Research On Bit-flipping Decoding Algorithms Of LDPC Codes For NAND Flash Memory

NAND flash memory has become a high competitive storage media in the current field of data storage owing to its many advantages,such as fast read-write speed,high storage density and low power consumption.With the rapid growth of storage density,the size of flash memory chips continues to shrink,and noise interference will affect the reliability of flash memory system.LDPC codes have strong error correction capability.They have been adopted by some vendors as error control technology for NAND flash memory to increase the reliability of flash memory system.However,the commonly used soft decision decoding algorithms for LDPC codes have the disadvantages of high decoding complexity and large decoding delay.The bit-flipping decoding algorithms of LDPC codes with low complexit y can effectively improve the reliability of flash memory system.In this thesis,the bit-flipping decoding algorithms of LDPC codes are investigated systematically.The improved gradient descent bit-flipping decoding algorithm is given.The threshold voltage distribution of flash memory cell is determined by analyzing the influe nce of several kinds of noise interference on threshold voltage of flash memory cel.The bitflipping decoding algorithm of LDPC codes for NAND flash memory is given to enhance the reliability of flash memory system.The main work of this thesis is as follows:1.The NAND flash memory structure,operation models and three types of errors are introduced in this thesis.The classification and representation of LDPC codes are described.The soft decision decoding algorithm and hard decision decoding algorithm of LDPC codes are summarized.2.After analyzing several typical bit-flipping decoding algorithms of LDPC codes,the gradient descent bit-flipping decoding algorithm of LDPC codes is improved by increasing the weight of posteriori reliability information.Simulation results show that the improved decoding algorithm has a better decoding performance than gradient descent bit-flipp ing decoding algorithm.3.The influences of cell to cell interference,data retention noise and random telegraph noise on the threshold voltage distribution of flash memory cell are analyzed.The threshold voltage distribution of flash memory cell under noises is determined according to the influence of these noises.The bit-flipping decoding algorithm of LDPC codes for NAND flash memory is given by using threshold voltage soft sensing method.Simulation results show that bit-flipping decoding algorithm can effectively improve the reliability of flash memory system.