It is characterized in that the information to be modulated in the phase difference, that belongs to the set of information possible according to the number of phases of the constellation to be used, an injective application between this set and the set of information possible to be modulated corresponding to the constellation with the maximum number of phases is carried out. This permits bits to be changed by a carrier without sending a new phase reference, to maximize the number of symbols of information within the frame and to implement a modulator and a low complexity modulator and demodulator. Applicable in multiuser digital communication that requires bits to be changed by the carrier of the constellation in one single data frame, in order to adapt to the maximum transmission capacity of the channel, to follow the amplitude variations thereof and to allow information to be transmitted from one to several users. (FIG. 1)