Research On Real-time Performance Of EDCA Mechanism Based On MAC Layer Of 802.11p

With the development of national economy and the industry,the ownership of cars continues to grow,cars and people’s lives are inseparable,and the related Vehicular Ad hoc Network has become a hot spot for research.The core of its Media Access Control（MAC）layer is Enhanced Distributed Channel Access（EDCA）,and the MAC layer determines the communication quality.In this paper,we study the real-time performance of the communication process of vehicles moving on the highway.Vehicles in motion,the network topology will change,the communication network is complex and variable,and eventually may not be able to complete the communication within the specified time,which leads to road safety problems.Since on highways,vehicles travel very fast and the vehicle communication performance is extremely demanding,it is necessary to establish an effective analytical model to analyze the vehicle communication performance.In response to this requirement,this paper proposes an analysis model and verifies the reliability of the model through simulation.The mean variance of the system service time,as well as the real-time packet delay and packet delivery rate are analyzed by the calculation results of the model.The main work of this paper is as follows.First of all,the performance of the access channel of vehicles running on the highway using the IEEE 802.11p EDCA mechanism with 4 access types（Access Categories,AC）is studied.The network connectivity model is established according to the research of the vehicle movement on the expressway.Taking into account the back-off rules of the EDCA mechanism,the back-off process of the EDCA mechanism is modeled.A one-dimensional Markov chain model is used to describe the AC0 queue with the highest priority,and then a two-dimensional Markov chain model is used to model the remaining lower priority ACs,taking into account saturation（vehicles always have data packets required Transmission）,non-saturation（the vehicle does not always have data packets to be transmitted）,and back-off freezing conditions.The mean value and variance of the service time are obtained by calculation,and the simulation result is compared with the theoretical value,and it is found to be more consistent.Secondly,a service system model of M/G/1/K is established,and the process of accessing the channel is regarded as a service process.Combined with the fixed-point steady-state fluid approximation（Pointwise Stationary Fluid Flow Approximation,PSFFA）method,the expressions of vehicle communication data packet delay and Packet Delivery Ratio（PDR）are derived.Finally,these performance parameters are calculated by an iterative method,and simulation software is used to obtain the results.The simulation results show that the communication delays of AC₀,AC₁,and AC₂ are all lower than 100ms specified in the 802.11p protocol,while AC₃ does not meet this requirement.Therefore,this model can ensure the timeliness of AC₀,AC₁,and AC₂messages for vehicle communication,And AC₃ messages cannot be guaranteed.