an apparatus and method for transmitting data on a plurality of frequency sub - channels for a plurality of successive symbol periods through a plurality of transmit antennas in an ofdm system . in the data transmitting apparatus , an s / p converter converts information symbols received from a data source to an information symbol vector , a coder generates at least one code symbol vector using the information symbol vector in at least one symbol period , an ifft unit generates at least as many transmission signal vectors as twice the number of code symbol vectors generated in the coder for the one symbol period , a plurality of p / s converters each convert the transmission signal vectors to a transmission signal stream , and a guard interval inserter inserts a guard interval into each of the transmission signal streams received from the p / s converters .