An efficient exhaustive search algorithm for elementary trapping sets of variable-regular LDPC codes

Abstract

In this paper, we propose a new characterization for elementary trapping sets (ETSs) of variableregular low-density parity-check (LDPC) codes. Recently, Karimi and Banihashemi proposed a characterization of ETSs, which was based on viewing an ETS as a layered superset (LSS) of a short cycle in the code's Tanner graph. A notable advantage of LSS characterization is that it corresponds to a simple LSS-based search algorithm (expansion technique) that starts from short cycles of the graph and finds the ETSs with LSS structure efficiently. Compared to the LSS-based characterization of Karimi and Banihashemi, which is based on a single LSS expansion technique, the new characterization involves two additional expansion techniques. The introduction of the new techniques mitigates two problems that LSS-based characterization/search suffers from: (1) exhaustiveness: not every ETS structure is an LSS of a cycle, (2) search efficiency: LSS-based search algorithm often requires the enumeration of cycles with length much larger than the girth of the graph, where the multiplicity of such cycles increases rapidly with their length. We prove that using the three expansion techniques, any ETS structure can be obtained starting from a simple cycle, no matter how large the size of the structure a or the number of its unsatisfied check nodes b are, i.e., the characterization is exhaustive. We also demonstrate that for the proposed characterization/search to exhaustively cover all the ETS structures within the (a, b) classes with a ≤ a max , b ≤ b max , for any value of a max and b max , the length of the short cycles required to be enumerated is less than that of the LSS-based characterization/search. We, in fact, show that such a length for the proposed search algorithm is minimal. We also prove that the three expansion techniques, proposed here, are the only expansions needed for characterization of ETS structures starting from simple cycles in the graph, if one requires each and every intermediate sub-structure to be an ETS as well. Extensive simulation results are provided to show that, compared to LSS-based search, significant improvement in search speed and memory requirements can be achieved.In an earlier paper [13], we complemented the results of [17]. By careful examination of NA structures, we demonstrated that they are all LSSs of some basic graphical structures which are slightly more complex than cycles. These basic structures were called prime with respect to the LSS property, or "LSS-prime" in brief, implying that they are not layered super sets of smaller ETSs. Results of [17] (and its corrections in [14]) along with those of [13] provided a simple LSS-based search algorithm that can find all the instances of ETSs of variable-regular LDPC codes, for any range of a and b values, in a guaranteed fashion, starting from the instances of LSS-prime structures. The LSS-based search algorithm, however, suffers from a major drawback. Although, the search algorithm itself is simple, to find the dominant ETSs in a guaranteed fa...

Characterization of Problematic Graphical Structures of LDPC Codes and the Corresponding Efficient Search Algorithms

“…In this thesis, we address the above challenges as described in the following. Our main contributions in this thesis, which are published or submitted for publication in [28], [29], [30], [31], [32], [33], [34], [35], [36], [37] can be summarized as follows:…”

“…• We propose a novel hierarchical graph-based expansion approach to characterize leafless elementary trapping sets (LETS) of variable-regular LDPC codes [30], [31], [32]. The new characterization, allows us to devise search algorithms that are provably efficient in finding all the instances of (a, b) LETS structures with a ≤ a max and b ≤ b max , for any choice of a max and b max , in a guaranteed fashion.…”