Concatenated LDPC-TCM coding for reliable storage in multi-level flash memories

In this paper, we present an efficient fault tolerant solution for multi-level per cell (MLC) flash memory that concatenates trellis coded modulation (TCM) with an outer low-density parity-check (LDPC) code. Traditional flash coding systems employ simple hard-decisions based codes, such as Bose-Chaudhuri-Hocquenghem (BCH) codes, that can correct a fixed, specified number of errors. Thanks to the Bahl, Cocke, Jelinek, and Raviv (BCJR) algorithm, the TCM decoder within the proposed design can provide soft decisions which make it possible to use the more powerful LDPC codes. Moreover, the error-correction performance is further improved since TCM can decrease the raw error rate of MLC and hence relieve the burden of outer LDPC code. The effectiveness of concatenated LDPC-TCM systems has been successfully demonstrated through computer simulations.