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In this paper, we propose a new method to recognizing and distinguishing among 8-PSK modulated signal and pi/4-shifted QPSK, from the samples of received noisy and faded PSK signal. The proposed algorithm is based on computation of relative Euclidean distance and relative phase difference among the received symbols in a frame. Prior to recognition, the constant modulus (CM) equalization is performed. Based on simulation results, this method can perform recognition task with high accuracy even at low signal to noise ratio (SNR) values and employing fairly small number of data samples. To the best of our knowledge in comparison with other methods of distinguishing among QPSK variants, this algorithm do well specially in practical systems which have a limited time to make the decision and use small number of samples of receiving signal.