Coded modulation to increase storage capacity of multilevel memories

Abstract

Multilevel memory cells and analog memory cells have the capability of storing considerably more than one bit per cell. A multilevel or an analog memory cell is modelled as an additive white Gaussian noise channel. Thus storing a certain number of bits in one cell corresponds to storing a finite number of amplitude levels. Due to the additive noise in the memory cell, the bits read from the memory cell are affected by an output error probability. This paper proposes using a coded modulation technique to increase the storage capacity of multilevel and analog memory cells by one extra bit per cell. The technique can also be applied to improve the read-out bit-error probability and provide unequal error protection for the bits to be stored. One and two-dimensional coded modulation schemes as well as uncoded modulation schemes are presented in detail. Complexity issues are also considered.