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 {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. 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, 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 and 1 will then be multiplied by the length-4 time-domain PN code and transmitted on OFDM symbols 0, 1, 2 and 3. NTT DoCoMo has already achieved 5Gbit/s transmissions to receivers travelling at 10 km/h using its 4G prototype system in a 100 MHz-wide channel. This 4G prototype system also uses a 12×12 antenna MIMO configuration, and turbo coding for error correction coding. Summary OFDMA with frequency spreading (MC-CDMA) OFDMA with time spreading (MC-DS-CDMA and MT-CDMA) OFDMA with both time and frequency spreading (Orthogonal Frequency Code Division Multiple Access(OFCDMA)) References 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