Modulation Techniques
Carrier frequency

A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel.

Carrier waves solve many other circuit, antenna, propagation, and noise problems. For example, a practical antenna must be approximately the size of one wavelength of the EM wave to be transmitted. If sound waves were broadcast in audible frequencies, the antenna would have to be more than one kilometer in height. The size requirement of the antenna can be significantly reduced by using higher frequencies which have shorter wavelengths.

An FM radio station typically has call letters associated with it, such as KPBS. However, a more practical way to think about a radio station is its carrier frequency such as 101.1 MHz, which is what the student tunes the radio to. For WLANs, the carrier frequency is 2.4 GHz or 5 GHz. Using carrier frequencies in WLANs has an added complexity, in that the carrier frequency is changed by frequency hopping or direct sequence chipping, to make the signal more immune to interference and noise.

The process of recovering the information from the carrier waves is called demodulation. It is essentially a reversal of the steps used to modulate the data. In general, as transmission or compressed modulation schemes become more complex and data rate goes up, immunity to noise decreases, and coverage goes down.