A Study On The Performance Analysis Of MAC Protocols For Vehicular AD HOC Networks

Vehicular ad hoc networks (VANETs) are a type of mobile ad hoc networks designed for communications between vehicles. VANET can not only decrease the occurring of traffic accidents and traffic jams, but also provide various recreation applications. Due to their extensive application prospect, VANET has received a lot of attention from both indutry and academia in many countries and become a hot research area in the field of communication networks. This thesis studies the medium access control (MAC) protocols for VANET and focuses on the performance analysis of MAC protocols. The main work and contributions include the following four aspects:Firstly, IEEE 802.11p enhanced distributed channel access (EDCA) mechanism is studied and a performance model is established for analyzing the access performance of the IEEE 802.11p EDCA mechanism. A 2-D Markov chain is first constructed to model the backoff procedure of an access category (AC) queue and establish a relationship between the transmission probability and collision probability of the AC queue. Then a 1-D discrete-time Markov chain is constructed to model the contention period of an AC queue and another relationship between the transmission probability and collision probability of the AC queue is established. Unlike most existing work, the 1-D Markov chain is extended to be infinite in modeling the contention period of an AC queue under both saturated condition and non-saturated condition. The two Markov models take into account all major factors that could affect the access performance of the IEEE 802.11p EDCA mechanism, including the saturation condition, standard parameters, backoff counter freezing, and internal collision. Based on the two Markov models, a performance model is further derived to describe the relationships between the parameters of an AC queue and the access performance of the AC queue in terms of the transmission probability, collision probability, normalized throughput and average access delay, respectively. The effectiveness and accuracy of the performance models are verified through simulation results.Secondly, the classic fair access problem in vehicular ad hoc networks is studied and a performance model is developed for analyzing the performance of an IEEE 802.11 distributed coordination function (DCF) based fair channel access protocol in a non-saturated state. In establishing the performance model, the relationship between the transmission probability of a vehicle and the minimum contention window size of the vehicle and the relationship between the velocity of a vehicle and the minimum contention window size of the vehicle in a non-saturated state are derived, respectively. Based on the analytical model, the minimum contention window size of a vehicle for a given velocity can be determined in order to achieve fair access among different vehicles. Moreover, an analytical model is also developed for analyzing the throughput performance of the fair channel access protocol in a non-saturated state. The effectiveness and accuracy of the analytical models is justified through simulation results.Then, a classic time division multiple access (TDMA)-based ADHOC MAC protocol is studied and a performance model is developed for analyzing the access performance of the ADHOC MAC protocol. In establishing the performance model, a Markov chain is first constructed to describe the number of vehicles which have acquired a timeslot successfully at the end of a frame when using the ADHOC MAC protocol. Based on the Markov model, an analytical model is then derived to describe the relationship between the frame length and the channel utilization. Based on the derived analytical model, an optimal frame length that maximizes the channel utilization is further obtained. Simulation experiments are conducted to verify the effectiveness and accuracy of the analytical model.Finally, considering that the number of vehicles changes frequently in a VANET, a dynamic frame length assignment based MAC protocol is proposed for reliable broadcast. Using this protocol, when the number of vehicles in the network decreases, the frame length can be decreased adaptively. When the number of vehicles increases, the frame length can be increased adaptively. Simulation results show that, compared with the VeMAC protocol, the channel access ratio of vehicles can be significantly increased, and the channel utilization of the network can be effectively improved when the initial frame length is larger than the number of vehicles in the network.