Content deleted Content added
No edit summary |
Sammi Brie (talk | contribs) Adding short description: "Multiple access scheme in telecommunications" |
||
| (7 intermediate revisions by 4 users not shown) | |||
Line 1:
{{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.
Line 11 ⟶ 12:
== 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
== 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<sub>i</sub>, on a subcarrier by a chip from the PN code, c<sub>i</sub>, or by using short PN codes and spreading each data symbol by an individual PN code — i.e. d<sub>i</sub> is multiplied by each c<sub>i</sub> and the resulting vector is placed on N<sub>freq</sub> subcarriers, where N<sub>freq</sub> 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 [[
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]].
Line 26 ⟶ 27:
As an example of how the 2D spreading on [[VSF-OFCDM]] works, if you take the first data symbol, ''d''<sub>0</sub>, and a spreading factor in the time ___domain, ''SF''<sub>time</sub>, of length 4, and a spreading factor in the frequency ___domain, ''SF''<sub>frequency</sub> of 2, then the data symbol, ''d''<sub>0</sub>, 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 km/h using its [[4G]] prototype system in a 100 MHz-wide channel. This [[4G]] prototype system also uses a 12×12 antenna [[MIMO]] configuration, and [[turbo coding]] for error correction coding.<ref>{{cite web|url
Summary
Line 32 ⟶ 33:
# [[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==
Line 47 ⟶ 51:
* 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]]
| |||