Evyatar Hemo – d-imbalance WOM Codes for Reduced Inter-Cell Interference in Multi-Level NVMs

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In recent years, due to the spread of multi-level nonvolatile memories (NVM), q-ary write-once memories (WOM) codes have been extensively studied.

By using WOM codes, it is possible to rewrite NVMs t times before erasing the cells. The use of WOM codes enables to improve the performance of the storage device, however, it may also increase errors caused by inter-cell interference (ICI). In this talk I will present WOM codes that restrict the imbalance between code symbols throughout the write sequence, hence decreasing ICI. I will first specify the imbalance model as a bound d on the difference between codeword levels. Then a 2-cell code construction for general q and input size will be proposed. An upper bound on the write count is also derived, showing the optimality of the proposed construction. In addition to direct WOM constructions, I will also present 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, I will show how the codes can be deployed within flash wordlines, and quantify their BER advantage using accepted ICI models.

This is a joint work with Prof. Yuval Cassuto.