| Signal to noise ratio(SNR) will be lower than-10 d B in spread spectrum communication and in deep space communication. This thesis studies the frame synchronization problem under the very low SNR with the design of a new frame maker and frame synchronization algorithm and completes the physical scheme.This thesis introduces the common frame synchronization methods to have the conclusion that marker largely determines synchronization performance based on the correlation detection, then analyzes the preferred synchronization sequence and the barker code. A composition of the 13-bit barker code and the 48-bit preferred synchronization sequence is adopted as the frame marker whose autocorrelation characteristics is briefly analyzed at the same time.With the above marker, this thesis designs the proposed “maximum- secondary correlation detection” algorithm, searching for local maximum correlation rather than the conventional maximum correlation detection and using a correlation array to calculate secondary correlation, which can reduce the complexity of long correlation and the difficulty of threshold selection and optimize the frame synchronization performance.The frame synchronization mathematical model based on the samples is established to analyze its performance. According to the offset between the received data and the local marker, the probability distribution function of the threshold decision, the maximum search and the secondary correlation detection can all be canculated. Besides, the correct detection probability of conventional algorithm and the proposed algorithm can both be canculated.Over the additive white Gaussian noise channel, Simulation shows that the false synchronization probability(FSP) will decrease with the increase of SNR when the threshold is fix and FSP will decrease with the increase of threshold in the same SNR for the above two algorithm. As for the proposed algorithm, FSP is 10% when SNR is-21 d B and FSP is 0.2% when SNR is-15 d B with the same threshold value 600, which has a 3 d B SNR gain compared with the conventional algorithm. Besides, the threshold sensitivity of the proposed algorithm is lower to reduce the difficulty of threshold selection.Two "multi-states" detection modes is designed to pay attention to both FSP and miss synchronization probability(MSP): scheme 1-“continuous N frame detection” and scheme 2-“fixed N frame accumulation”. As for scheme 1, it has a 3-d B SNR gain about FSP compared with the synchronization first acquire if N =K =3, M =5, however, MSP is greater than 4.2% if SNR is lower than-20 d B. The case N =7, K =4, M =5 is designed to improve MSP effectively and to guarantee a 3-d B SNR gain about FSP. The scheme 2 has a 1-d B more SNR gain than scheme 1. The two schemes can both achieve the protection for synchronization state.Selecting the hardware verification platform for “FMC30RF+ML605+PC”, we achieve the synchronization detection and verify the scheme 2’s realistic, at the same time, we randomly generate 16 frames data for sending circularly and receive the samples stream whose sampling frequency is 8 times greater than the bandwidth. |
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