In telecommunications, a carrier signal, carrier wave, 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 is usually a much higher frequency than the input signal. 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. Frequency modulation (FM) and amplitude modulation (AM) are common modes of modulating the carrier. In the case of 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). The frequency of a radio or television station is actually the carrier wave’s center frequency.