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SiPS ... design and implementation - IEEE Workshop on Signal Processing Systems

Extended-forward architecture for simplified check node processing in NB-LDPC decoders

Cédric Marchand, Emmanuel Boutillon, Hassan Harb, Laura Conde-Canencia, Ali Chamas Al Ghouwayel

Published Oct 1, 2017• 4 citations
Publisher

Abstract

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.

DOI
10.1109/sips.2017.8109992

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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.
This paper deals with low-complexity algorithms for the check node processing in nonbinary LDPC decoders. After a review of the state-of-the-art, we focus on an original solution to significantly reduce the order of complexity of the Extended Min-Sum decoder at the elementary check node level. The main originality of the so-called Bubble Check algorithm is the two-dimensional strategy for the check node processing, which leads to a reduction of the number of comparisons. The simulation results obtained for the Bubble Check show that this complexity reduction does not introduce any performance loss and that it is even possible to further reduce the number of comparisons. This motivated the search of a simplified architecture and led to the L-Bubble Check, which is the main contribution of the paper. The implementation of a forward/backward check node as a systolic architecture of L-Bubble elementary checks is also described. Finally, some FPGA synthesis results of a whole GF(64)-LDPC decoder implementation are presented.
Non-binary low-density parity-check (NB-LDPC) codes can achieve higher coding gain than binary LDPC codes when the code length is moderate at the cost of higher complexity. One major step of NB-LDPC decoding is check node processing. Previously, iterative forward-backward approaches are employed to implement this step. However, the storage of the intermediate results of the forward and backward computations requires large memory. In this paper, a novel check node processing scheme and corresponding VLSI architectures are proposed for the extended Min-sum NB-LDPC decoding. The proposed scheme only stores a limited number of sorted variable-to-check messages. Then the check-to-variable messages for different variable node are generated independently. For a (837, 726) NB-LDPC code over GF(25), the proposed architecture only requires 64% of the area of the previous design with the same throughput and error-correcting performance.
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 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 can achieve better error-correcting performance than their binary counterparts when the code length is moderate. Check node processing is one bottleneck in NB-LDPC decoding. Various techniques have been proposed to simplify the check node processing. Particularly, the computation complexity can be reduced by employing an iterative forward-backward scheme. However, this scheme requires large memory 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 chooses a limited number of variable-to-check messages, then the check-to-variable messages to all connected variable nodes are generated independently. Employing the proposed architecture, the area requirement of the check node processing for an (837, 726) NB-LDPC code over GF (25) can be reduced to 22.5% of that of the previous work with the same processing speed and error-correcting performance.