On optimal constellations for quasi-orthogonal space-time codes

Space-time block codes (STBC) exploit multiple-input multiple-output (MIMO) communication systems in order to obtain diversity for high link reliability. Unfortunately, it is not possible to construct STBC with transmission rate equal to one for more than two transmit antennas. A way to achieve higher transmission rates is to use quasi-orthogonal STBC (QSTBC). We can improve the performance of QSTBC through a special constellation rotation. We derive the optimal rotation analytically. Numerical simulations show that our analytical approach outperforms the regular QSTBC as well as approaches where a simulative method is used in order to obtain better performance in comparison to regular QSTBC. In addition, we study the impact of the rotation on the achievable portion of the outage capacity.