Histogram-Based Flash Channel Estimation

Abstract

Current generation Flash devices experience significant read-channel degradation from damage to the oxide layer during program and erase operations. Information about the read-channel degradation drives advanced signal processing methods in Flash to mitigate its effect. In this context, channel estimation must be ongoing since channel degradation evolves over time and is a function of the number of program/erase (P/E) cycles. This paper proposes a framework for on-line model-based channel estimation using limited channel measurements (reads). This paper uses a channel model characterizing degradation as a function of retention time and the amount of charge programmed and erased. For channel histogram measurements, equal-probability (equal-height) bin placement yields a good approximation to the original distribution using only ten bins (i.e. nine reads). With the channel model and binning strategy in place, this paper explores candidate numerical least squares channel estimation algorithms and ultimately demonstrates the effectiveness of the Levenberg-Marquardt algorithm, which provides both speed and accuracy, in an algorithm for voltage allocation.