Abstract: For the frequency selective fading characteristics of signals transmitted on a time-varying radio channel,the frequency diversity is an effective approach to controlling fading.Nowadays,the commonly used time-frequency-phase modulation causes the signals spectrum spread.Several methods of the frequency diversity which use constant amplitude multi-carrier are proposed,and they make the frequency diversity signals modulated to be of constant amplitude and higher spectrum efficiency.The methods can be applied to the communication system of multi-order frequency diversities.