Multiuser space-time/frequency code design

A significant body of results on space-time and space-frequency coding for single-user channels is available in the literature. In contrast, space-time/frequency coding for multiple-access channels (MACs) seems largely unexplored. Building on the framework in Gallager, IEEE Trans. IT, 1985 for characterizing the dominant error event regions in single-antenna additive white Gaussian noise (AWGN) MACs, we derive rate-dependent space-time/frequency code design criteria for fading multiantenna MACs with perfect channel state information at the receiver. It is demonstrated that, depending on the transmission rate tuple, joint designs taking the presence of multiple users explicitly into account may be necessary. Our results furthermore allow to identify the rate regions where, for each user, employing codes designed for the single-user case is optimal. Finally, we show that the number of receive antennas has a significant impact on the dominant error event regions and hence, plays an important role in the code design criteria. As a byproduct of our analysis, we find that the classical code design criteria (based on pan-wise error probabilities) are recovered using a completely different approach aimed at minimizing the probability of encountering a bad effective channel realization.