The carrier wave is a sinusoidal wave that is modulated by a message signal, m(t), prior to transmission.

The message signal modifies the carrier wave amplitude, frequency, or phase. The modulation process may vary these characteristics individually or in combination. The modified carrier signal, also referred to as the modulated wave, is transmitted to a receiver.

The process of demodulation recovers a replica of the original message signal from the carrier wave. In advanced communication systems, the carrier may be a moving signal, also known as a spread spectrum signal. When the characteristics of the carrier signal are deterministic and known by the receiver, virtually any type of carrier signal can be used.

The nominal frequency of the carrier wave is the carrier frequency. The carrier frequency is the center frequency of the spread spectrum signal.