Performance Optimization And QoS Analysis Of HCCA Scheme In VANET MAC Layer

The vehicular ad hoc network(VANET)is a very important part of the intelligent transportation systems(ITS).When vehiculars are having some emergencies,they have to send some urgent information with strict quality of service(QoS)requirements to other vehicles.How to satisfy these QoS requirements is one of the key points of VANET.The 802.11 p protocol follows the MAC layer scheme-HCCA(Hybrid Coordination Function Control Channel Access)of 802.11 e protocol to improve transmission performance.However,when applying the HCCA scheme in VANET environment,the system performance becomes unacceptable.The QoS requirements,including transmission delay,packet loss rate and bandwidth,will not satisfied.Firstly,we aim to improve the HCCA(Hybrid Coordination Function Control Channel Access)polling scheme at MAC layer of 802.11 p protocol and design a new data transmission scheduling scheme based on weighting algorithm.We also develop a mathematical model to systematically evaluate the performance of the HCCA scheme.The simulation results shows that,comparing with other schemes,this improved scheme has lower average delay and packet loss rate,the transmission redundancy is also reduced.Furthermore,above scheme could not satisfy the transmissions with very strict delay requirements.Thus,we optimize the above scheme by designing a delay guaranteed scheduling scheme(DG-HCCA),which works to schedule the network resource based on the Markov Decision Process(MDP).Besides,to guarantee the transmission delay and maintain the system stability a system feedback mechanism is also applied to monitor the system performance and adjust the system thresholds periodically.The simulation results demonstrate that most of the requirements could be satisfied;the packet loss rate is decreased and the channel capacity is also improved.Along with the traffic flow increases,not all of the transmission QoS could be fulfilled.The network may need to employ an admission control scheme to decline the data flow that the QoS could not be satisfied.In this thesis,we make a proposal of a hybrid admission control model to control the transmission data flow.By this model,the system parameters are monitored based on delay prediction scheme and the MDP model is also used on scheduling the transmission requirements.The simulation results show that the channel capacity is better than other schemes,the packet loss rate is reduced and the transmission QoS could also be fulfilled.Above all,our improvements to the HCCA scheme could preferably solve the urgent data transmission problem in the VANET environments with the QoS requirements be satisfied in a large scope.