Program Error Mitigation in MLC NAND Flash Memory with Soft Decision Decoders

Benefiting from strong decoding capabilities, soft-decision decoding has been used to replace hard-decision decoding in various communication systems, and NAND flash memory systems are no exception. However, soft-decision decoding relies heavily on accurate soft information. Owing to the incremental step pulse programming(ISPP), program errors(PEs) in multi-level cell(MLC) NAND flash memory have different characteristics compared to other types of errors, which is very difficult to obtain such accurate soft information. Therefore, the characteristics of the log-likelihood ratio(LLR) of PEs are investigated first in this paper. Accordingly, a PE-aware statistical method is proposed to determine the usage of PE mitigation schemes. In order to reduce the PE estimating workload of the controller, an adaptive blind clipping(ABC) scheme is proposed subsequently to approximate the PEs contaminated LLR with different decoding trials. Finally, simulation results demonstrate that(1) the proposed PE-aware statistical method is effective in practice, and(2) ABC scheme is able to provide satisfactory bit error rate(BER) and frame error rate(FER) performance in a penalty of negligible increasing of decoding latency.