| 摘要 |
Methods (figures 2,3) and apparatus (figure 1) are provided for detecting N-symbol codewords each being a permutation of one of a predefined plurality of N-symbol initial vectors whose symbols are ordered according to symbol value. The symbols of each codeword, each of which has one of q symbol values, are stored in respective q-level memory cells where q>2. The memory cells storing each codeword are read to obtain a read signal (10 figure 2) comprising N signal components corresponding to respective symbols of the codeword. The memory cell level which most-closely corresponds to each read signal component is then detected according to a first correspondence criterion dependent on reference signal levels for the q cell levels (12,13 figure 2). Unreliable read signal components are identified in dependence on a reliability indicator for each component 22. The reliability indicator is dependent on proximity of the component to the reference signal level for the most-closely corresponding memory cell level 22. For each unreliable read signal component, the next-most-closely corresponding memory cell level according to the first correspondence criterion is additionally detected 23. For each read signal, a set of ordered codeword vectors is defined. Each ordered codeword vector has N symbols corresponding to respective components of the read signal ordered according to signal level. The symbol values in each ordered codeword vector correspond to one possible combination of the detected memory cell levels for the read signal components. A set of candidate initial vectors 25 for a read signal comprises the intersection of the set of ordered codeword vectors for that read signal and said predefined plurality of initial vectors. If the set of candidate initial vectors for a read signal contains at least one vector, the codeword corresponding to that read signal is detected in dependence on the set of candidate initial vectors 26. |
