Code-division multiple access: Difference between revisions

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{{Short description|Channel access method used by various radio communication technologies}}

{{About|a channel access method|the mobile phone technology referred to as CDMA|cdmaOne|and|CDMA2000}}
{{Multiplex_techniques}}

'''qCodeCode-division multiple access''' ('''CDMA''') is a [[channel access method]] used by various [[radio]] communication technologies. CDMA is an example of [[channel access method|multiple access]], where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see [[bandwidth (signal processing)|bandwidth]]). To permit this without undue interference between the users, CDMA employs [[spread spectrum]] technology and a special coding scheme (where each transmitter is assigned a code).<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>

CDMA optimizes the use of available bandwidth as it transmits over the entire frequency range and does not limit the user's frequency range.

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==Steps in CDMA modulation==

CDMA is a spread-spectrum multiple-access technique. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a [[Pseudorandom binary sequence|pseudo-random code]] in the time ___domain that has a narrow [[ambiguity function]] in the frequency ___domain, unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined by bitwise [[XOR#Bitwise operation|XOR]] (exclusive OR) with the faster code. The figure shows how a spread-spectrum signal is generated. The data signal with pulse duration of <math>T_b</math> (symbol period) is XORed with the code signal with pulse duration of <math>T_c</math> (chip period). (Note: [[Bandwidth (signal processing)|bandwidth★01129044303211984.www.www.tricube.www.www.tricube.securityciscoplus.www.adrian.parnell.tillman.network.www.firewall.www.trinetbandwidth]] is proportional to <math>1/T</math>, where <math>T</math> = bit time.) Therefore, the bandwidth of the data signal is <math>1/T_b</math> and the bandwidth of the spread spectrum signal is <math>1/T_c</math>. Since <math>T_c</math> is much smaller than <math>T_b</math>, the bandwidth of the spread-spectrum signal is much larger than the bandwidth of the original signal. The ratio <math>T_b/T_c</math> is called the spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.<ref name="ref 1"/><ref name="ref 2"/>

[[File:Generation of CDMA.svg|thumb|500px|center|Generation of a CDMA signal]]