A multibit-per-cell memory model and nonbinary LDPC codes

Protecting nonvolatile memory systems in harsh radiation environments encountered in space missions is important and error correcting schemes can extend the lifetime of those memory systems. For example, recent research has shown that LDPC codes can extend the lifetime of nonvolatile memory under space radiation environment more than Bose-Chaudhuri-Hocquenghem (BCH) or Reed-Solomon (RS) codes at fixed codeword error rates. However, conventional memory models assume that bit errors are independent, but multibit errors were reported in satellite experiments. Moreover, memory feature sizes are shrinking and multibit-per-cell structures are becoming standard so radiation will increasingly lead to multibit errors. For these reasons, we can expect that the bit errors in memory systems will be correlated. In this work, we develop a mathematical multibit-per-cell memory model under a radiation environment. In this memory model, bit errors are correlated and the probability of errors depends on radiation parameters and time. For correlated bit errors, nonbinary codes can be more effective than binary codes. We will demonstrate that nonbinary LDPC code can outperform conventional BCH and RS codes in a correlated multibit error environment.