Multilevel NAND flash memories with superposition modulation: A non-orthogonal multi-user communication perspective

In this work, we propose a comprehensive theoretical framework for the multilevel NAND (NOT AND logic) flash memory, built upon the modified Student's t distribution where the distortion of the threshold voltage caused by the random telegraph noise, cell-to-cell interference and data retention noise are jointly considered. Based on the superposition modulation, we build a non-orthogonal multiuser communication model where a linear mapping is conducted between the verify voltages and binary antipodal symbols. Aimed at improving the storage efficiency, we propose an unequal amplitude mapping (UAM) solution by optimizing the weighting coefficients of verify voltages to intelligently adjust the width of each state. Moreover, the uniform storage efficiency region and sum storage efficiency of different labelings with various decoding schemes are discussed. Simulation results validate the effectiveness of our proposed UAM solution where an up to 20.9% storage efficiency gain can be achieved compared to the current used benchmark scheme. In addition, analytical and simulation results also demonstrate that the successive cancellation decoding outperforms other decoding schemes for all labelings.