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International Symposium on Medical Information and Communication Technology

An Improved Successive Cancellation List Flip Decoder for Polar Codes Based on Key Sets

Jingyun Bao, Shunjie Lin, Xingcheng Liu

Published Apr 14, 2021• 2 citations
Publisher

Abstract

Existing studies have documented that flipping operation can improve the performance of polar codes based on CRC-aided successive cancellation list (CA-SCL) decoding algorithm. However, the introduction of the flips greatly increases the complexity and delay of the decoder as well. In this paper, we analyzed the error distribution in the CA-SCL decoding and proposed a new efficient flipping set, key set, to reduce the complexity of the SCL-Flip decoding by narrowing down the size of the flipping set applicable. Different from the critical set which focuses on rate-1 nodes only, the key set can be constructed dynamically based on the results of the original CA-SCL decoding. In addition, we proposed an order-1 SCL-Flip decoder based on the key set to reduce the decoding complexity. Simulation results showed that the key set has a high accuracy of locating the first error and our proposed algorithm has almost no performance degradation compared with the existing SCL-Flip algorithms while the average complexity is reduced significantly.

DOI
10.1109/ismict51748.2021.9434898

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Existing studies have documented that flipping operation can improve the performance of polar codes based on CRC-aided successive cancellation list (CA-SCL) decoding algorithm. However, the introduction of the flips greatly increases the complexity and delay of the decoder as well. In this paper, we analyzed the error distribution in the CA-SCL decoding and proposed a new efficient flipping set, key set, to reduce the complexity of the SCL-Flip decoding by narrowing down the size of the flipping set applicable. Different from the critical set which focuses on rate-1 nodes only, the key set can be constructed dynamically based on the results of the original CA-SCL decoding. In addition, we proposed an order-1 SCL-Flip decoder based on the key set to reduce the decoding complexity. Simulation results showed that the key set has a high accuracy of locating the first error and our proposed algorithm has almost no performance degradation compared with the existing SCL-Flip algorithms while the average complexity is reduced significantly.
Several studies have documented that segmentation can improve the performance of polar codes based on CRC-aided successive cancellation list (CA-SCL) decoding algorithm. In this paper, we propose a new segmented CA-SCL decoder, referred to as improved segmented flipped successive cancellation list (ISF-SCL) decoder, with a reasonable segmentation strategy and a number of extra flipping attempts. Different from the existing methods, in which the uniform segmentation or LLR based segmentation scheme was adopted, a novel segmentation strategy that we firstly analyzed and proposed is based on the distribution of the correct path elimination. Hence, the segment effectiveness is promoted. In addition, to further improve the error correcting performance, a multi-path flipped strategy is proposed to restore the correct path eliminated from the list. Simulation results show that the performance of the ISF-SCL decoding algorithm is superior to the existing segmented CA-SCL decoding algorithms in the low signal-to-noise ratio regime, while the average computational complexity does not increase significantly.
Polar codes have garnered significant attention since they were proposed due to their capacity-achieving properties and their low-complexity decoding process, and they were selected as the coding scheme for the control channel in 5G mobile communication technology. A successive cancellation list (SCL) decoder, aided by a cyclic redundancy check (CRC), performs the competitive error correcting performance for polar codes compared to other sophisticated codes. The SCL-Flip decoding algorithm is proposed to enhance the error correction performance of the SCL. However, the SCL-Flip is hampered by a high decoding complexity due to the necessary selection and sorting process of path metrics (PMs). Additionally, it requires a large number of iterations to achieve a good error correction performance. In this paper, we propose the error segment bit-flip (ES-SCL-Flip) decoding algorithm, a simplified bit-flip approach for the SCL decoder based on analyzing the distributions of the first error bit. Firstly, we construct an index set with a high probability of the first erroneous bit. Second, a bit-flip criterion is introduced to prevent the costly operation of the PM selection and sorting without compromising the error correction performance. Finally, a search strategy is employed that utilizes segmented error indication to target and rectify the initial error segment sequentially. The simulation results demonstrate that the ES-SCL-Flip decoder provides an improved error correction performance with only a minimal increase in decoding complexity compared to the conventional CRC-aided SCL decoder. This algorithm attains a commendable equilibrium between the error correction performance and decoding complexity.
