A Class of q-Ary Unidirectional Error Correcting Codes for MLC Flash Memories

Error correcting codes (ECCs) are essential techniques to ensure reliability of NAND Flash Memories. BCH codes, ECCs which have been employed for multi-level cell (MLC) memories, don't reflect nonbinary error characteristics while error sources show them actually. Cell-to-cell interference (CCI), which occurs on a word unit, shifts threshold voltage distribution to the negative side. In addition, data retention, every cell suffers its effects, shifts the distribution to the positive side. Consequently, errors are either to upper levels, or lower levels in a word. Such errors, their values as an integer are either (+1) or (-1), can be regarded as unidirectional errors on q-ary symbols. This paper proposes codes capable of correcting the q-ary unidirectional errors. Evaluation results show the codes have less check bits than conventional BCH codes.