Distance-optimized space-time trellis codes

We propose methods for improving the performance of the space-time trellis codes for static or slow fading wireless channels. We use the 8-PSK symbol constellation with 8, 16 and 32-state trellis structure introduced by Throkh, Seshadri and Calderbank (1998), subject to the same power, bandwidth and trellis complexity for a communication system with two transmit antennas and one receive antenna. The modification to be done at the transmitter is either to rotate the constellation points in one of the transmit antenna relative to the other by some angle, or suitably divide (unequally) the total power between the two transmit antennas. We optimize the angle of rotation and power division by maximizing the d_free of the trellis codes. Simulation results show that, for AWGN channels, frame error rate performance can be improved by doing either rotation or power division at the transmitter.