Soft-decision decoding with cell to cell interference removed signal in nand flash memory

As the semiconductor feature size continuously decreases, the signal integrity of MLC (multi-level cell) NAND flash memory becomes degraded, which causes increased bit error rate and short retention time limit. Since it is well known that cell-to-cell interference (CCI) is one of the major sources of bit errors, several signal processing solutions to mitigate or remove the CCI have been developed. Even though these works show BER (bit error rate) performance improvement with hard-decision error correction, combining an interference canceller and soft-decision decoding still needs to be studied. In this research, we derive a mathematical formulation for computing the likelihood function of the CCI removed signal, and then propose a conditional LLR (log-likelihood ratio) to represent soft-reliability information for soft-decision error correction. The proposed soft-information computation scheme is applied to simulated NAND flash memory, and it is demonstrated that soft-decision error correction with the CCI removed signal significantly increases the retention time limit of MLC NAND flash memory.