On Asymptotic BER Performance of the Optimal Spatial-Temporal Power Adaptation Scheme with Imperfect CSI

This paper studies the asymptotic bit-error-rate (BER) performance of the joint spatio-temporal (S-T) power adaptation which has recently been proposed for beamforming space-time-coded multiantenna systems in the presence of imperfect channel state information at the transmitter (CSIT). We derive a closed-form asymptotic solution of the power adaptation, which is instrumental in characterizing the asymptotic BER performance. We show that with imperfect CSIT, the optimal S-T scheme gives exponential diversity gain in the low and medium SNR regions when the quality of CSIT is relatively good, but cannot increase the diversity order of the space-time-coded MIMO link at very high SNR. Compared to the previous design by treating imperfect CSIT as perfect to adjust the temporal power, the optimal S-T is more robust in retaining the exponential decrease in BER over a larger range of SNR. The results sharpen our understanding of the behavior of the optimal S-T strategy.