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.