationperiod. Node could send stable data flow such as beacon packets in its own slot. Otherburst data flow could send in the burst flow in order to get lower delay. H-MAC had acluster fusion mechanism that could adjust slot assignment in the reservation periodwith the changes of network topology. Simulation showed that H-MAC was moresuitable for VANET’s data link layer. It could achieve lower packet loss rate forsending beacon packet than802.11p protocol did and performed lower delay for other broadcast packets than RR-ALOHA.A broadcast contents windows Adjustment mechanism-NDCWA was proposedto solve the fixed content window problem coursed by broadcast. This mechanismadjusts content window according to the vehicle’s position in the network. In order tofurther improve the reliability of the MAC layer broadcast, this thesis designed abroadcast acknowledgment mechanism. This mechanism selected the neighbor vehiclewith the most urgency to acknowledge the reception if the transmission succeeded.Simulation showed that this improvement could achieve lower packet loss rate forsending broadcast packets than original802.11p protocol with acceptable sacrifice oftransmission delay.