Signal Detection For Flash Memory

As a class of non-volatile data storage system, flash memory has become more and more popular in the global semiconductor industry. Among all kinds of flash memory storage technologies, NAND flash memory is the dominant one. The NAND flash memory usage keeps growing thanks to its low cost and high density resulting from the continuous improvements in scaling technology and multilevel cell technology. However, as the storage density in flash memories continues to grow, various other factors such as cell-to-cell interference, continue to degrade the overall system performance. This thesis will discuss the NAND flash memory channel characteristics, including programming and erasing of the information and all kinds of interferences. And according to the channel characteristics we designed some schemes that can effectively eliminate the noises and interferences in the flash memory channel.First, we introduced the structure of the NAND flash memory, and the knowledge of the operating mechanisms about how to write and erase the memory cells, etc. And the noise disturbances towards flash memory during the processes of storage are also introduced. Then we present the channel model according to the basic knowledge explained above, and carried out the corresponding computer simulation. We introduced several optimization methods for Cell-to-Cell Interference on flash memory channel.Furthermore, we innovatively presented a new programming scheme, cross-programming, which program the cells on the odd word-lines first, then the ones on the even word-lines, aiming to diminish the C2 CI. This scheme can implement unequal error protection. By using various numbers of levels on different cells, an optimized scheme is also presented. The effectiveness of these two schemes has been well demonstrated through computer simulations and analysis.Inspired by spatial modulation in wireless communication, we presented two new programming schemes. In these schemes, input data are divided into two parts: position information and program information. Flash memory cells are divided into many small blocks. In each block, specific cells are chosen to program by position information and the data stored in these cells is detected by program information. The effectiveness of the spatial modulation program schemes has been well demonstrated through computer simulations and analysis.Besides that, a signal detection method which can diminish the noise disturbances in flash memory channel at the same time is proposed. In this method, dynamic threshold and balance coding algorithm are used to diminish retention process noise, whose distribution function is unknown. And some signal processing methods like pre-distortion are used to diminish random telephone noise and cell-to-cell interference, whose distribution functions are known. Compared with the method using the conventional fixed threshold, this method achieves better data-storage reliability performance.