Editing Frequency-hopping spread spectrum

{{Short description|Radio signal transmission method}}
{{Redirect|FHSS}}
{{Use American English|date = December 2019}}
{{Modulation techniques}}
{{Multiplex techniques}}
'''Frequency-hopping spread spectrum''' ('''FHSS''') is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. The changes are controlled by a code known to both [[transmitter]] and [[receiver (radio)|receiver]]. FHSS is used to avoid interference, to prevent eavesdropping, and to enable [[code-division multiple access]] (CDMA) communications.

The frequency band is divided into smaller sub-bands. Signals rapidly change ("hop") their carrier frequencies among the center frequencies of these sub-bands in a determined order. Interference at a specific frequency will affect the signal only during a short interval.<ref name="ref 1">{{cite book| title=Principles of Spread-Spectrum Communication Systems, 4th ed.| year=2018|last1=Torrieri|first1=Don}}</ref>

FHSS offers four main advantages over a fixed-frequency transmission:

# FHSS signals are highly resistant to [[narrowband]] [[Interference (communication)|interference]] because the signal hops to a different frequency band.
# Signals are difficult to intercept if the frequency-hopping pattern is not known.
# Jamming is also difficult if the pattern is unknown; the signal can be jammed only for a single hopping period if the spreading sequence is unknown.
# FHSS transmissions can share a frequency band with many types of conventional transmissions with minimal mutual interference. FHSS signals add minimal interference to narrowband communications, and vice versa.

== Usage ==

=== Military ===
[[Spread spectrum|Spread-spectrum]] signals are highly resistant to deliberate [[radio jamming|jamming]] unless the adversary has knowledge of the frequency-hopping pattern. Military radios generate the frequency-hopping pattern under the control of a secret [[TRANSEC|Transmission Security Key]] (TRANSEC) that the sender and receiver share in advance. This key is generated by devices such as the KY-57 Speech Security Equipment. United States military radios that use frequency hopping include the JTIDS/MIDS family, the [[HAVE QUICK]] Aeronautical Mobile communications system, and the [[SINCGARS]] Combat Net Radio, [[Link-16]].

=== Civilian ===
In the US, since the [[Federal Communications Commission]] (FCC) amended rules to allow FHSS systems in the unregulated 2.4&nbsp;GHz band, many consumer devices in that band have employed various FHSS modes. eFCC CFR 47 part 15.247 covers the regulations in the US for  902–928&nbsp;MHz, 2400–2483.5&nbsp;MHz, and 5725–5850&nbsp;MHz bands, and the requirements for frequency hopping.<ref>{{cite web |title=47 CFR § 15.247 - Operation within the bands 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz. |url=https://www.law.cornell.edu/cfr/text/47/15.247 |website=LII / Legal Information Institute |publisher=law.cornell.edu |access-date=17 December 2019 |language=en}}</ref>

Some [[walkie-talkies]] that employ FHSS technology have been developed for unlicensed use on the 900&nbsp;MHz band. FHSS technology is also used in many hobby transmitters and receivers used for [[radio-controlled model]] cars, airplanes, and drones. A type of multiple access is achieved allowing hundreds of transmitter/receiver pairs to be operated simultaneously on the same band, in contrast to previous FM or AM radio-controlled systems that had limited simultaneous channels.

==Technical considerations==
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one [[carrier frequency]]. But because transmission occurs only on a small portion of this bandwidth at any given time, the instantaneous interference bandwidth is really the same. While providing no extra protection against wideband [[thermal noise]], the frequency-hopping approach reduces the degradation caused by narrowband interference sources.

One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. The transmitter's data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel, and the segment can also have a [[checksum]] for integrity checking and further identification. The transmitter and receiver can use fixed tables of frequency-hopping patterns, so that once synchronized they can maintain communication by following the table.

In the US, [[Title 47 CFR Part 15|FCC part 15]] on unlicensed spread spectrum systems in the 902–928&nbsp;MHz and 2.4&nbsp;GHz bands permits more power than is allowed for non-spread-spectrum systems. Both FHSS and direct-sequence spread-spectrum (DSSS) systems can transmit at 1 watt, a thousandfold increase from the 1 milliwatt limit on non-spread-spectrum systems. The FCC also prescribes a minimum number of frequency channels and a maximum dwell time for each channel.

