Abstract: A modulation-assisted preprocessing scheme for non-binary LDPC reduces the computational complexity of non-binary LDPC decoding. A received signal is directly mapped onto a truncated symbol vector through a table lookup. The mapping table is built off-line based on the characteristics of a constellation structure. The receiver calculates and, if necessary, ranks the reliability of the symbols in the truncated vectors. This avoids unnecessary reliability calculation and ranking during on-line operation. The disclosed preprocessing methods offer lower computational complexity in terms of ranking and addition operations while offering the same error rate performance as conventional EMS preprocessing.