Improving LDPC performance via asymmetric sensing level placement on flash memory

Flash memory development through technology scaling and bit density has significant impact on the reliability of flash cells. Hence strong error correction code (ECC) schemes are highly recommended. With a strong error correction capability, low-density-parity code (LDPC) is now applied for the state-of-the-art flash memory. However, LDPC has long decoding latency when the raw bit error rates (RBER) are high. This is because it needs finegrained soft sensing between states to iteratively decode the raw data. In this work, we propose a smart sensing level placement scheme to reduce the LDPC decoding latency. The basic idea for the placement scheme is motivated by two asymmetric error characteristics of flash memory: the asymmetric errors at different states, and the asymmetric errors caused by voltage left-shifts and right-shifts. With understanding of these two types of error characteristics, the sensing levels are smartly placed to achieve reduced sensing levels while maintaining the error correction capability of LDPC. Experiment analysis shows that the proposed scheme achieves significant performance improvement.