A write-once memory (WOM) is a storage device that consists of cells that can take on q values, with the added constraint that rewrites can only increase a cell's value. A length-n, t-write WOM-code is a coding scheme that allows t messages to be stored in n cells. If on the ith write we write one of M i messages, then the rate of this write is the ratio of the number of written bits to the total number of cells, i.e., log 2 M i =n. The sum-rate of the WOM-code is the sum of all individual rates on all writes. A WOM-code is called a fixed-rate WOM-code if the rates on all writes are the same, and otherwise, it is called a variable-rate WOM-code. We address two different problems when analyzing the sum-rate of WOM-codes. In the first one, called the fixed-rate WOM-code problem, the sum-rate is analyzed over all fixed-rate WOM-codes, and in the second problem, called the unrestricted-rate WOM-code problem, the sum-rate is analyzed over all fixed-rate and variable-rate WOM-codes. In this paper, we first present a family of two-write WOM-codes. The construction is inspired by the coset coding scheme, which was used to construct multiple-write WOM-codes by Cohen et al. and recently by Wu, in order to construct from each linear code a two-write WOM-code. This construction improves the best known sum-rates for the fixed-and unrestricted-rate WOM-code problems. We also show how to take advantage of two-write WOM-codes in order to construct codes for the Blackwell channel. The two-write construction is generalized for two-write WOM-codes with q levels per cell, which is used with ternary cells to construct threeand four-write binary WOM-codes. This construction is used recursively in order to generate a family of t-write WOM-codes for all t. A further generalization of these t-write WOM-codes yields additional families of efficient WOM-codes. Finally, we show a recursive method that uses the previously constructed WOM-codes in order to construct fixed-rate WOM-codes. We conclude and show that the WOM-codes constructed here outperform all previously known WOM-codes for 2 t 10 for both the fixed-and unrestricted-rate WOM-code problems.

A Write Once Memory (WOM) is a storage medium with binary memory elements, called cells, that can change from the zero state to the one state only once. Examples of WOMs are punch cards, optical disks, and more recently flash memories. A t-write WOM-code is a coding scheme for storing t messages in n cells in such a way that each cell can change its value only from the zero state to the one state. The WOM-rate of a t-write WOM-code is the ratio of the total amount of information written to the WOM in t writes to the number of cells.In this paper we present a family of 2-write WOM-codes. It is shown how to construct from each linear code C a 2-write WOMcode. Then, we find 2-write WOM-codes that improve the best known WOM-rate with two writes. This scheme is proved to be capacity achieving when the parity check matrix of the linear code C is chosen uniformly at random. Finally, we show how to take advantage of 2-write WOM-codes in order to construct codes for the Blackwell channel.

A Write Once Memory (WOM) is a storage device that consists of cells that can take on q possible linearly-ordered values, with the added constraint that rewrites can only increase a cell's value. In the binary case, each cell can change from the level zero to the level one only once. Examples of WOMs include punch cards, optical disks, and more recently flash memories. A length-n, t-write WOM-code is a coding scheme that allows t messages to be stored in n cells. If in the i-th write we write one of M i messages, then the rate of the i-th write is the ratio of the number of bits written to the WOM to the total number of cells used, i.e., log 2 (M i )/n. The rate of the WOM-code is the sum of all individual rates in all writes.In this paper, we review a recent construction of binary twowrite WOM-codes. The construction is generalized for two-write WOM-codes with q levels per cell. Then, we show how to use such a code with ternary cells in order to construct three and four-write WOM-codes. This construction is used recursively in order to generate a family of t-write WOM-codes for all t. Another generalized construction is given which provides us with more ways to construct families of WOM-codes. Finally, we give a comparison between our codes and the best known WOMcodes in order to show that the WOM-codes constructed here outperform all previously known WOM-codes for 3 t 10.

Abstract-In this paper, we consider the joint coding constraint of forbidding the 101 subsequence and requiring all codewords to have the same Hamming weight. These two constraints are particularly applicable to SLC flash memory -the first constraint mitigates the problem of inter-cell interference, while the second constraint helps to alleviate the problems that arise from voltage drift of programmed cells. We give a construction for codes that satisfy both constraints, then analyze properties of best-case codes that can come from this construction.

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