  The huge amount of data produced in the fifth-generation (5G) networks not
only brings new challenges to the reliability and efficiency of mobile devices
but also drives rapid development of new storage techniques. With the benefits
of fast access speed and high reliability, NAND flash memory has become a
promising storage solution for the 5G networks. In this paper, we investigate a
protograph-coded bit-interleaved coded modulation with iterative detection and
decoding (BICM-ID) utilizing irregular mapping (IM) in the multi-level-cell
(MLC) NAND flash-memory systems. First, we propose an enhanced protograph-based
extrinsic information transfer (EPEXIT) algorithm to facilitate the analysis of
protograph codes in the IM-BICM-ID systems. With the use of EPEXIT algorithm, a
simple design method is conceived for the construction of a family of high-rate
protograph codes, called irregular-mapped accumulate-repeat-accumulate (IMARA)
codes, which possess both excellent decoding thresholds and
linear-minimum-distance-growth property. Furthermore, motivated by the
voltage-region iterative gain characteristics of IM-BICM-ID systems, a novel
read-voltage optimization scheme is developed to acquire accurate read-voltage
levels, thus minimizing the decoding thresholds of protograph codes.
Theoretical analyses and error-rate simulations indicate that the proposed
IMARA-aided IM-BICM-ID scheme and the proposed read-voltage optimization scheme
remarkably improve the convergence and decoding performance of flash-memory
systems. Thus, the proposed protograph-coded IM-BICM-ID flash-memory systems
can be viewed as a reliable and efficient storage solution for the
new-generation mobile networks with massive data-storage requirement.
