Spread spectrum
Passband modulation |
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Analog modulation |
Digital modulation |
Hierarchical modulation |
Spread spectrum |
See also |
Multiplexing |
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Analog modulation |
Statistical multiplexing (variable bandwidth) |
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Related topics |
In
Telecommunications
Spread spectrum generally makes use of a sequential
- 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 jamming (interference). Direct sequence (DS) is good at resisting continuous-time narrowband jamming, while frequency hopping (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, where 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 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 provide adequate security unless, at a minimum, long nonlinear spreading sequences are used and the messages are encrypted.
- Resistance to fading. 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. In direct-sequence systems, the signal can be detected by using a rake receiver.
- Multiple access capability, known as code-division multiple access (CDMA) or code-division multiplexing (CDM). Multiple users can transmit simultaneously in the same frequency band as long as they use different spreading sequences.
Invention of frequency hopping
The idea of trying to protect and avoid interference in radio transmissions dates back to the beginning of radio wave signaling. In 1899,
During World War II, Golden Age of Hollywood actress Hedy Lamarr and avant-garde composer George Antheil developed an intended jamming-resistant radio guidance system for use in Allied torpedoes, patenting the device under U.S. patent 2,292,387 "Secret Communications System" on August 11, 1942. Their approach was unique in that frequency coordination was done with paper player piano rolls, a novel approach which was never put into practice.[7]
Clock signal generation
This section needs additional citations for verification. (January 2020) |
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 (the desired clock frequency) and its harmonics.
Background
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
Spread-spectrum clocking avoids this problem by reducing 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. As 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.
Method
In PCIe, USB 3.0, and SATA systems, the most common technique is downspreading, via
Even though SSC compatibility is mandatory on SATA receivers,[10] it is not uncommon to find expander chips having problems dealing with such a clock. Consequently, an ability to disable spread-spectrum clocking in computer systems is considered useful.[11][12][13]
Effect
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 receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide.[14] 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 real-life interference problems is often debated,[9] as 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 higher 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.
See also
- Direct-sequence spread spectrum
- Electromagnetic compatibility (EMC)
- Electromagnetic interference (EMI)
- Frequency allocation
- Frequency-hopping spread spectrum
- George Antheil
- HAVE QUICKmilitary frequency-hopping UHF radio voice communication system
- Hedy Lamarr
- Open spectrum
- Orthogonal variable spreading factor(OVSF)
- Spread-spectrum time-domain reflectometry
- Time-hopping spread spectrum
- Ultra-wideband
Notes
- ^ Torrieri, Don (2018). Principles of Spread-Spectrum Communication Systems, 4th ed.
- ^ ISBN 9781466561991. Retrieved November 9, 2022 – via Google Books.
- ^ Tony Rothman, Random Paths to Frequency Hopping, American Scientist, January–February 2019 Volume 107, Number 1, Page 46 americanscientist.org
- ^ Jonathan Adolf Wilhelm Zenneck, Wireless Telegraphy, McGraw-Hill Book Company, Incorporated, 1915, page 331
- ^ Denis Winter, Haig's Command - A Reassessment
- zlotys for executing a model and as encouragement to further work." Cited in Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II, 1984, p. 27.
- ^ Ari Ben-Menahem, Historical Encyclopedia of Natural and Mathematical Sciences, Volume 1, Springer Science & Business Media - 2009, pages 4527-4530
- ^ "Spread Spectrum Clocking". Microsemi.
- ^ a b Item Media (19 March 2013). "Spread Spectrum Clock Generation – Theory and Debate". Interference Technology.
- ^ "CATC SATracer / Trainer Application Note: Spread Spectrum Clocking" (PDF). CATC. July 2, 2003. Retrieved 20 May 2023.
- ^ Western Digital Raid Edition III HDDs werden vom RAID Controller nicht erkannt (Thomas Krenn Wiki)
- ^ Intel Speichersystem SS4000-E: Festplatten, wie beispielsweise die Western Digital WD2500JS SATA, werden nicht erkannt. Woran liegt das? (Intel Reseller-Center)
- ^ SSC Toggle Utility – Barracuda 7200.9 at the Wayback Machine (archived 2010-04-29) (Seagate Knowledge Base)
- ^ American National Standard for Electromagnetic Noise and Field Strength Instrumentation, 10 Hz to 40 GHz—Specifications, ANSI C63.2-1996, Section 8.2 Overall Bandwidth
Sources
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
- NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management
- National Information Systems Security Glossary
- History on spread spectrum, as given in "Smart Mobs, The Next Social Revolution", ISBN 0-7382-0608-3
- ISBN 0-89093-547-5.
- Andrew S. Tanenbaum and David J. Wetherall, Computer Networks, Fifth Edition.