MultiCarrier Code Division Multiple Access (MCCDMA) is a multiple access scheme used in OFDMbased telecommunication systems, allowing the system to support multiple users at the same time.
MCCDMA 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.
Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), a signaltonoiseplusinterference 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 MCCDMA is to regard it as a directsequence CDMA signal (DSCDMA) which is transmitted after it has been fed through an inverse FFT (Fast Fourier Transform).
Rationale[edit]
Wireless radio links suffer from frequencyselective channel interference. If the signal on one subcarrier experiences an outage, it can still be reconstructed from the energy received over other subcarriers.
Downlink: MCCDM[edit]
In the downlink (one base station transmitting to one or more terminals), MCCDMA typically reduces to MultiCarrier 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 IFF.
Variants[edit]
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 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 multicarrier CDMA, called MCDSCDMA or MC/DSCDMA, performs spreading in the time domain, rather than in the frequency domain in the case of MCCDMA — for the special case where there is only one carrier, this reverts to standard DSCDMA.
For the case of MCDSCDMA 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} 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.
2dimensional spreading in both the frequency and time domains is also possible, and a scheme that uses 2D spreading is VSFOFCDM (which stands for variable spreading factor orthogonal frequency codedivision multiplexing), which NTT DoCoMo is using for its 4G prototype system.
As an example of how the 2D spreading on VSFOFCDM 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 length2 frequencydomain PN codes and placed on subcarriers 0 and 1, and these values on subcarriers 0 and 1 will then be multiplied by the length4 timedomain 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 using its 4G prototype system in a 100 MHzwide channel. This 4G prototype system also uses a 12x12 antenna MIMO configuration, and turbo coding for error correction coding.^{[2]}
Summary
1. OFDMA with frequency spreading (MCCDMA)
2. OFDMA with time spreading (MCDSCDMA and MTCDMA)
3. OFDMA with both time and frequency spreading (Orthogonal Frequency Code Division Multiple Access(OFCDMA))
References[edit]
 ^ http://citeseer.ist.psu.edu/atarashi02broadband.html Broadband Packet Wireless Access Based On VSFOFCDM And MC/DSCDMA (2002) Atarashi et al.
 ^ "DoCoMo Achieves 5 Gbit/s Data Speed". NTT DoCoMo Press. 20070209.
Literature[edit]
 N. Yee, J.P.M.G. Linnartz and G. Fettweis, "MultiCarrier CDMA in indoor wireless Radio Networks", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 109–113 (1993: first paper proposing the system and the name MCCDMA)
 K. Fazel and L. Papke, "On the performance of convolutionallycoded CDMA/OFDM for mobile communication system", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 468–472
 A. Chouly, A. Brajal, and S. Jourdan, "Orthogonal multicarrier techniques applied to direct sequence spread spectrum CDMA systems," in Proceedings of Global Telecommunications Conference (GLOBECOM'93), pp. 1723–1728, Houston, Tex, USA, November 1993.
 N.Yee, J.P.M.G. Linnartz and G. Fettweis, "MultiCarrierCDMA in indoor wireless networks", IEICE Transaction on Communications, Japan, Vol. E77B, No. 7, July 1994, pp. 900–904.
 J.P.M.G. Linnartz, "Performance Analysis of Synchronous MCCDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 1375–1387. PDF
 K. Fazel and S. Kaiser, MultiCarrier and Spread Spectrum Systems: From OFDM and MCCDMA to LTE and WiMAX, 2nd Edition, John Wiley & Sons, 2008, ISBN 9780470998212.
 Hughes Software Systems, Multi Carrier Code Division Multiple Access, March 2002.
 German Aerospace Center, Institute of Communications and Navigation, History of MultiCarrier Code Division Multiple Access (MCCDMA) and MultiCarrier Spread Spectrum Workshop, November 2006.
 Wireless Communication Reference Web Site, section about MCCDMA, 2001.
See also[edit]
 OFDMA, an alternative multiple access scheme for OFDM systems, where the signals of different users are separated in the frequency domain by allocating different subcarriers to different users.


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