CCF SHF(Small): Some Codes Applicable to Flash Memories and Computer Systems

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In the past six decades error control codes have been extensively used to improve the reliability in computer, communication and VLSI systems. Without the use of appropriate coding methods several of the common electronics devices (cell phones, computers, medical electronics, GPS, etc.) will not function in a reliable manner.In this proposal, first, some error control codes for multilevel flash memories are investigated. Even though the use of multilevel cells increases the density of the flash memories compared to single level cells it also imposes two major challenges. The first one is that the voltage difference between the states is narrowed and hence there is a high probability of errors due to noise. The error nature in such cases can be considered as symmetric errors of limited magnitude. The second challenge is that, in order to program a cell from an higher value to a lower value an entire block of data needs to be erased and then each and every cell needs to be carefully programmed to the desired values. In order to do this, electrons are injected to the floating gates using many iterations. Thus, this is a very time consuming operation. The overshoot of levels while writing can be considered as asymmetric errors of limited magnitude. Thus, by using asymmetric error correcting codes of limited magnitude the speed and reliability of the flash memories can potentially be improved. The concept of limited magnitude error correction and detection is new in coding theory area. Some fundamental theory and efficient code design methods are investigated for these types of limited magnitude errors. The other research problems are related to efficient design of unordered codes and balanced codes.Flash memories find wide applications in cell phones, cameras, computer systems, embedded systems, etc. The market value of the flash memory is in the order of tens of billions of dollars and will continue to grow in the coming years. The proposed codes is expected to help improve speed and reliability of these memory and computer systems.