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The average bit error rate (BER) performances of 16 square/star QAM and 16 CPSK/DPSK under Rayleigh fading are theoretically analysed and compared. 16 star QAM is inferior to 16 square QAM, which achieves the best performance, by ∼4 dB in the required average Eb/N0; however, when compared with differentially encoded square QAM, the performance inferiority is less than 2 dB.
Adachi et al. (Thu,) studied this question.