We analyze wireless communication systems employing multiple-transmit multiple-receive antennas in this thesis. Space-time codes have been proposed in the literature as an efficient means of coding over multiple transmit antennas. In particular, the rank and the determinant of code difference matrices have been shown to be important in the design of space time codes in quasi-static fading channels. In this thesis, we investigate the problem of maximizing the coding gain given by the minimum of the determinants of the code difference matrices. We show that equal singular values of the code difference matrices is a necessary and sufficient condition for obtaining optimal coding gain. We also show that equal singular values lead to robust codes. Finally, we present the construction of enhanced dimensional trellis coded modulation (EDTCM) codes based on equal singular values.