Direct-sequence spread spectrum: Difference between revisions

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

In [[telecommunication]], the term '''direct-sequence spread spectrum''' has the following meanings:

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

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

{{Modulation techniques}}

# A [[system]] (a) for generating spread-spectrum transmissions by [[phase]]-modulating a [[sine wave]] [[pseudorandom]]ly with a continuous [[string]] of [[pseudonoise]] [[code]] symbols, each of duration much smaller than a [[bit]] and (b) that may be time-gated, where the transmitter is keyed periodically or randomly within a specified [[time]] interval.

{{Multiplex_techniques}}

# A [[signal]] structuring technique utilizing a [[digital]] code [[sequence]] having a [[chip rate]] much higher than the [[information]] signal [[bit rate]]. Each information bit of a [[digital signal]] is transmitted as a pseudorandom sequence of chips.

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.

Put simply, 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, thereby spreading the energy of the original signal into a much wider band.

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>

The resulting signal resembles [[white noise]], like an audio recording of "static", except that this noise can be filtered out at the receiving end to recover the original data, by again multiplying the same pseudorandom sequence to the received signal (because 1 &times; 1 = 1, and -1 &times; -1 = 1).

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>

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.

==Transmission method==

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

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.

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]].

'''Comparison of DS SS and FH SS'''

*DS SS

**Flexible support of variable data rate

**High capacity is possible with enhancements (interference

**cancellation, adaptive antenna, etc.)

**Suffers from near-far effect ⇒ power control required

*FH SS

**Suitable for ad hoc networks (no near-far problem)

**Robust to interference

**Limited data rate

If an undesired transmitter transmits on the same channel but with a different spreading sequence, the despreading process reduces the power of that signal. This effect is the basis for the [[code-division multiple access]] (CDMA) method of multi-user medium access, which allows multiple transmitters to share the same channel within the limits of the [[cross-correlation]] properties of their spreading sequences.

*Bottleneck of DS SS

**Processing gain can be made larger by employing a longer PN sequence and more chips per bit

**Physical devices used to generate PN sequence impose practical limit on attainable processing gain

**Processing gain may be not enough to overcome effects of some jammers

==ReferencesBenefits==

* Resistance to unintended or intended [[Radio jamming|jamming]]

* [[Federal Standard 1037C]]

* Sharing of a single channel among multiple users

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

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

* Determination of relative timing between transmitter and receiver

==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 [[Zigbee]], 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]]

==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:RadioComputer modulationnetwork modestechnology]]

[[Category:Quantized radio modulation modes]]

[[Category:Wireless networking]]

[[Category:IEEE 802.11]]

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