为了进一步提高多级单元（multi-level-cell,MLC）闪存的耐久度和可靠性，提出了一种MLC闪存信道中基于互信息量（mutual information,MI）异构的polar码优化方法.该方法利用对数似然比（log-likelihood ratio,LLR）分布在MLC闪存信道和AWGN（additivewhite Gaussian noise）信道中的差异性，以MI重新拟合LLR分布，得到在闪存信道下等效的标准方差，从而进行高密度存储系统中的polar码优化设计.随后，分析了不同的polar码构造法对多级存储单元的纠错性能影响，并与所提的构造方法进行比较.仿真结果表明该文优化方法优于AWGN信道下传统的构造方法，当编程/擦除（program-and-erase,PE）循环为21 000次时，与蒙特卡罗法相比其误码率（bit error rate,BER）性能提升2个数量级，且在BER为2 × 10-5时可增加6 800次的编程/擦除循环.
In order to further improve the durability and reliability of multi-level-cell (MLC) flash memory, a polar code optimization method based on mutual information (MI) heterogeneity in the MLC flash channel is proposed. By exploiting the differences of log-likelihood ratio (LLR) distribution between MLC flash channels and AWGN (additive white Gaussian noise) channels, and employing MI re-fitting for obeying Gaussian distribution in AWGN channels, the method obtains its equivalent variance of AWGN channels. Thereafter, the polar code optimization design in the high-density storage system is performed according to the obtained new variance. This paper also analyzes the effects of other different polar code construction methods on the error correction of multi-level memory cells, and compares them with the proposed construction method. Simulation results show that the optimization method is better than the traditional construction methods in AWGN channels. It improves bit error rate (BER) by more than 2 orders of magnitudes compared with Monte-Carlo method when program-and-erase (P/E) cycles is 21 000, and it can increase the lifetime of MLC flash memory up to 6 800 P/E cycles at the BER of 2×10-5.
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