Two-Dimensional Differential Demodulation for 64-DAPSK Modulated OFDM Signals

A novel two-dimensional differential demodulation (2-D DD) algorithm, which exploits both the time and frequency dimensions by using the orthogonal frequency division multiplexing (OFDM) frame structure, was investigated for 64-differential amplitude and phase shift keying (DAPSK) modulated OFDM signals. Without changing the transmitter, the proposed algorithm based on graph attempts to search for a more confident bypass, probably containing multiple DD operations, to demodulate one 64-DAPSK symbol rather than conventional direct DD. In order to carry out the optimal path searching process, the soft decision metrics of the amplitude ratio and phase difference were derived by making use of a Gaussian approximation. Simulation results show that the proposed algorithm can obtain substantial performance gains with reasonable computational complexity for uncoded AWGN and frequency selective fading channels. In addition, it is advantageous to withstand the Doppler effect, which is hence suitable for high mobile (e.g., in a car or train) broadband wireless access (BWA) systems.