Level-estimation in multi-level cell memory

An apparatus for determining reference signal levels corresponding to respective levels of q-level memory cells, the cells storing respective symbols, each of which has one of q symbol values, of N-symbol codewords of a predefined codeword set, where q>2, the apparatus comprising:

• a memory controller for reading the memory cells storing a group of codewords to obtain respective read signals each comprising N signal components corresponding to respective symbols of a codeword;
• and
• a level estimator comprising control logic adapted to:
• order the components of each read signal according to signal level to produce an ordered read signal;
• order correspondingly-positioned components of the ordered read signals according to signal level to produce ordered component sets for respective component positions in a said ordered read signal;
• partition each ordered component set into subsets corresponding to respective memory cell levels, wherein the subsets of the ordered component sets contain respective numbers of components dependent on predefined probabilities of occurrence of different symbol values at different positions in a said codeword, wherein symbols of said codeword are ordered according to symbol value;
• and
• determine the reference signal level corresponding to a respective memory cell level in dependence on the signal components in the subsets corresponding to that memory cell level.