Research On Performance Evaluation And Improvement Of IEEE 802.11p Wireless Vehicle Network Channel Access Mechanism

The Wireless Vehicular Ad-Hoc Network is composed of vehicle nodes,which cooperate with each other to build a Network,and can sense the surrounding traffic environment and interact with each other in real time,improve road safety by preventing accidents.Since data related to safety applications needs to be received by all vehicles in the surroundings,safety messages such as accidents will be transmitted in a broadcast manner,so the nodes in VANET will periodically sense the surrounding environment by broadcasting beacon messages.The use of Medium Access Control(MAC)protocol directly determines the validity of messages transmission.IEEE 802.11p MAC protocol is used to control how nodes in VANET use limited channel resources,which directly determines the performance of channel utilization,delay and fairness.However,the performance of MAC protocol is also affected by the high-speed dynamic change of VANET:(1)the nodes in VANET have the high-speed moving characteristic,so the communication link between modes is very short;(2)In VANET,the change of node density is also unpredictable.In the road with high network load,the network topology will change dynamically with the change of nodes because the dynamically changing nodes join and exit communication frequently,the performance of the network such as access delay and total throughput is severely degraded;(3)the IEEE 802.11p channel access technology uses a constant contention window in the execution of the backoff algorithm,and the packet loss rate is serious because of the collision between the packets;(4)the wireless channel is unstable,which also has a great influence on the beacon communication of the vehicle network;(5)There is a lack of confirmation and retransmission mechanisms in the broadcast mode,and the reliability and real-time nature of broadcast safety messages cannot be guaranteed.There are two kinds of access protocols in IEEE 802.11p MAC layer:(1)based on IEEE 802.11 Distributed Coordination Function(DCF)mechanism;(2)Enhancement Distributed Channel Access(EDCA)mechanism based on IEEE 802.11e.In general,the beacon frame is broadcast by DCF access mechanism,while the EDCA mechanism can provide QoS for VANET.However,in the DCF based broadcast mode,the nodes have the following problems due to the constant contention window:when the density of nodes is very high,the probability of nodes simultaneously accessing the channel will increase greatly due to the selection of a constant contention window,and the fairness of the wireless channel cannot be guaranteed,but in the sparse network environment,the competition window with larger value is easy to cause long idle channel.In addition,the channel error is also an important cause of packet loss.In EDCA mechanism,both node density and contention window selection algorithm have important effects on the performance of EDCA access mechanism,especially when the node transmission is successful,the contention window of node will be restored to the minimum value,the conflict between the nodes will be intensified.This paper aims to improve the total throughput of IEEE 802.11p Wireless vehicle network,guarantee the fairness of channel usage and ensure the reliability of beacon message transmission,the performance of IEEE 802.11p Wireless vehicle network channel access mechanism is studied and optimized.The main work is as follows:(1)An overview of wireless vehicle networking,describing the communication structure of VANET,the challenges of technological development,and access technologies in VANET;wireless access to the vehicle environment,which focuses on the multi-channel coordination mechanism;IEEE 802.11p protocol,two access mechanisms of MAC layer,DCF and EDCA,are introduced.(2)Aiming at the DCF mechanism:a channel access method is proposed to dynamically adjust contention window and transmission rate.By observing the idle time slot and the probability of transmission failure,the node can distinguish the current channel state and congestion degree of the network,and further analyze the reasons of the failure of beacon broadcasting.When the wireless channel state is poor,the node dynamically adjusts the transmission rate,and when the congestion degree is high,the node dynamically adjusts the competition window.According to the judged network state,the node flexible uses the method of combining the dynamic adjustment transmission rate and contention window,which reduces the node to choose the competition window,which reduces the node to choose the same competition window,and better adapts to the dynamic change of the vehicle network environment.(3)Aiming at the EDCA mechanism:An adaptive contention window adjustment algorithm considering collision is proposed.When the node transfers successfully,the contention window does not regress to the minimum value,but adjust according to the last successful contention window;the node maintains a database table that records the number of conflicts and adjusts the value of the contention window based on the conflicts.(4)The VANET simulation platform is built by using NS3 network simulator to verify the performance of IEEE 802.11p broadcast mechanism and the IEEE 802.11p channel access mechanism based on DCF and EDCA proposed in(2)and(3).Finally,the NS3 simulation results show that the proposed algorithm can greatly improve the performance of IEEE 802.11p channel access mechanism.