Research On TDMA-based MAC Layer Low Collision Protocols In Vehicular Ad Hoc Networks

As an indispensable part of the intelligent transportation system(ITS),vehicular ad hoc networks(VANETs)is a dynamic private network realized by short distance communication technology between vehicle nodes or between vehicle nodes and road side infrastructure.One of its core functions is to ensure safety messages for vehicle nodes in the network.Owing to the high mobility of the vehicle nodes,the network topology changes dramatically.In order to timely and reliably transmit safety messages to avoid traffic accidents,it is required that the nodes quickly access the network and are able to stay in the network.Therefore,it is very practical to study how a node quickly access the network with low collisions and how to employ the network with low collisions after accessing the network.This thesis will design efficient MAC layer protocols for these two issues.In the VANETs,in order to enable the vehicle nodes to transmit safety messages in a timely and reliable manner,mainstream MAC layer protocols adopt a TDMA-based channel allocation method to divide channel resources into time slots,which ensure that channel resources can be utilized without collisions between nodes.On the one hand,since the VANETs is a distributed network,when multiple nodes attempt to access the network to occupy the same time slot,a large number of access collisions appear.On the other hand,when the node has access to the network,the higher relative speed between nodes will cause one node to continuously merge into the network composed of other nodes,resulting in the acquired time slot to be invalid,and the merge collision occurs.The occurrence of access collisions and merge collisions will lead to the failure to trans-mit safety messages in time,increasing the probability of traffic accidents.In order to meet the above challenges,this thesis proposes low collision access network protocols and merge network protocol after deeply analyzing the causes of access collision and merge collision.In the vehicle nodes access network phase,conventional protocols usually include the following processes.First,by listening to the channel,it is known that there are idle time slots;Then,sending its own occupation message in the randomly selected idle time slot;Finally,the channel is listened again,and the neighbor nodes broadcast informa-tion is received to confirm whether the reservation is successful.Once multiple nodes attempt to compete for the same time slot,it will cause collisions,and the conventional protocols still need to wait for a period of time before it can judge the collision,and then repeat the access process.In this thesis,a special signal is used to give different priority to the contending nodes,and a pre-aware collision solution was designed.Node can determine whether the access is successful within the time slot to be accessed,and do not need to wait until the neighbor message is collected.The simulation results show that the proposed schemes can effectively improve the speed of vehicle nodes accessing the network,especially in the higher node density.In the vehicle nodes merge network phase,conventional protocols divide the time slots according to the moving direction,relative speed,lanes,etc.of the vehicle nodes.Nodes can only use the corresponding time slot sets according to their own character-istics.So that vehicle nodes with relatively high speed occupy different time slots,and reduce the merge collision caused by the relative movement.In addition,other proto-cols employ the vehicle location and speed information to predict the potential merge collision,let the reference nodes actively release the occupied time slots,and reserve new time slots in advance to reduce the merge collisions.This thesis combines the above two ideas to solve the problem.First of all,in the time slot division,the time slot is not strictly grouped,so as to avoid the unbalance of traffic density leads to the relative shortage of time slots.Then,special signals are used to predict collision timely.Finally,adj ust the relevant nodes to their corresponding time slots to decrease potential merge collision.The simulation results show that the proposed scheme can effectively reduce the number of merge collisions and improve the throughput of safety messages.