Research On MAC Protocol And Related Technology For Vehicular Ad-hoc Networks

As specific Mobile Ad hoc networks(MAENT),Vehicular Ad hoc Network(VANET)is becoming increasingly important in Intelligent Transportation System(ITS).Through establishing the communication of Vehicle-to-Vehicle(V2V),Vehicle-to-Infrastructure(V2I)and Vehicle-to-Pedestrians(V2P),VANET can provide services to vehicles such as safety warning,traffic management and entertainment information,which is of great significance to promote traffic safety,traffic efficiency and driving experience.The abundant services of VANET need to be guaranteed by efficient and reliable Media Access Control(MAC)scheme.However,different from the other Ad hoc networks,the speeding cars bring VANET a series of questions,including frequently changed topology,unstable links,dramatically changed network loads and diverse applications and requirements,which make it impossible to transplant the traditional MAC protocols to the new network environment.Therefore,the research on MAC protocol and related technology is always the hot topics of VANET.Two main directions of research on VANET MAC protocol are performance modeling and optimized design.How to model and analyze the performance of the existing protocols,and how to optimize and design the more adaptive MAC protocol to VANET,which can make the nodes share the channel fairly,reliably and efficiently,the two problems are urgent to be solved in the field of VANET MAC.From these two aspects,this paper studies on the performance analysis of prioritized broadcast service in WAVE/IEEE 802.11 p and packet-value-function-based dynamic game theoretic MAC Protocol for VANET.IEEE 802.11 p is derived from the IEEE 802.11 e enhanced distributed channel access(EDCA)function.To meet the different requirements of VANET applications,IEEE 802.11 p provides four access categories(AC)with different priorities.This paper analyzes the EDCA function of the IEEE 802.11 p,and proposes an accurate model to capture the prioritized broadcast service in IEEE 802.11 p.In the modeling process,the concept of the variable conditional transmission probability is presented creatively.Almost all of existing studies on the model of EDCA function consider the transmission probability of AC as a constant,which is proved inapplicable in modeling prioritized broadcast service for VANET through the analysis in this paper.In fact,the transmission probability of AC varies greatly in different periods of channel contention,and is a variable against the fluctuated numbers of successive idle slots.In addition,unlike most previous work,almost all of the factors that can impact the performance of IEEE 802.11 p are taken into account in the modeling,which include four AC with different priorities,standard parameters,the unsaturated and saturated conditions,the backoff procedure where the transmission queue is empty,and internal collision.Thus,a more accurate model of prioritized broadcast service in IEEE 802.11 p is built up in this paper,and the effectiveness of the proposed performance model is faithfully verified by the simulation results.To enhance the utilization of channel resource and guarantee the transmission of safety-related information,IEEE 1609.4 specifies multichannel operations on top of IEEE 802.11 p.It specifies that all nodes synchronously and periodically tune to the CCH or switch to one of the SCHs,which significantly impacts the performance of the MAC.Unlike previous work only consider the extension on the access delay,the effect of transmission synchronization caused by channel switching is also presented quantitatively in this paper.First,the reason for transmission synchronization problems is analyzed.Second,the relationship between the access delay affected by channel switching and the EDCA model mentioned above is formulated and solved,thus the probability of transmission synchronization and the packet delivery rate affected by transmission synchronization are solved as well.Finally,the effectiveness of the model is verified by the simulation results.The existing studies usually optimize and design the VANET MAC protocol in terms of improving network throughput and channel utilization.However,the VANET applications are more sensitive to delay,especially safety-related applications have strict requirements to delay.In addition,due to the fast-changing states(location,speed,etc)of the vehicles,the packets of VANET applications are time sensitive,and the benefits of successful transmission will change with time.Therefore,the concept of the packet-value-fuction is introduced creatively in this paper.Considering the value of packet as a fuction of delay specified by the applications,this paper proposes a dynamic game theoretic MAC protocol for VANET,Which can ensure the allowed upper limit of message delivery delay is satisfied,but also maximize the utility of the applications.First,channel contention among vehicles based on packet-value-fuction is modeled as a stochastic game.Second,it is proved that the channel contention can be still formulated as a partially observed stochastic game process where the information about the delay is not available for others,and the equilibrium strategies are solved approximately.Finally,the superiority of proposed protocol over IEEE 802.11 DCF is verified by the simulation results.The stochastic game model is a multi-object Markov Decision Process,which becomes too complex to be solved with the vehicles increasing.First,this paper simplifies the game above with the particularity to channel contention.Second,a multi-agent reinforcement learning method combining k-means and actor-critic is presented,which improves the Nash-Q algorithm.Proposed method does not have to solve the equilibrium strategies before each iteration,instead amends Q value and the best response of the vehicles synchronously,thus guarantees all the vehicles converge to the same equilibrium.Finally,the effectiveness of proposed method are verified by the simulation results.