A joint source channel coding (JSCC) scheme based on source and channel low density parity check (LDPC) codes, which is named as the D-LDPC system, has attracted a lot of attention recently. However, it suffers from a high error-floor. By introducing a linking matrix connecting variable nodes of source LDPC and check nodes of channel LDPC, the error-floor can be lowered effectively. However, the water-fall performance was lost with the increasing of the size of linking matrix. In this paper, a detailed analysis about the impact of this linking matrix on the water-fall region is conducted. Some design principles for the linking matrix are proposed, by which several linking matrices for different source statistics will be designed to improve the water-fall performance from the perspective of both the source and channel LDPC codes. The extrinsic information transfer (EXIT) analysis and numerical simulations will be compared to verify the superiority of the proposed linking matrices.
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