Adaptive modulation in time and space for space time codes

Existing space time codes (STC) for multi-input and multi-output (MIMO) systems employ fixed modulation in time and the same modulation in space at all transmit (TX) antenna elements. Even existing adaptive modulation schemes use the same modulation in space at all TX elements. However, in practice, the fading channel coefficients are typically independent in both time and space. This is the motivation for this study, and the objective of this paper is to present a novel and simple STC trellis structure so that STC MIMO systems can employ adaptive modulation in both time and space at each TX element. The goal of our study is an improved spectral efficiency in bits/s/Hz. As typical adaptive modulation schemes, this paper assumes that channel state information (CSI) is available at each TX element. But, this paper considers a simple case, i.e., "good", "fair" or "bad" CSI, the corresponding modulation like 8PSK, QPSK or BPSK, and two TX and one receive (RX) element systems. Then, this paper verifies through simulation that the proposed scheme can improve spectral efficiency.