In this paper, we study the graphical structure of elementary trapping sets (ETS) of variable-regular low-density parity-check (LDPC) codes. ETSs are known to be the main cause of error floor in LDPC coding schemes. For the set of LDPC codes with a given variable node degree dl and girth g, we identify all the non-isomorphic structures of an arbitrary class of (a, b) ETSs, where a is the number of variable nodes and b is the number of odd-degree check nodes in the induced subgraph of the ETS. Our study leads to a simple characterization of dominant classes of ETSs (those with relatively small values of a and b) based on short cycles in the Tanner graph of the code. For such classes of ETSs, we prove that any set S in the class is a layered superset (LSS) of a short cycle, where the term 'layered' is used to indicate that there is a nested sequence of ETSs that starts from the cycle and grows, one variable node at a time, to generate S. This characterization corresponds to a simple search algorithm that starts from the short cycles of the graph and finds all the ETSs with LSS property in a guaranteed fashion. Specific results on the structure of ETSs are presented for dl = 3, 4, 5, 6, g = 6, 8 and a, b ≤ 10. The results of this work can be used for the error floor analysis and for the design of LDPC codes with low error floors.

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Persistent URL dx.doi.org/10.1109/ISTC.2014.6955086
Conference 2014 8th International Symposium on Turbo Codes and Iterative Information Processing, ISTC 2014
Citation
Karimi, M. (Mehdi), & Banihashemi, A. (2014). On characterization of elementary trapping sets of variable-regular LDPC codes. Presented at the 2014 8th International Symposium on Turbo Codes and Iterative Information Processing, ISTC 2014. doi:10.1109/ISTC.2014.6955086