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{{Short description|Spreading the frequency domain of a signal}}
{{Use American English|date = December 2019}}
{{Modulation techniques}}
{{Multiplex techniques}}

In [[telecommunications]], especially [[radio communication]], '''spread spectrum''' are techniques by which a [[signal]]  (e.g., an electrical, electromagnetic, or acoustic) generated with a particular [[Bandwidth (signal processing)|bandwidth]] is deliberately spread in the [[frequency domain]] over a wider [[frequency band]]. Spread-spectrum techniques are used for the establishment of secure communications, increasing resistance to natural [[Interference (communication)|interference]], [[Noise (electronics)|noise]], and [[radio jamming|jamming]], to prevent detection, to limit [[Spectral flux density|power flux density]] (e.g., in [[satellite]] [[downlink]]s), and to enable multiple-access communications.

==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]] 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.

==Invention of frequency hopping==
{{Further|Frequency-hopping spread spectrum}}

The idea of trying to protect and avoid interference in radio transmissions dates back to the beginning of radio wave signaling. In 1899, [[Guglielmo Marconi]] experimented with frequency-selective reception in an attempt to minimize interference.<ref name="kahn">{{Cite book|url=https://books.google.com/books?id=35ClAgAAQBAJ&dq=Marconi+1899&pg=PA158|title=How I Discovered World War II's Greatest Spy and Other Stories of Intelligence and Code|first=David|last=Kahn|authorlink=David Kahn (writer)|date=January 17, 2014|publisher=CRC Press|isbn=9781466561991 |accessdate=November 9, 2022|via=Google Books}}</ref> The concept of [[Frequency-hopping]] was adopted by the German radio company [[Telefunken]] and also described in part of a 1903 US patent by [[Nikola Tesla]].<ref>[https://www.americanscientist.org/article/random-paths-to-frequency-hopping Tony Rothman, Random Paths to Frequency Hopping, American Scientist, January–February 2019 Volume 107, Number 1, Page 46 americanscientist.org]</ref><ref>Jonathan Adolf Wilhelm Zenneck, Wireless Telegraphy, McGraw-Hill Book Company, Incorporated, 1915, page 331</ref> Radio pioneer [[Jonathan Zenneck]]'s 1908 German book ''Wireless Telegraphy'' describes the process and notes that [[Telefunken]] was using it previously.<ref name="kahn"/> It saw limited use by the German military in [[World War I]],<ref name="winter">Denis Winter, ''Haig's Command - A Reassessment''</ref> was put forward by [[Polish people|Polish]] engineer [[Leonard Danilewicz]] in 1929,<ref>Danilewicz later recalled: "In 1929, we proposed to the [[Polish General Staff|General Staff]] a device of my design for secret radio telegraphy which fortunately did not win acceptance, as it was a truly barbaric idea consisting in constant changes of transmitter frequency.  The commission did, however, see fit to grant me 5,000 [[zlotys]] for executing a model and as encouragement to further work." Cited in [[Władysław Kozaczuk]], ''Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II'', 1984, p. 27.</ref> showed up in a patent in the 1930s by Willem Broertjes ({{US Patent|1,869,659}} issued Aug. 2, 1932), and in the top-secret [[US Army Signal Corps]] [[World War II]] communications system named [[SIGSALY]].

During World War II, [[Golden Age of Hollywood]] actress [[Hedy Lamarr]] and avant-garde [[composer]] [[George Antheil]] developed an intended jamming-resistant radio guidance system for use in Allied [[torpedo]]es, patenting the device under {{US Patent|2,292,387}} "Secret Communications System" on August 11, 1942. Their approach was unique in that frequency coordination was done with paper player piano rolls, a novel approach which was never put into practice.<ref>Ari Ben-Menahem, Historical Encyclopedia of Natural and Mathematical Sciences, Volume 1, Springer Science & Business Media - 2009, pages 4527-4530</ref>

== Clock signal generation ==
{{more citations needed section|date=January 2020}}
[[Image:Aaronia Spectrum Analyzer Software.jpg|thumb|Spread spectrum of a modern switching power supply (heating up period) incl. waterfall diagram over a few minutes. Recorded with a NF-5030 EMC-Analyzer]]
Spread-spectrum clock generation (SSCG) is used in some [[synchronous circuit|synchronous digital systems]], especially those containing microprocessors, to reduce the spectral density of the [[electromagnetic interference]] (EMI) that these systems generate.  A synchronous digital system is one that is driven by a [[clock signal]] and, because of its periodic nature, has an unavoidably narrow frequency spectrum.  In fact, a perfect clock signal would have all its energy concentrated at a single frequency (the desired clock frequency) and its harmonics.

=== Background ===
Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the [[FCC]] in the United States, [[JEITA]] in Japan and the [[IEC]] in Europe).

Spread-spectrum clocking avoids this problem by reducing the peak radiated energy and, therefore, its electromagnetic emissions and so comply with [[electromagnetic compatibility]] (EMC) regulations. It has become a popular technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds and increasing integration of high-resolution LCD displays into ever  smaller devices. As these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. [[Active EMI reduction]] techniques such as spread-spectrum clocking are needed in these cases.

