Editing Code-division multiple access (section)

{{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}}
{{Confused|Carrier-sense multiple access}}

'''Code-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.

It is used as the access method in many [[mobile phone standards]]. [[cdmaOne|IS-95]], also called "cdmaOne", and its [[3G]] evolution [[CDMA2000]], are often simply referred to as "CDMA", but [[UMTS]], the 3G standard used by [[GSM]] carriers, also uses "wideband CDMA", or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies.  Many carriers (such as [[AT&T Corporation|AT&T]], [[U.S. Cellular|UScellular]] and [[Verizon (mobile network)|Verizon]]) shut down 3G CDMA-based networks in 2022 and 2024, rendering handsets supporting only those protocols unusable for calls, even to [[9-1-1|911]].<ref>{{cite web|url=https://www.cnn.com/2022/02/22/tech/att-3g-network-shutdown/index.html|title=AT&T is shutting down its 3G network. Here's how it could impact you|first=Samantha Murphy|last=Kelly|date=February 22, 2022|website=[[CNN]]}}</ref><ref>{{Cite web|last=Johnson|first=Allison|date=2021-03-30|title=Verizon will shutter its 3G CDMA by the end of 2022|url=https://www.theverge.com/2021/3/30/22358628/verizon-3g-cdma-shutdown-date-2022|access-date=2021-10-09|website=The Verge|language=en}}</ref>

It can be also used as a channel or medium access technology, like [[ALOHA]] for example or as a permanent pilot/signalling channel to allow users to synchronize their local oscillators to a common system frequency, thereby also estimating the channel parameters permanently.

In these schemes, the message is modulated on a longer spreading sequence, consisting of several chips (0es and 1es). Due to their very advantageous auto- and crosscorrelation characteristics, these spreading sequences have also been used for radar applications for many decades, where they are called [[Barker code]]s (with a very short sequence length of typically 8 to 32).

For space-based communication applications, CDMA has been used for many decades due to the large path loss and Doppler shift caused by satellite motion. CDMA is often used with [[binary phase-shift keying]] (BPSK) in its simplest form, but can be combined with any modulation scheme like (in advanced cases) [[quadrature amplitude modulation]] (QAM) or [[orthogonal frequency-division multiplexing]] (OFDM), which typically makes it very robust and efficient (and equipping them with accurate ranging capabilities, which is difficult without CDMA). Other schemes use subcarriers based on [[binary offset carrier modulation]] (BOC modulation), which is inspired by [[Manchester code]]s and enable a larger gap between the virtual center frequency and the subcarriers, which is not the case for OFDM subcarriers.