Analysis And Optimization Of Vehicle Safety Communication In The Transprotation Cyber Physical System Environment

The traffic accident is one of the worldwide public safety issue that creates the seriousdamage of life and property to the human society. To avoid the traffic accident, all the majorautomobile manufacturers dedicated to developing vehicle active safety applications duringthe past three decades. Since the lack of extensive interconection and interoperability betweenthe process of computation and physical environment, most of active safety technologies areimplemented on the sigle vehicle, which cannot achieve full coordination and optimization.The emergence of Cyber Physical System (CPS) concept opens up new solutions for aboveproblems. Transportation Cyber Physical System (T-CPS) is the application of CPS in thetransportation area, which integrates the sensoring, computing, control and communicationtechnologies to form the novel information, intelligent and humanized transportantion system.To reliaze the deep integration of cyber system and physical world in T-CPS, a vehicleactive safety technology based on Vehicular Ad Hoc Network (VANET) emerges as the timerequire. VANET is a new mobile ad hoc network for the transportation environment based onDedicated Short Range Communication (DSRC), which can be effectively integraded into theT-CPS framework. It provides Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I)communications to exchange safety-related messages that enables drivers to predict potentialdangers beyond their horizon awareness. Some researches indicate that VANET is moreeffective than the other traditional active safety technologies. Therefore, national governments,research insititutions and automobile manufacturers are focus on Vehicle SafetyCommunication (VSC) based on VANET.Whether the standard DSRC protocol is able to meet the requirements of the real-timeperformance and the stability of VSC, is the important issue of the protocol design and study.This dissertation discusses key technologies of VANET when applied in VSC. We study theprotocol model of VANET based on IEEE802.11p under the typical traffic environment, andanalyze the real-time performance and the stability according to proposed model. On the basisof theoretic model, we further investigate the optimazation solution and design the rateadaptation protocol in VSC environment compatible with the standard IEEE802.11p toimprove the reliability of communication. The contributions of our work are listed below:First, we establish the VSC synthetical model accoring to the vehicle distribution, thechannel condition and charactisristics of V2V safety communication in the highwayenvironment. Unlike other model just setting up for one level of VSC, this model includes thehighway environmental model, channel model, the PHY-layer and MAC-layer model of the IEEE802.11p and describe hidden terminals, virtual collosions and unsaturated networkconditions in detail. This model is an effective synthetical model in VSC, which provides thefoundation for deep understanding the protocol, analyzing various performance indicators andprotocol improvement;Second, on the basis of the above analytical model, we further propose the real-timeperformance and the stability analysis method of VSC. According to the M/G/1/k queuingtheory, we analyze the average delay of the standard IEEE802.11p protocol under the typicaltraffic environment. Meanwhile, we obtain the average packet loss rate via the area partitionof the highway. Simulation results verify that the error rate of our proposed model is less than5%. Morevoer, we analyze the impact of different factors on the real-time performance andthe stability of VSC. Numerical analysis show that the IEEE802.11p standard broadcast caneasily satisfies the delay constraints but is difficult to meet the reliability requirements;Third, we design and implement the vehicle safety communication rate adaptation(VSCRA) protocol. So far, there is little work on rate adaptation protocols for VSC whichadopts the single-hop broadcast mode. We first propose the cross-layer design framework forthe single-hop rate adaptation in VSC. The optimized protocol solves two major challenges ofrate adaptation in VSC. First, each node needs to accurately estimate channel conditions in thesingle-hop broadcast environment without any feedback. Second, each node shoulddifferentiate the interference losses from the fading losses as an important basis of the best bitrate selection. Simulation results prove VSCRA can improve the staility of VSC greatly;Fourth, we bulid the exprimental platform based on the real DSRC on-board unit. Theresults show that the offset of the packet loss rate at the same distance is less than4%, thus,the distance is a good indicatior of the large-scale channel fading. Then, we develop thesimulation platform based on the NS2simulator. The simulation is conducted to validate thecorrectness of the analytical model and the effectiveness of the proposed protocol underdifferent traffic environments. The simulation results indicate that the packet loss rate ofVSCRA averagely reduces34.16%,43.07%and60.76%than3,6and12Mbps, respectively.To sum up, this dissertation adopts theoretic model to analyze the limitation of VANETwhen applied on VSC and proposes the effective optimizied solutions based on the analyticalmodel. Researches in this thesis are helpful for design and improving the standard protocol invehicular environment with the theoretical value and practical significance.