LDPC Codes Based on Latin Squares: Cycle Structure, Stopping, and Trapping Set Analysis

Stefan Laendner, Olgica Milenković

It is well known that certain combinatorial structures in the Tanner graph of a low-density parity-check (LDPC) code exhibit a strong influence on its performance under iterative decoding. These structures include cycles, stopping/trapping sets, and parameters such as the diameter of the code. In general, it is very hard to find a complete characterization of such configurations in an arbitrary code, and even harder to understand the intricate relationships that exist between these entities. It is, therefore, of interest to identify a simple setting in which all the described combinatorial structures can be enumerated and studied within a joint framework. One such setting is developed in this paper, for the purpose of analyzing the distribution of short cycles and the structure of stopping and trapping sets in Tanner graphs of LDPC codes based on idempotent and symmetric Latin squares. The parity-check matrices of LDPC codes based on Latin squares have a special form that allows for connecting combinatorial parameters of the codes with the number of certain subrectangles in the Latin squares. Subrectangles of interest can be easily identified, and in certain instances, completely enumerated. This study can be extended in several different directions, one of which is concerned with modifying the code design process in order to eliminate or reduce the number of configurations bearing a negative influence on the performance of the code. Another application of the results includes determining to which extent a configuration governs the behavior of the bit-error rate curve in the waterfall and error-floor regions

B. N. Sindhu Tejaswini, Rajendra Prasad Lal, Vadlamudi Ch. Venkaiah

Alexander Gruner, M. Huber

This paper examines the construction of low-density parity-check (LDPC) codes from transversal designs based on sets of mutually orthogonal Latin squares (MOLS). By transferring the concept of configurations in combinatorial designs to the level of Latin squares, we thoroughly investigate the occurrence and avoidance of stopping sets for the arising codes. Stopping sets are known to determine the decoding performance over the binary erasure channel and should be avoided for small sizes. Based on large sets of simple-structured MOLS, we derive powerful constraints for the choice of suitable subsets, leading to improved stopping set distributions for the corresponding codes. We focus on LDPC codes with column weight 4, but the results are also applicable for the construction of codes with higher column weights. Finally, we show that a subclass of the presented codes has quasi-cyclic structure which allows low-complexity encoding.

Jing Sun, O.Y. Takeshita, Michael P. Fitz

Olgica Milenković, Stefan Laendner

In this paper, we introduce a family of structured low-density parity-check (LDPC) codes based on a class of idempotent, symmetric Latin and modified Latin squares. The parity-check matrices of the codes have a block structure with permutation blocks which insures that both their corresponding girth and minimum distance are at least equal to six. The storage requirement for codes from this class is reduced to only one parameter. We also propose a structured method for shortening the codes by removing columns that break a large number of cycles of length six. The storage requirements for the codes obtained using the described procedure consist in memorizing only two integers, while their performance under iterative decoding matches that of random-like codes of comparable length.

W.E. Ryan, Shu Lin