Spread spectrum
Encyclopedia
Spread-spectrum techniques are methods by which a signal
(e.g. an electrical, electromagnetic, or acoustic signal
) generated in a particular bandwidth is deliberately spread in the frequency domain
, resulting in a signal
with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference
, noise
and jamming
, to prevent detection, and to limit power flux density
(e.g. in satellite
downlinks).
in July 1900. Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." His patents covered two fundamentally different techniques for achieving immunity to interference, both of which functioned by altering the carrier frequency or other exclusive characteristic. The first had a transmitter that worked simultaneously at two or more separate frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuitry to respond. The second technique used a variable-frequency transmitter controlled by an encoding wheel that altered the transmitted frequency in a predetermined manner. These patents describe the basic principles of frequency hopping and frequency-division multiplexing, and also the electronic AND-gate
logic circuit.
Frequency hopping is also mentioned 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 several years earlier. Zenneck's book was a leading text of the time, and it is likely that many later engineers were aware of 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 (Germany 1929, , 1932). During World War II
, the US Army Signal Corps was inventing a communication system called SIGSALY
for communication between Roosevelt and Churchill, which incorporated spread spectrum, but due to its top secret nature, SIGSALY's 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". Lamarr had learned at defense meetings she had attended with her former husband Friedrich Mandl
that radio-guided missiles' signals could easily be jammed. The Antheil–Lamarr version of frequency hopping used a piano-roll to change among 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam. The patent came to light during patent searches in the 1950s when ITT Corporation
and other private firms began to develop Code Division Multiple Access
(CDMA), a civilian form of spread spectrum, though the Lamarr patent had no direct impact on subsequent technology. It was in fact ongoing military research at MIT Lincoln Laboratory, Magnavox
Government & Industrial Electronics Corporation, ITT and Sylvania Electronic Systems
that led to early spread-spectrum technology in the 1950s. Parallel research on radar systems and a technologically similar concept called "phase coding" also had an impact on spread-spectrum development.
Initial commercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System's very small aperture (VSAT) satellite terminal system for newspaper newswire services, Del Norte Technology's radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualcomm
's OmniTRACS system for communications to trucks. In the Qualcomm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one satellite to be used since the processing gain
of spread spectrum eliminated interference. The Del Norte system used the high bandwidth of spread spectrum to improve location accuracy.
In 1981, the Federal Communications Commission started exploring ways to permit more general civil uses of spread spectrum in a Notice of Inquiry docket. This docket was proposed to FCC and then directed by Michael Marcus of the FCC staff. The proposals in the docket were generally opposed by spectrum users and radio equipment manufacturers, although they were supported by the then Hewlett-Packard
Corp. The laboratory group supporting the proposal would later become part of Agilent.
The May 1985 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 Watt. FCC said at the time that it would welcome additional requests for spread spectrum in other bands.The resulting rules, now codified as 47 CFR 15.247 permitted Wi-Fi
, Bluetooth
, and many other products including cordless telephones. These rules were then copied in many other countries. Qualcomm was incorporated within 2 months after the decision to commercialize CDMA.
) signal is transmitted on a bandwidth considerably larger than the frequency
content of the original information.
Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. 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
).
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. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generator
s — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultra-wideband
(UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless Ethernet standard IEEE 802.11
uses either FHSS or DSSS in its radio interface.
Spread-spectrum clock generation (SSCG) is used in some 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 and its harmonics. 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 using one of the methods previously described to reduce 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. Since 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.
However, spread-spectrum clocking can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew
[expand how that happens, or refer to another article].
Note that this method does not reduce total 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 kHz wide. 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 interference is often debated, since 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 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.
An ability to disable spread-spectrum clocking in computer systems is considered useful for overclocking
, as spread spectrum can lower maximum clock speed achievable due to clock skew
.
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
(e.g. an electrical, electromagnetic, or acoustic signal
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
) generated in a particular bandwidth is deliberately spread in the frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
, resulting in a signal
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference
Interference (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...
, noise
Noise (electronics)
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
and jamming
Radio 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...
, to prevent detection, and to limit power flux density
Spectral flux density
In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light...
