Research On Frequency Synchronizationalgorithm In Vehicle Communication System

With the blowout-like development of the mobile Internet technology and the wide spread of private cars,the demand for real-time communication in the vehicle driving is increasing.Compared with the traditional wireless communication systems,the transmission characteristics of the channel in vehicle communication system are more complex,which are not only time-varing,multi-path and changing with the geographical environment,but also have a big doppler frequency shift because of the high speed movement.As the mainstream of the international vehicle communication protocol,IEEE 802.11p introduces OFDM in the physical layer,so that having a strict requirement on the carrier frequency synchronization.Traditional OFDM synchronization algorithm is divided into data-aided method,semi-blind synchronization and blind synchronization.This paper analyzes the data-aided algorithm,which is most widely used,and applies classic M&M algorithm to the communication system based on the data frame structure of 802.11p protocol.The simulation results show that the performance of the algorithm in the simple Gauss channel is pretty good,while in the complex vehicle communication channel is obviously decreased.So the traditional synchronization algorithm couldn’t adapt to the burst and special characteristics of the vehicle communication system.In view of the above problems,this paper mainly studies the frequency synchronization in vehicle communication system.In the study,the RA model and TU model prescribed in COST-207 channel are used to simulate the moving scene of vehicles in rural area and urban area.And the length of the data frame is designed based on the parameters of channel model.At the same time,the Maximization Expectation(EM)algorithm is introduced to solve the problem of the mutual dependence between the carrier frequency offset and the real-time channel impulse response in the vehicle communication system.The frequency synchronization algorithm based on EM is mainly divided into two steps.First,the proposed algorithm achieves a coarse estimation of the frequency offset based on the repetitiveness of the short training sequence in 802.11p and compensates the offset.Then regarding the channel impulse response as the implicit variable in the EM algorithm,the algorithm carries on the iterative process by utilize the connection between the frequency offset and channel impulse response.The algorithm adjusts the estimation of carrier frequency offset to the real-time condition of the wireless channel,so that realizes a better flexibility with time-varying vehicle communication system.Finally,this paper builds a simulation platform of vehicle communication system based on 802.11p in MATLAB to compare the proposed frequency offset estimation method with traditional algorithm.The results show that the performance of the frequency synchronization algorithm based on EM is significantly improved in time-varying vehicle communication system,and the proposed algorithm can also track the channel state in real time and achieve the channel estimation.