Modelling and characterization of NAND flash memory channels

• A NAND model incorporated distortions on RTN, cell-to-cell, retention is proposed.
• We derive the overall distribution for the three kinds of channel distortions.
• We mathematically formularize the threshold voltage distributions for MLC flash.
• Soft decisions can be extracted from channel output based on the results presented.
• Lookup table simplifies calculations of soft decisions and improves decoding speed.

The threshold voltage distribution after ideal programming in NAND flash memory cells is usually distorted by a combination of the random telegraph noise (RTN), cell-to-cell interference (CCI), and the retention process. To decide the original bits more accurately in this scenario, a precise channel model shall be utilized on the basis of the measured threshold voltages. This paper aims to characterize these various distortions occurring in multi-level cell (MLC) flash memories. A mathematical description of the overall distribution for the total flash channel distortion is presented. The final threshold voltage distribution for each symbol of MLC flash is also characterized, which is important for calculating the exact soft decisions of cell bits and the application of advanced flash error correction. The results of the theoretical analysis have been validated through Monte Carlo simulations of the flash channel.