Performance analysis of concatenated coding to increase the endurance of multilevel NAND flash memory

The increasing storage density of modern NAND flash memory chips, achieved both due to scaling down the cell size, and due to the increasing number of used cell states, leads to a decrease in data storage reliability, namely, error probability, endurance (number of P/E cycling) and retention time. Error correction codes are often used to improve the reliability of data storage in multilevel flash memory. The effectiveness of using error correction codes is largely determined by the model accuracy that exhibits the basic processes associated with writing and reading data. The paper describes the main sources of disturbances for a flash cell that affect the threshold voltage of the cell in NAND flash memory, and represents an explicit form of the threshold voltage distribution. As an approximation of the obtained threshold voltage distribution, a Normal-Laplace mixture model was shown to be a good fit in multilevel flash memories for a large number of rewriting cycles. For this model, a performance analysis of the concatenated coding scheme with an outer Reed-Solomon code and an inner multilevel code consisting of binary component codes is carried out. The performed analysis makes it possible to obtain tradeoffs between the error probability, storage density, and the number of P/E cycling. The resulting tradeoffs show that the considered concatenated coding schemes allow, due to a very slight decrease in the storage density, to increase the number of P/E cycling up to 2–2.5 times than their nominal endurance specification while maintaining the required value of the bit error probability.