(e.g. in satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
downlinks).
Frequency hopping
The concept of frequency hopping was first alluded to in the 1903 and filed by Nikola TeslaNikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
in July 1900. Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." His patents covered two fundamentally different techniques for achieving immunity to interference, both of which functioned by altering the carrier frequency or other exclusive characteristic. The first had a transmitter that worked simultaneously at two or more separate frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuitry to respond. The second technique used a variable-frequency transmitter controlled by an encoding wheel that altered the transmitted frequency in a predetermined manner. These patents describe the basic principles of frequency hopping and frequency-division multiplexing, and also the electronic AND-gate
AND gate
The AND gate is a basic digital logic gate that implements logical conjunction - it behaves according to the truth table to the right. A HIGH output results only if both the inputs to the AND gate are HIGH . If neither or only one input to the AND gate is HIGH, a LOW output results...
logic circuit.
Frequency hopping is also mentioned in radio pioneer Jonathan Zenneck
Jonathan 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 several years earlier. Zenneck's book was a leading text of the time, and it is likely that many later engineers were aware of 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 (Germany 1929, , 1932). 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....
for communication between Roosevelt and Churchill, which incorporated spread spectrum, but due to its top secret nature, SIGSALY's 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". Lamarr had learned at defense meetings she had attended with her former husband Friedrich Mandl
Friedrich Mandl
Friedrich Mandl was chairman of Hirtenberger Patronen-Fabrik, a leading Austrian armaments firm founded by his father, Alexander Mandl....
that radio-guided missiles' signals could easily be jammed. The Antheil–Lamarr version of frequency hopping used a piano-roll to change among 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam. The patent came to light during patent searches in the 1950s when ITT Corporation
ITT Corporation
ITT Corporation is a global diversified manufacturing company based in the United States. ITT participates in global markets including water and fluids management, defense and security, and motion and flow control...
and other private firms began to develop 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...
(CDMA), a civilian form of spread spectrum, though the Lamarr patent had no direct impact on subsequent technology. It was in fact ongoing military research at MIT Lincoln Laboratory, Magnavox
Magnavox
Magnavox is a US electronics company founded by Edwin Pridham and Peter L. Jensen, who invented the moving-coil loudspeaker in 1915 at their lab in Napa, California. They formed Magnavox in 1917 in order to market their inventions....
Government & Industrial Electronics Corporation, ITT and Sylvania Electronic Systems
Sylvania Electric Products
Sylvania Electric Products was a U.S. manufacturer of diverse electrical equipment, including at various times radio transceivers, vacuum tubes, semiconductors, and mainframe computers...
that led to early spread-spectrum technology in the 1950s. Parallel research on radar systems and a technologically similar concept called "phase coding" also had an impact on spread-spectrum development.
Commercial use
The 1976 publication of Spread Spectrum Systems by Robert Dixon, ISBN 0-471-21629-1, was a significant milestone in the commercialization of this technology. Previous publications were either classified military reports or academic papers on narrow subtopics. Dixon's book was the first comprehensive unclassified review of the technology and set the stage for increasing research into commercial applications.Initial commercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System's very small aperture (VSAT) satellite terminal system for newspaper newswire services, Del Norte Technology's radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualcomm
Qualcomm
Qualcomm is an American global telecommunication corporation that designs, manufactures and markets digital wireless telecommunications products and services based on its code division multiple access technology and other technologies. Headquartered in San Diego, CA, USA...
's OmniTRACS system for communications to trucks. In the Qualcomm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one satellite to be used since the processing gain
Process gain
In a spread spectrum system, the process gain is the ratio of the spread bandwidth to the unspread bandwidth. It is usually expressed in decibels ....
of spread spectrum eliminated interference. The Del Norte system used the high bandwidth of spread spectrum to improve location accuracy.
In 1981, the Federal Communications Commission started exploring ways to permit more general civil uses of spread spectrum in a Notice of Inquiry docket. This docket was proposed to FCC and then directed by Michael Marcus of the FCC staff. The proposals in the docket were generally opposed by spectrum users and radio equipment manufacturers, although they were supported by the then Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
Corp. The laboratory group supporting the proposal would later become part of Agilent.
