On the soft information extraction from hard-decision outputs in MLC NAND flash memory

In this work, we propose a scheme to extract soft information from hard-decision outputs in multi-level per cell (MLC) flash memory based on a cell-to-cell interference model. It will be shown that the soft information extracted in the form of log-likelihood ratio (LLR) by taking into account a dominant cell-to-cell interference term provides significant performance improvements when the error-control system is designed with an error-correcting code with iterative decoding algorithm, e.g. low-density parity-check (LDPC) code with the Belief-Propagation (BP) algorithm. To confirm the claims, we design error-control systems with a conventional Bose-Chaudhuri-Hocquenghem (BCH) code and an LDPC code with/without the soft information extraction. Performances of the systems are extensively evaluated and compared, which clearly shows that the soft information extraction based on the cell-to-cell interference model leads to a considerably better performance.