Multi-carrier code-division multiple access: Difference between revisions

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{{Short description|Multiple access scheme in telecommunications}}

'''Multi-carrier code-division multiple access''' ('''MC-CDMA''') is a [[multiple access]] scheme used in [[OFDM]]-based telecommunication systems, allowing the system to support multiple users at the same time over same frequency band.

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== Downlink: MC-CDM ==

In the downlink (one base station transmitting to one or more terminals), MC-CDMA typically reduces to Multi-Carrier Code Division Multiplexing. All user signals can easily be synchronized, and all signals on one subcarrier experience the same radio channel properties. In such case a preferred system implementation is to take N user bits (possibly but not necessarily for different destinations), to transform these using a Walsh [[Hadamard Transformtransform]], followed by an IFFT.

== Variants ==

A number of alternative possibilities exist as to how this frequency ___domain spreading can take place, such as by using a long PN code and multiplying each data symbol, d_i, on a subcarrier by a chip from the PN code, c_i, or by using short PN codes and spreading each data symbol by an individual PN code — i.e. d_i is multiplied by each c_i and the resulting vector is placed on N_freq subcarriers, where N_freq is the PN code length.

Once frequency ___domain spreading has taken place and the [[OFDM]] subcarriers have all been allocated values, [[OFDM]] modulation then takes place using the [[Fastfast Fourier Transformtransform|IFFT]] to produce an [[OFDM]] symbol; the [[OFDM]] guard interval is then added; and if transmission is in the downlink direction each of these resulting symbols are added together prior to transmission.

An alternative form of multi-carrier [[CDMA]], called MC-DS-CDMA or MC/DS-CDMA, performs spreading in the time ___domain, rather than in the frequency ___domain in the case of MC-CDMA — for the special case where there is only one carrier, this reverts to standard [[DS-CDMA]].

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# [[OFDMA]] with time spreading ([[MC-DS-CDMA]] and [[MT-CDMA]])

# [[OFDMA]] with both time and frequency spreading (Orthogonal Frequency Code Division Multiple Access([[OFCDMA]]))

==See also==

* [[OFDMA]], an alternative multiple access scheme for OFDM systems, where the signals of different users are separated in the [[frequency ___domain]] by allocating different sub-carriers to different users.

==References==

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* Wireless Communication Reference Web Site, section about [http://www.wirelesscommunication.nl/reference/chaptr05/mccdma/mccdma.htm''MC-CDMA''], 2001.

==See also==

* [[OFDMA]], an alternative multiple access scheme for OFDM systems, where the signals of different users are separated in the [[frequency ___domain]] by allocating different sub-carriers to different users.

{{cdma}}

{{Channel access methods}}

[[Category:Code division multiple access]]