Multi-carrier code division multiple access

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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. By spreading the data over a much wider bandwidth (in hertz) than the data rate (in bit/s), a signal-to-noise and interference ratio of less than 0 dB is feasible.

MC-CDMA applies DS-CDMA spreading in the frequency domain, where each user is assigned an individual pseudo-noise code to differentiate the signal from other users' signals.

On MC-CDMA, the data symbols are spread by multiplying them by the 'chips' of a pseudo-noise code (PN code). A number of 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 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, if the PN code chips consist of {1, -1} and the data symbol on the subcarrier is -j, then 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.

As an example of how the 2D spreading on VSF-OFCDM works, if you take the first data symbol, d₀, 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, 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 and 1 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 1 Gbps transmissions to receivers travelling at 20 km/h and 2.5 Gbps to stationary receivers using its 4G prototype system in a 100 MHz-wide channel. This 4G prototype system also uses a 4x4 antenna MIMO configuration, and turbo coding for error correction coding.

[edit] References

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

• N. Yee, J.P.M.G. Linnartz and G. Fettweis, "Multi-Carrier 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 MC-CDMA)

• K. Fazel and L. Papke, "On the performance of convolutionally-coded CDMA/OFDM for mobile communication system", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 468-472

• K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems, John Wiley & Sons, 2003, ISBN 0-470-84899-5

• 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 Spectrum Workshop, November 2006.

[edit] 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.