Space-time diversity systems based on unitary constellation-rotating precoders

We present a unified approach to constructing linear space-time (ST) block codes based on unitary constellation-rotating (ST-CR) precoders. We show that with an arbitrary number of M -transmit and N -receive antennas, ST-CR precoders achieve 1 symbol/sec rate and enjoy maximum diversity gainMN over both quasi-static and fast fading channels. We also compare real with complex rotations to delineate the tradeoff between performance and complexity. Based on a simplified decoder, we study diversity and coding gains as well as information-theoretic aspects of the proposed ST-CR scheme. Compared with ST orthogonally designed (ST-OD) codes, ST-CR precoding provides larger coding gain and maximum mutual information. Though ST-OD codes afford simpler decoding, the tradeoff between performance and rate versus complexity favors the ST-CR codes when M , N or the spectral efficiency of the system increase.