Exploiting Binary Equilibrium for Efficient LDPC Decoding in 3D NAND Flash

Abstract

3D NAND flash is prone to bit errors due to severe charge leakage. Modern SSDs adopt LDPC for bit error management, but LDPC can incur a high read latency through iterative adjustment to the reference voltage. Bit scrambling helps reduce inter-cell interference, and with it, ones and zeros equally contribute to raw data. We observed that as bit errors develop, the 0-bit ratio in raw data deviates from 50%. Inspired by this property, we propose a method for fast adjustment to the reference voltage, involving a placement step and a fine-tuning step. Our method uses only a few hundreds of bytes of RAM but improves the average read latency upon existing methods by up to 24%.