Direct-sequence spread spectrum: Difference between revisions

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{{Use American English|date = March 2019}}

{{Short description|Modulation technique to reduce signal interference}}

{{Use mdy dates|date = March 2019}}

{{Modulation techniques}}

{{Multiplex_techniques}}

In [[telecommunication]]s, '''direct-sequence spread spectrum''' ('''DSSS''') is a [[modulation]] technique. As with other [[spread spectrum]] technologies, the transmitted signal takes up more [[Bandwidth (signal processing)|bandwidth]] than the information signal that modulates the carrier or broadcast frequency. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Certain [[IEEE 802.11]] standards use DSSS signaling.

In [[telecommunications]], '''direct-sequence spread spectrum''' ('''DSSS''') is a [[spread-spectrum]] [[modulation]] technique primarily used to reduce overall signal [[Interference (communication)|interference]]. The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth.

==Features==

After the despreading or removal of the direct-sequence modulation in the receiver, the information bandwidth is restored, while the unintentional and intentional interference is substantially reduced.<ref name="ref 1">{{cite book| title=Principles of Spread-Spectrum Communication Systems, 4th ed.| year=2018|last1=Torrieri|first1=Don}}</ref>

# DSSS [[Phase modulation|phase-modulates]] a [[sine wave]] [[Pseudorandomness|pseudorandom]]ly with a continuous [[string (computer science)|string]] of [[pseudorandom noise|pseudonoise]] (PN) [[code]] symbols called "[[Chip (CDMA)|chips]]", each of which has a much shorter duration than an information [[bit]]. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the [[Chip (CDMA)|chip rate]] is much higher than the [[information]] signal [[Baud|bit rate]].

# DSSS uses a [[signaling (telecommunication)|signal]] structure in which the sequence of chips produced by the transmitter is already known by the receiver. The receiver can then use the same ''[[PN Sequences|PN sequence]]'' to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

[[Swiss people|Swiss]] inventor, [[Gustav Guanella]] proposed a "means for and method of secret signals".<ref>{{Cite web|title=Espacenet - Bibliographic data|url=https://worldwide.espacenet.com/publicationDetails/biblio?FT=D&date=19460806&DB=worldwide.espacenet.com&locale=en_EP&CC=US&NR=2405500A&KC=A&ND=7|access-date=2020-12-02|website=worldwide.espacenet.com}}</ref> With DSSS, the message symbols are modulated by a sequence of complex values known as ''spreading sequence''. Each element of the spreading sequence, a so-called ''chip'', has a shorter duration than the original message symbols. The modulation of the message symbols scrambles and spreads the signal in the spectrum, and thereby results in a bandwidth of the spreading sequence. The smaller the chip duration, the larger the bandwidth of the resulting DSSS signal; more bandwidth multiplexed to the message signal results in better resistance against narrowband interference.<ref name="ref 1"/><ref name="ref 2">{{cite book| title=Principles of Mobile Communication, 4th ed.| year=2017|last1=Stuber|first1=Gordon L.}}</ref>

==Transmission method==

Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of <code>1</code> and <code>−1</code> values, at a frequency much higher than that of the original signal.

Some practical and effective uses of DSSS include the [[code-division multiple access]] (CDMA) method, the [[IEEE 802.11#802.11b|IEEE 802.11b]] specification used in [[Wi-Fi]] networks, and the [[Global Positioning System]].<ref name="ref 3">{{cite book|title=Wireless Communications Principles and Practice, 2nd ed.| year=2002|last1=Rappaport|first1=Theodore}}</ref><ref name="ref 4">{{cite book| title=Global Positioning System: Signals, Measurements, and Performance, rev. 2nd ed.| year=2012|last1=Pratep|first1=Misra|last2=Enge|first2=Per}}</ref>

The resulting signal resembles [[white noise]], like an audio recording of "static". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading", mathematically constitutes a [[correlation]] of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.

