High Sum-Rate Three-Write and Nonbinary WOM Codes
Write-once memory (WOM) is a storage medium with memory elements, called cells, which can take on q levels. Each cell is initially in level 0 and can only increase its level. A t-write WOM code is a coding scheme, which allows one to store t messages to the WOM such that on consecutive writes every cell's level does not decrease. The sum-rate of the WOM code, which is the ratio between the total amount of information written in the t writes and number of memory cells, is bounded by log(t + 1). Our main contribution in this paper is a construction of binary three-write WOM codes with sum-rate approaching 1.885 for sufficiently large number of cells, whereas the upper bound is 2. This improves upon a recent construction of sum-rate 1.809. A key ingredient in our construction is a recent capacity achieving construction of two-write WOM codes, which uses the so-called Wozencraft ensemble of linear codes. In our construction, we encode information in the first and second write in a way that leaves a large number (roughly half) of the cells nonprogrammed. This allows us to use the above two-write construction in order to invoke a third write to the memory. We also give specific constructions of nonbinary two-write WOM codes and multiple writes, which give better sum-rate than the currently best known ones. In the construction of these codes, we build upon previous nonbinary constructions and show how tools such symbols relabeling can help in achieving high sum-rates.