Research On The Application Of Polar Codes In Multi-level Cell NAND Flash Memory

Polar code is a new channel coding scheme based on channel polarization which can achieve its channel capacity in any binary discrete memoryless channel with low coding and decoding complexity.Due to its excellent performance,Polar codes have attracted wide attention in applications.As a kind of non-volatile storage,multi-level cell NAND flash memory has become the mainstream of storage market because of its advantages of large storage capacity,low power consumption and storage cost.However,the reliability of data storage is reduced due to the increase of storage density,which greatly shortens the life span of flash memory.Error-correcting code technology can effectively improve the storage reliability of flash memory and prolong the service life of flash memory.Therefore,the application of Polar code in MLC NAND flash memory is studied in this thesis.The main research results are as the following:Firstly,a method based on threshold voltage distribution of flash memory cell is proposed to solve the construction of Polar codes in MLC NAND flash memory.The bhattacharyya parameters of each memory cell bit are calculated by using the variance characteristic of the threshold voltage distribution of flash memory cell,and a structure of Polar codes is then constructed.At the same time,a puncturing algorithm of Polar code based on flash memory cell bit storage reliability is presented.The algorithm calculates the corresponding storage reliability through the bhattacharyya parameter of each memory cell bit,and then chooses the flash cell bits with lower storage reliability as the punctured bits.The numerical simulation results show that the construction method based on threshold voltage distribution of flash memory cell has better BER performance in MLC NAND flash memory channel,compared with several construction methods of Polar codes in addictive white Gaussian channel(AWGN)channel;the puncturing method based on bits' storage reliability of flash flash memory cells has lower bit error rate under the same puncturing rates in comparison with random puncturing method and cut-off tree puncturing method.Secondly,considering the constructed Polar code have slightly worse performance than other construction methods at lower SNR,propose a construction algorithm of Polar codes based on parameter transformation of MLC NAND flash memory channel model.The algorithm reconstructs and integrates the channel variances to encode Polar codes by simulating the process of flash memory issuing storage and reading.In addition,a puncturing algorithm of Polar codes based onthe reading correctness of storage bits is proposed to further improve the storage efficiency.The simulation results show that,compared with other construction methods,the performance of the Polar codes constructed by the proposed method is greatly improved at low signal-to-noise ratio,which indicates that the method can more completely and accurately reflect the characteristics of MLC NAND flash channel model.At the same puncturing rates,the Polar codes punctured by the algorithm based on the memory bits' reading correctness have better performance than those codes with random puncturing method and cut-off tree puncturing method.