Abstract: Based on the pseudo-symplectic space over F(2v+1) q of characteristics 2, combining the definition of low density parity check (LDPC) codes with the knowledge of graph theory, two kinds of LDPC codes with larger girth are constructed. By the knowledge of bipartite graph and the girth properties of LDPC codes, both the girth of the code C(m1,2v+1,q) and the code C(m2,2v+1,q) are computed are 8. The girth is larger, the performance of LDPC codes is better. Finally, according to the properties of the check matrix and the linear relation between the column vectors of the check matrix, both the minimum distances of the two codes are obtained are 2q +2.

Key words: LDPC codes, pseudo-symplectic geometry, minimum distance, girth