Design of Rate-Compatible Protograph LDPC Codes for Spin-Torque Transfer Magnetic Random Access Memory (STT-MRAM)

Thanks to its superior features of non-volatility, fast read/write speed, high endurance, and low power consumption, spin-torque transfer magnetic random access memory (STT-MRAM) has become a promising candidate for the next generation non-volatile memories (NVMs) and storage class memories (SCMs). However, it has been found that the write errors and read errors caused by thermal fluctuation and process variation severely degrade the reliability of STT-MRAM. Moreover, process imperfection also causes a diversity of the raw bit error rate (BER) among different dies of STT-MRAM. In this paper, we propose the design of novel rate-compatible protograph low-density parity-check (RCP-LDPC) codes to correct memory cell errors and mitigate the raw BER diversity of STT-MRAM. In particular, to deal with the asymmetric property of the STT-MRAM channel, we first apply an independent and identically distributed (i.i.d.) channel adapter to symmetrize the STT-MRAM channel. We then present a modified protograph extrinsic information transfer (P-EXIT) algorithm for the symmetrized STT-MRAM channel. We further propose a combined guideline, including the modified P-EXIT algorithm, the asymptotic weight enumerator (AWE) analysis, as well as the actual error rate performance, for designing protograph LDPC codes with short information word lengths for STT-MRAM. By further applying a code extension approach, we design novel RCP-LDPC codes that can work with a single encoder/decoder. Simulation results show that our proposed RCP-LDPC codes outperform the well-known rate-compatible AR4JA protograph codes as well as the fixed-rate quasi-cyclic (QC) LDPC codes in terms of both the error rate performance and the convergence speed over the STT-MRAM channel.