Frequency Hopping Spread Spectrum Fhss Radio Modem

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frequency-hopping spread spectrum (FHSS)
Frequency-hopping spread spectrum (FHSS) transmission is the repeated switching of the carrier frequency during radio transmission to reduce interference and avoid interception.
FHSS is useful to counter eavesdropping, as well as to obstruct the frequency jamming of telecommunications and to enable code-division multiple access communications. It can also minimize the effects of unintentional interference.
The idea behind FHSS was discovered and rediscovered several times in the 20th century. German physicist and electrical engineer Jonathan Zenneck initially mentioned the concept in print in 1908.
Credit for FHSS belongs to actress Hedy Lamarr, however, who worked with composer George Antheil to bring the technology into existence during World War II.
Another more practical application of frequency hopping was created by Ray Zinn, co-founder of Micrel Inc. It enabled radio communication systems to function without synchronizing the receiver with a transmitter in low data rate wireless applications.
Examples include equipment monitoring and utility metering in environments such as the internet of things.
In FHSS, the transmitter hops between available narrowband frequencies within a specified broad channel in a pseudo-random sequence known to both sender and receiver.
A short burst of data is transmitted on the current narrowband channel, and then the transmitter and receiver tune to the next frequency in the sequence for the next burst of data. In most systems, the transmitter will hop to a new frequency more than twice per second.
Because no channel is used for long and the odds of any other transmitter being on the same channel at the same time are low, FHSS is often used as a method to allow multiple transmitter and receiver pairs to operate in the same space on the same broad channel at the same time.
Because a frequency band can be divided into subbands, user devices can rapidly change their carrier frequencies, or frequency hop, with minimal interference.
This wideband or broadband strategy provides a few benefits over fixed analog transmissions:
For those reasons, military radios commonly use spread-spectrum signals for spreading coded language because they need to be stealthy and resilient against jamming techniques. To disrupt an FHSS signal, an opponent's equipment would need to have insight into the frequency-hopping pattern.
Governments regulate the broadcast spectrum and often dictate some aspects of spread-spectrum communications.
For example, in North America, the industrial, scientific and medical waveband is divided into 75 hopping channels, and the devices using them may not transmit with more than 1 watt of power on any one channel.
These restrictions ensure a single device does not consume too much bandwidth or linger too long on a single frequency.
In the 2000s, the U.S. Federal Communications Commission (FCC) allowed FHSS systems to operate in the unregulated 2.4 gigahertz band in order to support the use of FHSS in 802.11b, 802.11g and 802.11n Wi-Fi deployments.
FCC Code of Federal Regulations 47 part 15.247 provides guidance for frequency hopping for several megahertz frequency bands as well.
Additional spread spectrum techniques and variations of FHSS are commonly used today. A few examples follow.
DSSS is a related technique to FHSS. It also spreads a transmitted signal across a wide channel, but it does so all at once instead of in discrete bursts separated by hops.
It can achieve higher throughput, but DSSS systems are more susceptible to interference and less effective as a spectrum-sharing method.
AFH is used in Bluetooth wireless technology to improve resistance to radio frequency interference by circumventing high-traffic frequencies in the hopping sequence. This type of adaptive transmission is simpler to implement with FHSS than with DSSS.
Chirp modulation is another form of frequency hopping that simply scans all available frequencies in consecutive order.
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"FHSS" redirects here. For other uses, see FHSS (disambiguation).
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver. FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.
The available 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 predetermined order. Interference at a specific frequency will only affect the signal during a short interval.[1]
FHSS offers three main advantages over a fixed-frequency transmission:
Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the frequency-hopping pattern. Military radios generate the frequency-hopping pattern under the control of a secret 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.
In the US, since the Federal Communications Commission (FCC) amended rules to allow FHSS systems in the unregulated 2.4 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 MHz, 2400–2483.5 MHz, and 5725–5850 MHz bands, and the requirements for frequency hopping [2]
Some walkie-talkies that employ FHSS technology have been developed for unlicensed use on the 900 MHz band. FHSS technology is also used in many hobby radio-controlled transmitters and receivers used for 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.
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, 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, FCC part 15 on unlicensed spread spectrum systems in the 902–928 MHz and 2.4 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 thousand-fold increase from the 1 milliwatt limit on non-spread-spectrum systems. The Federal Communications Commission (FCC) also prescribes a minimum number of frequency channels and a maximum dwell time for each channel.
In 1899 Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimise interference.[3]
The earliest mentions of frequency hopping in open literature are in US patent 725,605 awarded to Nikola Tesla on March 17, 1903, and in radio pioneer Jonathan Zenneck's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states 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.[4]
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.[5] Jonathan Zenneck's book Wireless Telegraphy was originally published in German in 1908, but was also translated into English in 1915 as the enemy started using frequency hopping on the front line. Zenneck was a German physicist and electrical engineer who had become interested in radio by attending Tesla lectures on “wireless sciences”. Wireless Telegraphy includes a section on frequency hopping, and, as it became a standard text for many years, it probably introduced the technology to a generation of engineers.[4]
A Polish engineer and inventor, Leonard Danilewicz, came up with the idea in 1929.[6] Several other patents were taken out in the 1930s, including one by Willem Broertjes (U.S. Patent 1,869,659, issued Aug. 2, 1932).
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. However, SIGSALY was a top-secret communications system, so its existence did not become known until the 1980s.
In 1942 actress Hedy Lamarr and composer George Antheil received U.S. Patent 2,292,387 for their "Secret Communications System"[7][8] an early version of frequency hopping using a piano-roll to change among 88 frequencies to make radio-guided torpedoes harder for enemies to detect or to jam, it was rejected by the US Navy then seized as 'alien property' in 1942 (Lamarr was Austrian) but filed away with no record of a working device being produced. Lamarr and Antheil's idea was 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, and has been cited numerous times since. In 1957 engineers at Sylvania Electronic Systems Division adopted the patented concept, combined with the recently invented transistor.[7] and in 1962 the US Navy finally utilised the technology during the Cuban Missile Crisis, Lamarr and Antheil's patent had since expired.[9]
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 U.S. Patent 6,996,399 for his "Wireless device and method using frequency hopping and sweep modes."
Adaptive frequency-hopping spread spectrum (AFH) as used in Bluetooth improves resistance to 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, by avoiding the "bad" frequency channels—perhaps those "bad" frequency channels are experiencing frequency selective fading, or perhaps some third party is trying to communicate on those bands, or perhaps those bands are being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good/bad channels.
However, if the radio frequency interference is itself dynamic, then the strategy of “bad channel removal”, applied in AFH might not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth Piconet), then they are mutually interfering and the strategy of AFH 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 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.
^ Torrieri, Don (2018). Principles of Spread-Spectrum Communication Systems, 4th ed.
^ "47 CFR § 15.247 - Operation within the bands 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz". LII / Legal Information Institute. law.cornell.edu. Retrieved 17 December 2019.
^ Kahn, David (2014). How I Discovered World War II's Greatest Spy and Other Stories of Intelligence and Code. Auerbach Publications. p. 158. ISBN 9781466561991.
^ a b eetimes.com: A short history of spread spectrum
^ Denis Winter, Haig's Command - A Reassessment
^ Danilewicz later recalled: "In 1929 we proposed to the 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 złotych 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.
^ a b "June 1941: Hedy Lamarr and George Antheil submit patent for radio frequency hopping". APS News. Vol. 20 no. 6. June 2011.
^ Wenner, Melinda (June 3, 2008). "Hedy Lamarr: Not just a pretty face". Scientific American.
^ Commissariat, Tushna (August 1, 2018). "A tale of two lives". Physics World.
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