Spreading Modulation for Multilevel Nonvolatile Memories

The aggressive scaling of NAND flash memories has caused significant degradation in their reliability and endurance. One of the dominant factors in this degradation is the inter-cell-interference, by which the programming of a cell can affect nearby neighboring cells corrupting the information that they store. This paper proposes a new data representation scheme, which increases endurance and significantly reduces the probability of error caused by inter-cell-interference. The method is based on using an orthogonal code to spread each bit across multiple cells, resulting in lower variance for the voltages being programmed in the cells. This new data representation method is also shown to present many of the advantages that spreading sequences bring to wireless communications. For example, multiple information sequences can be written on the same cells at different times without interfering with each other. It also allows storing additional information on an already programmed memory in such a way that the new information is hidden by the noise.