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, 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. di is multiplied by each ci and the resulting vector is placed on Nfreq subcarriers, where Nfreq 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, assume the PN code chips consist of 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. 2-dimensional spreading in both the frequency and time domains is also possible, and a scheme that uses 2-D spreading isReferences
Literature
* 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 * 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, "Multi-Carrier-CDMA in indoor wireless networks", IEICE Transaction on Communications, Japan, Vol. E77-B, No. 7, July 1994, pp. 900–904. * J.P.M.G. Linnartz, "Performance Analysis of Synchronous MC-CDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 1375–1387See 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. {{Channel access methods