考虑LDPC码字的NAND闪存信道量化  

LDPC code-dependent quantization for the NAND flash channel

作　　者：方泽群 马征[1] 周璇 庞琦珂 Zequn FANG;Zheng MA;Xuan ZHOU;Qike PANG(Key Lab of Information Coding and Transmission,Sichuan Province,School of Information Science and Technology,Southwest Jiaotong University,Chengdu 610031,China)

机构地区：[1]西南交通大学信息科学与技术学院,信息编码与传输四川省重点实验室,成都610031

出　　处：《中国科学：信息科学》2022年第8期1542-1559,共18页Scientia Sinica(Informationis)

基　　金：四川省科技计划重点项目(批准号:22NSFSC0759)资助。

摘　　要：随着尺寸工艺的提升和存储方法的演进,NAND闪存芯片的存储密度正在一步步提高.而存储密度的提高带来了芯片物理容错性能的下降,从而导致器件的寿命下降.如何保证NAND闪存的使用寿命是目前闪存器件的研究热点问题.纠错编码的软判决译码器应用到NAND闪存中能够充分提供数据可靠性保障,但其在数据读取时的量化方法是核心问题之一.针对目前最大互信息量化方法没有考虑纠错编码码字特性的缺点,本文提出使用密度进化量化算法来进行基于低密度奇偶校验码(low density parity check,LDPC)码字特征的量化.为了适应NAND信道非对称的特点,本文进一步提出首先利用比特翻转器来将实际非对称译码器输入对称化,然后通过使用固定比例方法进一步减少搜索最佳读取电压位置的复杂度.经在NAND闪存信道下的数值仿真比较,本文提出的量化算法可以比最大互信息量化算法更好地提升译码性能,能够显著提升闪存器件的寿命.With the improvement of scaling technologies and the evolution of storage strategies,the storage density of NAND flash memory is gradually growing.However,the increase in storage density increases error probability,which reduces the lifespan of NAND flash memory.Error correction coding technology is a powerful tool for ensuring data reliability in NAND flash memory,but how to maximize the error correction ability through quantization in the reading process is an important problem that must be addressed.In view of the shortcomings of the maximum mutual information(MMI)quantization,this study proposes to utilize the density evolution(DE)algorithm to perform quantization on the basis of the features of low-density parity-check codes.To meet the conditions of the DE algorithm,this research also proposes to implement a bit flipper in the NAND controller for symmetrizing the asymmetric input distribution of the decoder.Then,the constant ratio method is chosen to further reduce the complexity of searching for the best read-voltage thresholds.Through numerical simulation and verification,the proposed algorithm is proven to provide better decoding performance than the MMI quantization and significantly improve the lifetime of the NAND flash memory.

关 键 词：NAND闪存 非对称信道 LDPC码 量化 密度进化 

分 类 号：TP333[自动化与计算机技术—计算机系统结构] TN40[自动化与计算机技术—计算机科学与技术]