With frequency offset being taken into account, the performance of an existing coherent M-ary differential phase shift keying (MDPSK) demodulator, which bases on multiple symbols observation, is analyzed in this paper. It is revealed that the frequency offset will induce phase offset in proportion to the frequency offset in recovered carrier, and degrade the performance of demodulation. Aiming at this, a simple modified scheme is proposed to compensate for the phase offset; and theoretical performance analysis and simulation are also provided to show good agreement. Both theoretical and simulation results show that the modified scheme is superior to the original one under frequency offset.
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