=== Method ===
In PCIe, USB 3.0, and SATA systems, the most common technique is downspreading, via [[frequency modulation]] with a lower-frequency source.<ref>{{cite web |title=Spread Spectrum Clocking |url=https://www.microsemi.com/document-portal/doc_download/135439-white-paper-spread-spectrum-clocking |website=Microsemi}}</ref> Spread-spectrum clocking, like other kinds of [[dynamic frequency change]], can also create challenges for designers. Principal among these is clock/data misalignment, or [[clock skew]]. A [[phase-locked loop]] on the receiving side needs a high enough bandwidth to correctly track a spread-spectrum clock.<ref name="IM2013">{{cite web |author1=Item Media |title=Spread Spectrum Clock Generation – Theory and Debate |url=https://interferencetechnology.com/spread-spectrum-clock-generation-theory-and-debate/ |website=Interference Technology |date=19 March 2013}}</ref>

Even though SSC compatibility is mandatory on SATA receivers,<ref>{{cite web |title=CATC SATracer / Trainer Application Note: Spread Spectrum Clocking |url=https://cdn.teledynelecroy.com/files/appnotes/satracer_ssc_appnote.pdf |website=CATC |access-date=20 May 2023 |date=July 2, 2003}}</ref> it is not uncommon to find expander chips having problems dealing with such a clock. Consequently, an ability to disable spread-spectrum clocking in computer systems is considered useful.<ref>[http://www.thomas-krenn.com/de/wiki/Raid_Controller_erkennt_Western_Digital_Raid_Edition_III_HDDs_nicht Western Digital Raid Edition III HDDs werden vom RAID Controller nicht erkannt] (Thomas Krenn Wiki)</ref><ref>[http://www.intel.com/cd/channel/reseller/emea/deu/support/lcs/faq/334571.htm Intel Speichersystem SS4000-E: Festplatten, wie beispielsweise die Western Digital WD2500JS SATA, werden nicht erkannt. Woran liegt das?] (Intel Reseller-Center)</ref><ref>{{Webarchive |url=https://web.archive.org/web/20100429194819/http://seagate.custkb.com/seagate/crm/selfservice/search.jsp?DocId=214011&NewLang=en |title=SSC Toggle Utility – Barracuda 7200.9 |date=2010-04-29}} (Seagate Knowledge Base)</ref>

=== Effect ===
Note that this method does not reduce total [[radiation|radiated]] energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths. Typical [[measuring receiver]]s used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120&nbsp;kHz wide.<ref>American National Standard for Electromagnetic Noise and Field Strength Instrumentation, 10 Hz to 40 GHz—Specifications, ANSI C63.2-1996, Section 8.2 Overall Bandwidth</ref> If the system under test were to radiate all its energy in a narrow bandwidth, it would register a large peak. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits. The usefulness of this method as a means to reduce real-life interference problems is often debated,<ref name=IM2013/> as it is perceived that spread-spectrum clocking hides rather than resolves higher radiated energy issues by simple exploitation of loopholes in EMC legislation or certification procedures. This situation results in electronic equipment sensitive to narrow bandwidth(s) experiencing much less interference, while those with broadband sensitivity, or even operated at other higher frequencies (such as a radio receiver tuned to a different station), will experience more interference.

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits.  However, the spread-spectrum functionality may be disabled by the user in some cases. As an example, in the area of personal computers, some [[BIOS]] writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This might be considered a [[loophole]], but is generally overlooked as long as spread-spectrum is enabled by default.

== See also ==
*[[Direct-sequence spread spectrum]]
*[[Electromagnetic compatibility]] (EMC)
*[[Electromagnetic interference]] (EMI)
*[[Frequency allocation]]
*[[Frequency-hopping spread spectrum]]
*[[George Antheil]]<!--co-inventor -->
*[[HAVE QUICK]] military frequency-hopping UHF radio voice communication system
*[[Hedy Lamarr]]<!--co-inventor -->
*[[Open spectrum]]
*[[Orthogonal variable spreading factor]] (OVSF)
*[[Spread-spectrum time-domain reflectometry]]
*[[Time-hopping spread spectrum]]
*[[Ultra-wideband]]

== Notes ==
{{Reflist}}

== Sources ==
*{{FS1037C MS188}}
*[[NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management]]
*[[National Information Systems Security Glossary]]
*History on spread spectrum, as given in  "Smart Mobs, The Next Social Revolution", [[Howard Rheingold]], {{ISBN|0-7382-0608-3}}
*[[Władysław Kozaczuk]], ''Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two'', edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984, {{ISBN|0-89093-547-5}}.
* Andrew S. Tanenbaum and David J. Wetherall, ''Computer Networks'', Fifth Edition.

==External links==
* [https://www.eetimes.com/a-short-history-of-spread-spectrum/ A short history of spread spectrum]
* [http://www.telecomspace.com/cdma.html CDMA and spread spectrum] {{Webarchive|url=https://web.archive.org/web/20090416204621/http://www.telecomspace.com/cdma.html |date=2009-04-16 }}
* [http://sss-mag.com/index.html Spread Spectrum Scene newsletter]

{{Telecommunications}}
{{Authority control}}

{{DEFAULTSORT:Spread Spectrum}}
[[Category:Channel access methods]]
[[Category:Multiplexing]]
[[Category:Radio resource management]]
[[Category:Radio modulation modes]]
[[Category:Spectrum (physical sciences)]]