Spread spectrum

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Spread-spectrum techniques are methods in which energy generated at a single frequency is deliberately spread over a wide band of frequencies. This is done for a variety of reasons, including increasing resistance to natural interference or jamming and to prevent hostile detection.

Contents

1 History
2 Spread-spectrum telecommunications
3 Spread-spectrum clock signal generation
4 Notes
5 See also
6 External links

History

The concept of frequency hopping, a type of spread spectrum, was invented in 1942 during World War II by actress Hedy Lamarr and composer George Antheil, who received patent number 2,292,387 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 torpedoes harder for enemies to detect or to jam. The patent was little-known until recently because Lamarr applied for it under her married name of Hedy Kiesler Markey. Neither Lamarr nor Antheil made any money from the patent.

The Lamarr patent was a technological deadend and had no direct impact on subsequent technology as it was really ahead of its time. Military research at MIT Lincoln Laboratory, Magnavox Government & Industrial Electronics Corporation, and Sylvania Electronic Systems lead to early spread spectrum technology in the early 1950s. Parallel research on radar systems and a technologically similar concept called "phase coding" also had an impact on spread spectrum development.

The 1976 publication of Spread Spectrum Systems by Robert Dixon, ISBN 0471216291, 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 commmercial 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 radionavigation 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 Docket 81-413. This attracted much opposition from traditional spectrum users and manufacturers. The May 1985 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 W. FCC said at the time that it would welcome additional requests for spread spectrum in other bands.

The 1985 decision resulted in the rules that permitted Wi-Fi and Bluetooth and which were then copied in many other countries. Qualcomm was incorporated within 2 months after the decision to commercialize spread spectrum which is also called CDMA.

Spread-spectrum telecommunications

This is a technique in which a Signalling (telecommunication)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 signalling (telecommunication)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 (FHSS), direct-sequence spread spectrum (DSSS), PN spreading (using PN Sequences), time scrambling, chirp, and combinations of these techniques are forms of spread spectrum. 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.

Notes:

Invented by Hedy Lamarr, a Hollywood actress, in 1940
Techniques known since 1940s and used in military communication system since 1950s
"Spread" 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.
Two main techniques:
* Direct sequence (DS)
* Frequency hopping (FH)
Resistance to jamming (interference). DS is better at resisting continuous-time narrowband jamming, while FH 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 eavesdropping. 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 prevents the adversary from making sense of it. What's more, 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, therefore the adversary may be unable to determine if the signal exists at all. Note that these effects can also be achieved by using encryption and a very low-rate channel code, which can also be viewed as a spread-spectrum method, albeit more complex.
Resistance to fading. The high bandwidth occupied by spread-spectrum signals offer some frequency diversity, i.e. it is unlikely that the signal encounter severe multipath fading over its whole bandwidth, and in other cases the signal can be detected using e.g. a Rake receiver.
Multiple access capability. Multiple users can transmit simultaneously on the same frequency (range) as long as they use different spreading codes. See CDMA.

Spread-spectrum clock signal generation

Spread-spectrum clock generation (SSCG) is used in the design of 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, and would therefore radiate energy with an infinite spectral density. 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 Federal Communications Commission (FCC) in the United States, JEITA in Japan and the IEC in Europe).

To avoid this problem, which is of great commercial importance to manufacturers, spread-spectrum clocking is used. This consists of using one of the methods described in the Spread-spectrum telecommunications section in order to reduce the peak radiated energy. The technique therefore reshapes the system's electromagnetic emissions to comply with the electromagnetic compatibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a simple modification to the equipment.

Many personal computers have a BIOS setting to turn spread-spectrum clocking on or off. See external links at the bottom of this article.

It is important to note that this method does not reduce the total energy radiated by the system, and therefore does not necessarily make the system any less likely to interfere with sensitive equipment such as television and wideband receivers. It spreads the energy over a large frequency band which effectively reduces the electrical and magnetic field strengths that are measured within a narrow window of frequencies. Spread spectrum clocking works because the EMI receivers 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 of its energy at one frequency, then this energy would fall into a single frequency band of the receiver, which would register a large peak at that frequency. Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits. The usefulness of spread spectrum clocking as a method of actually reducing interference is often debated, but it is probable that some electronic equipment with sensitivity to a narrowband of frequencies will experience less interference, while other equipment with broadband sensitivity 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, 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 may be considered a loophole, but is generally overlooked as long as the default BIOS setting provided by the manufacturer has the spread spectrum feature enabled.

Notes

Source: some of this article is based on Federal Standard 1037C, the NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management, MIL-STD-188 and the National Information Systems Security Glossary.

Source: History on spread spectrum, as given in "Smart Mobs, The Next Social Revolution", Howard Rheingold, ISBN 0-7382-0680-3

External links