D-Imbalance WOM Codes for Reduced Inter-Cell Interference in Multi-Level NVMs

In recent years, due to the spread of multi-level nonvolatile memories (NVMs), q-ary write-once memory (WOM) codes have been extensively studied. By using WOM codes, it is possible to rewrite NVMs t times before erasing the cells. Use of WOM codes enables the improvement of the performance of the storage device; however, it may also increase errors caused by inter-cell interference (ICI). This paper presents WOM codes that restrict the imbalance between code symbols throughout the write sequence, hence decreasing ICI. We first specify the imbalance model as a bound d on the difference between codeword levels. Then, a two-cell code construction for general q and input size is proposed. An upper bound on the write count is also derived, showing the optimality of the proposed construction. In addition to direct WOM constructions, we derive closed form optimal write regions for codes constructed with continuous lattices. On the coding side, the proposed codes are shown to be competitive with known codes not adhering to the bounded imbalance constraint. On the memory side, we show how the codes can be deployed within flash wordlines, and quantify their bit-error rate advantage using accepted ICI models.