The eBCH-polar codes is a powerful way to construct polar codes. With high minimum Hamming distance, it outperforms ordinary Polar codes under successive cancellation list (SCL) decoder. However, the absence of cyclic redundancy check (CRC) bits in eBCH-polar codes makes it challenging to adopt SCL-flip (SCLF) decoding scheme. In this research, we propose a flip scheme activated by a Normalized Path Metric (PM) range and a mismatch criterion, which respectively leverage the properties of SCL decoding and dynamic frozen bits of eBCH-polar codes. This work also improves the selection the flipping bits by modifying metric to account for the structure of dynamic frozen bits and reduce the search space. Based on these modifications, we proposed SCLF-1 and SCLF-2 decoding algorithms for eBCH-polar codes. Simulation results show that both of the proposed decoding schemes achieve superior performance compared to other contemporary decoding algorithms for CRC aided polar (CA-polar) codes, demonstrating enhanced error rates performance within the same decoding attempt.
A successive cancellation list (SCL) decoder with the assistance of a cyclic redundancy check (CRC) can provide competitive decoding performance for polar codes compared with the state-of-the-art codes. In this paper, we adopt a bit-flip algorithm for the CRC-assisted SCL decoder to further improve the error-correcting performance. We first introduce a new bit-flip criterion for the SCL decoder. To support this criterion, we give the metric to identify the positions that need to be flipped. To verify the superiority of the proposed bit-flip criterion, we also consider a kind of SCL-flip decoder whose criterion is very similar with the existing one and we will prove that its performance is worse than the decoder with the proposed bit-flip criterion. The simulation results show that the decoder with the proposed bit-flip criterion outperforms both the standard CRC-assisted SCL decoder and the existing SCL-based bit-flip decoder.
Polar codes, as the coding scheme for the control channel in fifth-generation mobile communication technology (5G), have attracted widespread attention since their proposal. As a mainstream decoding algorithm for polar codes, the successive cancellation list (SCL) decoder usually improves the error correction performance by increasing the list size, but this method suffers from the problems of high decoding complexity. To address this problem, this paper proposes a layered-search bit-flipping (LS-SCLF) decoding algorithm based on SCL decoding. Firstly, a new flip-bit metric is proposed, which derives a formula to approximate the probability of an error occurring in an information bit. This formula introduces a perturbation parameter to improve the calculation accuracy. Secondly, a compromise scheme for determining the perturbation parameter is proposed. The scheme uses Monte Carlo simulation to determine an optimized parameter for the precise positioning of the first erroneous decoded bit under different decoding conditions. Finally, a layered search strategy is adopted to sequentially search the erroneous decoded bits from the low order to high order, which can correct up to multiple bits at the same time. Simulation results show that the proposed algorithm achieves improved error correction performance with a slight increase in decoding complexity compared to the generalized SCL-Flip (GSCLF) decoding algorithm. This algorithm also achieves a good balance between the error correction performance and decoding complexity.
By adopting successive cancellation list decoding (SCL), polar codes demonstrate competitive error correction performance over LDPC and Turbo codes. However, SCL decoding suffers from high computational complexity and long decoding latency, especially when the list size is very large. Successive cancellation flip (SCF), as another decoding algorithm that can achieve high error correction performance, has a complexity that is close to that of successive cancellation (SC) decoding. With the observation that SCL and SCF decoding are similar at giving more chances to inspect possible codewords simultaneously or sequentially, a novel hybrid decoder is proposed in this paper, which essentially combines the ideas of SCF and SCL decoders. Moreover, in order to compensate for the degradation of performance caused by the reduction of path splitting and further reduce the decoding latency, the convolutional polar codes are adopted with a designed bit-flipping set. Simulation results demonstrate that the proposed decoder achieves the reduction of decoding latency while attaining better performance than conventional CRC-aided SCL decoder.