Frame Synchronization Method for BDS B2a Signal Under the Constraint of Non-binary LDPC Code

Low-Density Parity-Check (LDPC) Code has become one of the best coding technologies at present because of its low complexity iterative decoding algorithm and its performance with further close to the Shannon limit. Non-binary LDPC codes designed in high order Galois fields have shown better potential than binary LDPC codes with the same code length and code rate, therefore it was applied for the navigation message encoding in BeiDou satellite navigation system (BDS). The navigation message contains time information, ephemeris parameters, clock correction terms and so forth, which is necessary for navigation solution. As the synchronization head was not encoded, it is hard to deal with data synchronization and navigation data decoding when the navigation receiver works at a low Signal-to-Noise Ratio (SNR). Aiming at this problem, we present a synchronization scheme for BDS B2a signal by utilizing the information from the process of decoding. The synchronizer is based on syndrome checks using the decoding result of the Extended Min-Sum (EMS) algorithm. Ratios of satisfaction of all check restrictions are computed firstly, and then the boundary value is obtained through MonteCarlo method. Simulation results show that the proposed algorithm performs well at relatively low SNR compared with traditional method.