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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== Rationale ==

Wireless radio links suffer from frequency-selective channel interference. If the signal on one subcarrier experiences an outage, it can still be reconstructed from the energy received over other subcarriers.

If the signal on one subcarrier experiences an outage, it can

still be reconstructed from the energy received over other subcarriers.

== 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 transform]], followed by an IFFT.

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 Transform]], 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]].

For the case of MC-DS-CDMA where [[OFDM]] is used as the modulation scheme, the data symbols on the individual subcarriers are spread in time by multiplying the chips on a PN code by the data symbol on the subcarrier. For example, assume the PN code chips consist of {1, -1−1} and the data symbol on the subcarrier is -−''j''. The symbol being modulated onto that carrier, for symbols 0 and 1, will be -−''j'' for symbol 0 and +''j'' for symbol 1.

2-dimensional spreading in both the frequency and time domains is also possible, and a scheme that uses 2-D spreading is [[VSF-OFCDM]] (which stands for variable spreading factor orthogonal frequency code-division multiplexing), which [[NTT DoCoMo]] is using for its [[4G]] prototype system.

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As an example of how the 2D spreading on [[VSF-OFCDM]] works, if you take the first data symbol, ''d''₀, and a spreading factor in the time ___domain, ''SF''_time, of length 4, and a spreading factor in the frequency ___domain, ''SF''_frequency of 2, then the data symbol, ''d''₀, will be multiplied by the length-2 frequency-___domain PN codes and placed on subcarriers 0 and 1, and these values on subcarriers 0 and 1 will then be multiplied by the length-4 time-___domain PN code and transmitted on [[OFDM]] symbols 0, 1, 2 and 3.<ref>http://citeseer.ist.psu.edu/atarashi02broadband.html Broadband Packet Wireless Access Based On VSF-OFCDM And MC/DS-CDMA (2002) Atarashi et al.</ref>

[[NTT DoCoMo]] has already achieved 5 {{nbsp}}Gbit/s transmissions to receivers travelling at 10&nbsp;km/h using its [[4G]] prototype system in a 100&nbsp;MHz-wide channel. This [[4G]] prototype system also uses a 12x1212×12 antenna [[MIMO]] configuration, and [[turbo coding]] for error correction coding.<ref>{{cite web|url = http://www.nttdocomo.com/pr/2007/001319.html|date=2007-02-09|publisher=[[NTT DoCoMo]] Press|title=DoCoMo Achieves 5&nbsp;Gbit/s Data Speed|access-date=2009-07-10|archive-url=https://web.archive.org/web/20080925084229/http://www.nttdocomo.com/pr/2007/001319.html|archive-date=2008-09-25|url-status=dead}}</ref>

Summary

1.# [[OFDMA]] with frequency spreading (MC-CDMA)

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

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

==See also==

1. [[OFDMA]] with frequency spreading (MC-CDMA)

* [[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.

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

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

==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]]