Carrier frequencies

In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave usually has a much higher frequency than the input signal does. 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, for example, a cable television system). The term is also used for an unmodulated emission in the absence of any modulating signal.

Most radio systems in the 20th century used frequency modulation (FM) or amplitude modulation (AM) to make the carrier carry information. The frequency of a radio or television station is actually the carrier wave's frequency. However, because the information transmitted by a radio signal is not at the carrier frequency itself but contained in sidebands on either side of the carrier, the energy of the carrier component is not useful in transmitting the information. Therefore, in many modern modulation methods the carrier is not transmitted. For example, in single-sideband modulation (SSB), the carrier is suppressed (and in some forms of SSB, eliminated). The carrier must be reintroduced at the receiver by a beat frequency oscillator (BFO).

Newer forms of radio communication (such as spread spectrum and ultra-wideband) do not use a conventional sinusoidal carrier wave, nor does OFDM (which is used in DSL and in the European standard for HDTV).

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