Improved Low-Density Parity Check Accumulate (LDPCA) Codes
We present improved constructions for Low-Density Parity-Check Accumulate (LDPCA) codes, which are rate-adaptive codes commonly used for distributed source coding (DSC) applications. Our proposed constructions mirror the traditional LDPCA approach; higher rate codes are obtained by splitting the check nodes in the decoding graph of lower rate codes, beginning with a lowest rate mother code. In a departure from the uniform splitting strategy adopted by prior LDPCA codes, however, the proposed constructions introduce non-uniform splitting of the check nodes at higher rates. Codes are designed by a global minimization of the average rate gap between the code operating rates and the corresponding theoretical lower bounds evaluated by density-evolution. In the process of formulating the design framework, the paper also contributes a formal definition of LDPCA codes. Performance improvements provided by the proposed non-uniform splitting strategy over the conventional uniform splitting approach used in prior work are substantiated via density evolution based analysis and DSC codec simulations. Optimized designs for our proposed constructions yield codes with a lower average rate gap than conventional designs and alleviate the trade-off between the performance at different rates inherent in conventional designs. A software implementation is provided for the codec developed.