Content deleted Content added
Lembit Staan (talk | contribs) →See also: rm spamsite |
Sammi Brie (talk | contribs) Adding short description: "Multiple access scheme in telecommunications" |
||
| (10 intermediate revisions by 6 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.
MC-CDMA spreads each user symbol in the frequency ___domain. That is, each user symbol is carried over multiple parallel subcarriers, but it is phase-shifted (typically 0 or 180 degrees) according to a code value. The code values differ per subcarrier and per user. The receiver combines all subcarrier signals, by weighing these to compensate varying signal strengths and undo the code shift. The receiver can separate signals of different users, because these have different (e.g. orthogonal) code values.
Line 8 ⟶ 9:
== 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.
== 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.
== 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]].
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,
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.
Line 29 ⟶ 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
Summary
==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==
Line 53 ⟶ 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]]
| |||