Decision-Directed Retention-Failure Recovery With Channel Update for MLC NAND Flash Memory

Abstract

To recover from the retention noise induced errors in nand flash memory, a retention-aware belief-propagation (RABP) decoding scheme for low-density parity-check codes is introduced. The RABP is a two-stage decoding scheme in which the memory cell's charge-loss effect is systematically compensated. In RABP decoding, instead of read retries for data recovery, the probable victim cells are first determined with the help of read-back voltage signal and the decoded bit decisions. Then, for such suspected victim cells, their log-likelihood-ratio regions are modified in such a way as to absorb the effect of cell voltage downshift caused by retention noise, and then a second round of belief-propagation (BP) decoding is performed afresh, often with decoding failure recovery. Furthermore, leveraging on the RABP decoded bit-error pattern, an RABP assisted channel update (RABP-CU) algorithm is proposed which re-estimates the latest cell voltage distribution parameters without incurring new memory sensing operations. This is achieved by minimizing the mean squared error between the measured and predicted bit error/erasure values. Through simulations, it is shown that the RABP decoder increases the retention time limit by up to 70% compared with single round of BP decoding. The proposed RABP-CU algorithm further extends the data retention time.