1. A transmitting apparatus for transmitting a data stream in a multiple antenna system, the transmitting apparatus comprising: a plurality of quantization codebooks corresponding to a number of data streams and formed by quantizing channel information between a transmitting antenna and a receiving antenna; a modulator for modulating at least one selected data stream based on an index of a stream mode in feedback information transmitted from a receiving apparatus (here, the stream mode is defined to determine a modulation method applied to each data stream); and a quantization precoder for precoding the modulated data stream based on a codeword index in the feedback information transmitted from the receiving apparatus among the quantization codebook corresponding to the stream mode of the index, and transmitting the precoded data stream to the receiving apparatus.