==Origins==
In 1899, [[Guglielmo Marconi]] experimented with frequency-selective reception in an attempt to minimise interference.<ref>{{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|page=158|publisher=[[Auerbach Publications]]|date=2014|isbn=9781466561991}}</ref>

The earliest mentions of frequency hopping in open literature are in [https://patents.google.com/patent/US725605 US patent 725,605], awarded to [[Nikola Tesla]] on March 17, 1903,<ref name="American Scientist">{{cite magazine|url=https://www.americanscientist.org/article/random-paths-to-frequency-hopping|title=Random Paths to Frequency Hopping|magazine=[[American Scientist]]|author = Rothman, Tony|date=Jan–Feb 2019|volume=107|issue=1|page=46|doi=10.1511/2019.107.1.46|access-date=27 March 2024}}</ref> and in radio pioneer [[Jonathan Zenneck]]'s book ''Wireless Telegraphy'' (German, 1908, English translation McGraw Hill, 1915),<ref name="Zenneck1915">{{cite book |last= Zenneck |title= Wireless Telegraphy |first= Jonathan |date= August 1915 |orig-date=1908 |translator-last1= Seelig |translator-first1= Alfred |publisher= McGraw-Hill |location= New York |edition = 5th |chapter = Receivers |page= 331 }}</ref>{{efn |name="Zennek1915" |"Furthermore the apparatus can be so arranged that the wave-length is easily and rapidly changed and then vary the wave-length in accordance with a prearranged program, perhaps automatically. (This method was adopted by the Telefunken Co. at one time.)" Zenneck describes additional methods of security, including synchronizing receiving to only a subset of transmission.}} although Zenneck writes that [[Telefunken]] had already tried it. Nikola Tesla doesn't mention the phrase "frequency hopping" directly, but certainly alludes to it. Entitled ''Method of Signaling'', the patent describes a system that would enable radio communication ''without any danger of the signals or messages being disturbed, intercepted, interfered with in any way''.<ref name="eetimes">{{cite web| url = https://www.eetimes.com/a-short-history-of-spread-spectrum/| website = [[EE Times]]| title = A short history of spread spectrum| date = January 26, 2012}}</ref>

The German military made limited use of frequency hopping for communication between fixed command points in [[World War I]] to prevent eavesdropping by British forces, who did not have the technology to follow the sequence.<ref name="winter">{{cite book |author=Denis Winter |title=Haig's Command - A Reassessment}}</ref> Jonathan Zenneck's book ''Wireless Telegraphy'' was originally published in German in 1908, but was translated into English in 1915 as the enemy started using frequency hopping on the front line.

In 1920, Otto B. Blackwell, De Loss K. Martin, and Gilbert S. Vernam filed a patent application for a "Secrecy Communication System", granted as [https://patents.google.com/patent/US1598673A U.S. Patent 1,598,673] in 1926. This patent described a method of transmitting signals on multiple frequencies in a random manner for secrecy, anticipating key features of later frequency hopping systems.<ref name="American Scientist" />

A [[Polish people|Polish]] engineer and inventor, [[Leonard Danilewicz]], claimed to have suggested the concept of frequency hopping in 1929 to the [[Polish General Staff]], but it was rejected.<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 [[Polish zloty|zloty]]s for executing a model and as encouragement to further work." Cited in {{cite book|author=Władysław Kozaczuk |author-link=Władysław Kozaczuk |title=Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II |date=1984 |page=27}}</ref>

In 1932, {{US Patent|1,869,659}} was awarded to Willem Broertjes, named "Method of maintaining secrecy in the transmission of wireless telegraphic messages", which describes a system where "messages are transmitted by means of a group of frequencies... known to the sender and receiver alone, and alternated at will during transmission of the messages".

During [[World War II]], the [[US Army Signal Corps]] was inventing a communication system called [[SIGSALY]], which incorporated spread spectrum in a single frequency context. But SIGSALY was a top-secret communications system, so its existence was not known until the 1980s.

In 1942, actress [[Hedy Lamarr]] and composer [[George Antheil]] received {{US patent|2,292,387}} for their "Secret Communications System",<ref name="apt-news">{{cite magazine|url=https://www.aps.org/publications/apsnews/201106/physicshistory.cfm|title=June 1941: Hedy Lamarr and George Antheil submit patent for radio frequency hopping|magazine=[[APS News]]|date=June 2011|volume=20|issue=6}}</ref><ref>{{cite web|url=https://www.scientificamerican.com/article/hedy-lamarr-not-just-a-pr/|title=Hedy Lamarr: Not just a pretty face|first=Melinda|last=Wenner|date=June 3, 2008|website=[[Scientific American]]}}</ref> an early version of frequency hopping using a [[Piano roll|piano-roll]] to switch among 88 frequencies to make radio-guided [[torpedo]]es harder for enemies to detect or jam. They then donated the patent to the [[United States Navy|U.S. Navy]].<ref>{{cite web |last1=Iancu |first1=Andrei |author1-link = Andrei Iancu|title=Remarks by Director Andrei Iancu at 2018 Military Invention Day |url=https://www.uspto.gov/about-us/news-updates/remarks-director-andrei-iancu-2018-military-invention-day |website=[[United States Patent and Trademark Office]] |date=19 May 2018 |publisher=[[United States Department of Commerce]] |access-date=25 February 2024}}</ref>

