In this paper, the authors have first given brief introduction to Multiple Inputs Multiple Outputs (MIMO) systems. After that different MIMO models and their capacity is discussed. They have compared bit error rate of different modulation techniques like Phase Shift Keying (PSK), Quadrature Amplitude Modulation (QAM) i.e. QAM 16, QAM 32, QAM 64 using Space Time Trellis Coding (STTC). The outage capacity and bit error rate of MIMO and MISO have also been compared. The Rank criterion is used for maximizing the rank of transmitting antennas matrix in STTC. The proposed technique increases spatial diversity and coding gain of MIMO channels.