Research On Multi-channel Protocol Design For VANETs

In the 5G era,vehicular ad hoc networks(VANETs)are going to provide more di-verse,ubiquitous,reliable and intelligent services.To efficiently support various advanced services,medium access control(MAC)protocols should provide high data rate,low la-tency and reliable communications by making full use of all the authorized channels.However,due to the ad hoc and high mobile nature of VANETs,the MAC protocol design for VANETs is a challenging task.Currently,many issues have not been well addressed,such as the multiple access of large number of nodes,protocol scalability,communica-tion negotiation,etc.Existing protocols often suffer from high access delays,low channel utilization and poor scalability.Thus,to satisfy the future applications of VANETs,ad-vanced MAC protocols should be designed in order to solve the above issues.Consequently,in this work,we focus on the design of multi-channel MAC protocols for VANETs.For the control channel(CCH),the reliable and low latency beacon delivery is carefully addressed.For service channels(SCHs),the protocol design focuses on the high data rate point to point transmissions of application oriented data packets.The main contributions are summarized as follows:1)To cope with the potential concurrent channel access to the CCH,a novel OFD-M based group contention MAC(OGCMAC)has been proposed,in which a new CCH architecture,a modified multi-carrier burst contention(MCBC)and a group contention strategy are established.The CCH architecture is proposed to avoid collisions by adopt-ing contention windows(CWs).Based on this architecture,the MCBC is modified to adapt to VANETs and the resource consumption of CWs is reduced.To accelerate the re-source assignment,a group contention strategy is developed by dividing the entire frame into several groups with different sizes.With the objective to maximize the ratio of the resources allocated free of collisions within a single frame,a greedy group partition al-gorithm is proposed to determine the appropriate group sizes according to the MCBC capability.Simulation results show that OGCMAC can considerably reduce the channel access delay and provide reliable beaconing.It is also shown that the proposed greedy group partition algorithm can reduce the resource waste and contention level.2)Considering the high mobile nature of VANETs,a scalable and cooperative MAC protocol,referred as SCMAC,has been proposed to support the periodic beaconing over the CCH.The main concerned aspects in SCMAC are communication reliability and pro-tocol scalability.The communication reliability is guaranteed by cooperative beaconing.When broadcasting a beacon,the node embeds the slot state information into the beacon so that surrounding nodes can determine the availability of each slot.Together with the contention window,collisions can be alleviated.The protocol scalability is implement-ed by the slot access method and the proactive slot reservation.On one hand,the slot access method reduces the average delay by using the smaller beaconing period as the node density is decreasing.On the other hand,the proactive slot reservation always keeps enough idle slots so that more nodes can quickly join the network.The beaconing period will increase and all nodes can gain chances to broadcast beacons.Simulation results show that SCMAC can achieve the reliable and scalable periodic beaconing in vehicular environments.The protocol performance is not sensitive to the node density variations either.3)For the application oriented data exchanges over SCHs,an adaptive high-throughput MAC Protocol,referred as AHTMAC,has been proposed to achieve high data rate point to point transmissions over SCHs.In AHTMAC,the data transmission range over SCHs is adjusted according to the beaconing range over CCH,so that a node is able to select a proper communication target and prepare the available resources for both of the com-munication nodes before sending the RTS.Thus,a high RTS success probability(RSP)is guaranteed so as to gain higher throughput.The adaptive resource reservation is im-plemented by the proposed adaptive resource sharing mechanism,where nodes occupy as many resources as possible at the first place and then share them with others proactively.This mechanism is implemented by the concise RTS/CTS handshake.As no extra beacon exchange is required,AHTMAC can dynamically adapt to rapid node density changes.Simulation results show that AHTMAC can achieve high RSP and high throughput.Ad-ditionally,AHTMAC gains higher communication reliability because of the RTS/CTS handshake and the proposed resource management policy.In summary,in this thesis,the multiple access of large number of nodes,proto-col scalability and communication negotiations have been carefully investigated.The proposed multi-channel MAC protocols can achieve lower access delays,higher com-munication reliability and throughput and maintain high performance in fast changing environments.