Robust Transmit Beamforming for Frequency-Selective Fading Channels with Imperfect Channel Feedback

In this paper, robust transmit beamforming (BF) for transmission over frequency-selective fading channels, taking into account the availability of imperfect channel state information at the transmitter (CSIT), is considered. In a realistic scenario, the CSIT differs from the true channel. The CSIT may be outdated due to channel variations or could be erroneous due to quantization effects and/or transmission errors. Transmit BF with reliable CSIT can improve the system performance significantly. On the other hand, pure space-time coding outperforms transmit BF if the CSIT is unreliable. The optimization criterion of the proposed robust BF scheme takes the reliability of the available CSIT in an optimum way into account. We show that the performance of the proposed robust BF scheme is upper and lower bounded by that of transmit BF with perfect CSIT and generalized delay diversity coding without instantaneous CSIT, respectively.