Research On Multi-Level Flash Data Representation And Error Control

Flash memory is a new type of semiconductor memory, which by virtue of high density, high memory speed, low cost and shock, as the current mainstream solid-state memory. However, with the people’s increasing demand for storage density, early single-level flash gradually being replaced by multi-level flash memory, it has a higher density and a greater inter-cell interference.The reduction of adjacent levels also makes mistakes more easily than the single-level flash occurred.this change makes the life of flash memory and data reliability is facing serious challenges, and error coding scheme is an effective way to improve the reliability of flash memory.Mostly trational error control techniques are designed by the basic which is based on the physical characteristics of the disk. If they are applied directly to a flash error control coding, not only inefficient, but also may be unexpected errors. The reason is that there are large differences between the disk and flash memory in the physical structure, access methods, and other aspects of the wrong type. And the evolution from the traditional error-correcting code directly out of the error correction code when applied to multi-level flash memory effect is not ideal, but it does not fit the physical characteristics to solve the multi-level flash memory-specific error, so the flash error control technology not only need to research on coding, data representation scheme also needs to be improved.This paper mainly studies data representation scheme and error control technology under multi-level flash memory. Based on the Rank Modulation Scheme, a recursive construction method of the free prefix codes was proposed. The main work of this paper is summarized as follows:1. The physical structure of flash memory storage is summarized. Then the major error correction technologies of NAND flash memory are introduced. And the operation for NAND flash memory model, the type of noise and BCH encoding scheme for multi-level NAND flash memory are analyzed in detail.2. For the channel model of Limited-Magnitude Errors, the relationship between the threshold voltage distribution and interference noise multi-level NAND flash memoryare calculated. And the bidirectional limited magnitude error correction algorithm based on the channel model is given. The simulation result shows that the error correction performance of bidirectional limited magnitude error correction algorithm is superior to BCH algorithm.3. For the problem of stored information errors caused by over programmed and the charge leakage in the multi-level flash memory, the new data representation scheme to store data with replacement is analyzed. And the recursive algorithm of rank modulation code constructed by free prefix tree is given. Experimental results show that the error handling capacity of rank modulation scheme is better than traditional data representation scheme, and the free prefix code can prolong the lifetime of the flash by reduce the average erase count effectively.