M.Sc Thesis

M.Sc StudentSolomon Amit
SubjectError-Correction WOM Codes: Concatenation and
Joint Design
DepartmentDepartment of Electrical and Computer Engineering
Supervisor ASSOCIATE PROF. Yuval Cassuto
Full Thesis textFull thesis text - English Version


Flash-based non-volatile memories (NVM) are the storage media of choice in most modern information applications, due to their low read and write latencies and increasing densities. However, in Flash technology data cannot be updated in-place, since removing charges from memory cells cannot be done at a fine granularity. Therefore it is impossible to update written data without first erasing a large data unit, which leads to degradation of write performance and acceleration of cells wear. Previous works have shown that WOM codes can mitigate this access limitation by enabling an update of the logical data repeatedly while nullifying the need to physically remove charges from the cells. However, using WOM codes may have negative effect on the devices’ data reliability.

In this thesis we construct error-correcting WOM (write-once memory) codes that guarantee correction of any specified number of errors in q-level memories. The constructions use suitably designed short q-ary WOM codes and concatenate them with outer error-correcting codes over different alphabets, using suitably designed mappings. With a new storage-efficiency measure we call EC-rate, we show that for common error types the codes save redundancy and implementation complexity over straightforward concatenation. In addition to constructions for guaranteed error correction, we extend the error-correcting WOM scheme to binary multilevel coding for random errors, and to soft-decision decoding without using higher-precision readout.