RS-Enhanced TCM for Multilevel Flash Memories

Multilevel flash memories store more than one bit per storage cell and are further characterized by large word (page) sizes and very low target error rates. In this paper, a high-rate error control scheme is presented that uses inner trellis-coded modulation (TCM) for storing two bits per cell with five possible charge levels. The coded subset-label bits and the uncoded signal-label bits of TCM are independently protected by separate outer Reed-Solomon (RS) codes. The resulting scheme permits multistage decoding. Errors made by the TCM decoder in the subset-label bits occur in bursts and are corrected by the associated first RS decoder prior to determining signal-label bits and correcting errors in those bits by the associated second RS decoder. The multi-stage decoding avoids the significant spread of errors from subset-label bits into the generally larger number of signal-label bits which is typical for conventional serial RS-TCM concatenation when the inner TCM system operates at relatively low SNR. The error performance of the proposed scheme is evaluated at low error rates by a mixed simulation-analytic method. It is shown that the proposed scheme exhibits highly favorable performance vs. complexity tradeoffs compared to the other schemes.