Enhanced LDPC Coding for 3-D TLC NAND Flash Memory: Leveraging RBER Difference From Intralayer Variation

NAND flash memory employs high code rate low-density parity-check (LDPC) codes to reduce the amount of redundant data that must be added. When the code rate is high, although the redundancy space is small, the error correction capability is inferior to medium or low code rate LDPC. RBER varies among the storage layers for 3D triple-level cell (TLC) NAND flash memory, which increases the frequency of read retry operations. Repeatedly initiating read retry seriously increases the decoding latency and decreases the performance of the 3D TLC NAND flash memory. To alleviate this problem, this article proposes Intra-Layer Variation aware LDPC coding, called LVLDPC. The proposed LVLDPC scheme establishes the correlation between inter-layer interference and raw bit error rate (RBER) based on a neural network model. By analyzing and predicting RBER through the neural network model, we are able to categorize RBER into distinct levels. Then, we then select LDPC codes with appropriate error correction capabilities to decode data with varying levels of RBER. Through this scheme, we don’t need to start read retry when RBER <1.56×10-2. The iteration number is reduced by 67% in total. This scheme only causes 1.15% space overhead, which is negligible. For the stage with high RBER, the number of iterations of LVLDPC is still large, and the extended LVLDPC scheme (eLVLDPC) is further proposed to reduce the use of high code rate and reduce the number of iterations by 19.1%, expanding the correctable RBER threshold to 2.68×10-2.