Beamforming with Imperfect CSI

With channel state information (CSI) at the transmitter, beamforming can be used for spatial diversity and multiple spatial access. Due to latency and feedback bandwidth limitation, the CSI at the transmitter is often known with some ambiguity. In this paper, we develop a robust method for downlink beamforming that takes the ambiguity of the CSI into consideration. It is shown by computer simulation that, compared with the existing method, the required signal-to-noise ratio (SNR) for a 1% bit-error rate (BER) is reduced by over 2 dB for a system with 4 transmit antennas and 2 users when the variance of the CSI is -20 dB. The performance gain increases with the number of transmit antennas when the number of users is fixed. The required SNR for a 1% BER is reduced by over 4 dB if the the number of transmitter antennas is 8. We also study the impact of power allocation on the downlink beamforming.