Research Of Routing Protocols In Vehicular Ad Hoc Networks

As the foundation of other functions in the Intelligence TransportationSystem (ITS), Vehicular Communication System is an important component ofITS. The present research indicates that the Mobile Ad Hoc Networks (MANET)has many advantages when applied in the Vehicular Communication System.Vehicular Ad Hoc Networks (VANET) is a special MANET designed for theinter-vehicle communication (IVC). It creatively applies the MANETtechnology in the inter-vehicle communication, so that the drivers can obtainthe information from other vehicles and the real-time state of roads in thebeyond line of sight scope. The object of VANET is to establish a platform ofIVC. The platform can not only improve traffic efficiency but also make thedrivers and passengers more safety and more comfortable.As a new research area, the routing protocols of VANET have a greatimpact on the performance of VANET. After analyzing the architecture andcharacteristics of VANET, This paper focuses on the study of the routingprotocols in VANET. First we analyzed the vehicular movement model; thenmodify the simulation tool of NS2 to add the obstacle element; after thesepreparations we analyzed the performance of the routing protocols in VANETthrough the simulation and optimize the GPSR protocol. The detail contents inthis paper are as follows:Part 1. Introduce the architecture and characteristics of VANET. Althoughthe VANET is a kind of MANET, it has been very different from the traditionalMANET and shows many new characteristics after applied in the VehicularCommunication System. Therefore it's necessary to re-analyze thecharacteristics of VANET. VANET's particular characteristics are mainly asfollows: the vehicles mobile regularly with a high speed, and also limited bythe traffic regulations; The buildings along the roads, the road situation andvelocity of vehicles make the quality of wireless channel change frequently; the engine in each vehicle can provide enough power to the electronic devicesplaced inside the vehicle, and the electronic devices also have powerfulcomputing ability and enough memory space; GPS device can supply accuratepositioning information and clock information for the vehicle, so the vehiclecan gains its position information and take on the clock synchronization. Someof these characteristics are very helpful to our research so they should beconsidered in this paper. For example, position-based routing protocols becomemore and more concerned because GPS can provide the position information.Because the engine in each vehicle can provide the enough power to theelectronic devices, we won't worry about the lack of power.Part 2. The vehicular mobility model was an important factor to affect thesimulation results, and the traditional Random WayPoint (RWP) mobilitymodel was no longer suitable for VANET. Vehicular mobility model underurban environment is more complex than traditional random mobility model.We should consider not only the urban environment but also the trafficregulations which have strong influence on the mobility of vehicles. Afteranalyzing several typical movement models, the paper set up a directionalgraph-based mobility. The road net structure is made a detailed division intolanes which are the basic elements in this model. The traffic regulations can bereflected though the lanes in the model. For example, if there is only a singlelane in a road we can know that the road is a one-way road. And the lanes canalso limit the vehicle's direction. In addition, the model has also divided thevehicle into two types to represent different vehicles. The paper has given thedetailed algorithm and develops a simple tool using C++ programminglanguage to generate the mobility documents as input files of NS2.Part 3. Besides considering vehicular movement characteristics, we shouldalso consider the influence of the urban environment especially the impact onthe radio when we study VANET. This paper takes NS2 as the simulatorplatform. NS2 is an object-oriented and discrete event network simulator whichwas developed by UC Berkeley University. It was built in C++ and provides a simulation interface through OTcl. It contains massive network components sothat it is very suitable for wireless network simulation, especially after theCMU project group extended the wireless functions. But there isn't an obstaclemodel which can represent the terrain in real world, so we must modify theNS2 to add the obstacle model to make the simulation environment approachthe real world. The paper first abstracted the urban buildings into the obstaclemodel, and then gave the algorithm weather the radio is blocked by obstacles.After the analysis of the wireless node structure, we focused our attentions tothe radio propagation models. These models are used to predict the receivedsignal power of each packet. At the physical layer of each wireless node, thereis a receiving threshold. When a packet is received, if its signal power is belowthe receiving threshold, it is marked as error and dropped by the MAC layer. Sowe should modify the radio propagation models to judge whether the single isblocked by obstacles.Part 4. After finishing the above preparations, we analyze the performanceof the routing protocols in VANET through the simulation. The resultsdemonstrate that the performance of the routing protocols in VANET decreasesrapidly respect to the ones in traditional MANET, especially in urbanenvironment. The existence of the buildings makes the wireless linkage changefrequently. We found that the GPSR protocol has shortcomings in urbanenvironment, and point out that it is caused by greedy forward and perimeterforward strategy. In this paper we give some optimum ideals to optimize GPSR.Through the simulation we found that the optimum GPSR has a higher packetdelivery ratio than GPSR.Through the work in this paper, we analysis the performance of routingprotocols in VANET and pointed out the key questions in the research ofrouting protocols in VANET especially in the urban environment. At the end ofpaper we proposed some improvement thoughts to GPSR. Another work in thispaper is establishing a platform for VANET simulation by re-developing NS2,which provides the solid foundation for the further research.