Novel Double Protograph LDPC Codes for Joint Source-Channel Coding Systems

In this paper, we first propose a novel joint source-channel block code (JSC-BC) in which two protograph-based low-density parity-check (P-LDPC) block codes are connected not only by a source-variable-channel-check (SVCC) linking matrix, but also by a source-check-channel-variable (SCCV) linking matrix that consists of a zero matrix and a lower or upper triangular (base) matrix with “1”s on its diagonal. Also, we simplify the traditional joint protograph extrinsic information transfer (JP-EXIT) algorithm and propose an “untransmitted protograph-based EXIT (UP-EXIT) algorithm” for calculating the source threshold of a JSC-BC. The proposed UP-EXIT algorithm is more efficient because a smaller protograph consisting of only the untransmitted VNs (i.e., the source VNs and the punctured channel VNs) and their connected check nodes need to be considered. By using the UP-EXIT algorithm, we search for candidate codes with high source thresholds. Then, we select those among the candidate codes also with low channel thresholds. Theoretical and simulation results show that the newly constructed codes outperform state-of-the-art double P-LDPC (DP-LDPC) block codes. Furthermore, we spatially couple the joint source-channel block code and obtain a spatially coupled joint source-channel code (SC-JSCC). Theoretical analyses and simulation results show that even with a smaller window size and lower decoding complexity, the SC-JSCC with the spatially coupled structure for each sub-block (source protomatrix, channel protomatrix, SCCV linking matrix, and SVCC linking matrix) can achieve better error performance than existing spatially-coupled DP-LDPC codes.