A waveform channel is a channel whose inputs are continuous functions of time. A passband channel is a waveform channel suitable for an input waveform that has a spectrum confined to an appropriately narrow interval of frequencies centered about a nonzero reference frequency, f0. A complex baseband channel is a waveform channel whose input waveform is a complex function of time that has a spectrum confined to an interval of frequencies containing the zero frequency. We shall see that every passband channel can be converted to or from a complex baseband channel by using standard techniques in the modulator and demodulator.

The function of a digital modulator for a passband channel is to convert a digital datastream into a waveform representation of the data that can be accepted by the passband channel. The waveform from the modulator is designed to accommodate the spectral characteristics of the channel, to obtain high rates of data transmission, to minimize transmitted power, and to keep the bit error rate small.

A passband modulation waveform cannot be judged independently of the performance of the demodulator. To understand how a modem works, it is necessary to study both the passband modulation techniques of this chapter and the passband demodulation techniques of Chapter 6. The final test of a modem is in the ability of the demodulator to recover the input datastream from the signal received by the demodulator in the presence of noise, interference, distortion, and other impairments.