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IEEE Transactions on Information Theory

Cyclic Partial Geometries and Their Associated LDPC and Constant-Weight Codes

Juane Li, Yi Gong, Xin Wei Xiao, Shu Lin, Khaled Abdel-Ghaffar

Published May 19, 2023• 2 citations
Publisher

Abstract

Partial geometries form an interesting branch in combinatorial mathematics, and recently they have been shown to be very effective in the construction of LDPC codes with distinct geometric and algebraic structures. This paper presents three specific cyclic classes of partial geometries. Based on these three classes of partial geometries, three classes of LDPC codes and three classes of constant-weight codes are constructed. Codes in these two categories are either cyclic or quasi-cyclic. Designs and constructions of these codes are straightforward and flexible without a need for extensive computer search. It is shown that long high-rate LDPC codes constructed based on the three classes of cyclic partial geometries perform well over the additive white Gaussian noise channel (AWGNC) with iterative decoding algorithm based on belief propagation. They can achieve low error-rates without visible error-floor and their decoding converges rapidly. These LDPC codes also perform well over the binary erasure channel (BEC) and are very effective in correcting phased-bursts of erasures. Based on cyclic partial geometries, a special type of constant-weight codes, called balanced codes, can be constructed. The constant-weight codes constructed based on partial geometries are either optimal or nearly optimal.

DOI
10.1109/tit.2023.3277860

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Partial geometries form an interesting branch in combinatorial mathematics, and recently they have been shown to be very effective in the construction of LDPC codes with distinct geometric and algebraic structures. This paper presents three specific cyclic classes of partial geometries. Based on these three classes of partial geometries, three classes of LDPC codes and three classes of constant-weight codes are constructed. Codes in these two categories are either cyclic or quasi-cyclic. Designs and constructions of these codes are straightforward and flexible without a need for extensive computer search. It is shown that long high-rate LDPC codes constructed based on the three classes of cyclic partial geometries perform well over the additive white Gaussian noise channel (AWGNC) with iterative decoding algorithm based on belief propagation. They can achieve low error-rates without visible error-floor and their decoding converges rapidly. These LDPC codes also perform well over the binary erasure channel (BEC) and are very effective in correcting phased-bursts of erasures. Based on cyclic partial geometries, a special type of constant-weight codes, called balanced codes, can be constructed. The constant-weight codes constructed based on partial geometries are either optimal or nearly optimal.
Low density parity check (LDPC) codes with iterative decoding based on belief propagation achieve astonishing error performance close to Shannon limit. No algebraic or geometric method for constructing these codes has been reported and they are largely generated by computer search. As a result, encoding of long LDPC codes is in general very complex. This paper presents two classes of high rate LDPC codes whose constructions are based on finite Euclidean and projective geometries, respectively. These classes of codes a.re cyclic and have good constraint parameters and minimum distances. Cyclic structure adows the use of linear feedback shift registers for encoding. These finite geometry LDPC codes achieve very good error performance with either soft-decision iterative decoding based on belief propagation or Gallager's hard-decision bit flipping algorithm. These codes can be punctured or extended to obtain other good LDPC codes. A generalization of these codes is also presented.
The use of partial geometries to construct parity-check matrices for binary low-density parity-check (LDPC) codes has resulted in the design of successful codes with a probability of error on the AWGN channel close to the Shannon capacity at bit error rate down to 10−1510^{-15}10−15 . Such considerations have motivated this further investigation. A new and simple construction of a type of partial geometries with a quasi-cyclic (QC) structure is given and their properties are investigated. Two new classes of this type of partial geometries, one based on prime fields and the other based on cyclic subgroups of prime orders of finite fields, are constructed. QC-LDPC codes with good error performances are constructed based on these two new classes of partial geometries. The trapping sets of the partial geometry codes were previously considered using the geometric aspects of the underlying structure to derive information on the size of allowable trapping sets. This topic is further considered here. Finally, there is a natural relationship between partial geometries and strongly regular graphs. The eigenvalues of the adjacency matrices of such graphs are well known, and it is of interest to determine if any of the Tanner graphs derived from the partial geometries are good expanders for certain parameter sets, since it can be argued that codes with good geometric and expansion properties might perform well on the AWGN channel under message-passing decoding.
Among the classes of LDPC codes that have been constructed and designed, the only class of LDPC codes that are cyclic is the class of codes constructed based on the incidence vectors of lines of finite geometries, called finite geometry LDPC codes. Cyclic finite geometry LDPC codes are known to have large minimum distances and can provide good error performance with very low error-floors using iterative decoding based on belief propagation. Their cyclic structure allows them to be efficiently and systematically encoded with simple shift-registers in linear time with linear complexity. An obvious question is whether, besides cyclic finite geometry LDPC codes, there are other cyclic codes with large minimum distances that can be efficiently decoded iteratively using channel soft information. In this paper, we present one such class of cyclic codes. Codes in this class are two-step majority-logic decodable and they are also constructed based on finite geometries. Two iterative decoding algorithms are devised for this class of cyclic codes and they provide significant coding gain over the two-step majority-logic decoding of codes in this class.
This paper presents two types of concatenated finite-geometry and finite-field LDPC codes which have the distinct features of both finite geometry and finite field LDPC codes, such as large minimum distances, no small trapping sets, fast decoding convergence, capable of correcting both random errors and bursts of erasures, flexibility in code construction, cyclic and quasi-cyclic structures. It is shown that these concatenated codes are globally coupled LDPC codes and they perform well over the AWGN and binary erasure channels with a two-phase iterative decoding scheme.