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An Angle Differential-QAM (ADQAM) scheme is proposed to solve phase ambiguity problem in non-data-aided continuous transmission system with square QAM constellation. Starting from the 16-ADQAM case, the authors derive differential encoding and decoding schemes in terms of two differential angles, and use a solar system analogy for explanation. The 16-ADQAM system incurs only about 0.5-dB performance degradation as compared to the coherent 16-QAM system under AWGN channel. Generalization to flat fading channel and higher-level ADQAM are straightforward.
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