A Study On MAC Protocols For Vehicular Ad Hoc Networks

A vehicular ad hoc network(VANET)is a special type of mobile ad hoc networks(MANETs),which consists of a number of vehicles and roadside units(RSUs).The vehicles and RSUs are configured into a VANET in an ad hoc manner.A VANET can provide short-distance communication between vehicles,which thus can realize a variety of applications,including traffic safety,traffic management and network entertainment.These applications can not only reduce the occurrence of traffic accidents,and but also improve traffic efficiency and service experience in the process of driving.For these reasons,VANET has attracted a lot of research interests in both academia and industry.Different from traditional MANET,the high mobility of vehicles makes the topology of VANET change frequently,which presents a big challenge to the design of MAC protocols for VANETs.Since existing MAC protocols for MANET cannot directly be employed in VANET,it is imperative to study and design an efficient and reliable MAC protocol for VANET.This thesis mainly studies the MAC protocol design for VANET.Three improved MAC protocols are proposed based on existing relevant MAC protocols,i.e.:a node density based MAC protocol(D-VeMAC),a node density based adaptive MAC protocol(A-VeMAC:)and a timeslot occupancy based MAC protocol(SO-MAC).The thesis first proposes a node density based MAC protocol,called D-VeMAC.In the timeslot reservation process,the D-VeMAC protocol no longer divides each frame into two disjoint timeslot sets at the middle of the frame in a fixed manner,but determines the timeslot set available for reservation according to the node densities in the left and right directions.If the node densities in the two directions are uniform,a node can only access the timeslot set associated with its direction;otherwise,if the node densities are not uniform,a node can not only access the timeslot associated with its direction,but also access the timeslot set associated with the opposite direction.The simulation results show that,compared with the VeMAC protocol,the D-VeMAC protocol is not only suitable to a uniform network scenario,but also can achieve better performance in a non-uniform network scenario.To address the limitation in performance improvement with D-VeMAC,the thesis proposes further a node density based adaptive MAC protocol,called A-VeMAC.The A-VeMAC protocol introduces a discrete random variable to represent the node density in the network,and decide whether to adjust the division of timeslot sets according to the value of this variable.The simulation results show that the A-VeMAC protocol can not only be applied to a uniform network scenario,but also be applied to a non-uniform network scenario.In a non-uniform network scenario,the A-VeMAC protocol can achieve a better performance improvement than the D-VeMAC protocol.To address the low channel utilization problem with a MAC protocol which allows a node to occupy only one timeslot in a sparse scenario,the thesis proposes a timeslot occupancy based MAC protocol,called SO-MAC.The SO-MAC protocol determines the probability to reserve a sub-timeslot based on the current timeslot occupancy.Once the current timeslot occupancy is larger than a threshold,a node will release all its occupied sub-timeslots.The simulation results show that the SO-MAC protocol can achieve higher channel utilization in both the highway and urban scenario,especially when the network load is small.