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Polar-coded forward error correction for MLC NAND flash memory

Abstract

With the ever-growing storage density, high-speed, and low-cost data access, flash memory has inevitably become popular. Multi-level cell (MLC) NAND flash memory, which can well balance the data density and memory stability, has occupied the largest market share of flash memory. With the aggressive memory scaling, however, the reliability decays sharply owing to multiple interferences. Therefore, the control system should be embedded with a suitable error correction code (ECC) to guarantee the data integrity and accuracy. We proposed the pre-check scheme which is a multi-strategy polar code scheme to strike a balance between reasonable frame error rate (FER) and decoding latency. Three decoders namely binary-input, quantized-soft, and pure-soft decoders are embedded in this scheme. Since the calculation of soft log-likelihood ratio (LLR) inputs needs multiple sensing operations and optional quantization boundaries, a 2-bit quantized hard-decision decoder is proposed to outperform the hard-decoded LDPC bit-flipping decoder with fewer sensing operations. We notice that polar codes have much lower computational complexity compared with LDPC codes. The stepwise maximum mutual information (SMMI) scheme is also proposed to obtain overlapped boundaries without exhausting search. The mapping scheme using Gray code is employed and proved to achieve better raw error performance compared with other alternatives. Hardware architectures are also given in this paper.

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Acknowledgements

This work was supported in part by National Natural Science Foundation of China (NSFC) (Grant Nos. 61501116, 61571105), Jiangsu Provincial NSF for Excellent Young Scholars (Grant No. BK20140636), Huawei HIRP Flagship (Grant No. YB201504), Fundamental Research Funds for the Central Universities, SRTP of Southeast University, State Key Laboratory of ASIC & System (Grant No. 2016KF007), ICRI for MNC, and Project Sponsored by the SRF for the Returned Overseas Chinese Scholars of MoE.

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Correspondence to Chuan Zhang.

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Song, H., Fu, J., Zeng, S. et al. Polar-coded forward error correction for MLC NAND flash memory. Sci. China Inf. Sci. 61, 102307 (2018). https://doi.org/10.1007/s11432-017-9394-y

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Keywords

  • polar coding
  • non-volatile memory
  • error correcting code
  • NAND
  • flash memory