Frequency-hopping spread spectrum
Encyclopedia
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier
among many frequency channels
, using a pseudorandom sequence known to both transmitter
and receiver
. It is utilized as a multiple access method in the frequency-hopping code division multiple access
(FH-CDMA) scheme.
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
, unless the adversary has knowledge of the spreading characteristics. Military
radios use cryptographic
techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.
By itself, frequency hopping provides only limited protection against eavesdropping and jamming. There is a simple algorithm that effectively discovers the sequence of frequencies. To get around this weakness most modern military frequency hopping radios employ separate encryption devices such as the KY-57
. U.S.
military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK
and SINCGARS
.
(FCC) amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various spread-spectrum modes.
Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtreme Radio Service
(eXRS). Despite the name's similarity to the FRS allocation, the system is a proprietary design, rather than an official U.S. Federal Communications Commission (FCC) allocated service.
using only one carrier frequency
. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extra protection against wideband thermal noise, the frequency-hopping approach does reduce the degradation caused by narrowband interferers.
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 have a checksum
for integrity and further identification. The transmitter and receiver can use fixed tables of channel sequences so that once synchronized they can maintain communication by following the table. On each channel segment, the transmitter can send its current location in the table.
In the US, FCC part 15 on unlicensed system in the 900 MHz and 2.4 GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission
(FCC) prescribes a minimum number of channels and a maximum dwell time for each channel.
In a real multipoint radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit
for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO
and DSSS systems. Beam steering
and directional antennas also facilitate increased system performance by providing isolation between remote radios.
's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken
had already tried it.
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.
A Polish
engineer, Leonard Danilewicz
, came up with the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes .
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.
The most celebrated invention of frequency hopping was that of actress Hedy Lamarr
and composer George Antheil
, who in 1942 received for their "Secret Communications System". This early version of frequency hopping used a piano-roll
to change between 88 frequencies, and was intended to make radio-guided torpedo
es harder for enemies to detect or to jam. The patent was rediscovered in the 1950s during patent searches when private companies independently developed Code Division Multiple Access
, a civilian form of spread-spectrum.
) improves resistance to radio frequency interference
by avoiding using 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.
In this case, there is a need to use strategies for dynamic adaptation of the frequency hopping pattern. 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.
The GSM cellular system uses frequency hopping, as a form of frequency interleaving, in view to avoid that two phone calls in adjacent cells constantly interfere with each other. However, this is not considered as spread spectrum, since the two phonecalls within the same cell never use the same frequency.
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
among many frequency channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
, using a pseudorandom sequence known to both transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. It is utilized as a multiple access method in the frequency-hopping code division multiple access
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(FH-CDMA) scheme.
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
- Spread-spectrum signals are highly resistant to narrowbandNarrowbandIn radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
interferenceInterference (communication)In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background. - Spread-spectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if the pseudorandom sequence was known.
- Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
Military use
Spread-spectrum signals are highly resistant to deliberate jammingRadio jamming
Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...
, unless the adversary has knowledge of the spreading characteristics. Military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
radios use cryptographic
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.
By itself, frequency hopping provides only limited protection against eavesdropping and jamming. There is a simple algorithm that effectively discovers the sequence of frequencies. To get around this weakness most modern military frequency hopping radios employ separate encryption devices such as the KY-57
KY-57
The Speech Security Equipment , TSEC/KY-57, is a portable, tactical cryptographic device in the VINSON family, designed to provide voice encryption for a range of military communication devices such as radio or telephone....
. U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK
HAVE QUICK
HAVE QUICK is a frequency-hopping system used to protect military UHF radio traffic.Since the end of World War II, U.S. and Allied military aircraft have used AM radios in the 225–400 MHz UHF band for short range air-to-air and ground-to-air communications...
and SINCGARS
SINCGARS
SINCGARS is a Combat Net Radio currently used by U.S. and allied military forces. The radios, which handle voice and data communications, are designed to be reliable, secure and easily maintained...
.
Civilian use
Since the Federal Communications CommissionFederal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various spread-spectrum modes.
Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtreme Radio Service
EXtreme Radio Service
eXtreme Radio Service is a proprietary personal communication technology marketed by in the United States. The radios, which are similar to other walkie-talkies, use the 900 MHz band, and employ frequency-hopping spread spectrum technology to attempt to address some of the perceived shortcomings...
(eXRS). Despite the name's similarity to the FRS allocation, the system is a proprietary design, rather than an official U.S. Federal Communications Commission (FCC) allocated service.
Technical considerations
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same informationInformation
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
using only one carrier frequency
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extra protection against wideband thermal noise, the frequency-hopping approach does reduce the degradation caused by narrowband interferers.
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 have a checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
for integrity and further identification. The transmitter and receiver can use fixed tables of channel sequences so that once synchronized they can maintain communication by following the table. On each channel segment, the transmitter can send its current location in the table.
In the US, FCC part 15 on unlicensed system in the 900 MHz and 2.4 GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) prescribes a minimum number of channels and a maximum dwell time for each channel.
