Efficient feedback signaling for adaptive space-time transmit diversity in mobile fading channels

We investigate the impact of feedback signaling on an adaptive space-time transmit diversity (STTD) scheme for wireless mobile communication systems. 4 transmit antennas are considered with 2 space-time block encoders, where at least 2 receive antennas are required for detection. In adaptive STTD systems, transmit power weights are adaptively allocated to all antennas, based on the feedback information. Using the same feedback information, asymmetric modulation methods can then be applied to each encoder in accordance with antenna weights. We show that depending on the number of feedback bits available, the performance of adaptive STTD with asymmetric modulations varies, while the same system with symmetric modulations has little gain by variable feedback bits. The optimum weights are determined for adaptation by numerical search, and are different according to the degree of feedback loading. Simulation results show that in adaptive STTD with asymmetric modulations the number of feedback bits can be reduced without performance degradation.