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SUMMARYAn angle differential quadrature amplitude modulation (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, we 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 compared with the coherent 16-QAM system under AWGN channel. Generalization of flat fading channel and higher-level ADQAM is straightforward.

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