Research On Selective-Repeat Cooperative Scheme Based On Distributed TDMA MAC For VANET

Road accidents are one of the leading causes of human disability and death,and represent a serious social problem on a global scale.Communications among vehicles can deliver the information of the driving status of vehicles,effectively reducing the frequency and the severity of road accidents.In addition,these communications can increase transportation efficiency and improve driving experience.With the rapid development of wireless communication technology and automotive industry,vehicle to vehicle communications become possible and evolve into the vehicular networks called Vehicular Ad Hoc Networks(VANET).VANET can operate in a distributed manner.However,high vehicles mobility,frequent topology changes,the absence of roadside infrastructure,and different applications requirements make VANET extremely challenging.The MAC protocols directly control packet transmission on wireless channel and have a major impact on the networks performance.IEEE 802.11 protocols cannot satisfy stringent requirements of VANET safety applications,which demand quick and reliable packet delivery.In addition,these protocols cannot provide an effective broadcast service.Thus they are not well suitable for VANET.Distributed TDMA(DTDMA)protocols can better adapt for VANET,and have a better performance in terms of channel utilization,network throughput,and protocol fairness etc,as compared to IEEE 802.11 protocols.However,since vehicles move with high speed and wireless signals are blocked by vehicles and roadside buildings,channel conditions in vehicular environment are worse than in general wireless environment,so that communication reliability becomes a serious problem in VANET.Considering limited channel resources and the rapid growth of the services provided by VANET in future,improving the networks performance simply via point-to-point wireless links becomes a bottleneck.On the other hand,via cooperation among nodes,cooperative communication can effectively mitigate wireless channel impairments to enhance communication reliability.But due to dynamic topology changes and the absence of central node,most cooperative schemes proposed for TDMA protocols cannot be directly applied in VANET.To enhance communication reliability in VANET,a new cooperative framework over DTDMA protocols is proposed in this thesis.The proposed cooperative framework does not rely on any central node,can well adapt to dynamic topology changes,and can be easily implemented with low overhead,thus providing a possible solution to the challenges in VANET.The main works are as follows:1)Propose a node-idle-time-slot based cooperative scheme referred to as Cooperative Distributed TDMA I(CoDTDMA-I).DTDMA protocols work via time-slot-assignment information included in packet header.After a node obtains a time slot,it occupies the time slot.For exchanging time-slot-allocation information and indicating the time slot occupied,even if a node has no application data to be transmitted,it still transmits a dummy packet during the time slot it occupies,thus inevitably introducing nodes idle time slots.CoDTDMA-?is proposed to further exploit these idle time slots.When a source node fails to transmit a packet,cooperative retransmission is performed by helper node during its idle time slot.Based on the mechanism of DTDMA protocols,cooperation is achieved without transmitting any controlling packets,and communication mode is not required to switch between transmitting and receiving during a time slot.However,the cooperation may introduce extra latency,referred to as cooperative latency.To solve the problem of cooperative latency,a cooperative selective-repeat solution is proposed.The thesis develops a mathematical model to analyze CoDTDMA-?.The closed-form expression for the probability of successful packet transmission and that for packet dropping rate are derived.Both analysis and simulation show that CoDTDMA-? can significantly improve the probability of successful packet transmission and reduce the rate of packet dropping.2)Develop a cooperative communication queuing model over DTDMA protocols.In CoDTDMA-?,since the cooperative selective-repeat solution is exploited,source node and helper node can transmit different packets from source node at the same time,and some packets may be transmitted in tandem by them.Therefore,the Pollackzek-Khinchin formula cannot be used in this case.Via the analysis of the second moment of the number of packets awaiting in the queue and being transmitted at source node,it can be proven that the number of packets depends on the probabilities of four special states.Based on the analysis of the relationships between the four special states,equations are listed and solved to get the four probabilities and the number of packets.Then,the number of packets awaiting in the queue at source node,the number of packets being transmitted,expected queuing delay and expected transmission delay are derived,such that a cooperative queuing model over DTDMA protocols is developed.Simulation is conducted to verify the analytical model.Both analysis and simulation show that CoDTDMA-? can reduce queuing delay and transmission delay significantly.3)Propose an unreserved-time-slot based cooperative scheme referred to as Cooperative Distributed TDMA?(CoDTDMA-?).Network members and topology are constantly changing in VANET.When network members within two-hop transmission range are not enough to use all the time slots in a frame,there are unreserved time slots.CoDTDMA-? is proposed to exploit the unreserved time slots to cooperatively retransmit the failed packet.To determine the helper node and the unreserved time slot for cooperation,a handshake protocol is designed in CoDTDMA-?.Since all controlling information is exchanged via piggyback mechanism,controlling packet is not introduced and communication mode is not required to switch between transmitting and receiving during a time slot.The cooperative selective-repeat solution is also exploited to solves the problem of cooperative latency.The thesis develops a mathematical model to analyze CoDTDMA-?.The closed-form expression for the probability of successful packet transmission and that for packet dropping rate are derived.Analysis and simulation show that CoDTDMA-? can increase the probability of successful packet transmission significantly,and reduce the rate of packet dropping and transmission delay effectively.Exploiting nodes idle time slots and unreserved time slots to perform cooperation,the proposed cooperative framework enhances the reliability of transmission,without additional cost in bandwidth and hardware devices.In addition,the proposed framework does not interrupt the normal packet transmissions in the networks.