Abstract
In this letter, protograph based quasi-cyclic (QC) low-density parity-check (LDPC) codes are designed for multi-level cell (MLC) NAND flash memories, where the exact voltage signal values are unavailable but rather the quantized voltage signals are measured for soft decoding. Existing LDPC codes optimized for symmetric, additive white Gaussian noise (AWGN)-like channels are not optimal for flash memory channels due to their asymmetries. The proposed approach exploits the mutual information (MI) between input and output of flash memory to model the quantized log-likelihood ratio (LLR) messages. The proposed protograph based codes, whose base matrix is constructed according to the degree sequences optimized by the modified extrinsic information transfer (EXIT) chart method for NAND flash memories, have a low-complexity QC encoder structure with a readily parallelizable protograph decoder structure. Moreover, a rate-compatible family of protograph LDPC codes can be conveniently generated according to the proposed nested photograph. Simulation results and theoretical analyses are provided to support the advantages of the proposed coding scheme.
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