JPL's Wireless Communication Reference WebsiteChapter: Analog and Digital Transmission

This scheme was first proposed at PIMRC '93 in Yokohama by Linnartz, Yee (U. of California at Berkeley) and Fettweis (Teknekron, Berkeley, currently at U. of Dresden, Germany). Independently, Fazal and Papke proposed a similar system. Linnartz and Yee showed that MCCDMA signals can also be detected with fairly simple receiver structures, using an FFT and a variable gain diversity combiner, in which the gain of each branch is controlled only by the channel attenuation at that subcarrier. At PIMRC '94 in The Hague, optimum gain control functions were presented. Results showed that a fully loaded MCCDMA system, i.e., one in which the number of users equals the spread factor, can operate in a highly time dispersive channel with satisfactory bit error rate. These results appeared in contrast to the behaviour of a fully loaded DSCDMA link that typically does not work satisfactorily with large time dispersion.
Since 1993, MCCDMA rapidly has become a topic of research. At the keynote address of the ISSSTA conference 1996, Prof. Hamid Aghvami predicted that the hottest topic in spreadspectrum, viz. multicarrier cdma, would attract 80% of the research by 1997. Around 2000, we see that MCCDMA has attracted tremendous attention, with entire conference sessions devoted to this. McCDMA is praised as a modulation solution that merges the insights due to Shannon (particularly those relating to CDMA) with insights due to Fourier (particularly those explaining why OFDM has advantages in a dispersive channel).
The MCCDMA method described here is NOT the same as DSCDMA using multiple carriers. In the latter system the spread factor per subcarrier can be smaller than with conventional DSCDMA. Such a scheme is sometimes called MCDSCDMA. This does not use the special OFDMlike waveforms to ensure dense spacing of overlapping, yet orthogonal subcarriers. MCDSCDMA has advantages over DSCDMA as it is easier to synchronize to this type of signals.
Figure: possible implementation of an MultiCarrier spreadspectrum transmitter. 
MCCode Division Multiple Access systems allow simultaneous transmission of several such user signals on the same set of subcarriers. In the downlink multiplexer, this can be implemented using an Inverse FFT and a Code Matrix.
Figure: FFT implementation of an MCCDMA base station multiplexer and transmitter.
The above transmitter can also be implemented as a DirectSequence CDMA transmitter, i.e., one in which the user signal is multiplied by a fast code sequence. However, the new code sequence is the Discrete Fourier Transform of a binary, say, Walsh Hadamard code sequence, so it has complex values.
Figure: Alternative implementation of a MultiCarrier spreadspectrum transmitter, using the Direct sequence principle.
The first paper on MultiCarrier CDMA appeared in 1993:
[93C4] 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. 109113. PDF 2.8MB  PDF LInnartz PIMRC 93 MC CDMA
N. Yee and J.P.M.G. Linnartz, "MultiCarrier in an indoor wireless radio channel", Memorandum UCB/ERL M94/6, U.C. Berkeley, 1994. http://www.eecs.berkeley.edu/Pubs/TechRpts/1994/ERL946.pdf (UCB bib data) (The original U.C. Berkeley Technical report)
[94C10] PS PDF PDF2 N. Yee and J.P.M.G. Linnartz, "Wiener filtering for MultiCarrier CDMA", IEEE / ICCC conference on Personal Indoor Mobile Radio Communications (PIMRC) and Wireless Computer Networks (WCN), The Hague, September 1923, 1994, Vol. 4, pp. 13441347.
PDF 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 13751387.