These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit the power flux density on satellite downlinks.
There are three popular methods of spread spectrum:
- Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
- Direct Sequence Spread Spectrum (DSSS) -- The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. 802.11b, 11.g, and 11.n use DSSS.
- time-hopping spread spectrum (THSS) is a type of spread spectrum technology in which the carrier is turned on and off by the pseudorandom code sequence.
- Chirp spread spectrum (CSS) is a spread spectrum technique that uses wideband linear frequency modulated chirp pulses to encode information. A chirp is a sinusoidal signal whose frequency increases or decreases over a certain amount of time.
- Orthogonal Frequency Division Multiplexing (OFDM) splits the signal and sends the signal fragments over different frequencies at the same time. 802.11a uses OFDM.