Research On Channel Access Control Technology In Vehicular Ad Hoc Networks

As a new Mobile Ad hoc Network(MANET), Vehicular Ad hoc Network(VANET) is applied to Intelligient Transport System(ITS). Through Vehicle-to-Vehicle(V2V) communication and Vehicle-to-Infrastructure(V2I) communication, VANETs can provide safety alarm and service applications to moving vehicles for the enhancement of driving safety and the comfort of drivers and passengers. Efficient and reliable Media Access Control(MAC) scheme can ensure the implenment of the abundant applactions of VANETs. However, compared to the traditional ad hoc networks, there are a lot of different characteristics of VANETs. For example, highly dynamic topology, variable traffic density and different transmission requirement of applacations, which make a lot of mature MANETs MAC protocols can not be used in this new communication scenario and challenge the design of the VANETs MAC protocol.Network connectivity is an important performance metric to indicate the communication quality of VANETs, especially in the high-way where the traffic density is low. If the connection between the vehicles is boroken, vehicles may have difficulties to delivery either service messages or safety messages. Then not only the service requirements of drivers and passengers cann’t be safisfied, but aslo the road safety. Moreover, when there are platoons in the network, the connectivity probability of VANETs will change. Hence there is great scientific and application value in the study on the connecitivity in the platoon-based VANETs and its impact on the VANETs MAC protocol design.Based on the above problems, this paper studies the channel access control technology in VANETs. According to the lack of the current Wireless Access in Vehicular Environments(WAVE) MAC framework defined by IEEE 802.11 p and IEEE 1609.4 standards, this paper presents the improved multi-channel coordination and multi-priority supported MAC protocols for VANETs. Moreover, the connectivity in platoon-based VANETs is studied and the connectivity-aware MAC protocol design is illustrated.With the fixed length of Control CHannel(CCH) interval and Service Channel(SCH) interval and the contention based medium access mechanism, the current mulit-channel MAC scheme defined by IEEE 1609.4 cannot satisfy the bandwidth requirments of either safety application or unsafety application. Based on the Coordinated Universal Time(UTC) scheme, a variable interval based multi-channel coordination protocol is proposed in this paper. This protocol can optimize the interval ratio between the CCH and the SCH according to the network conditions and improve the channel availability and the saturated throughput on the channels. Moreover, different from the existing research, a contention-free ACK scheme is firstly devised in this paper. This scheme can ensure the reliable delivery of the safey messages and achieve the SCHs reservation of the service messages and multi-channel coordination service through the interaction between the WSA(WAVE Service Announcement) packets and ACK packets.According to priority differentiation channel access characteristic defined in the EDCA(Enhanced Distributed Channel Access) protocol, a multi-priority p-persistent adaptive optimization algorithem is presented to satisfy the different service requirements of the variable service appliacations. The existing multi-priority adaptive optimization algorithems generally estimated the number of the active nodes to achieve the optimization of the network performance. Different from these optimization algorithems, this algorithem real-time optimizes and adjusts the transmission probabilities of different priority messages according to the detecting channel utilization status. Moreover, a 3-D multi-priority Markov chain model is presented to calculate the optimal transmission probabilities and the optimal minimum contention windows of different priority messages. Simulation and analytical results indicate that this algorithem can effectively ensure the prioritized transmition of the safety messages and improve the saturated throughput on CCH.The network connectivity will change greatly when there are platoons which drive as the special pattern in the VANETs. However, none of the previous stuides consider this problem. This paper firstly studies the network connectivity in platoon-based VANETs. The connectivity probabilities are analyzed for the platoon-based V2 V and platoon-based V2 I communication scenarios, respectiviely. The relationships between the connectivity probability and the key parameters are investigated, including the vehicle number, the traffic density, the platoon ratio, the coverages of vehicles and the coverage of RSUs in the VANETs. Analysis and simulation results show that the connectivity probabilities are increased by 10% or more in a platoon-based VANET than those in a VANET without platoons. Moreover, the connectivity probabilities in the two-way scenario are improved at most two times than that in the one-way scenario due to the forwarding packets between two-way vehicles.The QoS supported MAC scheme in VAENTs should both satisfy the requirement of the multi-priority service differentiation and achieve the optimal system performance. Especailly in the platoon-based VANETs, the WSA packets transmitted by the platoons and ordinary vehicles have different Quality-of-Service(QoS) requirements. In order to achieve these goals, a connectivity-aware multi-channel MAC protocol in platoon-based VANETs is presented in this paper. This protocol considers the impact of the center node role of the platoon leader and the network connectivity on the VANET MAC protocol design and network performance. Moreover, according to the current network status and network connectivity requirement, an adptive MAC protocol is designed, which can improve the system throughput of the network. Morover, a satisfaction-based multi-priority distingction scheme is proposed to classify the WSA packets to different priorities. This scheme maximizes the satisfactions of all the packets with proportional fairness. Furthermore, a multi-priority Markov chain is derived to obtain the relathionship between the connectivity probability and the system throughput and the optimal length of CCH interval.