Adaptive modulation for multi-antenna transmissions with channel mean feedback

Adaptive modulation has the potential to increase the system throughput significantly by matching transmitter parameters to time-varying channel conditions. However, adaptive modulation assuming perfect channel state information (CSI) is sensitive to CSI imperfections induced by estimation errors and feedback delays. In this paper, we design adaptive modulation schemes for multi-antenna transmissions based on partial CSI, that models the spatial fading channels as Gaussian random variables with non-zero mean and white covariance, conditioned on feedback information. Based on a two-directional beamformer, our proposed transmitter optimally adapts the beam directions, the power allocation between two beams, and the signal constellation, to maximize the transmission rate while maintaining a target bit error rate (BER). Numerical results demonstrate the rate improvement, and illustrate an interesting tradeoff that emerges between feedback quality and hardware complexity.