In-Memory Bit Error Rate Estimation Using Syndromes of LDPC Codes

Abstract

Modern AI systems entail steep energy costs due to massive-scale computations and data transfers; offloading parts of the computations to be performed in-memory holds great potential for reducing both. This paper studies a new architecture proposed for reliable in-memory computations. Its main component is a coding scheme that is designed for both in-memory error-rate estimation/detection and outside-of-memory error correction. Estimation and/or detection are used to decide when the error rate exceeds the tolerance of the computation, at which point error correction is invoked. The coding scheme is based on a nested bilayer LDPC construction, where in particular, the first layer comprises degree-1 variable nodes guaranteeing accurate bit-error rate (BER) estimation and detection. Towards that, we derive a closed-form maximum-likelihood BER estimator for irregular codes, and a gapped hypothesis testing framework for deciding when to decode given some prescribed error-rate tolerance. The performance analysis of the derived estimator includes a closed-form mean-squared-error expression with explicit dependence on the check-degree distribution. For the hypothesis testing the analysis shows the dependence of detection performance on the same degree distribution. Both results reveal an advantage of check-regular codes that minimize dominant error terms among codes with a given average check degree.