Abstract: A precoding technique is proposed for superposition coded modulation(SCM) single-carrier systems over inter-symbol interference(ISI) channels.The precoder can be efficiently implemented in the frequency domain and is introduced to shape the spectrum of transmitted signals.When the receiver implements frequency domain equalization,the precoder does not incur extra cost to the equalizer.It is proved in information theory that the ideally coded SCM system with the precoder implemented based on the "water-filling" principle can asymptotically achieve the ISI channel capacity.For practically coded SCM systems,a joint optimization problem is introduced for the design of both the precoder and the power allocation of SCM.Simulation results based on convolutional codes show that the joint optimization can considerably improve the system performance.