Highly Reliable Coding Methods for Emerging Applications: Archive and Enterprise Solid-State Drives (SSDs)

Abstract

This paper proposes highly reliable coding methods for applications in two extreme conditions. n-out-of-8 level cell (nLC) is proposed for archival applications which require significantly long data-retention time with small write/erase cycle. On the other hand, for applications with large write/erase cycle and short data-retention time (enterprise application, etc.), universal asymmetric coding (UAC) is proposed. nLC reduces the number of memory states to improve the reliability with low cost overhead. In 7LC, the bit-error rate (BER) reduction will be 79% after 1k-year data retention while seven memory states are efficiently used out of eight states. By considering nLC with error-correcting codes (ECCs), the optimum number of cell levels (n) can be determined to minimize the bit-cost with given acceptable data-retention time. In UAC, the coding method is changed according to the write/erase cycle and data-retention time to keep the BER low. As a result, BER is reduced by 52% at maximum, compared with the original random pattern.