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Syndrome based check node processing of high order NB-LDPC decoders

Philipp Schläfer, Norbert Wehn, Matthias Alles, Timo Lehnigk-Emden, Emmanuel Boutillon

Published Apr 1, 2015• 15 citations
Publisher

Abstract

Non-binary low-density parity-check codes have superior communications performance compared to their binary counterparts. However, to be an option for future standards, efficient hardware architectures must be developed. State-of-the-art decoding algorithms lead to architectures suffering from low throughput and high latency. The check node function accounts for the largest part of the decoders overall complexity. In this paper a new, hardware aware check node algorithm is proposed. It has state-of-the-art communications performance while reducing the decoding complexity. Moreover the presented algorithm allows for partially or even fully parallel processing of the check node operations which is not applicable with currently used algorithms. It is therefore an excellent candidate for future high throughput hardware implementations.

DOI
10.1109/ict.2015.7124675

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Non-binary low-density parity-check codes have superior communications performance compared to their binary counterparts. However, to be an option for future standards, efficient hardware architectures must be developed. State-of-the-art decoding algorithms lead to architectures suffering from low throughput and high latency. The check node function accounts for the largest part of the decoders overall complexity. In this paper a new, hardware aware check node algorithm is proposed. It has state-of-the-art communications performance while reducing the decoding complexity. Moreover the presented algorithm allows for partially or even fully parallel processing of the check node operations which is not applicable with currently used algorithms. It is therefore an excellent candidate for future high throughput hardware implementations.
This paper proposes a unified framework to describe the check node architectures of non-binary low-density parity-check (NB-LDPC) decoders. Forward-backward, syndrome-based, and pre-sorting approaches are first described. Then, they are hybridized in an effective way to reduce the amount of computation required to perform a check node. This paper is specially impacting check nodes of high degrees (or high coding rates). Results of 28-nm ASIC post-synthesis for a check node of degree 12 (i.e., a code rate of 5/6 with a degree of variable equal to 2) are provided for NB-LDPC over GF(64) and GF(256). While simulations show almost no performance loss, the new proposed hybrid implementation check node increases the hardware and the power efficiency by a factor of six compared with the classical forward-backward architecture. This leads to the first ever reported implementation of a degree 12 check node over GF(256), and these preliminary results open the road to high decoding throughput, high rate, and high-order Galois field NB-LDPC decoder with a reasonable hardware complexity.
Non-binary low-density parity check (NB-LDPC) codes are an extension of binary LDPC codes with significantly better performance.Although various kinds of low-complexity iterative decoding algorithms have been proposed, there is a big challenge for VLSI implementation of NBLDPC decoders due to its high complexity and long latency.In this brief, highly efficient check node processing scheme, which the processing delay greatly reduced, including Min-Max decoding algorithm and check node unit are proposed.Compare with previous works, less than 52% could be reduced for the latency of check node unit.In addition, the efficiency of the presented techniques is design to demonstrate for the (620, 310) NB-QC-LDPC decoder.
The Non-Binary Low Density Parity Check (NB-LDPC) codes constitutes an interesting category of error correction codes, and are well known to outperform their binary counterparts. However, their non-binary nature makes their decoding process of higher complexity. This PhD thesis aims at proposing new decoding algorithms for NB-LDPC codes that will be shaping the resultant hardware architectures expected to be of low complexity and high throughput rate. The first contribution of this thesis is to reduce the complexity of the Check Node (CN) by minimizing the number of messages being processed. This is done thanks to a pre-sorting process that sorts the messages intending to enter the CN based on their reliability values, where the less likely messages will be omitted and consequently their dedicated hardware part will be simply removed. This reliability-based sorting enabling the processing of only the highly reliable messages induces a high reduction of the hardware complexity of the NB-LDPC decoder. Clearly, this hardware reduction must come at no significant performance degradation. A new Hybrid architectural CN model (H-CN) combining two state-of-the-art algorithms - Forward-Backward CN (FB-CN) and Syndrome Based CN (SB-CN) - has been proposed. This hybrid model permits to effectively exploit the advantages of pre-sorting. This thesis proposes also new methods to perform the Variable Node (VN) processing in the context of pre-sorting-based architecture. Different examples of implementation of NB-LDPC codes defined over GF(64) and GF(256) are presented. For decoder to run faster, it must become parallel. From this perspective, we have proposed a new efficient parallel decoder architecture for a 5/6 rate NB-LDPC code defined over GF(64). This architecture is characterized by its fully parallel CN architecture receiving all the input messages in only one clock cycle. The proposed new methodology of parallel implementation of NB-LDPC decoders constitutes a new vein in the hardware conception of ultra-high throughput rate decoders. Finally, since the NB-LDPC decoders requires the implementation of a sorting function to extract P minimum values among a list of size Ns, a chapter is dedicated to this problematic where an original architecture called First-Then-Second-Extrema-Selection (FTSES) has been proposed.
When the code length is moderate, non-binary low-density parity-check (NB-LDPC) codes can achieve better error-correcting performance than their binary counterparts at the cost of higher decoding complexity. The check node processing is a major bottleneck of NB-LDPC decoding. Its computation complexity can be reduced by an iterative forward-backward scheme. Nevertheless, large memory is required in this scheme to store intermediate results. In this paper, a novel check node processing scheme and corresponding VLSI architectures are proposed for the Min-max NB-LDPC decoding. The proposed scheme first sorts out a limited number of the most reliable input messages. Then the forward and backward processes are carried out using only these messages. In addition, an innovative approach is developed to derive the output messages from only a small number of intermediate results without noticeable performance loss. Accordingly, both the memory requirement and computation complexity are greatly reduced. For an example (837, 726) NB-LDPC code over GF(25), our proposed check node processing architecture can achieve at least 34% area reduction and about the same throughput compared to previous efforts based on the original forward-backward scheme.
This paper focuses on low complexity architectures for check node processing in Non-Binary LDPC decoders. To be specific, we focus on Extended Min-Sum decoders and consider the state-of-the-art Forward-Backward and Syndrome-Based approaches. We recall the presorting technique that allows for significant complexity reduction at the Elementary Check Node level. The Extended-Forward architecture is then presented as an original new architecture for efficient syndrome calculation. These advances lead to a new architecture for check node processing with reduced area. As an example, we provide implementation results over GF(64) and code rate 5/6 showing complexity reduction by a factor of up to 2.6.
Non-binary low-density parity-check (NB-LDPC) codes show higher error-correcting performance than binary low-density parity-check (LDPC) codes when the codeword length is moderate and/or the channel has bursts of errors. The need for high-speed decoders for future digital communications led to the investigation of optimized NB-LDPC decoding algorithms and efficient implementations that target high throughput and low energy consumption levels. We carried out a comprehensive survey of existing NB-LDPC decoding hardware that targets the optimization of these parameters. Even though existing NB-LDPC decoders are optimized with respect to computational complexity and memory requirements, they still lag behind their binary counterparts in terms of throughput, power and area optimization. This study contributes to an overall understanding of the state-of-the-art on application-specific integrated-circuit (ASIC), field-programmable gate array (FPGA) and graphics processing units (GPU) based systems, and highlights the current challenges that still have to be overcome on the path to more efficient NB-LDPC decoder architectures.