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
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to th ...
and receiver. FHSS is used to avoid interference, to prevent eavesdropping, and to enable
code-division multiple access
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
(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 affect the signal only during a short interval.
FHSS offers four main advantages over a fixed-frequency transmission:
# FHSS signals are highly resistant to
narrowband
Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, narrowband sounds are sounds that occupy a narrow range of frequencies. In telephony, narrowband is usual ...
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
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 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
Single Channel Ground and Airborne Radio System (SINCGARS) is a Combat-net radio (CNR) used by U.S. and allied military forces. The CNR network is designed around three systems: SINCGARS, the high frequency (HF) radio, and the SC tactical satel ...
Combat Net Radio,
Link-16
Link 16 is a military tactical data link network used by NATO and nations allowed by the MIDS International Program Office (IPO). Its specification is part of the family of Tactical Data Links.
With Link 16, military aircraft as well as ships ...
.
Civilian
In the US, since the
Federal Communications Commission
The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
(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.
Some
walkie-talkies
A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, ...
that employ FHSS technology have been developed for unlicensed use on the 900 MHz band. FHSS technology is also used in many hobby transmitters and receivers used for
radio-controlled model
A radio-controlled model (or RC model) is a model that is steerable with the use of radio control. All types of model vehicles have had RC systems installed in them, including ground vehicles, boats, planes, helicopters and even submarines and ...
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
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a m ...
. 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
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
, 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
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...
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
Code of Federal Regulations, 'Title 47, Part 15(47 CFR 15) is an oft-quoted part of Federal Communications Commission (FCC) rules and regulations regarding unlicensed transmissions. It is a part of Title 47 of the Code of Federal Regulations ( ...
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 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.
Multiple inventors
In 1899
Guglielmo Marconi
Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian inventor and electrical engineer, known for his creation of a practical radio wave-based wireless telegraph system. This led to Marconi bei ...
experimented with frequency-selective reception in an attempt to minimise interference.
The earliest mentions of frequency hopping in open literature are i US patent 725,605 awarded to
Jonathan Zenneck
Jonathan Adolf Wilhelm Zenneck (15 April 1871 – 8 April 1959) was a German physicist and electrical engineer who contributed to researches in radio circuit performance and to the scientific and educational contributions to the literature of ...
's book ''Wireless Telegraphy'' (German, 1908, English translation McGraw Hill, 1915), although Zenneck writes that
Telefunken
Telefunken was a German radio and television apparatus company, founded in Berlin in 1903, as a joint venture of Siemens & Halske and the '' Allgemeine Elektrizitäts-Gesellschaft'' (AEG) ('General electricity company').
The name "Telefunken" ap ...
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''.
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.Denis Winter, ''Haig's Command - A Reassessment'' 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. Zenneck was a German physicist and electrical engineer who had become interested in radio by attending Tesla's 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.
A
Polish
Polish may refer to:
* Anything from or related to Poland, a country in Europe
* Polish language
* Poles, people from Poland or of Polish descent
* Polish chicken
*Polish brothers (Mark Polish and Michael Polish, born 1970), American twin scree ...
engineer and inventor,
Leonard Danilewicz Leonard Stanisław Danilewicz was a Polish engineer and, for some ten years before the outbreak of World War II, one of the four directors of the AVA Radio Company in Warsaw, Poland.
Cipher Bureau work
AVA designed and built radio equipment for t ...
, came up with the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes (, issued Aug. 2, 1932).
During World War II, the
US Army Signal Corps
The United States Army Signal Corps (USASC) is a branch of the United States Army that creates and manages communications and information systems for the command and control of combined arms forces. It was established in 1860, the brainchild of Ma ...
was inventing a communication system called
SIGSALY
SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest-level Allied communications. It pioneered a number of digital communications concepts, including th ...
, 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
Hedy Lamarr (; born Hedwig Eva Maria Kiesler; November 9, 1914 January 19, 2000) was an Austrian-born American film actress and inventor. A film star during Hollywood's golden age, Lamarr has been described as one of the greatest movie actresse ...
and composer
George Antheil
George Johann Carl Antheil (; July 8, 1900 – February 12, 1959) was an American avant-garde composer, pianist, author, and inventor whose modernist musical compositions explored the modern sounds – musical, industrial, and mechanical – of t ...
received for their "Secret Communications System", an early version of frequency hopping using a piano-roll to switch among 88 frequencies to make radio-guided
torpedo
A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, su ...
es harder for enemies to detect or jam. The U.S. Navy rejected the idea, then seized it as "alien property" in 1942 (Lamarr was Austrian) but filed it away with no record of a working device being produced. Lamarr's 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. In 1962, the US Navy finally utilized the technology during the
Cuban Missile Crisis
The Cuban Missile Crisis, also known as the October Crisis (of 1962) ( es, Crisis de Octubre) in Cuba, the Caribbean Crisis () in Russia, or the Missile Scare, was a 35-day (16 October – 20 November 1962) confrontation between the United ...
; Lamarr's and Antheil's patent had expired.
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 for his "Wireless device and method using frequency hopping and sweep modes."
Variations
Adaptive frequency-hopping spread spectrum (AFH) as used in
Bluetooth
Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limit ...
improves resistance to
radio frequency interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
by avoiding crowded frequencies in the hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with
DSSS
In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. The direct-sequence modulation makes the transmitted signal wider in bandwidth than t ...
.
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
A piconet is an ad hoc network that links a wireless user group of devices using Bluetooth technology protocols. A piconet consists of two or more devices occupying the same physical channel (synchronized to a common clock and hopping sequence). I ...
), 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
A cognitive radio (CR) is a radio that can be programmed and configured dynamically to use the best wireless channels in its vicinity to avoid user interference and congestion. Such a radio automatically detects available channels in wireless spe ...
, where the networks and the devices should exhibit
frequency-agile Frequency agility is the ability of a radar system to quickly shift its operating frequency to account for atmospheric effects, jamming, mutual interference with friendly sources, or to make it more difficult to locate the radar broadcaster through ...
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.
List of multiple discoveries
Historians and sociologists have remarked upon the occurrence, in science, of "multiple independent discovery". Robert K. Merton defined such "multiples" as instances in which similar discoveries are made by scientists working independently of eac ...
*
Maximum length sequence
A maximum length sequence (MLS) is a type of pseudorandom binary sequence.
They are bit sequences generated using maximal linear-feedback shift registers and are so called because they are periodic and reproduce every binary sequence (except th ...
*
Orthogonal frequency-division multiplexing
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital commun ...
Władysław Kozaczuk Władysław Kozaczuk (23 December 1923 – 26 September 2003) was a Polish Army colonel and a military and intelligence historian.
Life
Born in the village of Babiki near Sokółka, Kozaczuk joined the army in 1944, during World War II, at Bia ...