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It is possible to recognize the modulation type of an unknown signal in noise by its envelope characteristics. A quantity found to be distinctive to a given modulation type is the ratio (R) of the variance of the envelope to the square of the mean of the envelope. Equations of R for four modulation types, as a function of the carrier to noise ratio, are derived and used to set up a classification scheme. The R of a received signal is first computed. Then its modulation type is determined according to the range R falls in. Simulation experiments have confirmed some of the theoretical development as well as the effectiveness of the identification scheme.