Recently, multiple-input multiple-output (MIMO) systems that realize high bit rate data transmission with multiple antennas at both transmitter and receiver have drawn much attention for their high spectral efficiency. In MIMO systems, space division multiplexing (SDM) has been researched widely. In SDM, the input data symbols are transmitted from multiple transmit antennas at the transmitter, and the output data symbols are extracted by the signal processing at the receiver. In recent wireless communications, the environments that the number of transmit antennas is larger than that of receive antennas often exist. Under such environments, the MIMO system that transmits independent data streams from each transmit antenna simultaneously cannot separate the received signals, and the signal quality deteriorates largely. Therefore, we need the scheme that attains high quality and high throughput data transmission under such environments. In this paper, we propose a throughput maximization transmission control scheme for MIMO systems. The proposed transmission control scheme selects a transmission scheme (a set of transmit antennas, modulation schemes, and coding rates) with maximum throughput based on output signal to interference and noise ratio (SINR) and output signal to noise ratio (SNR). We show that the proposed transmission control scheme attains high throughput by our computer simulation.

Space-time block coded orthogonal frequency division multiplexing (ST-OFDM) has been proposed as an attractive solution for a high bit rate data transmission in a multipath fading environment. Space-frequency block coded OFDM (SF-OFDM) has been also proposed as another solution. These two systems utilize STBC with a 22 transmission matrix, using two transmit antennas. In ST-OFDM the block codes are formed over the space-time domains. In SF-OFDM the block codes are formed over the space-frequency domains. If we apply STBC with a 44 transmission matrix to OFDM, using four transmit antennas, we can expect the performance improvement. However, when the block codes are formed over space-time (frequency) domains with four transmit antennas, the conditions of the orthogonality become more strict. We can expect that if the block codes are formed over space-time-frequency domains with four transmit antennas, that is, if we implement space-time-frequency block coded OFDM (STF-OFDM), the condition of the orthogonality is more relaxed. In this paper, we apply STBC with a 44 transmission matrix to OFDM and propose STF-OFDM. We evaluate the performance of the three types of systems (ST-OFDM, SF-OFDM, STF-OFDM). We show that the best system with respect to the error rate performance differs in the different channel conditions. When the effect of the Doppler spread is large and the effect of the delay spread is small, SF-OFDM has the best error rate performance, and STF-OFDM and ST-OFDM follow in order. When the effect of the delay spread is large and the effect of the Doppler spread is small, ST-OFDM has the best error rate performance, and STF-OFDM and SF-OFDM follow in order. We also show that STF-OFDM is attractive in wireless communications. STF-OFDM is more tolerant than ST-OFDM with respect to the Doppler spread and SF-OFDM with respect to the delay spread, respectively.