On the robustness of space-time coding

Space-time (ST) coding has emerged as one of the most promising technologies for meeting the challenges imposed by the wireless channel. This technology is primarily concerned with two-dimensional (2-D) signal design for multitransmit antenna wireless systems. Despite the progress in ST coding, several important questions remain unanswered. In a practical multiuser setting, one would expect different users to experience different channel conditions. This motivates the design of robust ST codes that exhibit satisfactory performance in various environments. In this paper, we investigate the robustness of ST codes in line-of-sight and correlated Rayleigh fading channels. We develop the design criteria that govern the performance of ST codes in these environments. Our analysis demonstrates that full-diversity ST codes are essential to achieving satisfactory performance in line-of-sight channels. We further show that a simple phase randomization approach achieves significant performance gains in the line-of-sight case without affecting the performance in Rayleigh fading channels. In the correlated fading environments, we characterize the achievable diversity order based on the number of diversity degrees of freedom in the channel. This characterization supports experimental observations that suggest that the quasistatic model is not a worst-case scenario and establishes the necessary tradeoff between the transmission rate and performance robustness. Finally, we consider the design of ST codes using some prior knowledge about the channel spatio-temporal correlation function.