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IEEE AFRICON

A low complexity encoder construction for systematic quasi-cyclic LDPC codes

Yuval O. Genga, Olayinka Olaolu Ogundile, Olutayo O. Oyerinde, D.J.J. Versfeld

Published Sep 1, 2017• 5 citations
Publisher

Abstract

For practical applications in forward error correction, the importance of a systematic codeword cannot be overemphasized. Thus, this paper proposes the construction of a systematic quasi-cyclic (QC) LDPC code. This systematic structure is achieved by a row reduction technique different from the conventional Gaussian elimination method. This row reduction technique has the advantage of being easier to implement when compared to Gaussian row reduction techniques. The proposed row reduction technique maintains the quasi cyclic structure and the sparsity of the QC-LDPC parity check matrix while providing a low complexity approach to the construction of the generator matrix. In addition, the proposed construction exhibits a good BER performance for high rate codes.

DOI
10.1109/afrcon.2017.8095475

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For practical applications in forward error correction, the importance of a systematic codeword cannot be overemphasized. Thus, this paper proposes the construction of a systematic quasi-cyclic (QC) LDPC code. This systematic structure is achieved by a row reduction technique different from the conventional Gaussian elimination method. This row reduction technique has the advantage of being easier to implement when compared to Gaussian row reduction techniques. The proposed row reduction technique maintains the quasi cyclic structure and the sparsity of the QC-LDPC parity check matrix while providing a low complexity approach to the construction of the generator matrix. In addition, the proposed construction exhibits a good BER performance for high rate codes.
In this letter, we present a low-complexity encoding method for quasi-cyclic (QC) low-density parity-check (LDPC) codes in the case of non-full-rank parity-check matrices. Gaussian elimination can achieve systematic encoding, but it is usually too complex to implement. For QC-LDPC codes, efficient encoding methods have been presented by using shift-registers, but the encoded codewords are not systematic when parity-check matrices are non-full-rank. However, a systematic structure is important in practice. Therefore, we propose an encoding method which allows all information bits appear piecewise in the codeword, called piecewise systematic encoding.
This paper proposes a new technique for constructing the Quasi-Cyclic low density parity check (QC-LDPC) codes based on row division method. The new codes offer more flexibility in term of girth, code rates and codeword length. In this method of code construction, the rows are used to form as the distance graph. Then they are transformed to a parity-check matrix in order to acquire the desired girth. The new QC-LDPC codes show good bit-error rate performance as compared to the renowned Mackay codes for given values of Eb/Noby 0.12 dB at BER of10-7.
There is an important breakthrough in the field of forward error correction techniques, low-density parity-check (LDPC) codes. These codes have potential to approach the Shannon limit with reliable performance on a given channel, known as the channel capacity. Recently, researchers pay much more attention towards LDPC codes because its performance is better than Turbo codes. However, the LDPC code is the best when it is an irregular code and uses with a very large block-length. Although, the short block-length LDPC codes perform not so well, they are easy to implement in various practical applications. Therefore, there is still a demand of new development on the encoding side of LDPC codes for various ranges of code rate and code length so as to capable of using in many applications.This thesis proposes the development of quasi-cyclic (QC) LDPC codes for column weight 3. In the first part of this thesis, we propose a new construction algorithm of QC-LDPC codes for medium to large block-length by combining QC-LDPC codes of small block-length as their component codes, via Chinese remainder theorem (CRT). Such component codes were constructed by permuting each column block sequentially to attain the desire local girth. After combining all component codes to generate an expanded parity-check matrix, the resulting girth is greater than or at least equal to the maximum girth of the component codes. Simulation results show that our proposed construction method of the parity-check matrix significantly outperforms the other well-known existing methods in terms of low error-floor, simple structure, high performance, and can reduce encoding complexity.In addition, this thesis also proposes two new construction methods for QC-LDPC codes, namely a base matrix method and a subtraction based method. A base matrix based method is a simple, less computational complexity method for constructing the exponent matrix (3, K) of girth 8, 10 and 12 of QC-LDPC codes. Another method is a subtraction method, which has a similar exponent matrix as a base matrix method and has a girth at least 8. Results indicate that the LDPC codes constructed from these two methods have flexibility for arbitrary block-column length K and have a similar BER performance if compared to existing methods.
The existing design of quasi-cyclic(QC) low-density parity-check(LDPC) codes has not considered the problems of short length girths and the rows' dependency.The first problem leads to the BER performance of QC LDPC codes to be much poorer than that of randomly constructed LDPC codes,while the second problem leads to the difficulty of construction of generator matrices from parity-check matrices.To solve the first problem,the constraint conditions for designing the QC LDPC codes are proposed,and the dimension and shift factors of the circulant matrices of the given sparse parity-check matrix are adjusted according to the test results of both girth 4 and girth 6.To solve the second problem,an approach of irregular QC LDPC codes is proposed.The proposed approach is to replace some sub-matrices by zero matrices and identity matrices at special positions in the given sparse parity-check matrix so as to get a nonsingular square matrix for the construction of the generator matrix.Though adopting bidiagonal submatrices in parity-check matrix can solve the rows' dependency problem,many code words with low weights will occur,which leads to the BER performance can not be better by increasing code lengths.Examples are provided for the proposed design of QC LDPC codes,and computer simulation results show that the proposed QC LDPC codes achieve good BER performance.
This paper presents a reduced-complexity iterative decoding scheme for quasi-cyclic (QC) low-density parity-check (LDPC) codes which is devised based on the section-wise cyclic structure of their parity-check matrices. The decoding scheme significantly reduces the hardware implementation complexity of a QC-LDPC code decoder in terms of the number of message processing units and the number of wires required to connect the message processing units. A long high-rate high-performance QC-LDPC code is used to demonstrate the effectiveness of the proposed decoding scheme. Using this long code as the mother code, descendant QC-LDPC codes of various lengths and rates can be constructed by puncturing and masking the columns and rows of the parity-check matrix of the mother code. These descendant codes can be decoded by deactivating specific sets of message processing units of the mother decoder.
Presented in this paper is a reduced-complexity iterative decoding scheme for quasi-cyclic (QC) LDPC codes. This decoding scheme is devised based on the section-wise cyclic structure of the parity-check matrix of a QC-LDPC code. Using this decoding scheme, the hardware implementation complexity of a QC-LDPC decoder can be significantly reduced without performance degradation. A high-rate QC-LDPC code that can achieve a very low error-rate without a visible error-floor is used to demonstrate the effectiveness of the proposed decoding scheme. Also presented in this paper are two other high-rate QC-LDPC codes and a method for constructing rate -1/2 QC-LDPC codes whose Tanner graphs have girth 8. All the codes constructed perform well with low error-floor using the proposed decoding scheme.