IEEE COMMUNICATIONS LETTERS, VOL. 20, NO. 1, JANUARY 2016 13
Construction of High-Performance Array-Based Non-Binary
LDPC Codes With Moderate Rates
Shancheng Zhao and Xiao Ma
Abstract—Based on arrays, excellent low-density parity-check
(LDPC) codes with high rates have been constructed. In this let-
ter, we propose to construct high-performance nonbinary LDPC
(NBLDPC) codes with moderate rates based on arrays. The pro-
posed construction combines masking and column selection. First,
we construct a masking matrix M based on asymptotic thresh-
olds and certain error-prone substructures. Second, we generate
a number of column selection sets (CSS) based on Golomb rulers.
Based on M, we construct a parity-check matrix for each CSS.
Third, we compute the cycle distribution of each parity-check
matrix and select the one with the best cycle distribution as output.
To assess the effectiveness of the proposed construction, error per-
formances of several NBLDPC codes are presented. Comparisons
with other codes are also carried out to show the advantages of our
construction.
Index Terms—Array-based non-binary LDPC codes, inevitable
cycles, masking, moderate rates, short-to-moderate lengths.
I. INTRODUCTION
N
ON-BINARY low-density parity-check (NBLDPC)
codes were first introduced in [1] based on modulo
arithmetics and were redefined over finite fields in [2]. When
combined with higher-order modulations or used in burst-error
channels, NBLDPC codes with short-to-moderate block
lengths usually outperform binary LDPC codes of the same
bit lengths. Hence, in short-to-moderate block length regime,
NBLDPC codes are attractive candidates. In [3], Poulliat et al.
proposed to select non-zero elements to improve the error-floor
of NBLDPC codes with column weight 2, which are also
called NBLDPC cycle codes. Construction of NBLDPC cycle
codes with minimum lengths was proposed in [4]. In [5],
modular Golomb rulers were used to construct cyclic NBLDPC
codes.
From the implementation point of view, protograph-based
NBLDPC codes are preferred. Weight enumerators and asymp-
totic thresholds of protograph-based NBLDPC codes were
computed in [6], where good moderate-rate NBLDPC codes
were also constructed. To design long LDPC codes, asymp-
totic thresholds of [6] provide good guidelines. However, to
design finite-length codes, these thresholds should be used in
companion with other factors, such as cycle distribution and
Manuscript received June 26, 2015; revised October 15, 2015; accepted
October 19, 2015. Date of publication October 26, 2015; date of current version
January 7, 2016. This work was supported in part by the 863 Program under
Grant 2015AA01A709, and in part by the NSF of China Grant 61501206 and
Grant 91438101. The associate editor coordinating the review of this paper and
approving it for publication was E. Paolini.
S. Zhao is with the College of Information Science and Technology, Jinan
University, Guangzhou 510632, China (e-mail: zhaoday@mail2.sysu.edu.cn).
X. Ma is with the Department of ECE, Sun Yat-sen University, Guangzhou
510275, China (e-mail: maxiao@mail.sysu.edu.cn).
Digital Object Identifier 10.1109/LCOMM.2015.2494581
minimum weight. In [4], [7], NBLDPC codes with large girths
were constructed. Algebraic construction of NBLDPC codes
based on finite fields was proposed in [8]. This construction was
extended by Li et al. in [9] based on masking and quasi-cyclic
(QC) labeling to construct NB QC LDPC codes. Simulation
results showed that their codes perform better than some of the
protograph-based NBLDPC codes [6]. More on the construc-
tion and decoding of NBLDPC codes can be found in [10]–[13]
and the references therein.
High-rate array-based LDPC codes with thousands of bits
were shown to perform well [14]. In this letter, we attempt to
construct high-performance array-based NBLDPC codes with
moderate rates. Both constrained and unconstrained NBLDPC
codes are considered [6]. Our construction combines mask-
ing with column selection. Firstly, we construct a protograph
G with γ check nodes and ρ variable nodes. The adjacent
matrix M of G is taken as the masking matrix. We only
consider protograph without parallel edges. To construct uncon-
strained NBLDPC codes, the protograph having a competitive
asymptotic threshold [6] is selected. To construct constrained
NBLDPC codes, both asymptotic thresholds [6] and error-
prone substructures of the protograph are considered. The
error-prone substructures we considered are submatrices that
incur inevitable cycles of length 4i (i = 3, 4, 5) [15]. Secondly,
we generate a number of column selection sets (CSS) under the
constraint that the CSS or a subset of the CSS is a Golomb ruler.
Thirdly, for each CSS, we construct a parity-check matrix based
on column selection and masking. Finally, the cycle distribution
of each parity-check matrix is computed and the matrix with the
best cycle distribution is selected.
Compared with related work in [4], [5], [10], [16], the pro-
posed method is broader in viewpoint of construction. For
example, the proposed method can be used to reproduce the
NBLDPC cycle codes constructed in [4], [16]. Note that in the
proposed method asymptotic thresholds and error-prone sub-
structures are used. This is different from the construction in
[7], [9]. To assess the effectiveness of our construction, the
performance of several NBLDPC codes with moderate rates is
analyzed. Comparisons with other codes are carried out, which
show the superior performances of our codes. Particularly, our
64-ary constrained NBLDPC code performs 0.15 dB better than
a longer code recently constructed in [9].
II. A
RRAY-BASED NON-BINARY LDPC CODES
A. Array-Based Non-Binary LDPC Codes
An NBLDPC code is defined as the null space of a sparse
parity-check matrix H over the finite field F
q
. We assume that
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