==Transmission method==

The resulting effect of enhancing [[signal to noise ratio]] on the channel is called ''[[process gain]]''. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

Direct-sequence spread-spectrum transmissions multiply the symbol sequence being transmitted with a spreading sequence that has a higher rate than the original message rate. Usually, sequences are chosen such that the resulting spectrum is spectrally [[white noise|white]]. Knowledge of the same sequence is used to reconstruct the original data at the receiving end. This is commonly implemented by the element-wise multiplication with the spreading sequence, followed by summation over a message symbol period. This process, ''despreading'', is mathematically a [[correlation]] of the transmitted spreading sequence with the spreading sequence. In an AWGN channel, the despreaded signal's [[signal-to-noise ratio]] is increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process has reduced processing gain for that signal. This effect is the basis for the [[code division multiple access]] (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the [[cross-correlation]] properties of their PN sequences.

While a transmitted DSSS signal occupies a wider bandwidth than the direct modulation of the original signal would require, its spectrum can be restricted by conventional [[Pulse shaping|pulse-shape filtering]].

As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal [[Amplitude modulation|AM]] transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

If an undesired transmitter transmits on the same channel but with a different PNspreading sequence (or no sequence at all), the de-spreadingdespreading process hasreduces reducedthe processingpower gain forof that signal. This effect is the basis for the [[code -division multiple access]] (CDMA) propertymethod of DSSSmulti-user medium access, which allows multiple transmitters to share the same channel within the limits of the [[cross-correlation]] properties of their PNspreading sequences.

In contrast, [[frequency-hopping spread spectrum]] pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, the latter process results in a uniform frequency distribution whose width is determined by the output range of the [[pseudorandom number generator]].

==Benefits==

* Resistance to intendedunintended or unintendedintended [[Radio jamming|jamming]]

* Sharing of a single channel among multiple users

* Reduced signal/background-noise level hampers [[signals intelligence|interception]]

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==Uses==

* The United States [[GPS]], European [[Galileo positioning system|Galileo]] and Russian [[GLONASS]] [[satellite navigation]] systems; earlier GLONASS used DSSS with a single spreading sequence in conjunction with [[FDMA]], while later GLONASS used DSSS to achieve [[CDMA]] with multiple spreading sequences.

* DS-CDMA (Direct-Sequence Code Division Multiple Access) is a [[multiple access]] scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of [[CDMA]].

* [[Cordless telephone|Cordless phones]] operating in the 900&nbsp;MHz, 2.4&nbsp;GHz and 5.8&nbsp;GHz [[Band (radio)|bands]]

* [[IEEE 802.11b]] 2.4&nbsp;GHz [[Wi-Fi]], and its predecessor [[802.11-1999]]. (Their successor [[802.11g]] uses both [[Orthogonal frequency-division multiplexing|OFDM]] and DSSS)

* [[Automatic meter reading]]

* [[IEEE 802.15.4]] (used, e.g., as PHY and MAC layer for [[ZigBeeZigbee]], or, as the physical layer for [[WirelessHART]])

* [[Radio-controlled model]] Automotive, Aeronautical and Marine vehicles

* Spread spectrum [[radar]] for covertness and resistance to [[Radar jamming and deception|jamming]] and [[Spoofing attack|spoofing]]

==See also==

* [[Complementary code keying]]

* [[Frequency-hopping spread spectrum]]

* [[Linear -feedback shift register]]

* [[Orthogonal frequency-division multiplexing]]

* [[Barker code]]

==References==

{{reflist}}

* [https://ieeexplore.ieee.org/document/1095547/ The Origins of Spread-Spectrum Communications]

* {{FS1037C}}

* [[NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management]]

== External links ==

* [http://www.marcus-spectrum.com/page4/SSHist.html Civil Spread Spectrum History ]

{{cdma}}

[[Category:Computer network technology]]

[[Category:Quantized radio modulation modes]]

[[Category:Wireless networking]]