Optimal switching thresholds for space-time block coded rate-adaptive M-QAM

Channel state information (CSI) at the transmitter can be used to adapt transmission rate or antenna gains to time-varying fading channels in multi-antenna systems. However, the CSI that is fed back will get outdated if the channel is changing rapidly. We have developed a rate-adaptive M-QAM technique equipped with space-time block coding for multi-input multi-output (MIMO) systems and imperfect signal to noise ratio (SNR) feedback as opposed to the more complex full channel feedback in Ko et al. (2003). Based on the closed-form expressions for the average BER and average throughput derived in Ko et al, we derive herein optimal switching thresholds which maximize the average throughput under average BER constraint for MIMO fading channels. Our numerical results illustrate the immunity of our optimal thresholds to error due to time-delayed feedback.