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 [[spread spectrum]] [[modulation]] technique. Spread spectrum systems are such that they transmit the message bearing signals using a [[Bandwidth (signal processing)|bandwidth]] that is in excess of the bandwidth that is actually needed by the message signal. This spreading of the transmitted signal over a large bandwidth make the resulting [[wideband]] signal appear as a [[white noise|noise]] signal which allows greater resistance to intentional and unintentional interference with the transmitted signal.<ref name=Haykin4E>{{cite book|last1=Haykin|first1=Simon|title=Communication systems|date=2008|publisher=John Wiley & Sons|page=488|edition=4|url=https://archive.org/details/CommunicationSystems4thEditionSimonHaykinsolutionsManual|accessdate=11 April 2015}}</ref>

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.

One of the methods of achieving this spreading of the message signal is provided by DSSS modulation. In DSSS the message signal is used to modulate a bit sequence known as the [[Pseudorandom noise#PN code|Pseudo Noise (PN)]] code; this PN code consists of pulses of a much shorter duration (larger bandwidth) than the pulse duration of the message signal, therefore the modulation by the message signal has the effect of chopping up the pulses of the message signal and thereby resulting in a signal which has a bandwidth nearly as large as that of the PN sequence.<ref name=Haykin4E/> In this context the duration of the pulse of the PN code is referred to as the [[Chip (CDMA)|chip]] duration and the smaller this value, the larger the bandwidth of the resultant DSSS signal and the more immune to [[Interference (communication)|interference]] the resultant signal becomes.<ref name=Haykin4E>p499</ref>

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>

[[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>

Some of the uses of DSSS include the [[Code Division Multiple Access]] (CDMA) [[channel access method]] and the [[IEEE 802.11#802.11b|IEEE 802.11b]] specification used in [[Wi-Fi]] networks.<ref name=TRappaport>{{cite book|last1=Rappaport|first1=Theodore|title=Wireless Communications Principles and Practice|date=January 2010|publisher=Prentice-Hall, Inc|page=458|edition=2|url=http://www.pearson.ch/1471/9780130422323/Wireless-Communications-Principles-and.aspx|accessdate=11 April 2015}}</ref><ref>{{Citation

|title=Capacity, Coverage and Deployment Considerations for IEEE 802.11G | year =2005| pages =1| publisher =Cisco Systems, Inc| url =https://www.cisco.com/application/pdf/en/us/guest/products/ps430/c1244/ccmigration_09186a00801d61a3.pdf

}}</ref>

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>

==History==

{Expand section|date=April 2015}

==Features==

# DSSS [[Phase-shift keying|phase-shifts]] 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.

==Transmission method==

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.

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.

The resulting signal resembles [[white noise]], like an audio what is this going on?? recording of "static". However, this noise-like signal is 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 already knows the transmitter is using.

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.

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 PNspreading codesequence in conjunction with [[FDMA]], while latterlater GLONASS used DSSS to achieve [[CDMA]] with multiple PNspreading codessequences.

* 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}}

* [httphttps://ieeexplore.ieee.org/xpldocument/login.jsp?tp=&arnumber=1095547&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D1095547/ 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}}

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[[Category:Quantized radio modulation modes]]

[[Category:Wireless networking]]