Frequency-hopping ideas may have been rediscovered in the 1950s during patent searches when private companies were independently developing direct-sequence [[Code Division Multiple Access]], a non-frequency-hopping form of spread-spectrum.{{citation needed|date=July 2023}} In 1957, engineers at Sylvania Electronic Systems Division adopted a similar idea, using the recently invented transistor instead of Lamarr's and Antheil's clockwork technology.<ref name="apt-news" />{{dubious|date=October 2022}} In 1962, the US Navy utilized Sylvania Electronic Systems Division's work during the [[Cuban Missile Crisis]].<ref>{{cite magazine|url=https://physicsworld.com/a/a-tale-of-two-lives/|title=A tale of two lives|first=Tushna|last=Commissariat|date=August 1, 2018|magazine=[[Physics World]]}}</ref>

A practical application of frequency hopping was developed by [[Ray Zinn]], co-founder of Micrel Corporation. Zinn developed a method allowing radio devices to operate without the need to synchronize a receiver with a transmitter. Using frequency hopping and sweep modes, Zinn's method is primarily applied in low data rate wireless applications such as utility metering, machine and equipment monitoring and metering, and remote control. In 2006 Zinn received {{US patent| 6,996,399}} for his "Wireless device and method using frequency hopping and sweep modes."

== Variations ==
'''Adaptive frequency-hopping spread spectrum''' ('''AFH''') as used in [[Bluetooth]] improves resistance to [[co-channel interference|radio frequency interference]] by avoiding crowded frequencies in the hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with [[DSSS]].

The key idea behind AFH is to use only the "good" frequencies and avoid the "bad" ones—those experiencing [[frequency selective fading]], those on which a third party is trying to communicate, or those being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good and bad channels.

But if the radio frequency interference is itself dynamic, then AFH's strategy of "bad channel removal" may not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth [[Piconet]]), they are mutually interfering and AFH's strategy fails to avoid this interference.

The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibility with legacy Bluetooth devices was resolved in version 1.2 of the Bluetooth Standard (2003). Such a situation can often happen in the scenarios that use [[unlicensed spectrum]].

In addition, dynamic radio frequency interference is expected to occur in the scenarios related to [[cognitive radio]], where the networks and the devices should exhibit [[frequency-agile]] operation.

[[Chirp#Chirp modulation|Chirp modulation]] can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order to communicate.

Frequency hopping can be superimposed on other modulations or waveforms to enhance the system performance.

==See also==
* [[Dynamic frequency hopping]]
* [[List of multiple discoveries#20th century|List of multiple discoveries]]
* [[Maximum length sequence]]
* [[Orthogonal frequency-division multiplexing]]
* [[Radio-frequency sweep]]

==Notes==
{{Notelist}}

==References==
{{Reflist|30em}}

==Bibliography==
*{{cite journal |author=Tony Rothman |author-link=Tony Rothman |url=https://www.americanscientist.org/article/random-paths-to-frequency-hopping |title=Random Paths to Frequency Hopping |journal=[[American Scientist]] |volume=107 |issue=1 |date=Jan–Feb 2019 |page=46 |doi=10.1511/2019.107.1.46|url-access=subscription }}
*{{cite book |author=Władysław Kozaczuk |author-link=Władysław Kozaczuk |title=Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two |editor=Christopher Kasparek |editor-link=Christopher Kasparek |translator=Christopher Kasparek |translator-link=Christopher Kasparek |location=Frederick, MD |publisher=University Publications of America |date=1984 |isbn=0-89093-547-5}}

{{DEFAULTSORT:Frequency-Hopping Spread Spectrum}}
[[Category:Computer network technology]]
[[Category:Multiplexing]]
[[Category:Quantized radio modulation modes]]
[[Category:Radio frequency propagation]]
[[Category:Radio resource management]]
[[Category:Military radio systems]]

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