The May 1985 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 Watt. FCC said at the time that it would welcome additional requests for spread spectrum in other bands.The resulting rules, now codified as 47 CFR 15.247 permitted Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
, Bluetooth
Bluetooth
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...
, and many other products including cordless telephones. These rules were then copied in many other countries. Qualcomm was incorporated within 2 months after the decision to commercialize CDMA.
Spread-spectrum telecommunications
This is a technique in which a (telecommunicationTelecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
) signal is transmitted on a bandwidth considerably larger than the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
content of the original information.
Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectrum generally makes use of a sequential noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
-like signal structure to spread the normally narrowband
Narrowband
In 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...
information signal over a relatively wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
(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
Low probability of intercept
A low-probability-of-intercept radar is designed to be difficult to detect by passive radar detection equipment while it is searching for a target or engaged in target tracking...
(LPI
LPI
LPI may refer to:Places* Link%C3%B6ping SAAB Airport, Sweden * Linus Pauling Institute at Oregon State University* La Pyramide Inversée, a skylight constructed in an underground shopping mall in front of the Louvre Museum in France...
).
Frequency-hopping spread spectrum
Frequency-hopping spread spectrum
Frequency-hopping spread spectrum 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...
(FHSS), direct-sequence spread spectrum
Direct-sequence spread spectrum
In 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...
(DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum
Chirp spread spectrum
In digital communications, Chirp spread spectrum is a spread spectrum technique that uses wideband linear frequency modulated chirp pulses to encode information. A chirp is a sinusoidal signal whose frequency increases or decreases over a certain amount of time...
(CSS), and combinations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generator
Pseudorandom number generator
A pseudorandom number generator , also known as a deterministic random bit generator , is an algorithm for generating a sequence of numbers that approximates the properties of random numbers...
s — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultra-wideband
Ultra-wideband
Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...
(UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless Ethernet standard IEEE 802.11
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
uses either FHSS or DSSS in its radio interface.
Techniques
- 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 jammingRadio jammingRadio 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...
(interference). DS (direct sequence) is better at resisting continuous-time narrowband jamming, while FH (frequency hopping) 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, when 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 eavesdroppingEavesdroppingEavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...
. The spreading code (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 "encrypts" the signal and reduces the chance of an adversary's 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 — often significantly lower than the noise PSD — 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 intrinsically provide adequate security; "...just using spread-spectrum radio itself is not sufficient for communications security". - Resistance to fadingFadingIn wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. 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, and in other cases the signal can be detected using e.g. a Rake receiverRake receiverA rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" called fingers, that is, several correlators each assigned to a different multipath component...
. - Multiple access capability. Multiple users can transmit simultaneously on the same frequency (range) as long as they use different spreading codes. See CDMACode division multiple accessCode 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...
.
Spread-spectrum clock signal generation
Synchronous circuit
A synchronous circuit is a digital circuit in which the parts are synchronized by a clock signal.In an ideal synchronous circuit, every change in the logical levels of its storage components is simultaneous. These transitions follow the level change of a special signal called the clock...
, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
(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 and its harmonics. 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
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...
in the United States, JEITA
Jeita
Jeita is a Lebanese town located in the Keserwan District in the Mount Lebanon Governorate. The town is about north of Beirut. It is famous for the Jeita Grotto which is a popular tourist attraction, as well as the Nahr al-Kalb, a river that runs from a spring near the grotto emptying into the...
in Japan and the IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
in Europe).
Spread-spectrum clocking avoids this problem by using one of the methods previously described to reduce the peak radiated energy and, therefore, its electromagnetic emissions and so comply with electromagnetic compatibility
Electromagnetic compatibility
Electromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
(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. Since 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
Active EMI reduction
Active EMI Reduction refers to techniques such as spread spectrum clocking to reduce the effects of electromagnetic interference or EMI. It contrasts with "passive" techniques such as capacitors or shielding....
techniques such as spread-spectrum clocking are needed in these cases.
However, spread-spectrum clocking can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew
Clock skew
-In circuit design:In circuit designs, clock skew is a phenomenon in synchronous circuits in which the clock signal arrives at different components at different times...
