MCCDM Multi-Carrier Code Division Multiplex

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Multi-carrier code division multiple access

From Wikipedia, the free encyclopedia

Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access scheme used inOFDM-based telecommunication systems, allowing the system to support multiple users at the same

time.

MC-CDMA spreads each user symbol in the frequency domain. That is, each user symbol is carriedover multiple parallel subcarriers, but it is phase shifted (typically 0 or 180 degrees) according to acode value. The code values differ per subcarrier and per user. The receiver combines all subcarriersignals, by weighing these to compensate varying signal strengths and undo the code shift. Thereceiver can separate signals of different users, because these have different (e.g. orthogonal) codevalues.

Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), asignal-to-noise-plus-interference ratio (if defined as signal power divided by total noise plus

interference power in the entire transmission band) of less than 0 dB is feasible.

One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal (DS-CDMA)which is transmitted after it has been fed through an inverse FFT (Fast Fourier Transform)

Rationale

Wireless radio links suffer from frequency-selective channels. If the signal on one subcarrierexperiences 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 typicallyreduces 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 apreferred system implementation is to take N user bits (possibly but not necessarily for differentdestinations), to transform these using a Walsh Hadamard Transform, followed by an I-FFT.

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, di, on a subcarrier by a chip

from the PN code, ci, or by using short PN codes and spreading each data symbol by an individual

PN code i.e. diis multiplied by each c

iand the resulting vector is placed on N

freqsubcarriers,

Contents

1 Rationale 2 Downlink: MC-CDM 3 Variants 4 References 5 Literature 6 See also

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where Nfreq is the PN code length.

Once frequency domain spreading has taken place and the OFDM subcarriers have all been allocatedvalues, OFDM modulation then takes place using the IFFT to produce an OFDM symbol; the OFDMguard interval is then added; and if transmission is in the downlink direction each of these resultingsymbols are added together prior to transmission.

An alternative form of multi-carrier CDMA, called MC-DS-CDMA or MC/DS-CDMA, performsspreading in the time domain, rather than in the frequency domain in the case of MC-CDMA forthe 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 symbolson the individual subcarriers are spread in time by multiplying the chips on a PN code by the datasymbol on the subcarrier. For example, assume the PN code chips consist of {1, -1} and the datasymbol 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 orthogonalfrequency code-division multiplexing), which NTT DoCoMo is using for its 4G prototype system.

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, SFtime, of length 4, and a spreading factor in the

frequency domain, SFfrequency

of 2, then the data symbol, d0, will be multiplied by the length-2

frequency-domain PN codes and placed on subcarriers 0 and 1, and these values on subcarriers 0 and1 will then be multiplied by the length-4 time-domain PN code and transmitted on OFDM symbols

0, 1, 2 and 3.[1]

NTT DoCoMo has already achieved 5 Gbit/s transmissions to receivers travelling at 10 km/h usingits 4G prototype system in a 100 MHz-wide channel. This 4G prototype system also uses a 12x12

antenna MIMO configuration, and turbo coding for error correction coding.[2]

References

1. ^ http://citeseer.ist.psu.edu/atarashi02broadband.html Broadband Packet Wireless AccessBased On VSF-OFCDM And MC/DS-CDMA (2002) Atarashi et al.

2. ^ "DoCoMo Achieves 5 Gbit/s Data Speed". NTT DoCoMo Press. 2007-02-09.http://www.nttdocomo.com/pr/2007/001319.html.

Literature

N. Yee, J.P.M.G. Linnartz and G. Fettweis, "Multi-Carrier CDMA in indoor wireless RadioNetworks", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int.Conference, Sept. 1993, Yokohama, Japan, pp. 109113 (1993: first paper proposing thesystem and the name MC-CDMA)

K. Fazel and L. Papke, "On the performance of convolutionally-coded CDMA/OFDM formobile communication system", IEEE Personal Indoor and Mobile Radio Communications(PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 468472

A. Chouly, A. Brajal, and S. Jourdan, "Orthogonal multicarrier techniques applied to directsequence spread spectrum CDMA systems," in Proceedings of Global Telecommunications


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Conference (GLOBECOM'93), pp. 17231728, Houston, Tex, USA, November 1993.

N.Yee, J.P.M.G. Linnartz and G. Fettweis, "Multi-Carrier-CDMA in indoor wirelessnetworks", IEICE Transaction on Communications, Japan, Vol. E77-B, No. 7, July 1994, pp.900904.

J.P.M.G. Linnartz, "Performance Analysis of Synchronous MC-CDMA in mobile Rayleighchannels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp13751387. PDF

K. Fazel and S. Kaiser,Multi-Carrier and Spread Spectrum Systems: From OFDM and MC-CDMA to LTE and WiMAX, 2nd Edition, John Wiley & Sons, 2008, ISBN 978-0-470-99821-2.

Hughes Software Systems,Multi Carrier Code Division Multiple Access, March 2002.

German Aerospace Center, Institute of Communications and Navigation,History of Multi-

Carrier Code Division Multiple Access (MC-CDMA) and Multi-Carrier Spread SpectrumWorkshop, November 2006.

Wireless Communication Reference Web Site, section aboutMC-CDMA, 2001.

See also

OFDMA, an alternative multiple access scheme for OFDM systems, where the signals ofdifferent users are separated in the frequency domain by allocating different sub-carriers todifferent users.

Retrieved from "http://en.wikipedia.org/wiki/Multi-carrier_code_division_multiple_access"Categories: Channel access methods

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