Study On Synchronization Parameters Estimation Of QPSK Signal In VDES

VHF Data Exchange System(VDES)is a new generation of maritime communication system,which is produced for alleviating the load pressure of Automatic Identification System(AIS).VDES not only realizes the original design intention and basic functions of AIS,but also extends the existing AIS,which can comprehensively strengthen the performance of maritime communication and data transmission.Carrier synchronization is the core technology of VDES,synchronization performance can directly affects the correctness of symbol detection.In this paper,the synchronization algorithm of QPSK signal in VDES is researched,the main contents include:(1)A frequency-offset estimation algorithm of VDES signals based on cross-correlation is proposed.The local sequence of this algorithm is constructed by character sequence and different Doppler frequency offsets.It is used to get a coarse frequency-offset estimation by means of inter-correlation analysis with received signal.The coarse estimation compensated to received signal for eliminating the phase folding caused by argument calculation of the signal autocorrelation function.Finally,the proposed method overcomes the contradiction problem between high accuracy and wide range of frequency-offset estimation.Simulation results show that the frequency-offset estimation range of the algorithm reaches-4000Hz~4000Hz,which meets the requirements of the frequency-offset range of VDES,and its accuracy is very close to the Cramer-bound.(2)A frequency-offset estimation method of VDES signals based on interpolation DFT phase difference is introduced.Aiming at the sensitivity of DFT phase difference algorithm to frequency-offset relative deviation,algorithm in this paper proposes setting a threshold value for the deviation before calculating phase difference,and using Quinn and Rife interpolation to pre-estimate the deviation before and after the threshold.Thus the influence of frequency-offset relative deviation on the estimation performance is eliminated.Simulation results show that the frequency-offset estimation performance of the algorithm is very stable and the accuracy is improved.(3)A joint estimation algorithm of time-delay and frequency-offset with VDES frame structure is proposed.The algorithm can estimate the time-delay and frequency-offset simultaneously by cross-correlation matrix.As changing the estimation step three times,the complexity and synchronization time of the algorithm is greatly reduced,and the accuracy of parameter estimation is guaranteed.Simulation results show that frequency-offset performance of VDE-SAT downlink is better than that of ASM link,and time-delayestimation performance of two link is very similar.