An apparatus for transmitting a signal in a communication system includes: MT number of transmission antennas; a space-time encoder for generating MT number of transmission symbol streams by space-time encoding MT number of modulation symbol streams in accordance with a space-time encoding scheme determined by a predetermined control, and transmitting each of the MT transmission symbol streams through a corresponding transmission antenna from among the MT transmission antennas; and a controller for determining the space-time encoding scheme based on an iteration number of transmission, which indicates the number of times by which an information data bit stream corresponding to the MT modulation symbol streams has been transmitted.