Method of reducing the variance of a signal-to-noise ration estimate for a signal with differential phase and coherent amplitude modulation

Characterized in that in order to calculate the samples of the noise power affecting the system, the errors of amplitude and phase increment obtained in the receiver are combined carrying out a translation of those errors ( 1 ) and ( 4 ) to the corresponding point of the first ring ( 6 ) of the constellation as if this had been the one that was transmitted and without those errors undergoing any modification, in order later on to calculate the modulus squared of the error vector ( 9 ). It minimizes and equalizes the variance of the samples obtained for the different values, even and odd, of bits per carrier which the system uses. It reduces the variance of the estimation of the signal to noise ratio in a multiuser digital communications system. Said estimation is necessary for selecting the number of bits per carrier in order to be adapted to the channel and as back-up information in the event of using a receiver with diversity.