ELDPC: An Efficient LDPC Coding Scheme for Phase-Change Memory

Abstract

Low read latency, long lifetime, and high storage density have all been demonstrated in phase-change memory (PCM), making it an attractive contender for main memory. However, due to resistance drift per cell caused by long-term storage, data reliability becomes a major challenge. Low-density parity-check (LDPC) codes with improved error correction capability can be used in PCM to reduce bit error rates and thus improve data reliability. More interestingly, when the raw bit error rates (RBER) of various pages in PCM is compared at the same storage time, a considerable gap appears, resulting in high sensing and decoding latency. We propose eLDPC, an efficient LDPC coding scheme for reducing sensing and decoding latency, in this paper. We start with a preliminary experiment, which reveals that there is a significant variation in resistance drifts between adjacent distributions, resulting in a large RBER gap for different pages. Then, using a submatrix of the parity-check matrix to shorten the codeword length, eLDPC is inspired to encode pages with lower RBER. The original bit sequence is separated into even bit sequence (EBS) and odd bit sequence (OBS) for pages with higher RBER. eLDPC is used to encode EBS and OBS independently. By utilizing optimized soft information, EBS and OBS are eLDPC decoded. eLDPC can significantly improve error correction capability of LDPC hard decoding, effectively eliminating soft decoding processes and lowering decoding latency. The results of simulations show that eLDPC can greatly decrease decoding iterations and time.