Editing Spread spectrum (section)

==Telecommunications==
Spread spectrum generally makes use of a sequential [[noise]]-like signal structure to spread the normally [[narrowband]] information signal over a relatively [[wideband]] (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called [[low probability of intercept]] (LPI).<ref name="ref 1">{{cite book| title=Principles of Spread-Spectrum Communication Systems, 4th ed.| year=2018|last1=Torrieri|first1=Don}}</ref>

[[Frequency-hopping spread spectrum]] (FHSS), [[direct-sequence spread spectrum]] (DSSS), [[time-hopping spread spectrum]] (THSS), [[chirp spread spectrum]] (CSS), and combinations of these techniques are forms of spread spectrum. The first two of these techniques employ pseudorandom number sequences&mdash;created using [[pseudorandom number generator]]s&mdash;to determine and control the spreading pattern of the signal across the allocated bandwidth. Wireless standard [[IEEE 802.11]] uses either FHSS or DSSS in its radio interface.

* Techniques known since the 1940s and used in military communication systems since the 1950s "spread" a radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate.
* Resistance to [[radio jamming|jamming]] (interference). Direct sequence (DS) is good at resisting continuous-time narrowband jamming, while frequency hopping (FH) is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, where it will often not be much stronger than background noise.  By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.
* Resistance to [[eavesdropping]].  The spreading sequence (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it obscures the signal and reduces the chance of an adversary making sense of it. Moreover, for a given noise [[power spectral density]] (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower &mdash; often significantly lower than the noise PSD &mdash; so that the adversary may be unable to determine whether the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not provide adequate security unless, at a minimum, long nonlinear spreading sequences are used and the messages are encrypted. 
* Resistance to [[fading]].  The high bandwidth occupied by spread-spectrum signals offer some frequency diversity; i.e., it is unlikely that the signal will encounter severe [[Multipath propagation|multipath]] fading over its whole bandwidth. In direct-sequence systems, the signal can be detected by using a [[rake receiver]].
* Multiple access capability, known as [[code-division multiple access]] (CDMA) or code-division multiplexing (CDM).  Multiple users can transmit simultaneously in the same frequency band as long as they use different spreading sequences.