Study On Synchronization Technology Of OFDM Signal In VDE

VHF Data Exchange System(VDES)is a new maritime communication system which is developed on the Automatic Identification System(AIS),VDES can effectively alleviate the pressure of AIS communication,improve the ability of maritime data communication.VDES allows more than one coding rate and modulation scheme which can provide more flexibility in the system dimensioning and service availability.Within a dedicated channel,OFDM can be applied in VDES.In VDES,the received signal of satellite communication may have some problems such as large transmission delay and Doppler frequency offset,which may cause OFDM symbol timing offset(STO)and carrier frequency offset(CFO).We mainly study the symbol timing synchronization and carrier frequency synchronization method for OFDM.In this paper,the traditional methods are simulated and analyzed.Due to the shortcomings of the existing methods,the OFDM synchronization is studied.The main contents include:(1)Due to the maximum likelihood(ML)method which based on cyclic prefix(CP)has low accuracy in STO and CFO estimation under low SNR or multipath channel,a new ML method is proposed in this paper.The method designs a CP structure composed of two identical parts and introduces a timing metric function to eliminate the influence of inter symbol interference(ISI).Compared with ML method,the proposed method realizes the good timing performance in low SNR or multipath channel,and improves the frequency estimation mean square error(MSE)performance and expands the range of CFO estimation.(2)In this paper,a frequency synchronization method based on real training sequence is proposed for OFDM system to overcome the low synchronization precision and the small estimation range.The fractional frequency offset(FFO)can be obtained according to the conjugate characteristic of the training sequence in the time domain,and the integer frequency offset(IFO)can be estimated by the cyclic shift of the training sequence of the received signal in the frequency domain.The MSE of the frequency offset estimation is relatively low,and the range of FFO estimation is expanded,the correct detection probability of the IFO is higher than other methods.(3)A novel synchronization method based on training sequence is proposed to overcome the low precision of synchronization and fuzzy timing problem in the low SNR environment.The proposed method can achieve accurate STO estimation according to the conjugate property of training sequence in the time domain.FFO can be estimated by the phase angle of training sequence in the time domain,IFO can be estimated in the frequency domain.The IFO correct detection probability is higher and CFO estimation has good MSE performance.