Carrier wave

The purpose of the carrier is usually either to transmit the information through space as an electromagnetic wave (as in radio communication), or to allow several carriers at different frequencies to share a common physical transmission medium by frequency division multiplexing (as in a cable television system).

At the sending end, the information, in the form of a modulation signal, is applied to an electronic device called a transmitter.

In the transmitter, an electronic oscillator generates a sinusoidal alternating current of radio frequency; this is the carrier wave.

The alternating current is amplified and applied to the transmitter's antenna, radiating radio waves that carry the information to the receiver's location.

The frequency spectrum of a modulated AM or FM signal from a radio transmitter is shown above.

Carriers are also widely used to transmit multiple information channels through a single cable or other communication medium using the technique of frequency division multiplexing (FDM).

The information in a modulated radio signal is contained in the sidebands while the power in the carrier frequency component does not transmit information itself, so newer forms of radio communication (such as spread spectrum and ultra-wideband), and OFDM which is widely used in Wi-Fi networks, digital television, and digital audio broadcasting (DAB) do not use a conventional sinusoidal carrier wave.

The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a signal (C) at the carrier wave frequency f C , with the modulation contained in narrow frequency bands called sidebands (SB) just above and below the carrier. The entire signal range is the bandwidth (BW).