In a real multipoint radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit
Shannon–Hartley theorem
In information theory, the Shannon–Hartley theorem tells the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise. It is an application of the noisy channel coding theorem to the archetypal case of a continuous-time...
for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO
MIMO
In radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology...
and DSSS systems. Beam steering
Beam steering
Beam steering is about changing the direction of the main lobe of a radiation pattern.In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the RF signals driving the elements.In optical systems, beam steering may be accomplished by...
and directional antennas also facilitate increased system performance by providing isolation between remote radios.
Multiple inventors
Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Johannes ZenneckJonathan Zenneck
Jonathan Adolf Wilhelm Zenneck was a physicist and electrical engineer. Zenneck was born in Ruppertshofen, Württemberg. Zenneck contributed to researches in radio circuit performance and to the scientific and educational contributions to the literature of the pioneer radio art...
's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken
Telefunken
Telefunken is a German radio and television apparatus company, founded in Berlin in 1903, as a joint venture of Siemens & Halske and the Allgemeine Elektricitäts-Gesellschaft...
had already tried it.
The German military made limited use of frequency hopping for communication between fixed command points in World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
to prevent eavesdropping by British forces, who did not have the technology to follow the sequence.
A Polish
Poles
thumb|right|180px|The state flag of [[Poland]] as used by Polish government and diplomatic authoritiesThe Polish people, or Poles , are a nation indigenous to Poland. They are united by the Polish language, which belongs to the historical Lechitic subgroup of West Slavic languages of Central Europe...
engineer, Leonard Danilewicz
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...
, came up with the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes .
During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, the US Army Signal Corps was inventing a communication system called SIGSALY
SIGSALY
In cryptography, SIGSALY was a secure speech system used in World War II for the highest-level Allied communications....
, 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.
The most celebrated invention of frequency hopping was that of actress Hedy Lamarr
Hedy Lamarr
Hedy Lamarr was an Austrian-American actress celebrated for her great beauty who was a major contract star of MGM's "Golden Age".Lamarr also co-invented – with composer George Antheil – an early technique for spread spectrum communications and frequency hopping, necessary to wireless...
and composer George Antheil
George Antheil
George Antheil was an American avant-garde composer, pianist, author and inventor. A self-described "Bad Boy of Music", his modernist compositions amazed and appalled listeners in Europe and the US during the 1920s with their cacophonous celebration of mechanical devices.Returning permanently to...
, who in 1942 received for their "Secret Communications System". This early version of frequency hopping used a piano-roll
Player piano
A player piano is a self-playing piano, containing a pneumatic or electro-mechanical mechanism that operates the piano action via pre-programmed music perforated paper, or in rare instances, metallic rolls. The rise of the player piano grew with the rise of the mass-produced piano for the home in...
to change between 88 frequencies, and was intended to make radio-guided torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
es harder for enemies to detect or to jam. The patent was rediscovered in the 1950s during patent searches when private companies independently developed Code Division Multiple Access
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
, a civilian form of spread-spectrum.
Variations of FHSS
Adaptive Frequency-hopping spread spectrum (AFH) (as used in BluetoothBluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
) improves resistance to radio frequency interference
Co-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
by avoiding using 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
Piconet
The original piconet was a networking type used on RM Nimbus computers.These days, a piconet' is an ad-hoc computer network linking a user group of devices using Bluetooth technology protocols to allow one master device to interconnect with up to seven active slave devices...
), then they are mutually interfering and the strategy of AFH fails to avoid this interference.
In this case, there is a need to use strategies for dynamic adaptation of the frequency hopping pattern. 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
Cognitive radio
A cognitive radio is a kind of two-way radio that automatically changes its transmission or reception parameters, in a way where the entire wireless communication network -- of which it is a node -- communicates efficiently, while avoiding interference with licensed or licensed exempt users...
, 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.
The GSM cellular system uses frequency hopping, as a form of frequency interleaving, in view to avoid that two phone calls in adjacent cells constantly interfere with each other. However, this is not considered as spread spectrum, since the two phonecalls within the same cell never use the same frequency.
See also
- Direct-sequence spread spectrumDirect-sequence spread spectrumIn telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
- Dynamic frequency hoppingDynamic Frequency HoppingOne of the key challenges of the cognitive radio based wireless networks, such as IEEE 802.22 wireless regional area networks , is to address two apparently conflicting requirements: assuring Quality of Services satisfaction for services provided by the cognitive radio networks, while providing...
- List of multiple discoveries
- Maximum length sequenceMaximum length sequenceA maximum length sequence 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 that can be reproduced by the shift registers...
- Orthogonal frequency-division multiplexingOrthogonal frequency-division multiplexingOrthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
- Radio frequency sweepRadio frequency sweepRadio frequency sweep or "Frequency sweep" or "RF sweep" refer to scanning a radio frequency band for detecting signals being transmitted there. This is implemented using a radio receiver having a tunable receiving frequency...
- Spread spectrumSpread spectrumSpread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
External links
- FCC Part 15 Rules that cover frequency hopping
- Frequency hopping in unlicensed spectrum describes strategies for adaptive hopping in crowded spectrum, while considering the issues of radio etiquette and compliance with FCC Part 15 Rules