Carrier wave

The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. It consists of a strong 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.

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).

Carrierless modulation systems

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).

• OFDM may be thought of as an array of symmetrical carrier waves. The rules governing carrier-wave propagation affect OFDM differently from 8VSB.
• Some forms of spread spectrum transmission (and most forms of ultra-wideband) are mathematically defined as being devoid of carrier waves. Transmitter implementations typically produce residual carriers which may (or may not) be detectable or transmitted.

Carrier leakage

Carrier leakage is interference caused by cross-talk or a DC offset. It is present as an unmodulated sine wave within the signal's bandwidth, whose amplitude is independent of the signal's amplitude. See frequency mixers, to read further about carrier leakage or local oscillator feedthrough.

See also

• Carrier recovery
• Carrier system
• Carrier tone
• Frequency-division multiplexing
• Sideband