[expand how that happens, or refer to another article].
Note that this method does not reduce total radiated
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
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
Measuring Receiver
In telecommunication, a measuring receiver or measurement receiver is a calibrated laboratory-grade radio receiver designed to measure the characteristics of radio signals. The parameters of such receivers can usually be adjusted over a much wider range of values than is the case with other radio...
s used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. 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 interference is often debated, since 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 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
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
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
Loophole
A loophole is a weakness that allows a system to be circumvented.Loophole may also refer to:*Arrowslit, a slit in a castle wall*Loophole , a short science fiction story by Arthur C...
, but is generally overlooked as long as spread-spectrum is enabled by default.
An ability to disable spread-spectrum clocking in computer systems is considered useful for overclocking
Overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...
, as spread spectrum can lower maximum clock speed achievable due to clock skew
Clock skew
-In circuit design:In circuit designs, clock skew is a phenomenon in synchronous circuits in which the clock signal arrives at different components at different times...
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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...
- Open spectrumOpen spectrumOpen spectrum is a movement to get the Federal Communications Commission to provide more unlicensed, radio frequency spectrum that is available for use by all...
- Electromagnetic compatibilityElectromagnetic compatibilityElectromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
(EMC) - Electromagnetic interferenceElectromagnetic interferenceElectromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
(EMI) - Frequency allocationFrequency allocationUse of radio frequency bands of the electromagnetic spectrum is regulated by governments in most countries, in a Spectrum management process known as frequency allocation or spectrum allocation. Radio propagation does not stop at national boundaries...
- Frequency-hopping spread spectrumFrequency-hopping spread spectrumFrequency-hopping spread spectrum 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...
- Orthogonal variable spreading factor (OVSF)
- Process gainProcess gainIn a spread spectrum system, the process gain is the ratio of the spread bandwidth to the unspread bandwidth. It is usually expressed in decibels ....
- Spread-spectrum time-domain reflectometrySpread-spectrum time-domain reflectometrySpread-spectrum time-domain reflectometry is a measurement technique to identify faults, usually in electrical wires, by observing reflected spread spectrum signals. SSTDR is a type of time-domain reflectometry that can be advantageous to other systems due to the ability to use SSTDR in high-noise...
- Time-hopping spread spectrum
- HAVE QUICKHAVE QUICKHAVE 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...
military frequency-hopping UHF radio voice communication system
Sources
- NTIA Manual of Regulations and Procedures for Federal Radio Frequency ManagementNTIA Manual of Regulations and Procedures for Federal Radio Frequency ManagementThe NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management, also known as the Red Book, is a publication of the United States Office of Spectrum Management of the National Telecommunications and Information Administration or NTIA, and is the official source for all...
- National Information Systems Security Glossary
- History on spread spectrum, as given in "Smart Mobs, The Next Social Revolution", Howard RheingoldHoward Rheingold-See also:* Collective intelligence* Information society* The WELL* Virtual community-External links:***** at TED conference** a 48MB Quicktime movie, hosted by the Internet Archive...
, 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 KasparekChristopher KasparekChristopher Kasparek is a Scottish-born writer of Polish descent who has translated works by Ignacy Krasicki, Bolesław Prus, Florian Znaniecki, Władysław Tatarkiewicz, Marian Rejewski and Władysław Kozaczuk, as well as the Polish-Lithuanian Constitution of May 3, 1791.He has published papers on...
, Frederick, MD, University Publications of America, 1984, ISBN 0-89093-547-5.
External links
- HF Frequency Hopping
- HF VHF UHF Spread Spectrum Radio
- CDMA and spread spectrum
- Information about the use of spread spectrum for reduced AGP EMI
- Civil spread spectrum history
- Spread Spectrum Scene newsletter
- http://cas.et.tudelft.nl/~glas/ssc/techn/ "The principles of Spread Spectrum communication", hosted on TU Delft by Jack Glas
- Presentations at 4/08 George Mason University conference on unlicensed spread spectrum history
- Interview for the Indian press with Hedy Lamarr's (the inventor of spread spectrum) son, Anthony loder, on the impact of her invention