Enhanced Precision Through Multiple Reads for LDPC Decoding in Flash Memories

TL;DR

This paper demonstrates that multiple reads with optimized voltages significantly improve LDPC decoding in Flash memories, approaching full-precision performance and outperforming BCH codes, with insights into code design trade-offs and optimization strategies.

Contribution

It introduces a method to optimize word-line voltages for LDPC decoding in Flash, maximizing mutual information and analyzing the impact of quantization on error rates and code performance.

Findings

Optimized multiple reads improve FER performance.

Mutual information maximization aligns with minimal frame error rate.

Trade-offs exist in code design for different precision levels.

Abstract

Multiple reads of the same Flash memory cell with distinct word-line voltages provide enhanced precision for LDPC decoding. In this paper, the word-line voltages are optimized by maximizing the mutual information (MI) of the quantized channel. The enhanced precision from a few additional reads allows FER performance to approach that of full-precision soft information and enables an LDPC code to significantly outperform a BCH code. A constant-ratio constraint provides a significant simplification in the optimization with no noticeable loss in performance. For a well-designed LDPC code, the quantization that maximizes the mutual information also minimizes the frame error rate in our simulations. However, for an example LDPC code with a high error floor caused by small absorbing sets, the MMI quantization does not provide the lowest frame error rate. The best quantization in this case introduces more erasures than would be optimal for the channel MI in order to mitigate the absorbing sets of the poorly designed code. The paper also identifies a trade-off in LDPC code design when decoding is performed with multiple precision levels; the best code at one level of precision will typically not be the best code at a different level of precision.