Modeling and signal processing for flash memory

Thesis

This thesis examines the effects of noise and interference on the performance of NAND flash memory.

Chapter 3 studies the probabilistic input/output relation between the data stored and the read threshold voltage of a cell and generalizes it to a group of cells. It is then concluded that adjacent cells are correlated due to common aggressors. This motivates the study of adequate signal processing techniques to optimize the reliability performance.

Chapter 4 proposes two techniques that can reduce the error rate in light of the result of the previous chapter. The first is based on dividing a group of cells into sub-groups and detecting each sub-group independently. The second approximates the flash system model by a hidden Markov model, then uses the sum-product algorithm to detect the inputs. The soft outputs of the proposed detectors are passed on to the ECC soft decoder. It is shown that the second approach provides significant improvements. Then, it is shown that quantization negatively affects the performance of the sum-product algorithm more in comparison with the first approach. To partially mitigate this effect, an iterative detection/decoding strategy is proposed and shown to improve the performance.

Chapter 5 proposes a novel data representation scheme that provides a trade-off between reliability and the amount of data stored per cell, and partially mitigates the effects of device degradation. The scheme divides the stored data into two streams stored in the indices and levels of the non-erased cells, respectively, allowing the first to be detected without any knowledge about the channel. The simulation results show an improvement in the error rate while partially mitigating the need to track the channel parameters and the read references as the device degrade.

Authors: Yassine, H

• DOI: 10.5287/ora-gw6xg16yd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Keywords: Signal Processing; Flash memory; Inter-cell Interference; Index Modulation; Error Correcting Codes
• Subjects: Storage; Signal processing