Spread spectrum

From Free net encyclopedia

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.

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

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History

Spread-spectrum techniques were first discussed by movie actress Hedy Lamarr, born in Austria as Hedwig Maria Eva Kiesler and married to a defence mogul. While playing four-handed piano music, she thought of a way to eliminate radio interference in submarines. (She learned about the problem after being forced to attend defence meetings with her ex-husband while she was in Germany during WWII). She imagined a transmitter that sends a signal on various frequencies to receivers that demultiplex it, so that ships could not jam the signal. Subsequently, in the 1950s, she and composer George Antheil obtained a patent, and the US Navy was the first to develop a usable spread spectrum system.

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Spread-spectrum telecommunications

This is a technique in which a 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 wide 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 (FHSS), direct sequence (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 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 the adversary, 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.

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Spread-spectrum clock 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 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 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 on 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 TV and radio 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 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.

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