Direct-sequence spread spectrum: Difference between revisions

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

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

{{Modulation techniques}}

{{Multiplex_techniques}}

In [[telecommunicationtelecommunications]]s, '''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.

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>

The first known scheme for this technique was introduced by a [[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 bitssymbols are modulated by a [[Pseudorandomsequence binaryof sequence|pseudorandomcomplex bit sequence]]values known as a ''spreading sequence''. Each spreading-sequenceelement bit,of whichthe isspreading known assequence, a so-called ''chip'', has a much shorter duration (larger bandwidth) than the original message bitssymbols. The modulation of the message bitssymbols scrambles and spreads the piecessignal ofin datathe spectrum, and thereby results in a bandwidth size nearly identical to that 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">{{cite book| title=Principles of Spread-Spectrum Communication Systems, 4th ed.| year=2018|last1=Torrieri|first1=Don}}</ref><ref name="ref 2">{{cite book| title=Principles of Mobile Communication, 4th ed.| year=2017|last1=Stuber|first1=Gordon L.}}</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>

==Features==

# DSSS [[Phase-shift keying|phase-shifts]] a [[sine wave]] [[Pseudorandomness|pseudorandom]]ly with a continuous [[string (computer science)|string]] of 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]] [[Baud|bit rate]].

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

==Transmission method==

Direct-sequence spread-spectrum transmissions multiply the datasymbol sequence being transmitted bywith a pseudorandom spreading sequence that has a much higher bit rate than the original datamessage rate. TheUsually, resultingsequences transmittedare signalchosen resemblessuch bandlimitedthat the resulting spectrum is spectrally [[white noise|white]],. like an audio recordingKnowledge of "static".the same However, this noise-like signalsequence is used to exactly reconstruct the original data at the receiving end,. byThis multiplyingis itcommonly implemented by the sameelement-wise spreadingmultiplication sequencewith (becausethe {{nowrap|1spreading ×sequence, 1followed {{=}}by 1}},summation andover {{nowrap|−1a ×message −1symbol {{=}} 1}})period. This process, known as ''despreading'', is mathematically a [[correlation]] of the transmitted spreading sequence with the spreading sequence. thatIn thean receiverAWGN already knowschannel, the transmitterdespreaded is using. After the despreading, thesignal's [[signal-to-noise ratio]] is approximately increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

While a transmitted DSSS signal occupies a much wider bandwidth than athe simpledirect modulation of the original signal would require, its frequency spectrum can be somewhat restricted for spectrum economy by a conventional analog bandpass filter to give a roughly bell-shaped envelope centered on the [[carrierPulse frequency]]. In contrast, [[frequencyshaping|pulse-hoppingshape spread spectrumfiltering]] pseudorandomly retunes the carrier and requires a uniform frequency response since any bandwidth shaping would cause amplitude modulation of the signal by the hopping code.

If an undesired transmitter transmits on the same channel but with a different spreading sequence (or no sequence at all), the despreading process reduces the power of 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 spreading sequences.

==Benefits==

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* [[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==

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==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]]

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{{cdma}}

[[Category:Computer network technology]]

[[Category:Quantized radio modulation modes]]

[[Category:Wireless networking]]