A Research Of Multi-channel Coordination Mechanism For VANET

The initial motivation of the concept of VANET is to provide a unified solution for traffic safety issues,and at the same time it can provide an efficient unified solution platform for improving traffic effiency.Institute of Electrical and Electronics Engineers(IEEE)established two working groups,IEEE 802.11 p and IEEE 1609,to study the future development direction of VANET and formulate corresponding protocal specifications.On the one hand,the IEEE 1609.4 standard divides the wireless channel into two types of channels: control channel(CCH)and service channel(SCH).The CCH channel is used to transmit safety messages and control messages,and the SCH channel is used to transmit value-added services that are unrelated to security messages.However,the standard does not specify specific strategies for multi-channel cooperation and uses fixed-slot-length control channels and service channels.The design of fixed-slot lengths makes the CCH channel unable to provide sufficient bandwidth for carrying a large number of safety messages and control messages in a busy network environment.At the same time,in a vehicle environment with a relatively sparse node density,CCH channels will be wasted due to underutilization,and service channels that need to deliver large data traffic non-secure application messages will also suffer from bandwidth limitations.On the other hand,the IEEE 802.11 p protocol specifies that vehicle nodes must obtain channel resources through contention on both the CCH channel and the SCH channel.For the insufficiency of the WAVE MAC protocol defined by the above two standards,the industry urgently needs to explore the coordination of CCH and SCH channel resources and optimize the allocation strategy.In the condition of ensuring high reliability and low delay in transmitting safety messages,the saturation of the service channel should be improved as much as possible.In view of the shortcomings in the IEEE 1609.4 and IEEE 802.11 p standards,this paper studies the vehicle safety distance model and proposes a new vehicle safety distance-based WAVE MAC protocol(VSD MAC),which provides a new perspective for the study of the WAVE MAC protocol.The VSD MAC protocol inherits the division strategy of the CCH time slot and the SCH time slot in the IEEE 1609.4 standard,but has the following innovations: 1)By analyzing the motion characteristics of the vehicle in the car-following model,the definition of the VSD is proposed.And according to relationship between the actual vehicular distance and the safety distance,the probability of the vehicle to send safety alarm messages is deduced;2)The variable CCH and SCH time slots are designed on the base of the probability that vehicle sends safety alarm messages,thereby improving the channel utilization rate of the CCH and SCH channels and system saturation throughput;3)Different time slots of different CCH channels occupied by different vehicles can be used to enable different vehicles to achieve non-competitive access on the CCH channel,thus improving the transmission reliability of safety alarm messages.And by reserving the SCH channel resources on the CCH channel,non-competitive access and channel reservation of the SCH channel is achieved,which can reduce channel collisions and improve channel throughput.Through theoretical modeling and simulation analysis,it can be found that the VSD MAC protocol can obtain more reliable safety message transmission performance and higher saturation throughput than the WAVE MAC protocol defined by IEEE 1609.4 and IEEE 802.11p.