Method for sending information signals over a communication channel, and corresponding encoder

Pulse amplitude modulation scheme with the same spectral efficiency as QAM, that is twice the spectral efficiency of conventional PAM schemes, comprising the steps of: (1) forming a train of pulses q(t -kT/2) = p(t - k T/2)cos2 pi fo(t- k T/2), wherein p(t) is a Nyquist pulse for the symbol interval T and wherein f0 = 1/T (l+1/2) for some positive integer l, (2) modulating the amplitude of each of said pulses q(t - k T/2) with the value of the symbol ak in said stream, (3) sending said train of amplitude modulated pulses over said communication channel.