Linear precoding over time-varying channels in TDD systems

Linear transmit filters depend on current channel state information, which is available from the uplink channel estimation in time division duplex systems. For multiple antenna elements at the transmitter, we illustrate the influence of out-dated channel estimates on link level performance for symmetric and asymmetric traffic, comparing the transmit matched, zero-forcing, and Wiener filters. A Wiener predictor is proposed to improve channel knowledge of the transmitter. We observe an inherent robustness of the transmit matched filter and explain the occurrence of a minimum in the bit error ratio at a finite SNR for the transmit Wiener filter.