Differential Encoding for Quadrature-Amplitude Modulation

In this paper, the authors propose a novel differential encoder for QAM (Quadrature-Amplitude Modulation), by a look-up table instead of a rule. Using a table for differential encoding has the advantages of both low complexity and good error performance. They propose algorithms to construct the table for maximizing the minimum non-coherent distance and minimizing the number of different data bits between two near codewords. For 16QAM, they show that the differential encoder obtained by the proposed algorithms is optimal in terms of minimum non-coherent distance.