Post-FFT Beamforming in a Pilot-Assisted MIMO-OFDM System

It has been shown that by employing beamforming in the receiver of a Single-Input Multiple-Output-orthogonal frequency division multiplexing (SIMO-OFDM) system its performance has been greatly improved by suppressing co-channel interferences and mitigating fading effects of the channel. To get even greater performance applying a similar beamforming technique to the transmitter side of a Multiple-Input Multiple-Output (MIMO)-OFDM system was further employed. However, while in SIMO-OFDM systems both blind and reference-based methods have been investigated, in MIMO-OFDM systems only algorithms based on blind methods have been proposed so far. In this article we develop a reference-based joint transmit–receive beamforming technique, based on pilot symbols, in a MIMO-OFDM system. Post-FFT scheme is used in both sides as the beamforming method. Appropriate adaptive algorithms are developed to obtain the joint optimal beamforming weights in the receiver by minimizing the error signal between the estimated pilot symbols and their actual values. Receiver weights are used locally whereas transmitter weights are sent to the transmitter through a low-rate feedback channel similar to blind methods. The effect of various factors on the performance of the proposed beamforming structure is considered and it is shown that the joint transmit–receive beamforming is more effective than the sole receive beamforming although the improvement due to transmit beamforming is limited by the total transmit power constraint.